Journal of Contaminant Hydrology 164 (2014) 259–274

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Journal of Contaminant Hydrology

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Understanding the fate of sanitation-related nutrients in ashallow sandy aquifer below an urban slum area

P.M. Nyenje a,⁎, J.C.N. Havik b, J.W. Foppen a, A. Muwanga c, R. Kulabako c

a Department of Water Science and Engineering, UNESCO-IHE Institute for Water Education, P. O. Box 3015, Delft, The Netherlandsb Utrecht University, Faculty of Geosciences, P.O. Box 80125, 3508 TC Utrecht, The Netherlandsc Makerere University, P.O. Box 7062, Kampala, Uganda

a r t i c l e i n f o

⁎ Corresponding author. Tel.: +256 712 630482.E-mail addresses: p.nyenje@unesco-ihe.org, nyen

(P.M. Nyenje).

http://dx.doi.org/10.1016/j.jconhyd.2014.06.0110169-7722/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Article history:Received 25 November 2013Received in revised form 21 June 2014Accepted 23 June 2014Available online 30 June 2014

We hypothesized that wastewater leaching from on-site sanitation systems to alluvial aquifersunderlying informal settlements (or slums) may end up contributing to high nutrient loads tosurface water upon groundwater exfiltration. Hence, we conducted a hydro-geochemical study in ashallow sandy aquifer in Bwaise III parish, an urban slum area in Kampala, Uganda, to assess thegeochemical processes controlling the transport and fate of dissolved nutrients (NO3, NH4 and PO4)released from on-site sanitation systems to groundwater. Groundwater was collected from 26observation wells. The samples were analyzed for major ions (Ca, Mg, Na, Mg, Fe, Mn, Cl and SO4)and nutrients (o-PO4, NO3 and NH4). Data was also collected on soil characteristics, aquiferconductivity andhydraulic heads. Geochemicalmodeling using PHREEQCwas used todetermine thelevel of o-PO4 control by mineral solubility and sorption. Groundwater below the slum area wasanoxic and had near neutral pH values, high values of EC (average of 1619 μS/cm) and highconcentrations of Cl (3.2 mmol/L), HCO3 (11 mmol/L) and nutrients indicating the influencefrom wastewater leachates especially from pit latrines. Nutrients were predominantly present asNH4 (1–3 mmol/L; average of 2.23 mmol/L). The concentrations of NO3 and o-PO4 were, however,low: average of 0.2 mmol/L and 6 μmol/L respectively.We observed a contaminant plume along thedirection of groundwater flow (NE–SW) characterizedbydecreasing values of EC andCl, anddistinctredox zones. The redox zones transited fromNO3-reducing in upper flow areas to Fe-reducing in thelower flow areas. Consequently, the concentrations of NO3 decreased downgradient of the flow pathdue to denitrification. Ammonium leached directly into the alluvial aquifer was also partiallyremoved because the measured concentrations were less than the potential input from pit latrines(3.2 mmol/L). We attributed this removal (about 30%) to anaerobic ammonium oxidation(anammox) given that the cation exchange capacity of the aquifer was low (b6 meq/100 g) toeffectively adsorb NH4. Phosphate transportwas, on the other hand, greatly retarded and our resultsshowed that this was due to the adsorption of P to calcite and the co-precipitation of P with calciteand rhodochrosite. Our findings suggest that shallow alluvial sandy aquifers underlying urban slumareas are an important sink of excessive nutrients leaching from on-site sanitation systems.

© 2014 Elsevier B.V. All rights reserved.

Keywords:EutrophicationNutrientsTransportShallow groundwaterSlums

1. Introduction

Nutrients released from wastewater leaching from on-sitesanitation systems to groundwater can be of several orders of

je_p@yahoo.com

magnitude higher than theminimum required to cause surfacewater eutrophication (Nyenje et al., 2013b; Parkhurst et al.,2003; Robertson et al., 1998; Weiskel and Howes, 1992;Zurawsky et al., 2004). The principal nutrients that causeeutrophication are nitrogen (N) in the form of nitrate (NO3)and ammonium (NH4), and phosphorus (P) in the form oforthophosphate (o-PO4) (Correll, 1998; Thornton et al., 1999).Hence, several studies have attempted to understand the fate of

260 P.M. Nyenje et al. / Journal of Contaminant Hydrology 164 (2014) 259–274

wastewater-derived nutrients in shallow groundwater byassessing the geochemical and hydro-chemical processesgoverning their transport (e.g. Corbett et al., 2000; Griffioen,2006; Lawrence et al., 2000; Navarro and Carbonell, 2007;Ptacek, 1998; Zanini et al., 1998) or by modeling theseprocesses using reactive transport models like PHREEQC (e.g.Parkhurst et al., 2003; Spiteri et al., 2007; van Breukelen andGriffioen, 2004). These studies have shown that nutrientsleaching to groundwater can undergo a number of transforma-tions, which may regulate their transport depending onexisting hydro-chemical conditions. This may in turn limit orincrease the transport of nutrients and their impact todownstream surface water systems. For example, the transportof o-PO4 in groundwater is highly retarded owing to its strongadsorption affinity to metal oxides (esp. Fe and Mn), theco-precipitation of P with calcite and the precipitation of Pwithphosphate minerals such as hydroxyapatite (Golterman, 1995;Robertson et al., 1998; Shenker et al., 2005). However, o-PO4

can also be easily remobilized in anoxic groundwater followingreductive dissolution of Fe-/Mn-oxides (e.g. Zurawsky et al.,2004) or by inhibition of precipitation reactions due to thepresence of otherminerals (e.g. Cao et al., 2007) or the presenceof highly alkaline conditions (Reddy et al., 1999). N species aremainly regulated by redox processes. The main driver of theseprocesses in groundwater is the dissolved organic carbon (DOC)(Appelo and Postma, 2007). Under aerobic conditions, Noriginally present as NH4 inwastewater leachates can be readilyoxidized to NO3, which is relatively conservative and may endup in lakes causing eutrophication (e.g. Pieterse et al., 2005). Inhighly anaerobic conditions, however, N present as NO3 can beremoved from the aquifer by denitrification (e.g. Rivett et al.,2008).

So far, little information is available on the role of the abovementioned processes in determining the fate of nutrients inshallow alluvial aquifers underlying unsewered urban informalsettlements or slums. Such environments are characterized bydensely-populated informal settlements usually without accessto proper on-site sanitation systems (Enabor, 1998; Katukiza etal., 2012; Kulabako et al., 2010). Human excreta is predom-inantly disposed of via poorly constructed pit latrines whereasmost wastewater is disposed, untreated, over compounds ordirectly in excavated pits and drainage channels (e.g. Enabor,1998; Kulabako et al., 2010). A significant number of people inthese settlements also lack access to toilet facilities and somepractice open defecation. In Uganda for example, 1–2% of theurban population are still reported to engage in open defecation(WHO/UNICEF, 2013). These activities have been shown toheavily contaminate shallow groundwater with high levels offaecal coliforms and nutrients (e.g. ARGOSS, 2002; Cronin et al.,2007; Montangero and Belevi, 2007; Nyenje et al., 2013b). Moststudies in slum areas and other peri-urban areas have focusedon understanding and managing immediate health risksassociated with the contamination of drinking water supplies(e.g. springs and wells) by pathogenic organisms (ARGOSS,2002; Howard et al., 2003; Kimani-Murage and Ngindu, 2007;Kulabako et al., 2007; Lutterodt et al., 2014;Mkandawire, 2008).Nitrate contamination is also a widespread problem dealt within many papers investigating nitrate-transport processes, but ithas been mainly linked to agricultural practices (e.g. Andradeand Stigter, 2011; Stigter et al., 2011). However, research on thetransport of sanitation-relatednutrients in shallowgroundwater

in slum areas is lacking. This constrains efforts of safeguardingsurface water ecosystems from risks associated with eutrophi-cation when nutrient-rich groundwater exfiltrates to surfacewater systems. In this study, we attempted to investigate thegeochemical processes governing the transport and fate ofsanitation-related nutrients (NO3, NH4 and o-PO4) in a shallowsandy aquifer in Bwaise III parish, a slum area in Kampala,Uganda. Specifically our objectives were to: (i) characterizethe hydrogeology and geochemistry of the alluvial aquifer(ii) quantify the hydrochemistry, nutrients (P and N) andredox environment in the contaminated shallow groundwater,and (iii) establish the geochemical and hydro-chemical process-es controlling the fate of wastewater-derived contaminants andnutrients in the alluvial aquifer.

2. Description of the study area: hydrogeology andsanitation conditions

Bwaise III parish (54 ha) is a poorly sanitized urbaninformal settlement (or slum area) located in a low-lyingswampy area, North West of Kampala city, Uganda (Fig. 1). Ithas one of the highest population densities in the city (about300 persons/ha). There are 5 administrative zones in the parish(Fig. 1). This research focused on two zones: St. Francis andKatoogo. Details of the study area are described in Nyenje et al.(2013a, 2013b), Flynn et al. (2012), Katukiza et al. (2010) andKulabako et al. (2007). Here, we only provide additionalinformation relevant to this study. The slum area is situatedin a low-lying area with alluvial deposits, which are character-ized by a shallow groundwater table (b1 m) and clay soilsoverlain by fine sand and loamy top soils (Kulabako et al., 2007;Nyenje et al., 2013b). These deposits were derived fromweathering and erosion from upper areas and hill summits.The geology in upper areas is characterized by a deeplyweathered lateritic regolith (about 30 m thick) overlyingPrecambrian basement rocks consisting of predominantlyundifferentiated granite-gneiss rocks of the Buganda-ToroCover Formation (Taylor and Howard, 1996, 1999). Laterite isa highly weatheredmineral rich in iron and aluminium oxides.X-ray diffraction analyses show that the weathered regolith isdominated by kaolinite (Al2Si2O5(OH)4) as the most abundantclay mineral and quartz with small amounts of iron oxide(Flynn et al., 2012). Iron oxides have been reported to bepresent mainly in the crystalline forms as goethite (FeOOH)(Biryabareema, 2001; Taylor and Howard, 1999) and to a smallextent as heamatite (Fe2O3) (Flynn et al., 2012). The shallowgroundwater in the regolith aquifer is slightly oxic and rich innitrate and usually discharges as springs or diffuses to shallowalluvial aquifers in the low-lying areas. The area experiencestwo rainfall seasons (March–May and September–December)with a mean annual rainfall of about 1450 mm/y.

Most residents in the study area have access to piped watersupply, although some prefer to rely on springs tominimize costs(Kulabako et al., 2010). On-site sanitation is, however, very poorand as a result, shallow groundwater is heavily contaminated.Elevated concentrations of P (10–15 mg TP/L), N (15–30 mgTKN/L), Cl (40–80 mg/L), faecal coliforms (up to 28 × 108 cfu/100 mls) andhighvalues of EC (700–5000 μS/cm)have thereforebeen reported in the shallow groundwater (Flynn et al., 2012;Kulabako et al., 2007; Nyenje et al., 2013b). This contamination islargely attributed to the widespread use of un-improved

Fig. 1.Map of Bwaise III parish slum showing the groundwater monitoring network, the average water table contours and transects; transect 1 (GW20, GW18 andGW19) and transect 2 (GW24, GW25 and GW26), which are located downstream the slum area near Lubigi swamp.

261P.M. Nyenje et al. / Journal of Contaminant Hydrology 164 (2014) 259–274

traditional pit latrines for excreta disposal, the practice of opendefecation and flying toilets (use of polyethylene bags for excretadisposal), the poor solid waste management and the indiscrim-inate disposal of untreated grey water (Katukiza et al., 2012;Kulabako et al., 2007; Nyenje et al., 2013b). Most contaminationto groundwater is, however, believed to originate from pitlatrines. This is because over 80% of the residents depend on pitlatrines for excreta disposal. Only 1.3% use septic tanks (Katukizaet al., 2010) and less 5% practice open defecation (CIDI, 2006).Hence, there is a very high density of pit latrines in the study area(average of 10 pit latrines per hectare) with the majority locatedin Katoogo zone (29% of the total) and St. Francis (24% of thetotal) (Kulabako et al., 2004). These pit latrines are usuallyunlined at the bottom implying that the contents leach directlyinto the shallowgroundwater. Additionally, due to the highwatertable (b1 m), most pit latrines are elevated (see Fig. S1Supplementary material). Consequently, the pits fill up very fastand the pit contents are usually emptied manually into nearbychannels or adjacent pits. This practice is also attributed to thepoor accessibility in the area and the high costs of pit emptying.Hence, the contamination of groundwater from pit latrines isusually very high and leachate concentrations as high as 2.4 mg/Lfor o-PO4 and 57 mg/L for NH4 have been reported (Nyenje et al.,2013b). The open disposal of grey water is another possiblesourceof groundwater contamination in the studyarea.However,its contribution in relation to pit latrines is currently unknown.

3. Materials and methods

3.1. The monitoring network

The monitoring network consisted of 26 wells (Fig. 1).These included 19 wells installed in a randomly distributed

pattern in Bwaise III parish slum in St. Francis and Katoogozones, and 6 wells installed along two transects (each with 3wells) in an uninhabited area downstream of the slum area(Fig. 1). The elevation of each well was determined withrespect to a permanent benchmark (a reference point ofknown height above the mean sea level). The wells in theBwaise III slum area were installed specifically to understandthe chemical composition and the state of nutrients in theshallow groundwater. The well transects, on the other hand,gave an indication of the fate of ions and nutrients generatedfrom the slum area.

The well holes were drilled using a 50 mm Edelman handauger. The wells consisted of PVC pipes (50 mm internaldiameter) connected to slotted screen pipes. The pipes werecapped at the bottom to prevent sediments from entering thewell. Each well had varying screen lengths (0.5–1 m) and pipelengths (1–3 m) depending on the thicknesses of the aquiferand the overlying soil. The final depth of the wells was reachedafter striking the stiff clayey layer. This layer was difficult topenetrate by hand auger and indicated presence of consolidat-ed clayey soils acting as the bottom confining layer of theshallow aquifer. A cloth made of a nylon silk material waswrapped around the screen before installing the wells to act asa filter material for silts and clays, which would otherwise clogthe wells. This kind of installation also avoids changes in waterquality entering the well due to possible cation exchange withthe commonly used well materials such as the gravel pack(Appelo and Postma, 2007). A concrete base (equipped with ametallic cover lid) was constructed around the top of eachwellat the ground surface to avoid vandalism and contamination ofthewells from surrounding human activities. After installation,the wells were developed by pumping them dry daily for aweek until they reached full performance.

262 P.M. Nyenje et al. / Journal of Contaminant Hydrology 164 (2014) 259–274

Groundwater levels were recorded once every week fromFebruary to May 2011. These levels were used to constructflow contours using the Kriging interpolation technique ofthe Surfer 8 software (Golden Software Inc., Golden, CO,USA). The contours were used to estimate direction of flowand the head gradients.

3.2. Estimating hydraulic conductivity, K (m/day), and groundwaterflow

Hydraulic conductivity measurements (K, m/day) werecarried out using slug tests based on the Bouwer and Ricemethod (Bouwer, 1989). The application of this test includingthe step by step field procedure followed and the equationsused to calculate K, have been previously described byNyenje et al. (2013b). These tests were carried out at 5selected sites (GW16, GW1, GW4, GW10 and SW) (Fig. 1)evenly distributed across the study area. At each site, thetests were carried out in duplicate. Using the average K valueand the hydraulic gradient estimated from the water tablecontours, we were able to estimate the groundwater flowvelocity (m/day) using the Darcy equation.

3.3. Soil sampling and analysis

A total of 56 soil sampleswere collected during the augeringof well holes. The soil samples were collected from eachdistinctive layer encountered in the borehole profile. Distinc-tions between soil layers were made by examining the texture(by touching) and by visually inspecting the color of the soilsamples. The sampleswere kept in black polyethylene bags andtransferred to Makerere University Public Health and Environ-mental Engineering Laboratory, Kampala Uganda for dryingand initial analyses, and then to UNESCO-IHE Laboratory, TheNetherlands for further analyses. At Makerere University, thesamples were air dried for 2 weeks, sieved through a 2 mmsieve and analyzed for particle size distribution using thehydrometer method (Bouyoucos, 1962), pH measured on a2.5:1 water–soil suspension, organic carbon (OC) using theWalkley–Black method (Walkley and Black, 1934) and cationexchange capacity (CEC, meq/100 g) estimated by summingthe soil extractable cations (Ca, Mg, K and Na) extracted usingthe ammonium acetate method (Schollenberger and Simon,1945). At UNESCO-IHE, geo-available metals (especially theP-adsorbing metals: Fe, Ca, Al and Mg) were extracted with0.43 M HNO3 (Novozamsky et al., 1993; Rauret, 1998) andanalyzed using an Inductively Coupled Plasma spectrophotom-eter (ICP — PerkinElmer Optima 3000). Geo-available metalshere refer to the long-term mobilizable metals (Pettersen andHertwich, 2008).

3.4. Groundwater sampling and analysis

Groundwater samples were collected from the monitoringwells using a portable hand pump for 2–3 times a monthbetween February and May 2011. Before sampling, the wellswere purged at least three (3) well volumes to ensure that thesamples obtained were fresh and representative of siteconditions. For some wells, where recovery times were verylow (typically 4–6 h), they were purged a day before. Thisapproach has also been applied in other similar studies (e.g.

Carlyle and Hill, 2001). We also collected spring water, whichrepresented the inflowing shallow groundwater originatingfrom the upper regolith aquifer (see Nyenje et al., 2013a).Groundwater was collected in clean 500 ml polyethylenebottles. Immediately after sampling, the following parameterswere determined on-site: EC, pH, DO, Alkalinity (HCO3), NH4,o-PO4 and NO3. EC and temperatureweremeasuredwith an ECelectrode (TetraCon 325, WTW) connected to an EC meter(WTW 3310), pH with a pH electrode (SenTix 21, WTW)connected to pHmeter (WTW 3310) and DOwith a DO sensor(CellOx 325,WTW) connected to a DOmeter (WTW3310). Allmeterswere calibrated before takingmeasurements. HCO3wasdetermined by titrating with 0.2 M sulphuric acid. A portablecolorimeter (Hach DR890) was used to determine o-PO4, NH4

and NO3 on-site using standard Hach protocols. NO3 wasdetermined using the cadmium reduction method, NH4 usingNessler's method and o-PO4 using the ascorbic acid method.Samples for cations (K, Mg, Ca, Mg, Fe and Mn) and anions (Cland SO4) were filtered on-site through a 0.45 μm membranefilter into 25 ml scintillation vials and then kept cool at 4 °C.Cation samples were further preserved by adding two drops ofconc. nitric acid. These samples were shipped to UNESCO-IHEanalytical laboratory, Delft, The Netherlands, for determinationof cations using an Inductively Coupled Plasma spectrophotom-eter (ICP — PerkinElmer Optima 3000) and anions by IonChromatography (IC — Dionex ICS-1000). Dissolved OrganicCarbon (DOC) was also determined from the acidified samplesusing the Total Organic Carbon analyzer (Shimadzu, TOC-VPCN).

Some groundwater samples were unstable and changed tobrown color precipitates within 5 min after sampling. Hence,on-site analyses and filtrations were always done as fast aspossible (b5 min) to avoid possible changes in water quality(e.g. precipitation of Fe and Ca or conversion of NH4 to NO3

etc.). Given the complexity of collecting and preserving watersamples in slum environments, we considered an ion chargebalance error of 20% to be sufficient for this study.

3.5. Hydro-chemical data analysis

The processes likely governing the transport and trans-formation of nutrients were assessed by investigating thehydrochemistry and geochemistry of the shallow aquifer,the groundwater flow and the nutrient transport processes.The hydrochemistry of the shallow groundwater wascharacterized using the iso-concentration maps, redoxsensitive species and the water quality classification basedon the Stuyfzand classification method (Stuyfzand, 1989,1993). The Stuyfzand method was preferred to the tradi-tional piper diagram because it takes into account rare watertypes such as Ca-NO3 and NH4–HCO3, which were typical ofthe studied shallow groundwater. The iso-concentrationmaps were used to analyze the changes in hydrochemistryalong the groundwater flow path. Geochemical controls onnutrients transport were assessed by geochemical modelingand redox characterization.

3.6. Geochemical modeling

3.6.1. Mineral dissolution and precipitationThe PHREEQC code (version 2) (Parkhurst and Appelo,

1999) was used to carry out geochemical speciation and to

263P.M. Nyenje et al. / Journal of Contaminant Hydrology 164 (2014) 259–274

calculate the saturation indices (SIs) of phosphate minerals inorder to predict the mineral phases likely controlling thepresence of o-PO4 in groundwater (Appelo and Postma, 2007;Deutsch, 1997). Phosphate minerals considered were vivianite(Fe3(PO4)2·8H2O), hydroxyapatite (Ca5(PO4)3·OH) and man-ganese hydrogen phosphate (MnHPO4). Strengite was notconsidered because the aquifer was anoxic (see Section 4).Carbonates of Fe, Ca and Mn are also known to significantlyregulate o-PO4 concentrations by co-precipitation or by regu-lating metal ions via the common-ion effect (Deutsch, 1997;Reddy et al., 1999; Spiteri et al., 2007). Hence, the SI's of siderite(FeCO3), calcite (CaCO3) and rhodochrosite (MnCO3) were alsoconsidered. The SI's of Fe oxides and hydroxides (e.g. hematite)were not calculated because they have slow Kinetics (Deutsch,1997) and were therefore unlikely to regulate o-PO4 concen-trations in the shallow groundwater.

3.6.2. Phosphate adsorption on the aquifer materialThe PHREEQC codewas also used tomodel the adsorption of

orthophosphate (including the forms: PO43−, HPO4

2−, H2PO4−)

onto Fe-oxides using a surface-complexation process (Dzombakand Morel, 1990; Parkhurst and Appelo, 1999). We consideredFe-oxides to be the most capable of adsorbing P because Fe wasone of the most dominant geo-available metal (as shown lateron in results; Table 1) and because Fe oxides/hydroxides areconsidered as the principal sorbent of phosphorus in mostaquifers and in the PHREEQC model code. The sorption processproceeds by the following reaction (Parkhurst et al., 2003):

SiteOHþ PO3−4 þ 2Hþ ¼ SiteHPO−

4 þ H2O ð1Þ

Where Site represents the phosphorus sorption site(e.g. Fe-oxide). The equilibrium constant (log K) for theabove reaction depends on the type of the sorbing surface. Inthis study, we used the default database in the PHREEQCmodelcode, which contained thermodynamic data for the surfacecalled hydrous ferric oxide (Hfo, or ferrihydrite), derived fromDzombak and Morel (1990). Two binding sites are defined inthe database: a strong binding site (Hfo_s) and a weak bindingsite (Hfo_w). These sites were characterized by Dzombak andMorel (1990) as follows: 0.2 mol weak sites/mol Fe, 0.005 molstrong sites/mol Fe, a surface area of 600 m2/g Fe and a molarweight of 89 g Hfo/mol. The type of Fe-oxide used in this studywas goethite because it is the most abundant Fe-oxide in thestudy area (see Section 2). Goethite has a surface area, which isabout 10 times less than the default hydrous ferric oxide usedin the PHREEQC model code (Appelo and Postma, 2007). Wetherefore assumed 10 times fewer sorption sites in the model,

Table 1Average soil properties (with standard deviations) of the different stratigraphic un

Geo-available metals

Soil layer Depth (m) pH (−) OC (%) Fe (mg/kg) Al (mg/kg)

Top(n = 20)

0–0.9 6.8(±0.7)

0.8(±0.2)

399(±283)

156(±72)

Middle(n = 16)

0.5–2 6.7(±0.8)

0.7(±0.2)

373(±159)

125(±67)

Bottom(n = 19)

N2 5.2(±1.0)

0.6(±0.2)

308(±187)

74(±41)

i.e. 0.02 mol weak sites/mol Fe, 0.0005 mol strong sites/mol Feand a specific area of 60 m2/g. The total amount of iron oxidepresent was specified as the amount of Fe extracted using the0.43 M nitric acid extraction method. In Section 4, the modelresults of using the default hydrous ferric oxide as the sorbentwere also presented in order to provide an indication of theadsorption range of Fe oxides.

4. Results

4.1. Geology and soil characteristics

The soils in Bwaise III slum consisted of 3 distinct layers: 1) atop layer containing loamy soils mixed with fill material; 2) amiddle layer containingwhitish/grey sandswith small amountsof clay; and 3) a thick bottom layer containing stiff grey claysoils that extended to depths greater than 4 m below groundlevel (Fig. 2). Grain size analyses confirmed the distinction ofthe layers (Table 1) with soils in the middle layer having thehighest portion of sand (66%). The bottom layer had the highestproportion of clay (38%). The water table was always encoun-tered in the middle sandy layer implying that this layer formedthe shallow alluvial aquifer whereas the upper and bottomlayers acted as (semi) confining layers: confined at the bottomwith a stiff clay layer and semi-confined at the top with loamysoils. Water always gushed up whenever the middle layer wasstruck during augering. The average thickness of this aquiferwas 0.75 m (range of 0.5–1.5 m) (Table 1, Fig. 2). Geo-availablemetals were low (less than 0.2% by soil weight, Table 1)implying that their availability to interact with nutrients ingroundwater was low. The soil OC ranged from 0.6 to 0.8% byweight of soil (Table 1). The pH of the soils ranged from 5.2 to6.8 indicating acidic to neutral soils.

4.2. Groundwater flow

The hydraulic conductivity (K) of the aquifer ranged from1 to 5 m/day with a mean of 2.24 m/d (see Table S2 inSupplementary material). The water table was very shallow(average depth of 0.9 m below ground level (m.b.g.l) in dryseason and 0.5 m.b.g.l in the wet season) and fluctuated byabout 0.5 m. Groundwater consistently flowed from NorthEast to South West throughout the sampling period with acalculated residence time of about 60 years (see Fig. 1 andSupplementary material). The flow contours were moreclosely packed in the upstream areas than the downstreamindicating groundwater inflow from the upper areas.

its in Bwaise III slum.

Grain size distribution

Ca (mg/kg) CEC (meq/100 g) Sand (%) Silt (%) Clay (%)

663(±675)

8.4(±2.7)

58(±12)

16(±10)

26(±9)

481(±413)

5.8(±2.4)

66(±12)

13(±6)

21(±8)

140(±89)

6.4(±1.9)

50(±9)

12(±4)

38(±9)

Fig. 2. Hydro-geologic cross-section along profile x–x of the shallow alluvial aquifer in Bwaise III parish slum. The clayey sand formed the shallow aquifer in thearea. The bottom layer of clay was an impervious confining layer. The arrows represent the direction of groundwater flow.

264 P.M. Nyenje et al. / Journal of Contaminant Hydrology 164 (2014) 259–274

4.3. Hydrochemistry

4.3.1. Hydrochemistry and nutrients in shallow groundwaterTable 2 shows the descriptive statistics of the shallow

groundwater beneath the slum area. The complete list of allhydrochemical data is presented in Table S.1 (Supplementaryinformation). The shallow groundwater was highlymineralizedwith high EC values (average of 1619 μS/cm; Table 2, Fig. 3)compared to the EC of inflowing groundwater (spring water;average 502 μS/cm; Table 2) and precipitation (30 μS/cm).Some wells (e.g. GW5, GW7, GW15 and GW17) had extremelyhigh values of EC (up to 3000 μS/cm, Table S.1). These wellswere randomly spread within the slum area implying that anumber of potential point pollution sources, such as pit latrines,existed. For example, wells GW5 and GW15 were locatedimmediately downgradient of the pit latrines (personal obser-vation). The pH of the shallow groundwater was generally nearneutral (pH = 6.6–7.2) with exception of the spring and someupgradient wells (e.g. LS and SW), which were acidic (pH b 5)since they were likely directly recharged by the more oxic andacidic groundwater flowing from the upper regolith andfractured aquifers (see Flynn et al., 2012; Nyenje et al., 2013b).The average Cl concentration was 3 mmol/L (Table 2; Fig. 3),with highs of 14 mmol/L also measured (e.g. GW15; Table S.1).This average value is about 40 times more than the average Clconcentration in rainfall water (0.08 mmol/L; Table 2) and 2.3times the average Cl concentration of inflowing groundwater(Spring: 1.38 mmol/L). This suggested that the shallow ground-water beneath the slum area was heavily contaminated andmost likely from wastewater leaching from on-site sanitationsystems in the slum area.Wastewater contains high amounts oforganic matter, which when degraded, can lead to increasedconcentrations of nutrients (o-PO4, NH4, NO3) and ions such asCl, Na, Ca, K and HCO3 in groundwater (Navarro and Carbonell,2007; Ptacek, 1998). Consequently, the shallow groundwaterbeneath the slum area was characterized by high concentra-tions of carbon (1.1–1.9 mmol/L as DOC), nutrients (N and P)

and cations, particularly Na (up to 13.52 mmol/L) and Ca (up to6.13 mmol/L) andK (up to 15 mmol/L) (Table 2; Table S.1). NH4

was the dominant form of nitrogen (average of 2.23 mmol/L;range of 1–3 mmol/L) (Fig. 5; Table 2). NO3 was generallyabsent (b0.2 mmol/L), except in wells along the upper flowboundary and the spring,where high concentrations (average of2.2 mmol/L, Table 2) were measured. The high NO3 concentra-tions in the upgradient wells were consistent with low pHobservations, resulting from acid rain recharge in the catchment(Nyenje et al., 2013a) and the release of H+ following theconversion of NH4 to NO3 by nitrification. Phosphate (o-PO4)concentrations ranged from 1.3 to 13 μmol/L with an averagevalue of 6 μmol/L (Table 2, Fig. 3). Well GW2 was an exceptionbecause the o-PO4 concentrations were abnormally high (up to200 μmol/L; Fig. 3). This well was installed near an organicwaste dump subject to incineration with charcoal encounteredin the unfiltered groundwater. It is likely that P transport herewas affected by macro-pore flow. The dominant water typesin the shallow aquifer were NaHCO3 and CaHCO3. In someinstances KHCO3 (upgradient wells) and NH4HCO3 (alongNsooba channel) water types were encountered (Fig. 4B). Caand Na were the dominant cations.

4.3.2. Evolution of water quality and nutrients along the flow pathWells located along the transects downgradient of the slum

area (e.g. GW24, 25 and 26, about 200 m from the slumboundary) tended to have lower EC values (b1200 μS/cm) thanthosewithin the slumarea (Fig. 3). A fewwells located along theupper flow boundary (e.g. SW, LS and GW3) also tended toexhibit lower EC values than those within the slum area. Hence,we observed a spatial evolution of EC values along the directionof groundwater flow. This was supported by chloride measure-ments whereby Cl concentrations of the downstream wells(G24, 25 and 26) did not exceed 1.8 mmol/L whereas those inBwaise slum (Katoogo and St. Francis zones) ranged from 1.5(GW11) up to 9 mmol/L (GW15). The observed evolution of ECand Cl is consistent with what would be expected when

Table 2Descriptive statistics of the hydrochemistry of the shallow groundwater collected beneath Bwaise slum. Values of variables are given in mmol/L except forelectrical conductivity (EC, μS/cm), pH (−) and T (temperature, °C).

Shallow groundwater

Parameter Rainfall⁎ Spring Within the slum area Transect 1 Transect 2

EC 30 502 1619 1451 987pH 6.1 4.8 7.2 6.68 6.58T 24.2 25.9 26.5 26.6 26.3Na 0.03 0.950 4.213 2.49 2.023K 0.04 0.213 2.300 1.602 0.619Mg 0.01 0.123 0.800 0.963 0.479Ca 0.12 0.883 2.000 2.703 1.464NH4 0.05 0.190 2.230 1.843 1.071Cl 0.08 1.391 3.176 2.286 1.254HCO3 0.07 0 10.78 10.82 8.895SO4 0.02 0.032 0.364 0.662 0.084NO3 0.03 2.293 0.222 0.106 0.034⁎⁎o-PO4 0.004 0.0045 0.0068 0.0075 0.0079Fe 0 0.0029 0.316 0.619 0.800Mn 0.001 0.0072 0.0316 0.0366 0.054DOC (as C) – 0.41 1.07 1.68 1.93

⁎ Data from Nyenje et al. (2013a) and Kulabako et al., 2007⁎⁎ Well GW2 was not considered when computing the average values for o-PO4 because it had abnormally high concentrations of o-PO4.

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wastewater in the slumarea infiltrates the aquifer. The observedwater quality types along the direction of groundwater flow alsoshowed similar variations (Fig. 4). For example, wells alongupper flowboundary hadNO3 as the dominantN species (Fig. 4)whereas further downgradient, NH4 from wastewater leachatewas themost dominant. Similar trendswere also observedwiththe dominant cations (Ca and Na). Ca was the most dominantcation in the downgradient areas of the slum area (wells 20, 18,19. 24, 25 and 26) whereas in the upper areas, Na typedominated water started to appear. The wells GW2 and GW5,located at the extreme upper flow boundary had K as thedominant cation. A fewwells near the Nsooba channel had NH4

as the dominant cation, suggesting effects of inundation ongroundwater from Nsooba channel. This channel is heavilypolluted with high concentrations of NH4 (0.5–3 mmol/L) andtends to flood during heavy storms (Nyenje et al., 2013a, 2014).

The spatial variations in nutrients were a result of bothgroundwater flow and a number of transformation processes.For example, the most dominant form of dissolved N in theshallow alluvial aquifer was NH4. But as we moved upgradient,NO3 species progressively started to appear (Fig. 3). Phosphatesdid not show significant trends along the flow path. Concen-trationswere in most cases less than 10 μmol/L implying that Pwas regulated by various processes.

4.3.3. The redox environmentFig. 4A shows the distribution of the most sensitive redox

species; NO3, Mn, Fe and SO4. With the exception of a fewupgradient wells (e.g. SW1, SW2, LS10, spring, GW5), theconcentrations of NO3 were relatively low (0–0.38 mmol/L)(Fig. 3, Table S.1). On the other hand, high concentrations ofFe (up to 1.74 mmol/L, Fig. 3, Fig. 4 and Table S.1), and to alimited extent Mn (0–0.09 mmol/L), were measured espe-cially in the downgradient wells indicating that the shallowaquifer was generally anoxic. Consequently, NH4 was also thedominant N species as mentioned above.

A close examination of NO3 variations (Fig. 3, Fig. 4B) showedthat high NO3 concentrations occurred along the upper flow

boundary (e.g. wells LS10, SW1, SW2, GW5 and the spring),which gradually reduced (from well GW14) towards thedowngradient areas in Katoogo zone and outside the slum area.This implied that denitrification took place along the direction offlow beneath the slum area. However, Fe concentrations had themost remarkable results where a significant contrast existedbetween the upgradient wells and the downgradient wells. Inmost upgradient wells (e.g. SW1, SW2, LS10, GW5, GW2, GW3,GW4 and the spring), Fe concentrations were below 0.1 mmol/L(5 mg/L), whereas in most downgradient wells and a fewupgradient wells (e.g. GW14), concentrations were generallyhigher than 0.2 mmol/L (11 mg/L) with values reaching as highas 1.74 mmol/L (94 mg/L) measured (Fig. 4A; Table S.1). Thissuggested that the redox environment was predominantlyFe-reducing, which is consistent with field observations thatmost groundwater samples turned brown within 5 min aftersampling. This change in color of sampleswasmost likely causedby oxidation of Fe2+ to Fe3+ due to exposure of the samples tothe atmosphere. In addition, we observed brown rusty depositsin the soil at the interface between themiddle and bottom layer.These deposits were probably pockets of oxidized Fe. Similarobservations were made in previous studies (e.g. Nyenje et al.,2013b).

A few wells (e.g. GW15, GW14, GW9, and GW7) hadrelatively high concentrations of both Fe and NO3 indicatingthat these wells were located at the transition betweennitrate-reducing to Fe/Mn-reducing environments. However,this occurrence could also have been caused by the mixedgroundwater types during sampling given that our wellswere fully penetrating.

SO4 was generally present in high concentrations (rangeof 0–4.8 mmol/L, mean 0.36 mmol/L) indicating that condi-tions were not yet sulfate-reducing (Fig. 4, Table S.1). This isalso consistent with our field observations of the absence ofthe H2S smell. In the last transect (wells GW24, GW25 andGW26), however, groundwater seemed to be depleted of SO4

(Fig. 4) suggesting that sulfate-reducing conditions werestarting to occur.

Fig. 3. Evolution of average values of EC (μS/cm) and average concentrations of Cl, NO3, NH4, o-PO4 and Fe (in mmol/L except o-PO4 which is in μmol/L) in BwaiseIII slum. Values were interpolated using Kriging techniques. Well GW5 was omitted from the interpolations of EC and Cl because it had abnormally high values.The blue lines represent the drainage network. The scale bar displays meters.

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Although our results indicated that the redox conditions inthe alluvial aquifer were Fe-reducing, the concentrations ofMn2+ were relatively low compared to Fe2+, even thoughMn-oxides are reduced at a higher redox potential than Feoxides. The limited presence of Mn in these rather stronglyreducing conditions (Fe-reducing) suggests that Mn mineralswere less abundant in the aquifer soils. It is also likely thatMn2+ ionswere regulated by the solubility ofMn-minerals likerhodochrosite (MnCO3) since HCO3 was a dominant ion in theshallow aquifer. Rhodochrosite has been shown in manystudies to act as a sink of Mn2+ in anoxic aquifers (e.g. Jensenet al., 2002; Massmann et al., 2004; Matsunaga et al., 1993).

4.4. Geochemical modeling results

4.4.1. Mineral saturation indicesThe relevant mineral saturation indices (SIs) calculated

using the PHREEQC code are presented in Fig. 5. Samples N18,N35 and N34 (Table S.1) were excluded from SI calculationsbecause their pH measurements were considered to beerroneous. One of the most interesting observations was thevery high partial pressure of CO2 (pCO2), which generallyranged from 10(−1.0) to 10(−0.6) atm. This is about 300–1000times the atmospheric pCO2 gas pressure of 10(−3.5). Giventhat Bwaise slumwas a reclaimed wetland, both the decaying

Fig. 4. (A) Distribution of redox species (means) and (B) classification of the most dominant water quality types in shallow groundwater in Bwaise slum.

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organic matter related to wetland vegetation (0.6–0.8% assoil OC) and the large input of organic matter via wastewaterleaching from pit latrines (up to 1.9 mmol/L as DOC), canexplain the observed high CO2 pressures. As noted inliterature, it was also evident in our study that increases inpCO2 corresponded with a decrease in the SI values of calciteand the subsequent decrease in pH values (Fig. 5, Fig. 6). Onthe other hand, the decrease in the pCO2 resulted in anincrease in pH and subsequent increase of SI of calcite

(Fig. 6). Indeed, some of our samples were supersaturatedwith respect to calcite at low values of pCO2 (Fig. 6). Hence,the CO2-partial pressures likely played an important role inregulating calcite in groundwater.

Groundwater was generally near saturation with respectto calcite and rhodochrosite (−0.6 b SI b 0.7). Most sampleswere, however, under-saturated with respect to hydroxyap-atite (Ca5 (PO4)3OH) (SI b −2), suggesting that this mineralwas not present and did not regulate o-PO4 concentrations. If

Fig. 5. Distribution of mean saturation indices of minerals likely to regulate fate of o-PO4 and the mean partial pressures of CO2 (pCO2).

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this mineral phase was present, it is unlikely that it wouldhave remained undersaturated because the groundwaterresidence time was long (60 years). The well GW2 was anexception because it had a high SI value of hydroxyapatite(N2; Fig. 5) owing to the abnormally high o-PO4 concentra-tions. This pointed to localized geochemical conditions likelycaused by contamination via macro pores.

A number of samples were slightly supersaturated withrespect to siderite (0.6 b SI b 1) especially in themore reducedzones where Fe concentrations were high. This suggested thatthis carbonate phase did not control the concentrations of Fe in

those areas. In the upgradient less reduced areas, groundwaterwas undersaturated with respect to siderite (FeCO3), which isconsistent with the observed low concentrations of Fe in theseareas (Fig. 3, Fig. 4). Most groundwater samples wereconsistently supersaturated with respect to vivianite (SI N 1)suggesting that this mineral did not regulate o-PO4 concentra-tions. Mobile Fe2+ was likely supplied through reductivedissolution of Mn and Fe-oxides and hydroxides. Groundwaterwas also consistently oversaturated with respect to MnHPO4

with SI values ranging from 1.8 to 2.2 except in the upper flowareas where SI values ranged from −0.6 to 0.6. This suggests

5

6

7

8

9

10

-4 -3 -2 -1 0

pH

Log (pCO 2)

(a)

-3

0

3

-4 -3 -2 -1 0

SI C

alci

te

Log (pCO 2)

(b)

Fig. 6. The pH of groundwater and saturation index (SI) of calcite plotted against the partial pressure of CO2 (pCO2).

269P.M. Nyenje et al. / Journal of Contaminant Hydrology 164 (2014) 259–274

that MnHPO4 could have regulated o-PO4 concentrations onlyin the upgradient parts of the aquifer.

4.4.2. PHREEQC model results of phosphorus sorptionThe PHREEQC model code was used to simulate the

sorption of o-PO4 to the Fe-oxides present in the aquifer asgoethite. A plot of the modeled amount of adsorbed P and theequilibrium o-PO4 concentrations in the aquifer is shown inFig. 7. The figure shows that the amount of goethite presentwould get saturated at equilibrium o-PO4 concentrations ofabout 8 × 10−6 μmol/L, with a maximum P sorption capacityof 2.1 mmol/Kg. Here, saturation refers to a statewhere there isa rapid increase in the equilibrium orthophosphate concentra-tionswhile the amount of P sorbed remains relatively constant.The average concentration of o-PO4 measured in the aquiferwas 6 μmol/L and this value is much higher than theconcentrations that would be present (8 × 10−6 μmol/L) ifsorption to Fe-oxides controlled P concentrations in theaquifer. This implied the sorption sites on the Fe-oxides presentin the aquifer as goethite (≅400 mg/Kg: Table 1) were alreadyexhausted (or saturated) and that the adsorption of P toFe-oxides did not play a significant role in regulating Pconcentrations in the aquifer. In case ferrihydrite was thedominant Fe-oxide present, the results (in Fig. 7) show that theamount of P adsorbed would be 10 times more (21 mmol/Kg)but the equilibrium o-PO4 concentrations would remain thesame.

5. Discussion

5.1. Evidence of pollution from on-site sanitation

The shallow alluvial sandy aquifer (about 1 m thick) washighly permeable with a mean hydraulic conductivity of2.24 m/day, which suggests that it was prone to contamina-tion. It was confined at the bottom by a stiff clay layerimplying that contaminants spread mostly laterally in thedirection of groundwater movement. Groundwater consis-tently flowed in the direction NE–SW with a calculated meanresidence time of about 60 years.

Despite the presence of a large number of point pollutionsources (essentially pit latrines — average number of 10 perhectare), which would otherwise suggest that diffuse pollutionwas taking place, our field findings showed that a contaminantplume existed. This plume was characterized by a front ofpolluted groundwater moving downstream in the direction ofgroundwater flow. Our results showed that the plume originat-ed fromwastewater infiltrating from on-site sanitation systems,particularly pit latrines (the dominant formof excreta disposal—see Section 2). This is because we observed an evolution of ECvalues and Cl concentrations along the direction of groundwaterflow. The breakthrough of EC peaks have for a long time beenused to characterize contaminant plumes from wastewaterloadings (e.g. Robertson, 2008; Robertson et al., 2012). Thechloride ion is also a good conservative tracer of wastewaterleaching from pit latrines (Graham and Polizzotto, 2013). Theshallowgroundwater in the slumareawas characterized byhighvalues of EC (up to 3000 μS/cm) and high concentrations of Cl(up to 9 mmol/L), which decreased along the direction of flow.The lowest concentrations (EC b 1200 μS/cm, Cl b 1.5 mmol/L)occurred in the last transect of monitoring wells (GW24, GW25and GW26) located near the swamp (Fig. 3). This transect,therefore represented relatively unpolluted shallow groundwa-ter. More to this, the concentrations of ions (e.g. Ca, Na, Cl, HCO3

and o-PO4) in the shallow groundwater beneath the contami-nating slum area were always much higher than correspondingconcentrations in the inflowing groundwater (spring) andprecipitation (Table 2). The contaminant plume was alsodominated by HCO3-type water (up to 30 mmol HCO3/L)indicating that the shallow aquifer received high organic loadsfrom infiltrating wastewater. Organic matter is considered to bethe principal source of HCO3 in non-calcareous soils (Appelo andPostma, 2007).

5.2. Presence of redox zones

Two distinct redox zones and a transition zonewere evidentin the contaminant plume. In the upper flow zones, ground-water contained elevated concentrations of NO3 (and Mn tosome extent) whereas Fe was largely absent. In the lower flowzones in Bwaise slum, Fe (and some Mn ions) was dominant

0

3

6

9

12

15

18

21

24

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0E+00 1.0E-06 2.0E-06 3.0E-06 4.0E-06 5.0E-06 6.0E-06

sorb

ed P

(mg/

Kg)

Sorb

ed P

(mm

ol/K

g)

o-PO4 (µmol/L)

FerrihydriteGoethite

Cmax 0.07 mmol/kg = 2.1 mgP/kg

Fig. 7. A plot of dissolved o-PO4 concentrations in groundwater and modeled phosphorus concentrations adsorbed on the Fe-oxides in the aquifer material.

270 P.M. Nyenje et al. / Journal of Contaminant Hydrology 164 (2014) 259–274

whereas NO3 was absent. Hence, the shallow groundwater wasnitrate/Mn-reducing in the upper flow areas and Fe-reducing inthe lower flow areas. The lower flow areas included large partsof Bwaise slum and areas downgradient of the slum area. Thestrong Fe-reducing conditions were reflected by the highconcentrations of dissolved Fe2+ (Fig. 3). Between the upperand lower flow zones, groundwater contained all the threeredox species Fe, Mn and NO3 suggesting the redox state ofgroundwater here was in transition (Mn-reducing). Thesechanges in redox zones can be explained as follows: Whenpartly oxidized nitrate-rich groundwater flowing from theupper regolith aquifers enters Bwaise III slum (see Nyenje et al.,2013a), it mixes with wastewater leachates from on-sitesanitation. This will deplete oxygen during the degradation ofthe high organic carbon in the aquifer, hence resulting in anoxicconditions that are responsible for the observed decline innitrate concentrations (or denitrification). We think that thesource of carbon responsible for depleting oxygen anddenitrifying nitrate originated from both the high DOC inwastewater leaching into the aquifer (1–1.9 mmol/L C; Table 2)and the soil organic carbon present in the wetland soils (0.6 to0.8%; Table 1). DOC concentrationsmore than 0.4 mmol/L C areusually considered sufficient to denitrify nitrate (Rivett et al.,2008). Likewise, soil organic carbon contents in the range of0.08 to 0.16% are reported to be adequate to denitrify largequantities of nitrate (Hiscock et al., 1991; Trudell et al., 1986).Following further degradation of organic carbon from DOC andthe soil, the aquifer becomesmore reducing and in our study, itwas Fe-reducing. In between the nitrate-reducing and Fe-reducing zones, a Mn-reducing zone exists, which in this studywas evident by the presence of Mn2+ ions. Fig. 8 thereforeshows a schematic of the nutrient transport processes and theredox zones observed in the studied shallow aquifer.

5.3. Fate of nutrients (N and P)

The fate of nitrogen in the studied aquifer seemed to belargely controlled by changes in redox zones. In the upperflow boundary (which received nitrate-rich groundwaterfrom the regolith aquifer), NO3 was dominant while NH4 waslargely absent. As groundwater flowed into Bwaise III slumand other downgradient areas, NH4 became the dominant Nspecies whereas NO3 disappeared. This suggested that NO3

was removed by denitrification (NO3 converted to free N2

gas) as groundwater became more reducing (Fe-reducing)whereas NH4 was likely added following wastewater infil-tration from on-site sanitation systems in the slum area. Ourfindings are consistent with earlier studies in shallow alluvialaquifers where denitrification is believed to be the mainprocess responsible for the removal of nitrate from ground-water originating from upland areas (e.g. Hoffmann et al.,2006; Rivett et al., 2008; Stigter et al., 2011). NH4 concentra-tions ranged from 1 to 3 mmol/L (average of 2.23 mmol/L).These concentrations were, slightly lower than the potentialinput from pit latrines of 3.2 mmol/L (or 57 mg/L; Nyenje etal., 2013b), which indicated that a 30% partial removal of NH4

also likely took place. This removal, however, cannot beexplained by cation exchange because of the low CEC of thesandy aquifer (b6 meq/100 g; Table 1). The partial removalof NH4 was therefore likely due to ANaerobic AMMoniumOXidation (ANAMMOX) as demonstrated in a related studyby Robertson et al. (2012). Anammox is a bacteria-facilitatedbiological process that converts NH4 directly to N2 gas underanaerobic conditions. The presence of the anammox bacteriacan be detected by extracting the DNA of microbialcommunities using the quantitative polymerase chain reac-tion (qPCR). Evidence for the presence of anammox bacteria

ALLUVIAL SAND AQUIFER(Confined by top soils and a s�ff clay layer at the bo�om)

STIFF CLAY CONFINING LAYER

NO3-rich groundwater

REGOLITH AQUIFER

SpringHigh NO3 > 2mMLow pH < 5

SLUM AREA

Pit latrine leachateNH4 = 3.2 mMP O4 = 26 µM

Nitrate reducingzone

Mn- reducingzone

StronglyFe - reducing zone(o-PO4 regulated by adsorption to calcite)

Fe2+, NH4, PO4 < 10 MMn2+, NO3

NO3

NsoobaChannelNH4 = 0.5 – 3 mMPO4 = 3.6 – 25 µMNO3 = 0 – 0.26 mM

Fractured/ weathered regolith

N2

µ

Fig. 8. A schematic of the fate of nutrients and transport processes in the shallow aquifer beneath the slum area (modified from Nyenje et al. (2013a). Not to scale.

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in this study was, however, not carried out. In summary, theshallow sandy aquifer contributed to an almost completeremoval of NO3 originating from the nitrate-rich inflowinggroundwater due to denitrification, and to a 30% partialremoval of NH4 (1–3 mmol/L was left) originating fromon-site sanitation (particularly, pit latrines) due to anaerobicoxidation. The concentrations of NO3 and NH4 in Nsoobadrainage channel where groundwater exfiltrates ranged from0 to 0.26 mmol/L and 0.5 to 3 mmol/L, respectively (seeNyenje et al., 2013a, 2014). These concentrations are indeedclose to those measured in the shallow groundwater in thisstudy.

The fate of o-PO4, however, seemed to be governed by arange of complex processes because its concentrations weregenerally low and did not follow a particular trend along thegroundwater flow path (Fig. 3). Measured o-PO4 concentra-tions ranged from 1.3 to 13 μmol/L with a mean of 6 μmol/L.These values were very low compared to the potential inputfrom pit latrines (2.4 mg/L or 26 μmol/L; Nyenje et al., 2013b).This implies that at least 75% of P originally present inwastewater effluents was removed within short distanceduring transport from the source of pollution to the aquifer.Previous studies show that most P is retained either in thevadose zone immediately below the source of pollution or inthe aquifermaterial beneath the pollution source (Nyenje et al.,2013b; Robertson et al., 1998; Zanini et al., 1998; Zurawsky etal., 2004). Despite the high o-PO4 removal in the studiedaquifer, the concentrations measured (average of 6 μmol/L)were still slightly higher than the minimum required to causeeutrophication (0.075 mg TP/L or 2.4 μmol/L; Dodds et al.,1998). This groundwater can therefore still impair surfacewater upon groundwater exfiltration. The concentrations ofo-PO4 in Nsooba channel where groundwater dischargedwere,however, much higher (3.4–25.3 μmol/L; Nyenje et al., 2014)than those found in groundwater. This was probably due toadditional P loads from greywater effluents to surfacewater asnoted by Nyenje et al. (2014).

The attenuation of P following wastewater loading has for along time been attributed to the strong adsorption affinity of Pto metal oxides and calcite, and to mineral precipitationreactions with Fe, Mn, Ca and Al (Golterman, 1995; Reddy etal., 1999; Zanini et al., 1998). In this study, we used geochemicalmodeling to investigate the relative importance of these twoprocesses in regulating P concentrations in the shallow aquifer.Results from the PHREEQC model indicated that mineralprecipitation reactions played an important role in limitingo-PO4 transport in groundwater. Normally, minerals whose SIvalues are near zero or near saturation (SI = 0 ± 0.5) areconsidered capable of controlling mineral compositions ingroundwater. Along the upper flow boundary of the studiedslum area, conditions were slightly oxic and groundwater wasnear saturation with respect to MnHPO4 and calcite implyingthat these mineral phases controlled the presence of o-PO4 viaprecipitation (forMnHPO4) and co-precipitation (for calcite). Inthe more reducing downgradient areas (the slum area),groundwater was near saturationwith respect to rhodochrosite(MnCO3) and calcite (CaCO3) suggesting that these mineralsregulated P concentrations by co-precipitation (Fig. 3). Calcite iswidely reported to control o-PO4 concentrations in contami-nated groundwater (e.g. Cao et al., 2007; Golterman andMeyer,1985; Griffioen, 2006; Richardson and Vaithiyanathan, 1995;Spiteri et al., 2007). Calcite was likely regulated by the highpartial pressures of CO2 (10−1 to 10−0.6) due to the highorganic content in the aquifer. Mn carbonates, on the otherhand, also have a high potential to precipitate because of theirlow solubility (Boyle and Lindsay, 1986). Groundwater in ourstudy was supersaturated with respect to vivianite, siderite andMnHPO4 and undersaturated (SI b −2) with respect to hy-droxyapatite implying that these minerals were either notpresent or they were not reactive. In other studies, however,vivianite and hydroxyapatite are widely reported to regulate Pin wastewater plumes from on-site sanitation (Golterman andMeyer, 1985; Ptacek, 1998; Spiteri et al., 2007; Zanini et al.,1998). There are a number of possible reasons why these

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minerals,were not reactive in this study. First, groundwaterwasstrongly reducing (Fe-reducing) implying that there wascontinuous enrichment of Fe2+ and Mn2+, which probablyresulted in supersaturation levels of vivianite and siderite (dueto reduction of Fe III oxides to Fe2+) and MnHPO4 (due toreduction of Mn IV oxides). Second, the high SI values ofMnHPO4 were likely maintained by the carbonate phase of Mn(rhodochrosite), whose SI values were always near saturation(Fig. 5). Third, the concentrations of o-PO4 in groundwaterwerelow.

Apart of mineral precipitation reactions, sorption to theaquifer material could also have contributed to the high Pattenuation observed in this study (N75%). The adsorption ofo-PO4 is largely attributed to presence of Fe-oxides, althoughother sorbents like Al and Mn oxides and calcite are alsoconsidered important (Golterman, 1995). We used geochemicalmodeling to investigate the relative importance of the availableFe-oxides (about 400 mg/kg as goethite, Table 1) in regulatingthe transport of o-PO4 by adsorption. Results from the PHREEQCmodel simulations suggested that if sorption to Fe-oxideswas animportant process, then equilibrium o-PO4 concentrations in theaquifer would have been 8 × 10−6 μmol/L (Fig. 7). However,the measured average equilibrium o-PO4 concentration in theaquifer was 6 μmol/L. This suggested that the Fe-oxide sorptionsites present in the aquifer were few to cause a substantialdecrease in P concentrations by adsorption. The shallowgroundwater was largely anoxic (Fe-reducing) implying thatreductive dissolution of Fe oxides could have depleted theadsorption capacity of the aquifer. Fig. 7 also shows that the Psorption capacity of the aquifer (Cmax) was about 0.07 mmol/Kg(2.1 mg P/Kg). This value is much lower than the amount ofavailable P in the sediments (18–50 mg/Kg; Nyenje et al., 2013band Kulabako et al., 2008). These findings suggest that o-PO4

could have been regulated by the adsorption of P to calcite oreven clay minerals as suggested by de Mesquita Filho andTorrent (1993). But we think that calcite played an importantrole here as explained in the following text. Kulabako et al.(2008) investigated the adsorption capacity of the aquifer in thesame study area by performing sorption and fraction experi-ments on a number of soil samples (Fig. 1). Results fromKulabako's study showed that the aquifer, on the contrary,had a strong sorption capacity with a maximum sorbedphosphorus potential (Cmax) of 600 mg/Kg and a Langmuirconstant of 563 L/g. This study further performed multi-variatestatistical correlations and found out that the Langmuir Cmaxwasmore correlated with Ca, pH, organic carbon and availableP (r2 = 0.8). pH is already known to affect P sorption wherebysorption increaseswith decreasing pH (Reddy et al., 1999)whileorganic carbon could have been related to presence of organic P.AlthoughKulabako's studydid not determine the content of Al inthe soil, , P adsorption to Al-oxides is generally reported to begreatly inhibited in presence of high organic matter (up to1.9 mmol/L as DOC in this study) (Borggaard et al., 1990;Janardhanan and Daroub, 2010). We therefore conclude fromthese findings that the P sorption capacity of the aquifersediment was related to Ca content and that sorption ontocalcite could be contributing the high P retention in the studiedaquifer. This conclusion is supported by the high Ca and HCO3

concentrations we measured in the shallow groundwater(Table 2, Table S.1) and the high Ca content in the sediment(Table 1). Groundwater was also near saturation with respect to

calcite suggesting that calcite was present. Unfortunately, it isnot possible to quantify the amount of calcite or carbonatecontent in the aquifer because it was not determined. Likewise,few mineralogical studies have been carried out in the studyarea.

In general, our findings suggest that the removal of nutrientsin the lateritic alluvial sandy aquifers underlying the studiedslum area was significant due to strong Fe-reducing conditionsassociated with long residence times. These conditions favoredN removal by denitrification (for NO3) and anaerobic oxidation(for NH4), and o-PO4 removal by co-precipitation with Ca andMn carbonates — a natural attenuation process that may bepreferred in urban poor areas without an end-of-pipe treat-ment. Hence, shallow alluvial aquifers with long residencetimes provide an important sink of nutrients even in heavilycontaminated environments like unsewered urban slum areas.

6. Conclusions

We attempted to understand the processes governing thefate of sanitation-related nutrients (NO3, NH4 and o-PO4) in ashallow sandy aquifer underlying a poorly sanitized urbanslum area in Kampala Uganda. Below are the most importantconclusions from our study:

1. The shallow sandy aquifer in the slum area was heavilycontaminated as indicated by the high values of EC and highconcentrations of Cl, Na, Ca, HCO3, DOC and nutrients.Nutrients were predominantly present as NH4 (1–3 mmol/L;average of 2.23 mmol/L)whereas the concentrations of o-PO4

(average of 6 μmol/L) and NO3 (b0.2 mmol/L) were low. Weattributed this contamination to wastewater infiltrating frompit latrines.

2. Field data revealed the presence of a contaminant plume,which was characterized by decreasing values of EC and Cldowngradient of the groundwater flow line.

3. Distinct redox zones were identified along the groundwaterpath (NE–SW), whereby the groundwater in the upper flowzones was NO3/Mn-reducing whereas within the slum areaand further downstream, it was strongly Fe-reducing.Hence, groundwater was anaerobic and we attributed thiscondition to the long residence time (60 years) and the highorganic loading (up to 1.9 mmol/L as DOC) originating fromwastewater infiltration.

4. Variations in redox zones seemed to affect the mobility ofsanitation-derived nutrientswhereby nitrogen speciesweresignificantly removed from the aquifer. The high concentra-tions of NO3 (2.2 mmol/L) flowing from the upper regolithareas was almost completely removed by denitrification(b0.2 mmol/L) in the alluvial aquifer beneath the slum area.Ammonium leaching directly into the aquifer from the slumarea was also partially removed (by about 30%) because themeasured concentrations were less than the potential inputfrom pit latrines. Because the cation exchange capacity ofthe aquifer material was low (b6 meq/100 g), we conclud-ed that anaerobic ammonium oxidation (anammox) wasresponsible for the partial removal of NH4.

5. The concentrations of o-PO4 in the shallow aquifer below theslum area were generally low (average of 6 μmol/L) com-pared to the potential input frompit latrines (26 μmol/L). Ourfindings showed that P was strongly attenuated by two

273P.M. Nyenje et al. / Journal of Contaminant Hydrology 164 (2014) 259–274

processes: the adsorption of P to calcite in the aquifer and theco-precipitation of P with calcite and rhodochrosite. Theadsorption of P to Fe-oxides present in the aquifer was foundto be low because of the low Fe content (400 mg/Kg) in theaquifer sediments.

6. Our findings suggest that heavily contaminated groundwa-ter discharging from poorly sanitized slum environmentsbuilt on low-lying lateritic alluvial sandy aquifers may notbe of major concern to eutrophication when groundwaterexfiltrates to surface water systems. This is because theseaquifers are highly anaerobic, due to long groundwaterresidence times, and can therefore effectively attenuatenutrients by precipitation and adsorption with mineralcarbonates for phosphorus, and by denitrification andanaerobic oxidation which converts NO3 and NH4 to freenitrogen.

Supplementary data to this article can be found online athttp://dx.doi.org/10.1016/j.jconhyd.2014.06.011.

Acknowledgments

We thank the residents of Bwaise III parish slum whoallowed us to freely install monitoringwells in their homes andfor guarding them at the same time. Special thanks to our fieldassistants who helped us with sampling and sensitization ofthe locals about the research project. Laboratory assistants bothatMakerere University and UNESCO-IHE are acknowledged fortheir assistance in analyzing the soil and water samples. Thework presented here was funded by the Netherlands Ministryof Development Cooperation (DGIS) through the UNESCO-IHEPartnership Research Fund. It was carried out jointly withUNESCO-IHE, Makerere University, and the Kampala CityCouncil in the framework of the Research Project ‘Addressingthe Sanitation Crisis in Unsewered Slum Areas of AfricanMega-cities’ (SCUSA). It has not been subjected to peer and/orpolicy review by DGIS, and, therefore, does not necessarilyreflect the view of DGIS.

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