Estimating groundwater recharge beneath irrigated farmland using

environmental tracers fluoride, chloride and sulfate

Dan Lin & Menggui Jin & Xing Liang & Hongbin Zhan

Abstract Accurate recharge estimation is essential foreffective groundwater management, especially in the NorthChina Plain, where irrigation return flow is significant tovertical recharge but brings difficulty for recharge estimation.Three environmental tracers (F−, Cl− and SO4

2−) were used toestimate vertical recharge based on the mass balance andcumulative methods. Four boreholes were dry-drilled to 5–25m depth beneath irrigated farmland and one was drilled to5m beneath non-irrigated woodland; soil samples werecollected in all boreholes at set depths. The results indicatedthat F−, Cl− and SO4

2−were suitable tracers beneath the non-irrigated woodland, yielding recharge rates of 16.9, 18.8 and19.4mm/year, respectively. Recharge estimation was notstraightforward when taking account of crop type, irrigationand/or fertilizer use. After comparing with previous research,conclusions were drawn: Cl− was an appropriate tracer forirrigated farmland when taking account of Cl− input fromirrigation and absorption by crops; recharge rates were 65.9–126.8mm/year. However, F− was a more suitable tracer forirrigated regions where account is made of the proportion ofprecipitation to irrigation return flow, provided low F−

concentrations can be measured reliably.

Keywords China . Groundwater recharge . Unsaturatedzone . Environmental tracers . Fluoride

Introduction

Relatively accurate recharge estimation is essential for ef-fective management of groundwater and becomes especially

important in (semi) arid regions (de Vries and Simmers 2002;Lu et al. 2011). However, despite intensive research byscientists around the world for many decades, accuratelyestimating recharge is still a very challenging task. This ispartially due to the complex hydrological processes occurringin the vadose zone and the intrinsic nature of soil (e.g.heterogeneity) in addition to changes in precipitation resultingfrom global climate changes. Human activities bring inadditional difficulties, especially irrigation and application offertilizer and pesticides.

Tracer techniques, using historical tracers (tracers fromprevious human activities or significant events, like nuclearexperiment during 1960s), environmental tracers and appliedtracers (tracers injected for research purposes), are effectivefor recharge estimation but the applicability of commonlyused tracers varies with the environmental conditions. In(semi) arid regions, the chloride mass-balance technique isconsidered to be applicable and is an effective approach forestimating groundwater recharge in rain-fed areas (Gates etal. 2008; Liu et al. 2009; Scanlon et al. 2006; Wang et al.2006; Zagana et al. 2007). However, groundwater rechargein (semi) arid areas is affected by land use/land cover (Huangand Pang 2011) and this method is no longer simple to use ina groundwater-fed agricultural area (Scanlon et al. 2010a).

Previous research on this subject mainly consideredsituations of natural ecosystems and rain-fed agriculture, andconcluded that recharge rates usually varied with land use(Liu et al. 2009; Lu et al. 2011; Scanlon et al. 2006, 2007,2010a, b; Wang et al. 2008a)—for instance, the change fromno recharge to 24 mm/year recharge when the naturalecosystems was turned to rain-fed agriculture in the southernHigh Plains of USA, as recorded by chloride profiles (Scanlonet al. 2007), while the recharge rate declined from 100 to55 mm/year when natural sparse grass was turned over towinter wheat in the Loess Plateau of China (Huang and Pang2011). Moreover, recharge rates are not the same for differentregions, under different irrigation regimes and lithology, asestimated by applying artificial tracers (3H and Br−) todifferent land uses (Wang et al. 2008a). Previous studiesindicated that recharge rates in groundwater-fed agriculturewere larger than those in rain-fed areas (Scanlon et al. 2010b;Wang et al. 2008a) because irrigation return flow makes up alarge part of vertical recharge.

The purpose of this study was to test commonenvironmental tracer approaches for estimating rechargein an agro-ecosystem (both non-irrigated and irrigated

Received: 25 August 2012 /Accepted: 15 June 2013

* Springer-Verlag Berlin Heidelberg 2013

D. Lin :M. Jin ()) :X. Liang :H. ZhanState Key Laboratory of Biogeology and Environmental Geologyand School of Environmental Studies,China University of Geosciences, Wuhan, 430074, Chinae-mail: mgjin@cug.edu.cnTel.: +86-27-67883461Fax: +86-27-87436235

H. ZhanDepartment of Geology and Geophysics,Texas A&M University, College Station, TX 77845-3115, USA

Hydrogeology Journal DOI 10.1007/s10040-013-1015-y

with groundwater), based on a case study at Shijiazhuang onthe North China Plain (NCP) where groundwater is generallythe main water supply and water shortage is the mostsignificant bottleneck that restricts agriculture and hinderseconomic progress. Three widely found tracers, fluoride (F−),chloride (Cl−) and sulfate (SO4

2−), were employed for thispurpose. From the basis of this investigation, it is possible toidentify the most suitable and relatively accurate method forvertical groundwater recharge estimation in an agro-ecosys-tem, which has not been attempted before in great detail. Themotivation for choosing NCP as the study site is that NCPcontains about one fifth of the country’s agricultural land, butit has less than one sixth of the country’s available water (Liu1989). Agriculture is the major water user in NCP, consistingof approximately 70%of the total water use. In addition, mostof farmland in NCP is irrigated by groundwater pumped fromlocal aquifers, hereafter called ‘groundwater-fed agriculture’.Long-term over-exploitation of groundwater has resulted insignificant decline of the water table in the past few decades inmost parts of NCP (Fei et al. 2009). Increasing thickness ofthe vadose zone plus water-saving agricultural practices usedsince 1990s in NCP may have greatly affected the rechargeprocesses and net vertical groundwater recharge (Jin et al.2000;Wang et al. 2008a). Thus, it is essential to find a suitablemethod for estimating the vertical recharge rate beneathirrigated farmland inNCP. The findings of this research can beapplied to other agro-ecosystems, where recharge of irrigatedfarmland is of concern.

Background of the study area

The study area is located in a groundwater depressioncone at Shijiazhuang (114°20′∼114°50′E, 38°00′∼38°10′N), in the piedmont plain of NCP. The topography inclinesgently to the east, with elevation from 85 to 54 m abovesea level (m asl; Fig. 1). Because of the temperatecontinental monsoon climate, precipitation is highlyvariable both spatially and temporally. Mean annualprecipitation is 525.7 mm, 70 % of which falls betweenJuly and September. Annual precipitation from 1960 to2010 at Shijiazhuang is shown in Fig. 2. Average annualtemperature is 13.5 °C (1960–2010) and average annualwater-surface evaporation is 1,677.8 mm (1972–2008;according to statistical data of 17 observing stations inShijiazhuang city; Han et al. 2009).

Cultivating soil in the study area belongs to thecinnamon soil type, and the vadose zone is mainly madeof alluvial and proluvial sediments. The water table varieswith the seasons and has been declining since 1970s(Fig. 1). The water-table depth is greater than 50 m at thecenter of the depression cone.

Precipitation is the main source of recharge under naturalconditions with piedmont lateral flow as the secondary sourceat the study site, while beneath farmland, irrigation returnflow makes another portion of recharge. Pumping andevapotranspiration are the main discharge. Assuming thevertical recharge as the main recharge in the study area,ignoring the difference between lateral recharge and lateral

discharge, the conceptual model of vertical recharge can beexpressed by Fig. 3a. The vadose zone inNCP is an extremelycomplex layered heterogeneous media rather than a singlehomogeneous porous media (Fig. 1). Thus, a modifiedconceptual model is presented in Fig. 3b.

Farmland with winter wheat and summer maize, acommon crop rotation in the study site, is irrigated about 6times per year in the normal precipitation years (Wang etal. 2008b) and it is irrigated about 7 and 5 times per yearin drought and wet years, respectively. The total irrigationamount would be 390 mm/year assuming the averageirrigation amount is 65 mm every time (usually 60–70 mmevery time).

In this study, the recharge coefficient, R/(P + I), isintroduced to represent the percolation of precipitation andirrigation, which is mainly impacted by precipitation (orirrigation) amount, precipitation (or irrigation) intensity,lithology of vadose zone, depth to water table, terrain andvegetation, and where R, P, and I are recharge, precipitation,and irrigation return flow, respectively (Zhang et al. 2011).

Theory and methods

The chloride mass-balance technique and chloride profileswere commonly used to estimate recharge from precipitationin previous research (Gates et al. 2008; Scanlon et al. 2007;Wang et al. 2006). An improvedmass-balance technique hasbeen suggested for use beneath irrigated agro-ecosystems(Scanlon et al. 2009, 2010a, b; Stonestrom et al. 2003).

The basic assumptions of the mass-balance technique andthe cumulative method are as follows: (1) the tracer isconservative (non-reactive); (2) atmospheric input consistingof dry and wet fallout is considered to be constant with timeover long periods; (3) water flow in the thick vadose zone isconsidered to be vertically non-diffusive (piston flow) belowthe root zone. Tracers in both recharged water and soilmoisture of each layer follow the mass conservationprinciple (see Fig. 3b). Therefore, vertical recharge rate andsoil moisture age can be calculated according to the differentconcentrations of tracers in the soil moisture and rechargedwater.

F−, Cl−, and SO42− mainly come from atmospheric

deposition (including wet precipitation and dry fallout),irrigation water and fertilizer in the study area. The chemicalbehaviour of Cl− is considered to be stable under naturalconditions. The chemical properties of F− and Cl− are similarsince they both belong to the halogen group; however, F−

would be easily adsorbed by certain types of soil (Fluhler etal. 1982). The results of the column experiment of Fluhler etal. (1982) showed that whatever the medium, F− concentra-tion in effluent would reach the concentration of inflow aftera period of time and it would not always reach the maximumadsorption even if it was at an equilibrium state. Therefore,F− in outflow and inflow of the vadose zone should balancesince the time for F− to reach an equilibrium state could beignored over long periods of time. SO4

2− was also suspectedto be adsorbed by soil. Thus, a concept of water washingcoefficient (WWC; Nan and Li 2000) was introduced to

Hydrogeology Journal DOI 10.1007/s10040-013-1015-y

analyze the behavior of F− and SO42− in the profile according

to Eq. (1).

WWCi ¼ ci−1ci

ð1Þ

where WWCi is water washing coefficient of tracers inlayer i; ci−1 and ci are the concentrations of tracers in layeri−1 and i, respectively (mg/kg); layer i−1 is deeper thanlayer i. Assuming tracer input stays constant over longperiods of time, WWC should be 1 if none was adsorbed,while WWC should be 0 if all was adsorbed. Thus, if tracerwould be adsorbed by soil, WWC should decline with depth.Any adsorption of F− and SO4

2− in the vadose zone was notevident since WWCs of F− and SO4

2− do not decline with

depth (Fig. 4). Further discussion about F− and SO42−will be

presented later in section Salt inventories and sources.Assuming vertical recharge includes atmospheric de-

position (precipitation and dry fallout) and irrigationreturn flow, the mass-balance equation should be:

R ¼ cP⋅P þ Dþ cI⋅Icsw

ð2Þ

where R is average vertical groundwater recharge rate overlong term (mm/year); P is precipitation (mm/year); D issteady anion concentration in dry fallout, mg/(m2/year); I isirrigation amount (mm/year); cP is anion concentration inprecipitation (mg/L); cI is anion concentration in irrigationreturn flow (mg/L); csw is anion concentration in soil water ata certain depth (mg/L).

Fig. 2 Annual precipitation from 1960 to 2010 at Shijiazhuang (Data from China Meteorological Data Sharing Services System)

Fig. 1 The lithology of the vadose zone (details of boreholes CKB14, CKB2 and GAO1 were obtained from Geology Bureau of HebeiProvince 1981)

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If one considers the ratio of each layer’s thickness to theentire profile as a weight to calculate the mean recharge ratein the profile, then the weighted mean recharge rate is:

R ¼ R1⋅ z1−zrð Þ þ⋯þ Rn⋅ zn−zn−1ð Þzn

ð3Þ

where Rn is the average vertical groundwater recharge rateover the long term in layer n (mm/year); zr is the thicknessof root zone (cm); zn is the depth to bottom of layer n (cm).

Assuming the total anion deposition is constant withtime, the soil moisture age t(z) at the point of interest

or the cumulative method can be expressed as follows(Edson 1998):

t zð Þ ¼

Z z

0θ⋅cswdz

P⋅cP þ Dþ I ⋅cIð4Þ

where z is the depth of sampling (mm); θ is water content atthe point of interest (cm3/cm3); t(z) is usually approximatedby the sample interval as follows:

t zð Þ≈X

θi⋅ cswð Þizi� �

P⋅cP þ Dþ I ⋅cIð5Þ

where i is the interval number; θi is the water content ofinterval i; zi is the thickness of interval i (mm). Dry falloutis ignored in this study because it is much less than wetfallout (Liu et al. 2009).

Sample collection and analysis

Sample collectionFour boreholes (DH01, TY01, NC01 and GC01) were drilledwithout drilling fluid beneath winter wheat and summermaize farmland which had been irrigated with groundwaterfor decades. The boreholes were respectively in the west,middle and east of the depression cone (Figs. 1 and 5).Sediment samples were collected at given depths in August2010 (DH01, TY01 and GC01) and October 2011 (NC01).The borehole depths ranged from 5 to 25 m. In addition, a

: : : : : : : : : : : : : : : : : : : :

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: : : : : : : : : : : : : : : : : : : :

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: : : : : : : : : : : : : : : : : : :

ba

Fig. 3 a Simple conceptual model and b modified conceptual model, of vertical water flow and solute transport in a thick vadose zone. Ris the average vertical groundwater recharge rate over the long term, mm/year; P is precipitation, mm/year; I is irrigation amount, mm/year;D is steady anion concentration in dry fallout, mg/(m2/year); ET is evapotranspiration for the atmospheric boundary conditions, mm/year;zET (or zr) is the extinction depth of evapotranspiration; z is the depth and z1, z2, …, zn−1, zn is the depth to bottom of Layer n (cm); cP isanion concentration in precipitation, mg/L; cI is anion concentration in irrigation return flow, mg/L; csw is anion concentration in soil waterat a certain depth, mg/L; Rn is average vertical groundwater recharge rate over the long term in Layer n, mm/year

Fig. 4 The water washing coefficient (WWF) of F− and SO42− in

four borehole profiles

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borehole of 5 m depth (DH02) was selected as a comparisonbeneath non-irrigated woodland. Undisturbed soil sampleswere collected by sample rings for soil bulk density testing anddisturbed samples were collected in polythene bags for otheranalysis. For water content (g/g) measurements, samples ofDH01, TY01, NC01 and GC01 were collected usingaluminum moisture boxes and capped as quickly as possible,

weighed in the field immediately, then taken back to thelaboratory and oven-dried to constant weight at a temperatureof 105 °C; while water content (cm3/cm3) of DH02 was testedby soil moisture meter in situ (MPM-160, Australia, ICTInternational Pty Ltd.). Both rainwater and groundwater weresampled twice, in August and September 2010 for rainwaterand in August 2010 and October 2011 for groundwater. In

Fig. 5 Locations of the study area and boreholes

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addition, data from previous studies were collated andsummarized (see Table 1).

Sample analysis in the laboratoryIn all, 150 mL of deionized water was added to 30 g of air-dried sediment. The mixture was placed in a reciprocalshaker for 3 min in an Erlenmeyer flask and then centrifugedat 4,000 r/min for 10 min to separate liquid from sediments.The supernatant fluid was filtered (0.45 μm) and stored at4 °C before being analyzed by ion chromatography (DionexICS-1100, Thermo Fisher Scientific Inc.). Water-extractableconcentration of anions was expressed on a mass basisas mg of anion per kg of dry sediment andwas also expressedas mg of anion per liter of pore water. The test error(calculated by Eq. 6) of the ion chromatography wascalculated and the relative error of Cl− (3.1 %) was slightlylarger than F− (1.5 %) and SO4

2− (1.3 %). Anion concentra-tions of water samples were also analyzed by the Dionex ICS-1100 (Table 1). Since the recharge rates in this study representthe long-term value, the mean values of precipitation andgroundwater chemistry (Table 1) are used in the estimations.

Rerr ¼ cP−ctestctest

ð6Þ

where Rerr is test error, %; ctest is sample concentrationmeasured by Dionex ICS-1100, mg/L; cP is the concen-tration of parallel processing of sample, mg/L.

Recharge estimation by environmental tracers

The distribution of anions (F−, Cl

−and SO4

2−)

in the profilesFigures 6 and 7 present the vertical distribution of watercontent, sand, silt, clay and the concentrations of F−, Cl−

and SO42− for the different boreholes. Water content

increased when sand content decreased or clay increased.Extremely low water content lead to high values of anionconcentration (mg/L) in some layers. However, anionconcentration exhibited uniform distribution in the samelithologic layer. Tracers behaved nearly the same in siltand clay layers, while the concentrations of tracers(mg/L) increased in sand layers. F−, Cl− and SO4

2−

were conservative in clay layers and their concentra-tions in the profiles showed similar distribution, exceptin root zones. This suggests that it may be desirable touse F− and SO4

2− together with Cl− to estimate therecharge rate, and the most suitable depth for rechargeestimation should be at least below the root zone. Aninteresting observation is that concentrations of F−, Cl−

and SO42− were much larger in the vadose zone

beneath irrigated farmland than beneath non-irrigatedwoodland (Figs. 6a and 7), which may be caused bythe farmland irrigation. In addition, the concentrationof F− clearly reduced with depth in the Profile NC01while Cl− and SO4

2− exhibited relatively uniformdistribution over depth. This implies that there isspatial variability in the study area and NC01 will bediscussed separately later.

Sensitivity analysis on irrigation return flowThe concentrations of Cl− and SO4

2− in irrigation waterpumped from local aquifers were much larger than theircounterparts in precipitation except for F− (Table 1). If theirrigation term (cI·I) is much larger than the precipitation term(cP·P), the smaller one can be ignored in Eq. (2). Theirrigation amount, unfortunately, cannot be precisely deter-mined due to incomplete historic data sets for the site. Thus,sensitivity analysis was necessary to see whether theirrigation amount was a sensitive factor or not. The sensitivitywas calculated according to Eq. (7) and the amplitudevariation mainly depended on cI. The results showed that

Table 1 Anion (F−, Cl− and SO42−; mg/L) concentrations in precipitation and groundwater samples

Sample (literature source) F− Cl− SO42− Sampling time Sample site

Rainwater-1 0.3 5.6 7.3 Aug 2010 ZhengdingRainwater-2 0.3 5.6 10.9 Sept 2010 ZhengdingRainwater-3 (Han et al. 2009) – 5.33 – Xiaobi, Dahe, LuquanRainwater-4 (Liu et al. 2009) – 6.796 – 2003–2004 ZhengdingMean value of rainwater above 0.3 5.8 9.1

Groundwater-1 0.6 51.8 200.5 Aug 2010 Chencun, Dahe, LuquanGroundwater-2 (Li 2011) – 48.7 136.1 2006 Baichigan, Dahe, LuquanGroundwater-3 (Wang et al. 2006) – 43.42 – – Xiaobi,Dahe, LuquanGroundwater-4 (Zhang et al. 2004) – 42.56 – May 2001 Baichigan, Dahe, LuquanGroundwater-5 (Wang et al. 2006) – 48.76 – Sept 2002 Dahe, LuquanGroundwater-6 (Wang et al. 2006) – 52.02 – May 2001 Zhaolingpu, ShijiazhaungMean value of groundwater above for DH01 0.6 47.9 168.3

Groundwater-7 for TY01 and GC01 0.4 48.8 175.2 Aug 2010 North Gaoying, ZhengdingGroundwater-8 for NC01 – 83.5 200.0 Oct 2011 Nancun, Gaocheng

If there is no reference with the sample name, it is “this study”. DH01 located in Chencun, Dahe, Luquan; TY01 located in North Gaoying,Zhengding; NC01 located in Nancun, Gaocheng; the distances from Baichigan, Xiaobi and Zhaolinpu to Chencun are within 6 km

Hydrogeology Journal DOI 10.1007/s10040-013-1015-y

when the irrigation amount increased or decreased by 25 %,the corresponding recharge rate varied by less than 25 %when calculated using F−, Cl− and SO4

2− and the change ofrecharge rate calculated using F−was the least (see Table 2). Itappeared that all of the three tracers were suitable to estimaterecharge rate.

AR ¼X

Rt−X

RbXRb

¼ AI%⋅cI⋅IcP⋅P þ Dþ cI⋅I

ð7Þ

where AR is amplitude variation of the recharge rate; AI isamplitude variation of the irrigation amount; Rt is thecorresponding recharge rate when irrigation amount

changes (test recharge rate); Rb is real recharge rate (basicrecharge rate).

Salt inventories and sourcesSalt inventories (kg/ha) beneath each profile were calcu-lated (calculation method see Scanlon et al. 2009) and theresults are in Table 3. Profiles had variable salt concen-trations, with overall larger Cl− and SO4

2− concentrationsthan F− .

Cl− is a conservative ion, and its concentrationgenerally can only be modified by the original concentra-tion in precipitation (irrigation water) and evapotranspira-tion prior to recharge (Dogramaci et al. 2001). Thus,variations in SO4

2−/Cl− or F−/Cl− ratios indicate net

Fig. 7 Cl− , F− and SO42− changes with depth for borehole DH02 beneath non-irrigated woodland. Values are shown for mg/L (solid

lines) and mg/kg (dash lines)

Fig. 6 Cl−, F− and SO42− changes with depth in boreholes a DH01; b TY01; c GC01; d NC01 Values are shown for mg/L (solid lines) and

mg/kg (dash lines). WC is water content

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changes to SO42− or F− due to biogeochemical processes

in the vadose zone prior to recharge, or within the aquifer.F−/Cl− ratios ranged from 0.039 to 0.150, while SO4

2−/Cl−

ratios ranged from 1.760 to 5.218. Both total F− and SO42−

inventories were correlated with Cl− inventories with R2

values of 0.9916 and 0.9965 in the entire profiles (Fig. 8a,d,boreholes DH01, DH02, TY01 and GC01), which couldindicate that all three salts came mostly from the samesource. However, Borehole NC01 seemed to have differentsources of salts from the other four boreholes (DH01, DH02,TY01 and GC01). When including NC01 in the correlationfunction the R2 values of total F− and SO4

2− inventoriesslightly dropped to 0.9904 and 0.9960, respectively. Thoughthe correlations between F− (or SO4

2−) and Cl− inventories inthe profiles (except NC01) above 1.2 m were somewhatbetter than for profiles beneath 1.2 m (Fig. 8b,e andFig. 8e,f), both correlations were good with R2 valuesgreater than 0.990.

Vertical recharge estimationThe mass-balance technique and the cumulative methodwere chosen to estimate vertical recharge rate in the studyareas using three environmental tracers (F−, Cl− and SO4

2−).The mass-balance technique can be used to calculate moisture

fluxes and recharge rates and to examine modes of moisturetransport through the vadose zone, while the cumulativemethod can be used to reconstruct the paleoclimaticvariations and recharge history according to tracers andsoil moisture contents.

Irrigated profiles (DH01, TY01, NC01, GC01)There should be a certain depth, beneath which downwardwater flow is not affected by evapotranspiration in terms ofboth quantity and quality, and soil water below that depthwill definitely reach the water table after some time. Abovethat depth, evapotranspiration will affect residual soil-waterquantity and salinity (Herczeg et al. 2001). Experimentalstudy by Feng and Liu (1998) demonstrated that the wateruptake by winter wheat roots mainly happens above thedepth of 1.2 m. In addition, Zhang (1999) pointed out thatthe maximum rooting depth of summer maize was 1.2 m inTaihang Mountain Piedmont Plain, the western part of NCP.Thus, data below 1.2 m (regarded as the maximum depth ofevapotranspiration in the vadose zone) to the bottom of theboreholes were selected to estimate the vertical recharge ratebeneath irrigated profiles.

The results (except NC01) showed that there were somedifferences in recharge rates calculated using F− (rangefrom 28.3 to 90.0 mm/year), Cl− (range from 78.2 to

Table 2 Sensitivity analysis of irrigation return flow

Borehole AI (%) F− Cl− SO42−

R (mm/year) AR (%) R (mm/year) AR (%) R (mm/year) AR (%)

DH01 0 37.4 122.1 287.125 43.0 14.9 148.3 21.5 354.0 23.3

−25 31.8 −14.9 95.9 −21.5 220.2 −23.3TY01 0 28.3 78.2 88.5

25 31.8 12.4 95.0 21.5 109.2 23.4−25 24.8 −12.4 61.3 −21.5 67.8 −23.4

GC01 0 90.0 150.5 271.525 101.2 12.4 182.9 21.5 335.0 23.4

−25 78.8 −12.4 118.0 −21.5 208.1 −23.4NC01 0 20.2 135.8 68.5

25 22.7 12.4 165.1 21.5 84.5 23.4−25 17.7 −12.4 106.6 −21.5 52.5 −23.4

Table 3 Salt inventories beneath each profile

Borehole Total depth (m) Cl− (kg/ha) F− (kg/ha) F−/Cl− SO42−(kg/ha) SO4

2−/Cl− Note

DH01 18 11,008.6 822.1 0.075 21,284.2 1.93 Entire profileDH02 5 2,613.5 178.7 0.068 6,622.9 2.53TY01 18 8,841.8 368.7 0.042 27,933.7 3.16GC01 25 104,993.8 4,064.7 0.039 184,428.8 1.76NC01 5 2,960.5 445.1 0.150 15,447.2 5.22

DH01 16.8 10,277.3 734.1 0.071 19,392.4 1.89 Profile beneath 1.2 mDH02 3.8 2,043.6 108.0 0.053 5,397.3 2.64TY01 16.8 8,348.7 305.4 0.037 26,809.8 3.21GC01 23.8 101,290.8 3,554.9 0.035 174,376.1 1.72NC01 5 2,515.1 270.4 0.108 14,141.5 5.62

DH01 1.2 731.3 88.0 0.120 1,891.7 2.59 Profile above 1.2 mDH02 1.2 569.9 70.7 0.124 1,225.6 2.15TY01 1.2 493.1 63.3 0.128 1,124.0 2.28GC01 1.2 3,703.0 509.8 0.138 10,052.6 2.71NC01 5 445.4 174.7 0.392 1,305.6 2.93

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150.5 mm/year) and SO42− (range from 88.5 to 287.1 mm/

year) beneath irrigated farmland (see Table 4).

Non-irrigated profile (DH02)Data below 1.2 m in the vadose zone were used to estimatethe vertical recharge rate beneath the non-irrigated profile.The results showed that the vertical recharge rates and thepercentages of precipitation calculated by F−, Cl− and SO4

2−

were nearly the same (Table 5). This finding indicated that F−,Cl− and SO4

2− could be chosen to estimate the recharge ratebeneath non-irrigated areas.

Soil moisture age in the profilesSoil-moisture age was estimated by the cumulative methoddescribed in the preceding. The results for irrigated areas(Table 4) varied among tracers, with soil-moisture ageof: 209.9 years for F−, 50.7 years for Cl− and30.2 years for SO4

2− at the bottom end of DH01;

109.5 years for F−, 40.0 years for Cl− and 38.2 yearsfor SO4

2− at the bottom end of TY01; and 179.2 yearsfor F−, 47.8 years for Cl− and 31.4 years for SO4

2− atthe bottom end of GC01. Furthermore, a depth of 5 m(the depth of shallowest borehole) was selected for thepurpose of easy comparison, with soil moisture age of35.3–134.5 years for F−, 12.6–13.4 years for Cl− and13.3–21.1 years for SO4

2−. The range using F− waslargest and that of Cl− was narrowest, suggesting that F− hasless reliability and Cl− is the best tracer for soil moisture ageestimation. The differences between SO4

2− and Cl− for DH01and GC01were less than 1 year, suggesting that SO4

2− can beused for the soil-moisture age calculation beneath irrigatedareas. The soil moisture age for DH02 (non-irrigated area,Table 5) was 113.3 years for F−, 85.7 years for Cl− and138.4 years for SO4

2−. The soil moisture ages of DH02 weremuch larger than those of DH01, indicating that irrigationimpacted soil-water infiltration greatly.

Fig. 8 Inventories of F− and SO42− versus Cl− for sampled boreholes: a and d in entire profiles; b and e profiles below 1.2 m; and c and f

profiles above 1.2 m

Table 4 Recharge parameters beneath long term irrigated farmland

TracerBorehole Recharge parameter F− Cl− SO4

2−

DH01 Weighted mean recharge rate below 1.2–18 m (mm/year) 37.4 122.1 287.1Recharge coefficient (%) 4.1 13.3 31.4Soil moisture age at depth 5 m (year) 78.1 13.4 13.5Soil moisture age at depth 18 m (year) 209.9 50.7 30.2

TY01 Weighted mean recharge rate below 1.2–18 m (mm/year) 28.3 78.2 88.5Recharge coefficient (%) 3.1 8.5 9.7Soil moisture age at depth 5 m (year) 35.3 13.3 16.4Soil moisture age at depth 18 m (year) 109.5 40.0 38.2

GC01 Weighted mean recharge rate below 1.2–25 m (mm/year) 90.0 150.5 271.6Recharge coefficient (%) 9.8 16.4 29.6Soil moisture age at depth 5 m (year) 89.1 12.6 13.3Soil moisture age at depth 25 m (year) 179.2 47.8 31.4

NC01 Weighted mean recharge rate below 1.2–5 m (mm/year) 20.2 135.8 68.5Recharge coefficient (%) 2.2 14.8 7.4Soil moisture age at depth of 5 m (year) 134.5 13.4 21.1

Hydrogeology Journal DOI 10.1007/s10040-013-1015-y

DiscussionThe presence of Cl−—average 3,465 mg/(m2/year)—isnecessary for winter wheat production (Tian 1998). Thus,the actual recharge rate, RCl(Act), should be less than thecalculated recharge rate—RCl(Cal); see Eq. (8).

RCl Actð Þ ¼ cP Clð Þ⋅P þ Dþ cI Clð Þ⋅I−Xcsw þ Y

< RCl Calð Þ

¼ cP Clð Þ⋅P þ Dþ cI Clð Þ⋅Icsw

ð8Þ

where X (mg/(m2/year)) and Y (mg/L) are the Cl− taken up bywinter wheat from input water and soil moisture, respectively.Since data below the root zone were used in the calculations,the value of Y in Eq. (8) should be zero. Assuming the Cl− thatis essential for growth of winter wheat only comes from inputwater (X=3465 mg/(m2/year), Y=0), the modified rechargerates (ranging from 65.9 to 126.8 mm/year, as shown inTable 6) were similar to those calculated using F− (Table 4).

For the purpose of comparison, another profile (NC01)was studied using 2H and 18O distribution in the profiletogether with historical precipitation data (B. Ma, ChinaUniversity of Geosciences, personal communication, 2012).The average recharge rate was 147.9 mm/year, derived frominfiltration information recorded between 2002 and 2010from depths of 1.15 to 3.85m, which was similar to the resultcalculated using Cl− (135.8 mm/year). This finding en-hanced the credibility of the recharge estimations using Cl−.

Since boreholes DH01 and DH02 were close to eachother, they had similar terrain, lithology, water table,precipitation, etc. Moreover, their soil-moisture distributionswith depth were also similar to each other (Figs. 6a and 7).Thus, one could assume that the recharge coefficients ofDH01 and DH02 should be similar to each other as well.Using the values in Tables 4 and 5, the percentages ofprecipitation recharge and irrigation recharge were calculated

(see Table 7), hereafter called ‘recharge source division’.Previous study on recharge estimation using 3H as a tracer, inLuancheng (about 40 km southeast of Borehole DH01, seeFig. 5), gave the result that precipitation contributed slightlymore than half (55.2 %) of the vertical recharge (Wang et al.2008a). In Table 7 (this study), only the value calculatedusing F− showed a similar conclusion, i.e. precipitationcontributed 45.2 % of the vertical recharge. Hence, it can beinferred that the recharge source division calculated using F−

was relatively credible, especially beneath irrigated farmland.However, the detection limit for F− was not sufficiently lowto enable accurate results for F− in the extracted liquid(Fig. 6), which limited the method’s application.

An interesting question is why the recharge ratescalculated using F−, Cl− and SO4

2− were different. Thismay be explained as follows. When the vertical rechargewas made up of only precipitation, all of the three tracerswould be suitable for recharge calculations. When crop typeis considered, and irrigation and fertilizer application takeplace, complications arise because extra sources of F− or Cl−

or SO42− must be considered, leading to different results

using these three tracers. The average recharge coefficientbeneath groundwater-fed farmland estimated by the im-proved INFIL3.0 model was 9 % in this area (Tan 2012).INFIL3.0 is a grid-based, distributed-parameter, determinis-tic water-balance watershed model that calculates thetemporal and spatial distribution of daily net infiltration ofwater across the lower boundary of the root zone. As acomparison, the average recharge coefficient of F− (5.7 %)and modified average recharge coefficient of Cl− (10.8 %)both would be applicable. In all of the calculations, tracercontribution from fertilizer was regarded as secondary to thatprovided by precipitation and crop irrigation. Though themain element in fertilizer is (NH4)2HPO4, impurity and otherelements (including small amount of SO4

2− or F−) infertilizer are quite common.

Hence, the recharge rate and the recharge coefficientcalculated using F− and the modified recharge rates—according to Eq. (8), see Table 6—calculated by Cl− beneathirrigated farmland are promising in the study area. Therecharge rate calculated using F− was 16.9 mm/year (3.2 %of P) for non-irrigated area and ranged from 20.2 to90.0 mm/year (2.2∼9.8 % of P + I) for irrigated areas. Therecharge rate calculated using Cl− was 18.8 mm/year (3.6 %of P) for non-irrigated area and ranged from 65.9 to126.8 mm/year (7.2∼13.9 % of P + I) for irrigated areas. Ingeneral, the results calculated using F− and Cl− were similarto each other. While considering the recharge source

Table 5 Recharge parameters beneath non-irrigated woodland

TracerBorehole Recharge parameter F− Cl− SO4

2

−

DH02 Weighted mean recharge ratebelow 1.2–5 m (mm/year)

16.9 18.8 19.4

Recharge coefficient (%) 3.2 3.6 3.7Soil moisture age at depth of5 m (year)

113.3 85.7 138.4

Table 6 Modified recharge rates calculated using Cl− beneath irri-gated farmland

BoreholeDH01 TY01 GC01 NC01 Note

Modified weightedmean recharge ratebelow 1.2 m to thebottom (mm/year)

102.6 65.9 126.8 120.2 Y=0

Recharge coefficient(%)

11.2 7.2 13.9 13.1 –

Table 7 Recharge source division beneath irrigated farmland, der-ived from borehole DH01

TracerF− Cl− SO4

2− Input 3H inLuancheng(Wang et al. 2008a, )

Precipitation(%)

45.2 18.3 6.8 55.2

Irrigation returnflow (%)

54.8 81.7 93.2 44.8

Hydrogeology Journal DOI 10.1007/s10040-013-1015-y

division, the results using F−would be better when comparedto previous research, with 45.2 % of the vertical rechargecoming from precipitation. It is a great challenge to estimaterecharge rate using F− and SO4

2− beneath irrigated farmland,according to the results of NC01, in which F−was presumablyadsorbed by the soil. However, if the laboratory method usedto measure soil F− could be improved, F− could be a goodtracer in places where it was not adsorbed by soil. In thisstudy, the uncertain source of SO4

2− limited its application asa tracer. Thus, if 34S and 18O in SO4

2−weremeasured, it couldbe used to estimate recharge rate beneath irrigated farmland.

Conclusions

Environmental tracers were found to be very useful tools toestimate recharge rate beneath natural ecosystems. However,difficulties were faced when they were used beneath agro-ecosystems because of extra sources of tracers coming fromagricultural activities. Thus, the results of previous researchwere cited as a comparison for suitable tracer searching. Inthis study, water flows in the vadose zone were assumed to bevertical and the estimation of recharge was based on the mass-balance technique and the cumulative method. The followingconclusions can be made from this study.

1. F−, Cl− and SO42− are applicable in estimating vertical

recharge for non-irrigated areas, but the recharge calcula-tion should be associated with data taken from below theroot zone.

2. Because of the extremely complex layered heteroge-neous vadose zone, the recharge rates were calculatedusing a weighted average by taking account of thelithology thickness to increase the reliability of the results.

3. Considering crop type, irrigation and fertilizer application,the estimation of recharge rates and recharge coefficientswas no longer straightforward for irrigated farmland. Thisinvestigation showed that Cl−was a suitable tracer beneathirrigated farmland of the study area, with recharge ratesranging from 65.9 to 126.8 mm/year (or 7.2∼13.9 % of thesummation of precipitation and irrigation).

4. When considering the recharge source division (i.e. theproportion of recharge from precipitation and that fromirrigation), F−was a more suitable tracer. In the area whereDH01 was drilled, 55.2 % of the input water was fromprecipitation and it made up 45.2 % the vertical recharge.In addition, F− was the least sensitive to the variation inirrigation amount. Thus, F− could still be a potentiallyreliable tracer, especially where there is uncertainty in theamount of irrigation.

Acknowledgements This study was supported by the National BasicResearch Program of China (2010CB428802) and the National NaturalScience Foundation of China (41172218). We would like to thank allthe members of the project group for their help on field and laboratoryworks. The leading author thanks the China Scholarship Council forthe financial support to her 18-month visit at the University of Arizona.We gratefully acknowledge the editors and the reviewers (Dr. JiuJimmy Jiao, Dr. Reed Maxwell and two anonymous reviewers) fortheir constructive comments and suggestions for this article.

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