(Methyl)Mercury, Arsenic, and Lead Contaminationof the World’s Largest Wastewater Irrigation System:the Mezquital Valley (Hidalgo State—Mexico)

Stéphane Guédron & Céline Duwig & Blanca Lucia Prado & David Point &Marizol Giovana Flores & Christina Siebe

Received: 21 February 2014 /Accepted: 18 June 2014 /Published online: 26 July 2014# Springer International Publishing Switzerland 2014

Abstract In the Mezquital valley, untreated wastewater(45m3 s−1) fromMexico City is used for the irrigation ofaround 900 km2 of agricultural soil. High concentrationsof metals including methylmercury (3.8±2.5 ng l−1) andlead (0.16±0.05 mg l−1) were measured in anoxicwastewater canals. Downstream, dissolved, and partic-ulate polymetallic (Hg, Pb, Cr…) concentrations de-creased by factors 10 to 1,000 in the Tula River (whichreceived a mix of fresh and wastewater) due to thedilution and oxidation of surface water, and to the

decrease of contaminants concentration in wastewaterdownstream irrigated soils. However, dissolved andparticulate methylmercury concentrations (0.06 to0.33 ng l−1 and 1.6 to 4.5 μg kg−1, respectively)remained elevated in comparison to other naturalhydrosystems. The monitoring of an irrigation eventand the distribution of metals in a soil profile irrigatedfor more than 80 years showed that metals were retainedin the draining tilled layer. The oxic conditions andslightly acidic pH (~6.5) in this layer were found favor-able for metal adsorption and co-precipitation withredox-sensitive elements (Fe, Mn) and suggestively formercury demethylation. In the downstream Tula Riverand groundwater, almost all metallic concentrationsremained below guideline thresholds. Only, dissolvedAs and Pb concentrations remained two to five timesabove thresholds for drinking water, highlighting a po-tential health risk for approximately 500,000 peoplewho use groundwater as water supply.

Keywords Wastewater irrigation .Metal fractions .

Bioavailability . Soil . Groundwater

1 Introduction

High costs of water treatment force many countries inthe developing world to reuse untreated or only partiallytreated effluents (Qadir and Scott 2010). It is thereforecommon that each region, decides on the affordabletreatment in combination with water management op-tions to allow minimizing the health risks in each

Water Air Soil Pollut (2014) 225:2045DOI 10.1007/s11270-014-2045-3

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11270-014-2045-3) contains supplementarymaterial, which is available to authorized users.

S. Guédron (*)ISTerre, Université Grenoble 1, IRD—UMR 5275 (IRD/UJF/CNRS), BP 53, 38041 Grenoble, Francee-mail: stephane.guedron@ird.fr

C. DuwigLTHE, IRD—Université Grenoble 1, UMR 5564 (IRD/UJF/CNRS), BP 53, 38041 Grenoble, France

B. L. Prado : C. SiebeInstituto de Geología, Universidad Nacional Autónoma deMéxico, Cd. Universitaria, D.F., CP 04510 Mexico,Mexico City, Mexico

D. PointGéoscience Environement Toulouse—Université PaulSabbatier, UMRGET, 14Av. Edouard Belin, 31400 Toulouse,France

S. Guédron :D. Point :M. G. FloresLaboratorio de Calidad Ambiental, Universidad Mayor deSan Andrés, Cota Cota, La Paz, Bolivia

particular case (WHO 2006). Thus, the use of sewageeffluents for irrigating agricultural land is less expensiveand is expanding worldwide (Drechsel and Evans 2010;Feigin et al. 1991) in arid and semi-arid regions. How-ever, pollutants and pathogens contained in the waste-water pose a risk to crop and groundwater consumers,and surface water systems are often contaminated eitherby direct or indirect discharge.

Among the pollutants that pose a risk to humanhealth in wastewater irrigation systems are severalmetals and metalloids, discharged by multiple industrialand urban sources into the sewer system and also incor-porated by surface runoff from streets (Jamali et al.2007; Oliveira et al. 2007; Qishlaqi et al. 2008; Zhuet al. 2012). The sewage also contains large organicmatter (OM) contents, which provides rapid oxygenconsumption and may favor the biologically mediatedinterconversion of many contaminants (i.e., Hg, As, Cr)into their highly toxic reduced or organo-metallic forms.Indeed, some contaminants can be biotically or abioti-cally reduced (As, Cr, Mn, Sb) (Huang et al. 2009;Vaiopoulou and Gikas 2012; Yang et al. 2007), whileothers like Hg can be methylated. Recent publicationsshowed that the highly neurotoxic methylated form ofHg (i.e., monomethyl–mercury—MMHg) that canbioaccumulate and biomagnify in food webs (Boudou2002; Ponce and Bloom 1991; Watras 1992; Masonet al. 1995), was produced in very high concentrationsinside a waste water treatment plant (0.7–17 ng l−1)(Gbondo-Tugbawa et al. 2010) or in surface waters ofurban watersheds (Balogh and Nollet 2008; Baloghet al. 2005; Mason and Sullivan 1998).

In the Mexico basin, water resource management is amajor issue, since it concentrates 18 % of Mexicanpopulation (>112 million inhabitants) and 45 % of in-dustrial activities (INEGI 2013; Ruffat 2006). TheMezquital valley is the largest example of urban waste-water reuse for agricultural irrigation in the world(Gallegos et al. 1999; Lucho-Constantino et al. 2005),where 90 % of untreated wastewater from Mexico Cityis used to irrigate around 900 km2 of highly productiveagricultural land (Jiménez and Chávez 2002). Someareas have been irrigated for more than 100 years. Thewastewater flux (45 m3 s−1) represents more than 13times the natural aquifer recharge, leading to the in-crease of the Tula River discharge (Jiménez et al.2000). In parallel, the increased groundwater rechargehas created a new, shallow aquifer, which constitutes themain source of water for the approximately 500,000

people in the valley as well as for industrial use (DFID1998). The Mexican water commission also proposes touse this groundwater as a new source for the drinkingwater supply of Mexico City (Jiménez and Chavez2004).

The use of wastewater for irrigation has increasedmaize production by 150 % (Jiménez and Chávez2001), but the polymetallic (Pb, Cd, Cr, Cu) contam-ination of soils of the Mezquital valley has beenhighlighted by several authors (Herre et al. 2004;Lucho-Constantino et al. 2005; Siebe 1995; Siebeand Fischer 1996; Siebe-Grabach 1994). Althoughsoils are well documented for total concentrations ofmetals in surface samples, only little information isavailable concerning distribution and speciation ofmetals and metalloids between dissolved and particu-late phases in the water resources (i.e., the irrigationnetwork, the surface and groundwater). Indeed, mostof the chemical analyses were performed on unfil-tered wastewaters from Mexico City and evidencedhigh heavy metal (Cd, Cr, Cu, Pb) and B concentra-tions (Carrillo and Cajuste 1995; Jiménez and Chavez2004; Lucho-Constantino et al. 2005). However, noinformation exists on the behavior of the differentaqueous species, particularly for Hg.

The general objective of this study was to assess thestate of contamination and the fate of metals (Hg, Pb,Cr) and metalloids (As) from the raw wastewater canalsto the hydrographic network and irrigated soils of theMezquital valley. A special emphasis was placed on Hg(i.e., total Hg and MMHg) and As which are poorlydocumented and may present a major threat for localpopulation and cattle via local crops or groundwaterconsumption. At first, a regional sampling of the hydro-graphic network was performed from the outlet of thetwomain wastewater canals (Emisor Profundo and GranCanal del Desagüe) to the River Tula and severalgroundwater re-emergences to measure the change inspeciation and distribution of contaminants. Second, wemonitored a wastewater irrigation event in an agricul-tural field (irrigated with wastewater for more than80 years), to study the changes in speciation of dissolvedmetals with redox variations in surface and soil porewaters and the amount of contaminants leached duringthe irrigation period. Finally, a discussion was made onthe implications of the use of wastewater for irrigationon water and crop quality and the possibility to reuse theMezquital recharged shallow aquifer as drinking watersupply.

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2 Sampling Sites, Material, and Methods

2.1 Environmental Settings and Sampling Strategy

The Mezquital valley is located in the Hidalgo stateapproximately 80 km North of Mexico City (Fig. 1).The Tula River is the main stream with a mean annualflow of 12.7 m3 s−1 (Jiménez and Chavez 2004). Themean temperature is 16±7 °C with an average annualrainfall at elevations around 2,000 m of 578mm (Friedelet al. 2000). This valley is a large agricultural productionarea where approximately 900 km2 are irrigated withwastewater (Jiménez et al. 2000). Irrigation is doneapproximately every 35 days by flooding soil surfacewith 150 to 200 mm water column. The annual waterapplication rates vary between 1,500 and 2,000 mmaccording to crop requirements, soil textures, depths,and water availability throughout the year (Gutiérrez-Ruíz et al. 1994).

Various locations in the Mezquital valley water-shed, in particular along the hydrological network,were sampled for sediments, surface water, andgroundwater (sampled in re-emergence and wellsduring May 4 and 5, 2011). These sites werechosen as a function of the topography and waste-water storage and distribution areas (Fig. 1). Alongthe irrigation network, five sites were sampled fordissolved elements, suspended solids, and surfacesediments: (1) the El Salto Station (ESS—N19°56′52.9″, W99°17′44.4″, 2130 m a.s.l.) which is theoutlet of the underground main drainage canal ofthe basin of Mexico; (2) the outlet of the Endhódam (EDO—N20°09 ′24.2″ , W099°21 ′30.2″ ,2010 m a.s.l.) which is the largest wastewaterstorage dam used for irrigation after particle sedi-mentation; (3) the irrigation canal at Las Palmas(LPC N20°07′50″, W099°12′31″) which is abranch of the Gran Canal del Desagüe used forirrigation of the Eastern agricultural district includ-ing Las Palmas (LP) plot without particle sedimen-tation; (4) the Tula River downstream ESS atTezontepec (TT—N20°11′39.8″, W099°16′48.1″,1962 m a.s.l.) which is a mix between naturalwaters (~20 %), spring water produced by theartificial groundwater recharge produced throughwastewater irrigation (<1 %), and wastewater(~80 %); and (5) the Tula River at Mixquiahuala( TM – N20 ° 1 4 ′ 2 5 . 2 ″ , W 0 9 9 ° 1 3 ′ 4 9 . 5 ″ ,1921 m a.s.l.) receiving water from the Salado

River which drains the South East district intowhich the Gran Canal del Desagüe discharges.Three springs or wells were also sampled to col-lect groundwater: (1) a spring located betweenTezontepec and the Tula River (SGR—N20°11′15.2″, W099°16′46.6″, 1955 m a.s.l.) which is are-emergence of the semi-confined shallow aquiferof the Mezquital valley used for recreation andaquaculture; (2) the Cerro Colorado spring( CC S—N2 0 ° 0 7 ′ 1 3 . 3 ″ , W 0 9 9 ° 1 5 ′ 4 3 . 4 ″ ,2052 m a.s.l.) which is a re-emergence of theupper aquifer of the Mezquital valley used fordrinking water after being chlorinated; and (3)the Las Palmas aquifer (LPA—N20°07′54.20″,W099°13′49.49″) which was sampled in the wellof an inhabitant of Tlahuelilpan at around 8.5 mdepth in the upper aquifer of the Mezquital valley.

The agricultural field of Las Palmas ranch (LP),located in the irrigation District 03 (Fig. 1), irrigatedwith wastewater and cultivated for alfalfa, grass, andcorn production, was studied and sampled during anirrigation event (fromMay 2 to 3, 2011) for soil, surfacewater, and soil pore-water. Las Palmas site (LP) isrepresentative of a long term irrigated area since it hasbeen irrigated with wastewater for about 80 years with aperiodicity of irrigation of about once a month (Fig. 1).This site was compared with a site that has never re-ceived wastewater irrigation, Santiago (STG—N20°09′56.7″, W99°06′24.8″, 2126 m a.s.l.), used for cornproduction under natural rain water located in SantiagoTezontlale (STG—Fig. 1). STG site was considered asthe reference agricultural site in order to compare thegeochemical background for metals in this region. Bothsoil (LP and STG) are Haplic Phaeozems according toWRB classification (IUSS 2007); these soils are thedominant soil units in the region. A restricted set ofcrops grown on wastewater irrigated field was sampledin an adjacent field in order to evaluate the soil–planttransfer by comparison of their total concentrations withthe “bioavailability” (EDTA extraction) and leachability(MQ water extraction) of metals contained in soil.

Finally, an additional sediment sampling wasperformed at Las Palmas site to recover sedimentsfrom the upper aquifer. Samples were recoveredfrom two drillings: (1) at 24 m depth (LPA M9– N20°07′29.14″, W099°13′02.07″, 2076 m a.s.l.),and (2) at 35 and 38 m depth (LPA M24 and M25– N20°07′21.85″, W099°12′24.17″, 2093 m a.s.l.),respectively.

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2.2 Sample Collection

In the following section, all analytical procedureswere conducted using ultra clean sample handling

to avoid field and laboratory contamination ofl ow- l eve l s amp l e ex t r a c t s and a c ro s s -contamination of high-level samples (Cossa andGobeil 2000).

Fig. 1 Scheme of the wastewater transfer network from MexicoCity to the Mezquital valley. Locations of water sampling sitesalong (1) the surface network; wastewater at El Salto station(ESS), Endhó dam outlet (EDO), and Las Palmas irrigation canal(LPC), Tula river water at Tezontepec (TT) and at Mixquiahuala(TM) and (2) the groundwater sampled at re-emergences; spring

located near Tezontepec (SGR), Cerro Colorado spring (CCS), andLas Palmas Aquifer (LPA). Soil profiles were performed at LasPalmas corn field (green square—soil irrigated with waste waterfor almost 80 years) where surface and soil pore water weresampled during an irrigation event and at Santiago Tlezontlalecorn field (STG—pristine reference site irrigated with rain waters)

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2.2.1 Surface Water, Groundwater, and Total SuspendedSolids Sampling

Duplicates or triplicates were sampled at each samplingsite after being filtered with Sterivex®-HV 0.45-μmsterile filters. Aliquots for total dissolved Hg ((THg)D)and dissolved monomethylmercury ((MMHg)D) analy-sis were collected in FEP bottles, and acidified with HCl0.5 %v/v (Millipore® Seastar) (Parker and Bloom2005). Aliquots for trace metal and metalloid analysiswere filtered as described above for Hg, stored in clean15-ml polypropylene tubes and acidified with HNO3

2 %v/v (Millipore® Seastar). Dissolved organic carbon(DOC) samples were stored in Pyrex® bottles (previ-ously burned at 550 °C) and acidified (HCl 1 %v/v,Millipore Seastar). Aliquots of suspended material forthe determination of total particulate Hg ((THg)P) andtotal metals and metalloids were obtained by filtrationthrough hydrophilic Teflon® membranes (LCR,Millipore®), 0.45-μm pore size, 47-mm diameter(Guédron et al. 2011) or by freeze-drying a knownvolume of water. Total suspended solids (TSS) weredetermined gravimetrically.

2.2.2 Soil Pore-Water (Rhizons) Sampling

The study of temporal variations of Hg species, metals,and metalloids dissolved concentration in pore-waterduring the irrigation event was performed at site LPusing HCl (1 %) and Milli-Q®-rinsed micro-porous(0.45 μm) polymer tube samplers (rhizon® SMS, Rhi-zosphere Research Products) inserted close to the fieldoutlet (1) vertically into the 10 first cm of soil and (2)horizontally in a “soil pit” at 5 and 15 cm depth(Seeberg-Elverfeldt et al. 2005). Samples were thenstored as described above. It should be noted thatrhizons preferentially recover water from soilmacropores (Guedron et al. 2012; Harvey et al. 1995).During the irrigation event, surface water was also sam-pled during the first stage of flooding (irrigation) event.

2.2.3 Soil, Sediment, and Plant Sampling

Surface sediment (~0–10 cm) and deep sediment sam-ples were collected using a Teflon shovel and stored insterile polyethylene bags in a cooler at 4 °C during thesampling operations. Soil profiles were sampled system-atically every 10 cm, down to 0.8 m depth for theSantiago site and 1.1 m depth for Las Palmas site, using

an auger and collected in sterile polyethylene bags. Allsamples were homogenized on site and stored in thefridge at 4 °C until freeze drying. We could not recovermaize plants and grains on both LP and STG sampledsoils because we collected soil samples in the earlysowing season. However, we sampled courgette fruitand flower (Cucurbita pepo sp.) and stems with leavesof alfalfa (Medicago sativa sp.) in an adjacent field, twospecies used for human consumption and to feed cattle,respectively. Plants were MQ rinsed before beingfreeze-dried.

2.3 Analytical Methods

2.3.1 Water Analyses

Samples were analyzed for [(THg)D], and [(MMHg)D]by cold vapor atomic fluorescence spectrometry(CVAFS) using a Tekran® (Model 2500) mercury de-tector after conversion of all Hg species into Hg0

(Bloom and Fitzgerald 1988). The principles of themethods used are from the Bloom and Fitzgerald(1988) gold amalgamation method for (THg)D, andfrom the Filippelli et al. (1992) hydride-generationmethod modified by Cossa et al. (2009) for (MMHg)D.For the latter, the mercury hydrides (methyl and inor-ganic mercury) formed with NaBH4, were separated bycryogenic chromatography (GC separation withChromosorb W/AW-DMCS 60/80 mesh impregnatedwith 15 % OV-3) under an helium flux. The accuracyof analyses was checked using certified reference mate-rials (CRMs) ORMS-4 (22.0±1.6 ng l−1—National Re-search Council of Canada) for (THg)D, and ERM-AE670 (175.1±62 μmol l−1—IRMM—European Com-mission) for (MMHg)D. The measurement error (1SDfor triplicate measurements of replicate samples) wasusually about 10 % and always better than 15 % for[(THg)D], and [(MMHg)D], respectively. The detectionlimits (3SDblk) were 0.05 ng l−1 for (THg)D and0.01 ng l−1 for (MMHg)D.

Samples were analyzed for nine major and tracemetals (Fe, Al, Cd, Cr, Cu, Mn, Ni, Pb, and Zn) andtwo metalloid and non-metal (As and Se) by ICP-AOS(Varian 720-ES) using internal spikes and multi-elements CRMs RotiStar (28 elements, Roth®2650.1),SLRS-4, and CASS-5 (National Research Council Can-ada) for the accuracy of analysis. DOC concentrationwas determined using a Non Dispersive Infra-Red CO2

spectrometer (NDIR, Shimatzu®) after humid oxidation

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in a sodium persulfate solution at 100 °C. The accuracyof analyses was checked using CRM MISSISSIPI–03(Environment Canada). The measurement error wasalways below 10 %. Eh, pH, and electric conductivitymeasurements were determined using a multi-parameterprobe set (WTW, multi 340i) at the sampling locationused for rhizon and surface water sampling.

2.3.2 Sediment, Soil, Suspended Solid, and PlantAnalysis

Solid samples (sediment, soils, suspended solids, andplants) were freeze-dried and crushed in a planetary mill(Pulverisette 7 premium line model, Fritsch®) to obtaina size smaller than 63 μm for elemental analysis(Guédron et al. 2009). The grain size distribution wasmeasured in air dried and sieved samples (<2 mm) usingthe pipette method (Day 1965). The proportions of threemajor size classes (clay<2 μm; silt 2–63 μm; and sand63–2,000 μm), as well as the median grain size, weredetermined from size distributions. Total particulate or-ganic carbon ([TOC]) and nitrogen ([N]) concentrationswere determined from the dry combustion of soil pow-der sample aliquots using a Fisons 1500 CHNS auto-analyzer. Total particulate Hg concentrations ([(THg)P])were determined by atomic absorption spectrophotom-etry using an AMA 254 analyzer (ALTEC—CAS) fol-lowing catalytic decomposition and gold amalgamation(Guédron et al. 2009; Roos-Barraclough et al. 2002)with a relative precision of ±10 % determined fromtriplicates. Concentrations obtained for repeated analy-ses of CRMs never exceeded the published range ofconcentration (0.090±0.012 and 0.091±0.008 μg g−1

for CRMs 7002, and MESS-3, respectively). The detec-tion limit was 5 ng g−1. MMHg concentrations on solidswere determined using about 100 mg of solid samplesfirst extracted with 5 mL HNO3 (6 N) in 15-ml Falcontubes (VWR) at 60 °C during 2 h. Sample aliquots of theextraction solution were then transferred into 40-mLglass vials equipped with teflon septa filled with25 mL of MQ water, and derivatized at pH 4.0 using180 μL of a daily prepared 1 % Sodium Propylboratesolution (Sigma Aldrich). The derivatized sample tubeswere then analyzed using a purge and trap-gaschromatograph-atomic absorption spectrophotometryanalyzer (MERX System, Brooks Rand®). MMHg con-tents for all samples were quantified using the standardaddition technique. MMHg measurements traceabilityand accuracy were checked against BCR 580 certified

sediment material. MMHg concentrations measured forthis material were within 10 % of the certified values(69±4 ng Hg g−1).

Aqua regia digestion (HCl/HNO3 1/3v/v ratio for7 days at 70 °C) was performed to assess the quasi totalamount of metals (Al, Ca, Mg, Fe, Cr, Cd, As, Cu, Mn,Ni, Pb, Se, and Zn) in each sediment or soil samples andon a certified reference material, namely a channel sed-iment (BCR 320R—European Commission—Joint Re-search Centre) for QA/QC. For a given sample, diges-tions were performed on separate aliquots of dry soil(~400 mg) at a 1:100 solid-to-liquid ratio (Bloom et al.2003) to ensure the efficiency of the digestion. Milli-Qrinsed plants were digested using the same protocol withaddition of H2O2 to ensure a complete digestion. Inparallel, water extraction (MQ) was performed on byadding 40 ml of de-ionized water to a sample aliquot(Guédron et al. 2013). Samples were shaken for 2 h.EDTA extraction was performed by adding 40 ml0.05 M EDTA solution buffered at pH 7, and left foragitation for 5 h (Guédron et al. 2013). All reagents weretested to insure digestion blanks being always close orbelow detection limits of the analytical device. Theconcentration of major and trace elements was deter-mined by ICP-AES (see description above) with a rela-tive analytical error for each sample (from triplicatemeasurements) lower than 10 %.

2.3.3 Soil Hydraulic Conductivity

Bulk density was determined at the soil surface bytaking an intact soil sample of known volume anddetermining its dry weight which allows the calculationof the total porosity. Saturated hydraulic conductivitywas measured in the field with a Guelph permeameter(Model 2800K1, Soil moisture Equipment Corp.®) withthe constant head method. Both parameters were deter-mined in 12 replicates.

2.4 Statistical Treatment

Kolmogorov–Smirnov test was used to evaluate thenormality of the data.We also used non-parametric tests,the Mann–Whitney rank sum test (U test), or theKruskal–Wallis one-way analysis of variance on ranks(H test), to compare two, or more than two sets of data,respectively, and pairwise multiple comparison accord-ing to Dunn’s method to isolate the set or sets thatdiffered from the others (Webster 2001).

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Linear regressions were performed only when nor-mality test, constant variance test, and alpha power ofthe performed regression (α test, p>0.8) passed. Whendatasets did not follow a Normal distribution, we per-formed a spearman rank order correlation test (Rho test).Correlation coefficient (R) and p values are reported forthe computed linear regressions. In general, results arepresented as the mean±SD. All statistical analyses wereperformed using the package Sigmastat® (Systat Soft-ware Inc.) and a p value of 0.05 was chosen to indicatestatistical significance.

3 Results

3.1Metal(loïd) Contamination of the Irrigation Networkof the Mezquital Valley

Wastewaters sampled in the two main canals (i.e., GranCanal del Desagüe—LPC and Emisor Profundo—ESS)were anoxic (Eh=−170±110 mV) with a high particu-late (8 to 14 %) and dissolved (16 to 78 mg l−1) organicmatter content (Fig. 2) and electric conductivity (1,208±268 μS cm−1). These wastewater canals are source ofmetals since concentrations were up to 40 times higher(depending on the elements) for both dissolved andparticulate phases than those of the downstream part ofthe network (i.e., Tula River and groundwater re-emergences—Fig. 2 and S.I.1). Higher dissolved andparticulate Hg, MMHg, Pb, and Cr (also Cu, Ni, Cd,Zn—see S.I.1) concentrations were found in water orig-inating from the “open air” Gran Canal del Desagüe(LPC site) than in the underground ESS canal. Thesedissolved and particulate concentrations sharply de-creased at the outlet of the Endhó dam (EDO) in parallelwith the sharp decrease of TOC probably due to partic-ulate OM recycling and/or settlement in the dam. Down-stream the dam, along the Tula River, a subsequentdissolved and particulate concentrations decrease oc-curred when anoxic wastewaters were mixed in the Tulastream and became oxic with a gradual increase in pH(Fig. 2). The dilution with fresh water and mineralparticles from the watershed downstream the dam mayhave also contributed to the decrease in dissolved andparticulate concentrations. Finally, smaller dissolvedconcentrations were found in oxic groundwaters sam-pled at re-emergences where pH decreased around 7.

Particulate metal concentrations followed the distri-bution of TOC and showed a mirrored distribution with

TSS, with a decrease in concentration in oxic watersalong the Tula stream (Fig. 2). Metal concentrations insediment were also higher at the LPC canal than at ElSalto station (ESS) and at other sites along the Tulastream where they remained in the same range. Themain difference between upstream irrigation canal andthe Tula stream was a significant increase in the sandyfraction along the stream in parallel with a decrease inthe silty–clay fraction and TOC content. In downstreamirrigation canals, a slight increase in TSS and metalconcentrations on suspended particles and sedimentwas observed between TT and TMwhich was attributedto inputs of wastewaters from the Gran Canal delDesagüe and from eroded soil particles transported byEl Salado River which drains the large South Eastirrigation district.

Besides metals, only dissolved As concentrationsincreased significantly from wastewater canals togroundwater re-emergences (Fig. 2). Dissolved As con-centrations differed between locations of groundwaterre-emergences with lower concentrations measured atthe shallow LPA aquifer and higher at the Cerro Colo-rado spring (CCS) and the semi-confined shallow aqui-fer of the Mezquital valley (SGR). In groundwater sam-pled at re-emergences, filtered Pb concentrations alsoremained elevated (LPA, 0.094±0.005 mg l−1; CCS,0.038±0.006 mg l−1; and SGR, 0.023±0.005 mg l−1).

3.2 Dissolved Metal(loïds) in Soil Water During anIrrigation Event at Las Palmas Plot

The irrigation of the Las Palmas field (May 2 and 32011) consisted of a complete flooding of soil by awastewater column of approximately 200 mm. The flowwas permanent during~3 h (13:00 to 16:00 PM), thenthe outlet flow stopped and the water column complete-ly percolated the soil within 30 min after the closure ofthe irrigation inlet (16:00 PM). Since the sampling de-vice was installed at the outlet of the field, collection ofsamples for runoff waters started 3 h after the opening ofthe field inlet connection to the wastewater canal(10:00 AM).

During the irrigation event, pH (~7.5) of enteringanoxic wastewater (Eh<−200 mV) remained broadlyconstant with time, while an increase of around 1 pHunit occurred in pore-water (PW) during the percolationof anoxic wastewaters within the 5 h following thebeginning of the flooding. Pore-water pH and Ehreturned gradually to initial values after the end of

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irrigation with the oxidation and equilibration of waterwith soil components (Fig. 3). Fe, Cr, Pb, THg, andMMHg concentrations in the filtered PW graduallydecreased with time after the end of irrigation suggest-ing a rapid adsorption on the soil matrix and oxidation–precipitation of reduced elements (i.e. Fe, Cr, Mn—Fig. 3 and S.I.2). Indeed, the most rapid decrease in

concentration for dissolved PW elements concernedredox-sensitive elements (i.e., Fe, Al, Mn, and Cr—Fig. 3 and S.I.2), while the decrease of divalent element(i.e., Zn, Pb, Hg, Cu, Ni) concentrations was morespread. Only dissolved As and Se concentrationsremained broadly constant with time (Fig. 3 and S.I.2).This rapid decrease in concentrations simultaneously

Fig. 2 Dissolved, particulate, and sediment Fe, Cr, THg, MMHg,Pb, and As concentrations, pH, Eh, and grain size distributionalong the irrigation network from two of the wastewater (blacksymbols) emissaries (LPC: Gran Canal del Desagüe and ESS:

Emisor Profundo), the Endhó dam outlet (EDO), the Tula river(gray symbols) at Tezontepec (TT) and Mixquiahuala (TM) andgroundwater (white symbols) sampled at re-emergences (CSS andSGR) or wells (LPA)

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measured at various depths in PW (i.e., −5 and −15 cm)with rhizon samplers reflected the rapid percolation ofwastewater in surface soil horizons (i.e., 0–60 cm) dueto the high infiltration potential (soil surface porosity of50 % and saturated hydraulic conductivity of 210−5 m s−1) of these surface horizons.

3.3 Metal(Loids) in Las Palmas and Santiago SoilProfiles and Plants

The distribution of metal(loïds) onto the soil solid phaseof LP soil profile illustrated the important sorption ca-pacity of the surface horizon since all studied metals(i.e., Hg, MeHg, Cr, Pb) were concentrated in the first60-cm layer dominated by clay (58 %) size fraction (siltand sand=20 and 22 %, respectively). Then, concentra-tions dropped sharply down to 60 cm depth (Fig. 4 andS.I. 3). In comparison, the reference agricultural Santi-ago soil (STG - under natural rain water), which pre-sented a grain size distribution dominated by silt in the0–60 cm horizon (clay, silt, and sand=41, 47, and 12 %,respectively—data not shown) and major geogenic ele-ment (i.e. , Fe, Al, K, Mg, Sr, Ba, and Mn)

concentrations (Fig. 4 and S.I. 3), showed a homoge-nous distribution with depth of the studied metals withsmaller concentrations especially in the first 60 cm layer.In addition to metal distribution, Las Palmas soil profilewas characterized by a two to three times larger TOCcontent in the 0- to 60-cm layer than at the Santiago site.These soils were characterized by a great abundance ofFe, Al, and Ca being typical of Fe and Al oxides andcarbonates (Fig. 4 and S.I. 3). The fraction extracted byEDTA defined as the “bioavailable” fraction showedthat less than 5 % of Hg, Cr, and Pb were potentially“bioavailable” for plants especially in the tilled layerwhile the leachable (MQ extraction) was negligible ifquantifiable.

In opposition to metals, As concentration (2–5 μg g−1) onto the soil solid phase remained broadlyconstant with depth along the soil profile (Fig. 4) and indeeper sediment (1.5–4 μg g−1) of the Mezquital valleyupper aquifer (S.I. 4). The EDTA extracted As fractionwas lower than 1 % total As concentrations while asignificant labile fraction reaching 30 % was measuredin the LP profile with a gradual increase from 60 cmdepth to deeper layers (Fig. 4).

Fig. 3 Variations with time of pH, Eh, dissolved element (DOC,Fe, Cr, Pb, THg, MMHg, and As) concentrations, during theirrigation event of May 2, 2011 at Las Palmas site (LP), in surfacewaters (SW—inlet (empty symbols): waters from the irrigation

canal and outlet (dotted symbols): runoff water from the field)and pore-water (PW-down triangles) sampled with rhizon sam-plers at the field outlet

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Fig. 4 Soil grain size distribution, percentage total organic carbon(TOC) content, carbon–nitrogen ratio (C/N), Fe, THg,MMHg, Cr,Pb, and As concentrations obtain after aqua regia (black symbols),EDTA (gray symbols), and MQ (white symbols) extractions, forthe Las Palmas soil profile (triangles) which has been irrigatedwith wastewater for almost 80 years and for the Santiago soil

profile concentrations obtain after aqua regia (black symbols),EDTA (gray symbols), and MQ (white symbols) extractions, forthe Las Palmas soil profile (triangles) which has been irrigatedwith wastewater for almost 80 years and for Santiago soil profilethat has never received wastewater irrigation (under natural rainwater)

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4 Discussion

4.1 Wastewater: A Source for Metal(loïd)icContaminants

The large dissolved concentrationsmeasured for most ofthe studied metallic contaminants is mainly related tothe highly reducing conditions and to large DOC andTOC content in wastewater which may favor the reduc-tion of Fe and Mn oxy(hydr)oxides and their sorbed orcomplexed metals. Among dissolved metals, the moststriking contamination was evidenced for dissolvedMMHg with mean concentrations of 3.8±2.5 ng l−1 inwastewater canals reaching 20 ng l−1 in the irrigationcanal at LP site. These large dissolved MMHg concen-trations suggest that wastewater probably act as a “bio-reactor” for the genesis of organo-metallic species. Inwastewaters, filtered MMHg accounted for 10 to 20 %of (THg)D (Fig. 5) with concentrations being in thehighest range of reported concentrations for wastewatersin treatment plants (0.08–6 ng l−1) (Balogh and Nollet2008; Gbondo-Tugbawa et al. 2010; Gilmour andBloom 1995). Such MMHg concentrations are higherthan reported concentrations for surface waters con-taminated by industrial (Feng and Qiu 2008; GarcíaBravo et al. 2014; Liu et al. 2012; Nriagu and Pacyna1988; Ullrich et al. 2007) or mining activities (Fenget al. 2003; Guédron et al. 2011; Horvat et al. 2003)where (MMHg)D rarely exceed 1 ng l−1. In suchanoxic wastewaters, conditions are combined formethylmercury production since the large DOC, sul-fate, and Hg contents are favorable for the methyla-tion attributed to the activity of anaerobic bacteria(Benoit et al. 2001; Compeau and Bartha 1985;Fleming et al. 2006; Gilmour et al. 1998; Gilmouret al. 2013; Han et al. 2008; Kerin et al. 2006). Asfor the other studied metallic contaminants (i.e., Zn,Al, Cr, Pb, Cu, As, or Hg…), the concentrations offiltered MMHg was greatly correlated with DOC (R=0.88, p=0.9) and Fe (R=0.78, p=0.6) suggesting thatspeciation may be dominated by colloidal and dis-solved OM and clusters of FeS (Luther and Rickard2005) in anoxic wastewater. Despite sulfur was notanalyzed, other studies reported high dissolved sulfurconcentrations (de Velasquez et al. 1998; García et al.2006; Jiménez and Chavez 1997) and the strong andirritating smell typical of H2S during field samplingreinforce the suggestion of abundant reduced sulfurspecies in the anoxic water.

Considering the entire set of metallic and metalloidiccontaminants in wastewaters, total concentrations (dis-solved + particulate) for wastewater (Table 1) werealways below or close to Mexican guidelines for waste-water to be disposed into soils (DOF 2003).

4.2 Soil: a Geochemical Barrier for ContaminantsIncoming with Wastewater

Time series of dissolved elements in surface and soil porewater performed during the irrigation event at Las Palmassite (Fig. 3) evidenced the high sorption capacity of thesesoils rich in OM and smectite clays for divalent (i.e., Cd,Cu, Pb, Hg, Ni, and Zn) and trivalent (Cr, Al) metals(Gallegos et al. 1999; Siebe 1995; Siebe and Fischer1996; Siebe-Grabach 1994) incoming with wastewaters.Only As and Se were weakly retained. The major mech-anisms favoring dissolved wastewater metallic contami-nants retention in soils are suggestively (1) their rapidadsorption on soil OM, (2) the coagulation of metal ionswith fulvic acids (FA) (Schnitzer and Kerndorff 1981),and/or (3) the oxidative precipitation of redox-sensitiveelements probably combined with the co-precipitation ofother metals when wastewaters become oxic during wa-ter percolation in soil. Indeed, in anoxic wastewaters,dissolved redox-sensitive elements occur under their re-duced (i.e., Fe(II), Mn(II), Cr(III), As(III), Sb(III)) form.During irrigation events, when water percolates, the in-crease in redox potential to suboxic or oxic conditionswith the decrease in pH from 8 to about 6.5, might leadFe(III), Cr(III), Al, Pb, and Cu to form water-insolublecomplexes with FA, in partly hydroxylated forms, andtheir accumulation in the soil (Schnitzer and Kerndorff1981). Under such abundance of DOC and with a highionic strength, the order of elemental concentration de-crease in soil solution observed during the irrigationevent (May 2 to 3, 2011—Fig. 3), over a pH rangebetween 6 to 7, is in great accordance with the order ofeffectiveness of metal ions to precipitate FA described bySchnitzer and Kerndorff (1981) being Fe, Al, Cr > Zn,Cu, Pb > Ni, Hg > Mn (Fig. 3 and S.I. 2).

As for canals and stream waters, the great correlationfound between dissolved metals and Fe, Al, and DOCconcentrations for both field surface water and porewater are in accordance with the major role of Fe/Al,Mn oxides, and OM as carrier phases for the transport ofdissolved contaminants. A similar observation is made inthe soil profile (Fig. 4) where the vertical distribution ofmetals follow that of TOC with higher concentrations in

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the surface horizon enriched in TOC followed by a sharpdecrease in the horizon with the larger clay content downto 60 cm depth. In the surface horizon, the distribution ofmetals with depth is broadly homogeneous being typicalin tilled layers (Guédron et al. 2013). Among the studiedelements, the ratio of MMHg/THg (~0.1 %) in the tilledhorizon is one to two orders of magnitude lower thanthose measured on wastewater TSS suggesting that theoxidizing conditions related to the great drainage capac-ity might favor microbiologically mediated oxidativedemethylation process during the aerobic OM minerali-zation (Drott et al. 2008; Marvin-DiPasquale et al. 2000;Oremland et al. 1991).

In opposition to studied metals and OM, As distribu-tion in the soil profile showed a broadly constant concen-tration (2 to 5 μg g−1) with depth, remaining in the samerange as in deeper sediments (S.I.4). Such range of Asconcentration on solids is typical of theworld geochemicalAs background in soil (mean=6 μg g−1) (Mandal andSuzuki 2002), where under oxidizing conditions, arsenatesare the dominant stable As species being strongly sorbedonto clays, Fe and Mn oxides/hydroxides and OM (JainandAli 2000; Sharma and Sohn 2009;Wilson et al. 2010).

4.3 Attenuation of the Metal(loïd)ic Contaminationin the Downstream Tula River by Oxidation and SoilFiltration

In the Tula stream, As, Mn, Hg, and Pb concentrationswere above European guidelines for surface waters

(E.U. 2009) (Table 1). The sharp decrease in both dis-solved and particulate metal concentrations between thewastewater canals and the Tula stream can be attributedto four main probable causes: (1) the oxidation andsuccessive precipitation of reduced elements in the Tulastream, (2) their dilution in the Tula stream by uncon-taminated natural waters with fresh particles from pris-tine areas of the watershed (Gallegos et al. 1999), (3) theadsorption and precipitation of dissolved elements dur-ing percolation of water in irrigated soils, and (4) theparticle deposition in the Endhó dam located betweenthe outlet of the Emisor Profundo at ESS and the TulaRiver. Since dilution with fresh water dose not accountfor more than ~20 % and concentrations monitored atthe outlet of the Endhó dam were still elevated, theoxidation of the Tula surface water and the soil whichacts as a “sink” for contaminants can be considered asthe major contributor for this decrease.

First, the redox transition between anoxic wastewa-ters and the oxic Tula stream (Fig. 2), which receives amixture of fresh water and wastewater downstream theirrigation fields, certainly resulted in a change in ele-ment speciation. Indeed, while main carrier phases fordissolved elements was suggestively dominated by col-loidal and dissolved OM and clusters of FeS in anoxicwastewater (see Section 4.1), best correlations werefound between DOC and filtered metals (Cd, Fe, Mn,Ni, Pb, and Hg – Rho test, p<0.05) in the Tula Riversuggesting that under oxidizing conditions dissolvedOM takes over FeS as carrier phase for filtered metals.

Fig. 5 Dissolved MMHg/THg ratio, LOGKD for Fe, and Al, andmean DOC and dissolved THg concentration vs MMHg along theirrigation network from the wastewaters (black symbols) emissar-ies (LPC:Gran Canal del Desagüe and ESS: Emisor profundo) and

reservoir (EDO: Endhó dam outlet), the Tula river (gray symbols)at Tezontepec (TT) and Mixquiahuala (TM) and groundwater(white symbols) re-emergences (CSS and SGR) or wells (LPA)

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Therefore, following the oxidation of wastewater, asignificant part of dissolved metals might (co)-pre-cipitate with particulate OM and/or Fe, Al, and Mn(hydr)oxides after the oxidative dissolution of FeS.This suggestion is corroborated by the downstreamincrease of log partition coefficients (logKD=log[M]p− log[M]D) for Fe and Al with the dissolvedmetals decrease in concentration from EDO waste-waters to the Tula stream (Fig. 5), reflecting theiradsorption onto organic–Fe(III) complexes and/orco-precipitation on suspended solids or on neo-formed oxides. In addition, along the Tula stream,this increase in logKD for Fe, Al, and other metalsbetween TT and TM reflects the inputs of freshparticles originating from the irrigated South Eastirrigation district (including Las Palmas site) andtransported by El Salado River which flows intothe Tula upstream Mixquiahuala (Fig. 1).

Second, downstream irrigated fields, runoff watersflowing out the irrigation district and reaching the Tulastream are oxic with large TSS content but depleted ind i s so lved Fe , A l , and Mn sugges t ing the(co)precipitation of Fe, Al and Mn oxides, and thesorption of remaining dissolved metals on eroded parti-cles and their successive sedimentation. This is

evidenced by a significant decrease in particulate Fe,Al, and TOC concentration between canals and the Tulastream with an increase in Fe and Al content in thesediment compartment (Fig. 2). Larger metal concentra-tions measured on suspended particles and sediments inthe LPC irrigation canal than at ESS are suggestivelyattributed to eroded particles from the bordering irrigat-ed soils. Al and Fe concentration on suspended particlesand sediments at LPCwere also significantly larger thanat ESS being typical of the bordering Phaeozem parti-cles enriched in Fe/Al minerals. The increase of thesemetal concentrations in sediment between TT and TMalso reflects the contribution of these eroded particles inthe Tula stream downstream the confluence of Saladoand Tula Rivers.

4.4 Wastewater Irrigation: Environmental and HealthIssues: Implications for Crops Production and WaterResources

4.4.1 Implication for Agricultural Crop Production

THg, Cr, and As (Table 2) measured in crops (i.e.,Alfalfa and Courgette) grown in soil of LP were lowand within the reported average concentrations for same

Table 1 Mean metal and metalloïd total concentration (=particulate+dissolved) in wastewater irrigation canals and in the Tula stream andmean dissolved concentration in groundwater re-emergences

Concentration Emissaries (ESSand LPC)

Mexican WastewaterGuidelines

Tula river (TTand TM)

Surface waterguideline

Groundwater (LPA,CSS, and SGR)

Drinking waterguideline

Al (mg L−1) 1.82±0.03 5 0.64±0.03 0.50 0.12±0.03 0.20a

As (mg L−1) 0.013±0.007 0.2 0.026±0.013 0.025 0.017±0.007 0.010a,b

Cd (mg L−1) 0.001±0.001 0.2 <DL 0.002 <DL 0.003a

Cr (mg L−1) 0.015±0.001 1.0 0.002±0.001 0.050 <DL 0.050a,b

Cu (mg L−1) 0.038±0.002 4 0.006±0.001 0.05 0.001±0.001 2.0a,b

Mn (mg L−1) 0.37±0.01 2.0 0.13±0.01 0.25 0.008±0.001 0.4a,c

Ni (mg L−1) 0.019±0.003 2.0 0.003±0.003 0.04 0.001±0.001 0.020a

Pb (mg L−1) 0.14±0.01 0.5 0.103±0.146 0.02 0.051±0.028 0.010a,b

Se (mg L−1) 0.005±0.006 – 0.006±0.005 – 0.003±0.002 0.040a,b

Zn (mg L−1) 0.80±0.01 10 0.029±0.003 0.3 0.38±0.53 3a,c

THg (ng L−1) 363.4±18.1 10,000 100.4±24.9 50 0.60±0.21 600a/25b

N 9 4 6

N refers to the number of samples. Water guidelines for irrigation refer to Mexican guidelines for wastewater discharges into soil (DiarioOficial de la Federación 2003). Surface water guidelines refer to European guidelines (1996 and 2009)a Drinking water guidelines refer to health based guidelines given by WHO (2011) for drinking waterb Drinking water guidelines refer to health based guidelines given by bUS-EPA (2001; 2009) for drinking waterc Refers to indicative value (WHO) when no health-based guidelines exists

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species grown in this valley (Cajuste et al. 1991; Siebe1995) or in E.U. and U.S.A. (Kabata-Pendias andPendias 2001). Such relatively low total THg, Pb, Cr,and As concentrations are in accordance with previousobservations suggesting a little translocation of contam-inants to plants (Flores et al. 1997; Siebe 1994; Siebe-Grabach 1994). Only Pb concentrations in alfalfa (2.4±0.3μg g−1) were larger than reported concentrations (0.1to 0.4 μg g−1) for the same district (Siebe 1994, 1995;Siebe-Grabach 1994). Such difference can be related tothe washing protocol (HCl 3 %) used by these authorswhich have certainly removed larger sorbed or particu-late Pb from the surface than our MQ washing proce-dure. However, Hg, Cr, Pb, and As, concentrations werebelow the maximum values allowed in Dutch and Ger-man standards for cereals, forage, and fresh vegetablesas reported by Lucho-Constantino et al. (2005). Al-though we did not measure metal(loïd)s concentrationsin corn and oat, other authors (Cajuste et al. 1991; Floreset al. 1997; Solis et al. 2005) reported low concentra-tions or uptake of these metal(loïd)s in this valley. Suchconcentrations suggest that metals are weaklytranslocated in leaves, flower, and grains of these plants.Therefore, both cattle (via Alfalfa) and human (cour-gette, oat, corn…) exposure to metals via the consump-tion of crops may be low to moderate.

4.4.2 Implications for Surface Water Quality and Biota

Although metal concentrations sharply decreased in theTula stream, Pb, As, and MMHg concentrationsremained high in both dissolved or particulate phase.Especially, dissolved and particulate MMHg (0.06–0.33 ng l−1 and 1.6–4.5 μg kg−1, respectively) were inthe high range of concentrations of other hydrosystemswhere large bioaccumulation and biomagnification inbenthic organisms and fish were reported (Boudou et al.2005; Cossa et al. 2009; Mason et al. 2006). Albeit THg

concentration were far below the probable effect con-centration (PEC) for Hg (1,060 ng.g−1); the concentra-tion above which harmful effects are likely to be ob-served in sediment-dwelling organisms (MacDonaldet al. 2000), such PEC are not yet available for MMHg.In addition, the remaining significant pool ofpolymetallic contaminants (including MMHg, Cr,Pb…) in the sedimentary compartment, even if dilutedwith fresh particles, suggest a potential risk of trophictransfer and biomagnification especially in benthic or-ganisms and fish such as bottom feeder fish (Boudou2002; Mason et al. 2000; Vignati et al. 2010; Watraset al. 1998).

4.4.3 Groundwaters of the Tula Watershed:a Sustainable Source for Drinking Water?

In opposition to surface water, metal concentrationsmeasured in groundwater sampled at re-emergenceswere low except for dissolved arsenic (5 and25 μg l−1) and lead which were above WHO (2011)and USEPA (2001, 2009) guidelines for drinking water(10 μg l−1 and for As and Pb, Table 1).

Higher As concentrations measured in groundwaterthan in wastewater can result of an additional As leachingfrom irrigated soils (Fig. 4) and deeper sediments of theaquifer (~4 μg g−1, S.I. 4). Indeed, although As concen-trations measured in the LP soil and deep sedimentremained within ranges of the local and world geochem-ical background (Banning et al. 2012; Mandal andSuzuki 2002), the water extraction experiments (Fig. 4)evidence that almost 30 % of the total As pool wasleachable at LP site. Such leaching of As might also beenhanced under the measured alkaline pH (Berg et al.2001; Kim et al. 2009). Besides, the weak retention of Asobserved during the monitoring of pore-water (irrigationevent at Las Palmas) is likely due to the small adsorptioncapacity of As onto OM and/or oxy(hydr)oxides due to

Table 2 Metal, metalloid concentrations (in microgram per gram) obtained after aqua regia digestion and total mercury concentration forcourgette fruit and flower, alfalfa, and corn grown at Las Palmas site

THg Pb Cr As

Courgette fruit (Cucurbita pepo sp.) 0.011±0.001 10.1±0.5 <DL 0.88±0.03

Courgette flower (Cucurbita pepo sp.) 0.027±0.006 1.8±0.2 1.30±0.09 1.08±0.03

Alfalfa (Medicago sativa sp.) 0.025±0.005 2.4±0.3 0.03±0.11 1.26±0.19

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electrostatic repulsion and the occurrence of colloid-facilities transport (Huang et al. 2002) which is enhancedby the important amount of DOC and anionic surfactants(Friedel et al. 2000; Siebe and Cifuentes 1995) comingwith wastewaters. The abundance of DOC in wastewa-ters can also enhance competitive sorption facilitating thedesorption of geogenic As in groundwater (Simeoni et al.2003; Grafe et al. 2002). However, the origin of As ingroundwater remains unclear and needs furtherinvestigations.

In opposition to As, the very high Pb concentrationsmeasured in wastewater have resulted in concentrationsbeing four to five times above guidelines concentrationsfor drinking water (Table 1) in the semi-confined shal-low aquifer, although its retention is high in soils andsediments (Fig. 4). The great correlation between fil-tered Pb and Fe concentrations (R=0.9, P<0.01) and theabsence of correlation with DOC orMn suggests that Pbis likely complexed with colloidal Fe hydrous oxides inthe upper aquifer (Hochella et al. 2005; Hermann andNeumann-Mahlkau 1985).

Finally, besides the documented contamination ofsuperficial groundwaters of the Mezquital valley withfecal bacteria and nitrate (Gallegos et al. 1999), As andPb contamination have to be considered as a potentialthreat for local population who drink this water (Berget al. 2001; Kim et al. 2009; Polya and Charlet 2009)since both As and Pb intoxication are directly related todrinking water consumption and in a lesser extent to diet(Fitzgerald and Clarkson 1991; Kim et al. 2009; Charletet al. 2012). In addition, further ecotoxicological andmedical investigations have to be performed in ground-water and crops to evaluate the potential toxicity of themixture of contaminants, including microbiological(i.e., fecal and pathogens bacteria and total coliform)and pharmaceutical contaminants, resulting from waste-water irrigation.

5 Conclusions

Mezquital valley organic rich soils showed a large fil-tering capacity for metallic contaminants. Our resultsshowed a low availability (EDTA) of metal retained insoil for crops and evidenced limited exposure to metalsfor both cattle and humans via crop consumption. Be-cause of the dispersion of contaminants through the useof wastewater for the irrigation, the metal(loïd)ic con-tamination of the Mezquital valley hydrographic

network, soils and plants remains moderate. However,remaining elevated MMHg concentrations in the Tulastream is of concern and presents a potential risk oftrophic transfer and biomagnification especially in ben-thic organisms and fish such as bottom feeder fish.Finally, the high dissolved As and Pb concentrationsmeasured in groundwaters being above guidelines fordrinking water, also suggests a potential health risk forthe 500,000 people consuming groundwater and poten-tially for the Mexico city inhabitants if the project to usethis groundwater as a new source for the drinking watersupply of Mexico City is adopted. Therefore, furtherinvestigations should focus on the speciation of Asand Pb in deep sediments and their reactivity and mo-bility to understand the origin and fate of these contam-inants in the Mezquital valley aquifers.

Acknowledgments This research work was funded by theDGAPA-PAPIIT (IA100513) project, the CONACyT (CB83767), and the IRD. The authors acknowledge the enthusiasticcooperation received from the Cornejo family from Las PalmasRanch.

References

Balogh, S. J., & Nollet, Y. H. (2008). Methylmercury input to theMississippi River from a large metropolitan wastewater treat-ment plant. Science of the Total Environment, 406(1–2), 145–153.

Balogh, S. J., Nollet, Y. H., & Offerman, H. J. (2005). A compar-ison of total mercury andmethylmercury export from variousMinnesota watersheds. Science of the Total Environment,340(1–3), 261–270.

Banning, A., Cardona, A., & Rüde, T. R. (2012). Uranium andarsenic dynamics in volcano-sedimentary basins—an exem-plary study in north-central Mexico. Applied Geochemistry,27(11), 2160–2172.

Benoit, J. M., Gilmour, C. C., & Mason, R. P. (2001). Theinfluence of sulfide on solid phase mercury bioavailabilityfor methylation by pure cultures of Desulfobulbuspropionicus (1pr3). Environmental Science & Technology,35(1), 127–132.

Berg, M., Tran, H. C., Nguyen, T. C., Pham, H. V., Schertenleib,R., & Giger, W. (2001). Arsenic contamination of ground-water and drinking water in Vietnam: a human health threat.Environmental Science & Technology, 35, 2621–2626.

Bloom, N. S., & Fitzgerald, W. F. (1988). Determination of vola-tile mercury species at the picogram level by low-temperaturegas chromatography with cold-vapor atomic fluorescencedetection. Analytica Chimica Acta, 208, 151–161.

Bloom, N. S., Preus, E., Katon, J., & Hiltner, M. (2003). Selectiveextractions to assess the biogeochemically relevant fraction-ation of inorganic mercury in sediments and soils. AnalyticaChimica Acta, 479, 233–248.

Water Air Soil Pollut (2014) 225:2045 Page 15 of 19, 2045

Boudou, A. (2002). Fish contamination by metals: bioaccumula-tion and trophic transfer mechanisms. Revue de MédecineVétérinaire, 153, 437.

Boudou, A., Maury-Brachet, R., Coquery, M., Durrieu, G., &Cossa, D. (2005). Synergic effect of gold mining and dam-ming on mercury contamination in fish. EnvironmentalScience & Technology, 39(8), 2448–2454.

Cajuste, L. J., Carrillo, R., Cota, E., & Laird, R. J. (1991). Thedistribution of metals fromwaste-water in the mexican valleyof Mezquital. Water, Air, & Soil Pollution, 57–8, 763–771.

Carrillo, R., & Cajuste, L. J. (1995). Behavior of trace-metals insoils of Hidalgo, Mexico. Journal of Environmental Scienceand Health, 30, 143–155.

Charlet, L., Chapron, Y., Faller, P., Kirsch, R., Stone, A. T., &Baveye, P. C. (2012). Neurodegenerative diseases and expo-sure to the environmental metals Mn, Pb, and Hg.Coordination Chemistry Reviews, 256(19–20), 2147–2163.

Compeau, G. C., & Bartha, R. (1985). Sulfate-reducing bacteria—principal methylators of mercury in anoxic estuarine sedi-ment. Applied and Environmental Microbiology, 50(2), 498–502.

Cossa, D., & Gobeil, C. (2000). Mercury speciation in the LowerSt. Lawrence estuary. Canadian Journal of Fisheries andAquatic Sciences, 57, 138–147.

Cossa, D., Averty, B., & Pirrone, N. (2009). The origin of meth-ylmercury in open Mediterranean waters. Limnology andOceanography, 54(3), 837–844.

Day, P. R. (1965). Particle fraction and particle size analysis. In C.A. Black (Ed.), Methods of soil analysis, part 1. Agronomy(Vol. 9, pp. 545–567). Madison: Am. Soc. Agronomy.

de Velasquez, M. T. O., Manero, O., Cardoso, J., & Martenez, G.(1998). New polymeric coagulants tested in water and waste-water. Environmental Technology, 19(3), 323–329.

DFID. (1998). Impact of wastewater reuse on groundwater in theMezquital Valley, Hidalgo State, Mexico. In CNA, & BGS(Ed.), Technical report (London School of Hygiene andTropical Medicine ed). Mexico: University of Birmingham:Comision Nacional del Agua British Geological Survey.

DOF. (2003). NOM 001-SEMARNATECOL-2003. Establece loslimites maximos permisibles de contaminantes en lasdescargas de aguas residuales en aguas y bienes nacionales(Maximum allowable limits of pollutants of wastewater dis-charges in water bodies national goods (water bodies andsoil)). In Norma Oficial Mexicana. Mexico: Diario Oficial dela Federacion. in Spanish.

Drechsel, P., & Evans, A. E. V. (2010). Wastewater use in irrigatedagriculture. Irrigation and Drainage Systems, 24(1–2), 1–3.

Drott, A., Lambertsson, L., Bjorn, E., & Skyllberg, U. (2008).Potential demethylation rate determinations in relation toconcentrations of MeHg, Hg and pore water speciation ofMeHg in contaminated sediments.Marine Chemistry, 112(1–2), 93–101.

E.U. (2009). European communities environmental objectives (sur-facewaters) regulations 2009. Dublin: EuropeanCommunities.

Feigin, A., Ravina, I., & Shalhevet, J. (1991). Irrigation withtreated sewage effluent. Management for environmental pro-tection (Advanced series in agricultural sciences, Vol. 17).Berlin; New York: Springer.

Feng, X., & Qiu, G. (2008). Mercury pollution in Guizhou,Southwestern China—an overview. Science of the TotalEnvironment, 400(1–3), 227–237.

Feng, X., Qiu, G., Wang, S., & Shang, L. (2003). Distribution andspeciation of mercury in surface waters in mercury miningareas in Wanshan, Southwestern China. Journal de PhysiqueIV, 107, 455–458.

Filippelli, M., Baldi, F., Brinckman, F. E., & Olson, G. J. (1992).Methylmercury determination as volatile methylmercury hy-dride by purge and trap gas chromatography in line withFourier transform infrared spectroscopy. EnvironmentalScience & Technology, 26(7), 1457–1460.

Fitzgerald, W. F., & Clarkson, T. W. (1991). Mercury andmonomethylmercury: present and future concerns.Environmental Health Perspectives, 96, 159–166.

Fleming, E. J., Mack, E. E., Green, P. G., &Douglas, C. N. (2006).Mercury methylation from unexpected sources: molybdate-inhibited freshwater sediments and an iron-reducing bacteri-um. Applied and Environmental Microbiology, 72(1), 457–464.

Flores, L., Blas, G., Hernandez, G., & Alcala, R. (1997).Distribution and sequential extraction of some heavy metalsfrom soils irrigated with watsewater from Mexico City.Water, Air, & Soil Pollution, 98, 105–117.

Friedel, J. K., Langer, T., Siebe, C., & Stahr, K. (2000). Effects oflong-term waste water irrigation on soil organic matter, soilmicrobial biomass and its activities in central Mexico.Biology and Fertility of Soils, 31, 414–421.

Gallegos, E., Warren, A., Robles, E., Campoy, E., Sainz, M. G.,Bonilla, P., et al. (1999). The effect of waste water irrigationon groundwater quality in Mexico. Water Science andTechnology, 42(2), 45–52.

García Bravo, A., Cosio, C., Chevalley, P.-A., Zopfi, J.,Amouroux, D., Spangenberg, J., Ungureanu, V.-G., &Dominik, J. (2014). Extremely elevated methyl mercurylevels in water, sediment and organisms in a Romanianreservoir affected by release of mercury from a chlor-alkaliplant. Water Research, 49(1), 391–405.

García, L., Biswas, A. K., Braga, B., Tortajada, C., & Rodriguez,D. J. (2006). Water Quality Management in the Americas. InA. K. Biswas, & C. Tortajada (Eds.), Water Quality Issues inLatin America (pp. 1–15): Springer.

Gbondo-Tugbawa, S. S., McAlear, J. A., Driscoll, C. T., & Sharpe,C. W. (2010). Total and methyl mercury transformations andmass loadings within a wastewater treatment plant and theimpact of the effluent discharge to an alkaline hypereutrophiclake. Water Research, 44(9), 2863–2875.

Gilmour, C. C., & Bloom, N. S. (1995). A case study of mercuryand methylmercury dynamics in a Hg-contaminated munic-ipal wastewater treatment plant.Water, Air, & Soil Pollution,80(1), 799–803.

Gilmour, C. C., Riedel, G. S., Ederington, M. C., Bell, J. T., Benoit,J. M., Gill, G. A., et al. (1998). Methylmercury concentrationsand production rates across a trophic gradient in the northernEverglades. Biogeochemistry, 40(2–3), 327–345.

Gilmour, C. C., Podar, M., Bullock, A. L., Graham, A.M., Brown,S. D., Somenahally, A. C., Johs, A., Hurt, R. A., Bailey, K.L., & Elias, D. A. (2013). Mercury methylation by novelmicroorganisms from new environments. EnvironmentalScience & Technology, 47, 11810–11820.

Grafe, M., Eick, M. J., Grossi, P. R., & Saunders, A. M. (2002).Adsorption of arsenate and arsenite on ferrihydrite in thepresence and absence of dissolved organic carbon. Journalof Environmental Quality, 31(4), 1115–1123.

2045, Page 16 of 19 Water Air Soil Pollut (2014) 225:2045

Guédron, S., Grangeon, S., Lanson, B., & Grimaldi, M. (2009).Mercury speciation in a tropical soil association; conse-quence of gold mining on Hg distribution in FrenchGuiana. Geoderma, 153, 331–346.

Guédron, S., Grimaldi, M., Grimaldi, C., Cossa, D., Tisserand, D.,&Charlet, L. (2011). Amazonian former goldmined soils as asource of methylmercury: evidence from a small scale water-shed in French Guiana. Water Research, 45(8), 2659–2669.

Guedron, S., Huguet, L., Vignati, D. A. L., Liu, B., Gimbert, F.,Ferrari, B. J. D., et al. (2012). Tidal cycling of mercury andmethylmercury between sediments and water column in theVenice Lagoon (Italy). Marine Chemistry, 130–131, 1–11.

Guédron, S., Grangeon, S., Jouravel, G., Charlet, L., & Sarret, G.(2013). Atmospheric mercury incorporation in soils of anarea impacted by a chlor-alkali plant (Grenoble, France):contribution of canopy uptake. Science of the TotalEnvironment, 445–446(0), 356–364.

Gutiérrez-Ruíz, M., Siebe, C., & Sommer, I. (1994) EnvironmentalAspects of Land Application ofWastewater fromMexico CityMetropolitan Area: A Bibliographical Review and Analysis ofImplications. In 15th World Congress of Soil Sci., Acapulco,Mexico, 3, 318–330.

Han, S., Obraztsova, A., Pretto, P., Deheyn, D. D., Gieskes, J., &Tebo, B. M. (2008). Sulfide and iron control on mercuryspeciation in anoxic estuarine sediment slurries. MarineChemistry, 111(3–4), 214–220.

Harvey, J. W., Chambers, R. M., & Hoelscher, J. R. (1995).Preferential flow and porewater solutes in segregation ofwetland sediment. Estuaries, 18(4), 568–578.

Hermann, R., & Neumann-Mahlkau, P. (1985). The mobility ofzinc, cadmium, copper, lead, iron and arsenic in groundwateras a function of redox potential and pH. Science of the TotalEnvironment, 43(1–2), 1–12.

Herre, A., Siebe, C., & Kaupenjohann, M. (2004). Effect ofirrigation water quality on organic matter, Cd and Cu mobil-ity in soils of CentralMexico.Water Science and Technology,50(2), 277–284.

Hochella, M. F., Jr., Moore, J. N., Putnis, C. V., Putnis, A.,Kasama, T., & Eberl, D. D. (2005). Direct observation ofheavy metal-mineral association from the Clark Fork RiverSuperfund Complex: implications for metal transport andbioavailability. Geochimica et Cosmochimica Acta, 69(7),1651–1663.

Horvat, M., Nolde, N., Fajon, V., Jereb, V., Logar, M., Lojen, S.,et al. (2003). Total mercury, methylmercury and selenium inmercury polluted areas in the province Guizhou, China.Science of the Total Environment, 304(1–3), 231–256.

Huang, P. M., Bollag, J. M., & Senesi, N. (2002). Interactionsbetween soil particles and microorganisms: impact on theterrestrial ecosystem (IUPAC Series on Analytical andPhysical Chemistry of Environmental Systems, Vol. 8).New York: John Wiley and Sons.

Huang, X., Liao, X., & Shi, B. (2009). Adsorption removal ofphosphate in industrial wastewater by using metal-loadedskin split waste. Journal of Hazardous Materials, 166(2–3),1261–1265.

INEGI (2013). Censo de Población y Vivienda 2010. InstitutoNacional de Estadística y Geografía. http://www.inegi.org.mx/ Accessed 11 Oct 2013.

IUSS. (2007). World Reference Base for Soil Resources, firstactualization 2007. In Food and Agriculture Organization of

the United Nations (Ed.), Reports on world soil resources(Vol. 103). Rome: FAO.

Jain, C. K., & Ali, I. (2000). Arsenic: occurrence, toxicity andspeciation techniques. Water Research, 34(17), 4304–4312.

Jamali, M. K., Kazi, T. G., Afridi, H. I., Arain, M. B., Jalbani, N.,& Memon, A. R. (2007). Speciation of heavy metals inuntreated domestic wastewater sludge by time saving BCRsequential extraction method. Journal of EnvironmentalScience and Health Part A: Toxic/Hazardous Substances &Environmental Engineering, 42(5), 649–659.

Jiménez, B., & Chavez, A. (1997). Treatment of Mexico Citywastewater for irrigation purposes. EnvironmentalTechnology, 18, 721–730.

Jiménez, B., & Chávez, A. (2002). Low cost technology forreliable use of Mexico City’s wastewater for agriculturalirrigation. Environmental Technology, 9, 84–107.

Jiménez, B., & Chavez, A. (2004). Quality assessment of anaquifer recharged with wastewater for its potential use asdrinking source: “El Mezquital Valley” case. Water Scienceand Technology, 50(2), 269–276.

Jiménez, B., Chávez, A., Barrios, E., & Pérez, R. (2000). Impactand potential of reused water in the Mezquital Valley. Water21 IWA, 6, 34–36.

Kabata-Pendias, A., & Pendias, H. (2001). Trace elements in soilsand plants. 3rd Edn. Boca Raton (USA): CRC Press 2000.

Kerin, E. J., Gilmour, C. C., Roden, E., Suzuki, M. T., Coates, J.D., & Mason, R. P. (2006). Mercury methylation by dissim-ilatory iron-reducing bacteria. Applied and EnvironmentalMicrobiology, 72(12), 7919–7921.

Kim, K.-W., Bang, S., Zhu, Y., Meharg, A. A., & Bhattacharya, P.(2009). Arsenic geochemistry, transport mechanism in thesoil-plant system, human and animal health issues.Environment International, 35(3), 453–454.

Liu, B., Yan, H., Wang, C., Li, Q., Guédron, S., Spangenberg, J.E., et al. (2012). Insights into low fish mercury bioaccumu-lation in a mercury-contaminated reservoir, Guizhou, China.Environmental Pollution, 160, 109–117.

Lucho-Constantino, C. A., Prieto-García, F., Del Razo, L. M.,Rodriguez-Vázquez, R., & Poggi-Varaldo, H. M. (2005).Chemical fractionation of boron and heavy metals in soilsirrigated with wastewater in Central Mexico. Agriculture,Ecosystems & Environment, 108, 57–71.

Luther, G., & Rickard, D. (2005). Metal sulfide clustercomplexes and their biogeochemical importance in theenvironment. Journal of Nanoparticle Research, 7(4),389–407.

MacDonald, D. D., Ingersoll, C. G., & Berger, T. A.(2000). Development and evaluation of consensus-based sediment quality guidelines for freshwater eco-systems. Archives of Environmental Contamination andToxicology, 39, 20–31.

Mandal, B. K., & Suzuki, K. T. (2002). Arsenic round the world: areview. Talanta, 58(1), 201–235.

Marvin-DiPasquale, M., Agee, J., McGowan, C., Oremland, R. S.,Thomas, M., Krabbenhoft, D., et al. (2000). Methyl-mercurydegradation pathways: a comparison among three mercury-impacted ecosystems. Environmental Science & Technology,34(23), 4908–4916.

Mason, R. P., & Sullivan, K. A. (1998). Mercury and methylmer-cury transport through an urban watershed. Water Research,32(2), 321–330.

Water Air Soil Pollut (2014) 225:2045 Page 17 of 19, 2045

Mason, R. P., Reinfelder, J. R., & Morel, F. M. M. (1995).Bioaccumulation of mercury and methylmercury. Water,Air, & Soil Pollution, 80, 915–921.

Mason, R. P., Laporte, J. M., & Andres, S. (2000). Factors con-trolling the bioaccumulation of mercury, methylmercury, ar-senic, selenium, and cadmium by freshwater invertebratesand fish. Archives of Environmental Contamination andToxicology, 38(3), 283–297.

Mason, R. P., Heyes, D., & Sveinsdottir, A. (2006).Methylmercury concentrations in fish from tidal waters ofthe Chesapeake Bay. Archives of EnvironmentalContamination and Toxicology, 51(3), 425–437.

Nriagu, J. O., & Pacyna, J. M. (1988). Quantitative assessment ofworldwide contamination of air, water and soils by tracemetals. Nature, 333(12), 134–139.

Oliveira, A. d. S., Bocio, A., Trevilato, T. M. B., Takayanagui, A.M. M., Doming, J. L., & Segura-Munoz, S. I. (2007). Heavymetals in untreated/treated urban effluent and sludge from abiological wastewater treatment plant. EnvironmentalScience and Pollution Research, 14(7), 483–489.

Oremland, R. S., Culberstone, C. W., & Winfrey, M. R. (1991).Methylmercury decomposition in sediments and bacterialclusters: involvement of methanogens and sulfate reducersin oxidative demethylation. Applied and EnvironmentalMicrobiology, 57(1), 130–137.

Parker, J. L., & Bloom, N. S. (2005). Preservation and storagetechniques for low-level aqueous mercury speciation.Science of the Total Environment, 337(1–3), 253–263.

Polya, D., & Charlet, L. (2009). Rising arsenic risk? NatureGeoscience, 2, 383–384.

Ponce, R. A., & Bloom, N. S. (1991). Effect of pH on thebioaccumulation of low-level, dissolved methylmercury byrainbow-trout (oncorhynchus-Mykiss). Water, Air, & SoilPollution, 56, 631–640.

Qadir, M., & Scott, C. A. (2010). Non-pathogenic trade-offs ofwastewater irrigation. In P. Drechsel, C. A. Scott, & M.Redwood (Eds.), Wastewater Irrigation (pp. 101–126).London: International Water Management Institute (IWMI).

Qishlaqi, A., Moore, F., & Forghani, G. (2008). Impact of untreat-ed wastewater irrigation on soils and crops in Shiraz subur-ban area, SW Iran. Environmental Monitoring andAssessment, 141(1–3), 257–273.

Roos-Barraclough, F., Givelet, N., Martinez-Cortizas, A.,Goodsite, M. E., Biester, H., & Shotyk, W. (2002). Ananalytical protocol for determination of total mercury con-centration in solid peat samples. Science of the TotalEnvironment, 292, 129–139.

Ruffat, S. (Ed.). (2006). Mexico, au risque de son développement(Géoconfluences, Vol. Brève n°2). Lyon: ENS LSH.

Schnitzer, M., & Kerndorff, H. (1981). Reactions of fulvicacid with metal ions. Water, Air, & Soil Pollution, 15,97–108.

Seeberg-Elverfeldt, J., Schlüter, M., Feseker, T., & Kölling, M.(2005). Rhizon sampling of porewaters near the sediment-water interface of aquatic systems. Limnology andOceanography: Methods, 3, 361–371.

Sharma, V. K., & Sohn, M. (2009). Aquatic arsenic: toxicity,speciation, transformations, and remediation. EnvironmentInternational, 35(4), 743–759.

Siebe, C. (1994). Acumulación y disponibilidad de metalespesados en suelos regados con aguas residuales en el

Distrito de Riego 03, Tula, Hidalgo. Revista Internacionalde Contaminación Ambiental, 10, 15–21.

Siebe, C. (1995). Heavy metal availability to plants in soils irri-gated with wastewater from Mexico City.Water Science andTechnology, 32(12), 29–34.

Siebe,C.,&Cifuentes,E.(1995).Environmentalimpactofwaste-water i r r igat ion in central Mexico: an overview.International Journal of Environmental Health Research,5, 161–173.

Siebe, C., & Fischer, W. R. (1996). Effect of long-term irrigationwith untreated sewage effluents on soil properties and heavymetal adsorption of Leptosols and Vertisols in CentralMexico. Zeitschrif t für Pflanzenernährung undBodenkunde, 159(4), 357–364.

Siebe-Grabach, C. (1994). Accumulation, mobility, and availabil-ity of heavy metals in soils of central Mexico that weresubjected to long-term irrigation with municipal wastewater.Stuttgart: University of Hohenheim.

Simeoni, M. A., Batts, B. D., & McRae, C. (2003). Effect ofgroundwater fulvic acid on the adsorption of arsenate byferrihydrite and gibbsite. Applied Geochemistry, 18(10),1507–1515.

Solis, C., Andrade, E., Mireles, A., Reyes-Solis, I. E., Garcia-Calderon, N., Lagunas-Solar, M. C., et al. (2005). Distributionof heavy metals in plants cultivated with wastewater irrigatedsoils during different periods of time. Nuclear Instruments andMethods in Physics Research B, 241, 351–355.

Ullrich, S. M., Ilyushchenko, M. A., Tanton, T. W., &Uskov, G. A. (2007). Mercury contamination in thevicinity of a derelict chlor-alkali plant—part II: con-tamination of the aquatic and terrestrial food chain andpotential risks to the local population. Science of theTotal Environment, 381(1–3), 290–306.

USEPA. (2001). Water quality criterion for the protection ofhuman health: methylmercury (p. 303). Washington, DC:Office of Science and Technology Office of Water U.S.Environmental Protection Agency. EPA-823-R-01-001.

USEPA. (2009). National primary drinking water regulations (p.6). Washington, DC: Office of Science and TechnologyOffice of Water U.S. EPA 816-F-09-0004.

Vaiopoulou, E., & Gikas, P. (2012). Effects of chromium onactivated sludge and on the performance of wastewater treat-ment plants: a review. Water Research, 46(3), 549–570.

Vignati, D. A. L., Dominik, J., Beye, M. L., Pettine, M., &Ferrari, B. J. D. (2010). Chromium(VI) is more toxicthan chromium(III) to freshwater algae: a paradigm torevise? Ecotoxicology and Environmental Safety, 73(5),743–749.

Watras, C. (1992). Mercury and methylmercury in individualzooplankton: implications for bioaccumulation. Limnologyand Oceanography, 37(6), 1313–1318.

Watras, C. J., Back, R. C., Halvorsen, S., Hudson, R. J. M.,Morrison, K. A., & Wente, S. P. (1998). Bioaccumulationof mercury in pelagic freshwater food webs. Science of theTotal Environment, 219(2–3), 183–208.

Webster, R. (2001). Statistics to support soil research and theirpresentation. European Journal of Soil Science, 52(2), 331–340.

WHO. (2006). World Health Organisation guidelines for the safeuse of wasterwater excreta and greywater (Vol. 3). Geneva:World Health Organization.

2045, Page 18 of 19 Water Air Soil Pollut (2014) 225:2045

WHO. (2011). In WHO (Ed.), Guidelines for drinking-waterquality (Vol. Fourth edition, p. 564). Geneva: World Healthorganisation.

Wilson, S. C., Lockwood, P. V., Ashley, P. M., & Tighe,M. (2010). The chemistry and behaviour of antimonyin the soil environment with comparisons to arsenic: acritical review. Environmental Pollution, 158(5), 1169–1181.

Yang, J. K., Song, K. H., Kim, B. K., Hong, S. C., Cho, D. E., &Chang, Y. Y. (2007). Arsenic removal by iron and manganesecoated sand.Water Science and Technology, 56(7), 161–169.

Zhu, H., Yan, B., Cao, H., &Wang, L. (2012). Risk assessment formethylmercury in fish from the Songhua River, China:30 years after mercury-containing wastewater outfalls wereeliminated. Environmental Monitoring and Assessment,184(1), 77–88.

Water Air Soil Pollut (2014) 225:2045 Page 19 of 19, 2045