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1. INTERNATIONAL JOURNAL and Technology (IJCIET), ISSN 0976 – 6308 International Journal of Civil Engineering OF CIVIL ENGINEERING AND (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME TECHNOLOGY (IJCIET)ISSN 0976 – 6308 (Print)ISSN 0976 – 6316(Online)Volume 3, Issue 2, July- December (2012), pp. 465-476 IJCIET© IAEME: www.iaeme.com/ijciet.aspJournal Impact Factor (2012): 3.1861 (Calculated by GISI) © IAEMEwww.jifactor.com INTRINSIC VULNERABILITY ANALYSIS TO NITRATE CONTAMINATION: IMPLICATIONS FROM RECHARGE IN FATE AND TRANSPORT IN SHALLOW GROUNDWATER (CASE OF MOULARES-REDAYEF MINING BASIN) Nadia Khelif 1, Imed Ben Slimène 2 and M.Moncef Chalbaoui3 1 (Assitant Professor, Faculty of Science of Gafsa, 2100 Sidi Ahmed Zarroug, Gafsa, Tunisia) 2 (Ph.D, Faculty of Science of Gafsa, 2100 Sidi Ahmed Zarroug, Gafsa, Tunisia) 3 (Hydrogeologist Professor, Institute of Arts and Trades, 9100 Sidi Bouzid, Tunisia) ABSTRACT In many rapidly urbanizing cities, groundwaters are constantly affected by anthropogenic factors such as landscaping, additional abstractions, reduction in catchment perviousness, etc. Population growth has been uninterrupted and accelerating phenomena in parts of Moulares-Redayef basin, where urbanization is increasing at an unprecedented rate. Urban agglomeration is causing radical changes in groundwater recharge and modifying the existing mechanisms. The Moulares city and majority of the phosphate laundries are sited on unconfined or semi confined aquifers depend upon wadis water for most of their water supply and disposal of most of their liquid effluents and solid residues. There has also been an inevitable rise in waste production. Drainage of surface water has been disrupted as the small natural channels and low lying areas have been in filled, often with municipal waste. In agricultural areas, fertilizer application is the main source of nitrate contamination of groundwater. To develop fertilizer management strategies to combat this problem, arable land in studied area, the mining basin was evaluated using geographic information system techniques for intrinsic groundwater vulnerability to nitrate contamination. The DRASTIC method was modiﬁed to adapt it to the Moulares-Redayef environment and used for the evaluation. The rating for the net recharge factor was also modiﬁed to a dilution factor for contaminants, rather than as a transporter. However, in the pastures, vulnerability did not exhibit a clear relationship with the frequency of wells exceeding the standard. This suggests that the modiﬁed DRASTIC method is applicable for fertilizer application management in ﬁelds and in the shores of the wadis. In addition, this method will be useful for deciding the arrangement of arable land taking into consideration the potential risk of fertilizer-induced nitrate contamination of groundwater. 465
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEMEKeywords: DRASTIC, Fertilizer application, Groundwater management, GIS Hydrogeology,Nitrate contamination.I. INTRODUCTION Groundwater constitutes an important source of water for domestic, industrial,agricultural and other purposes. The ever-increasing demand for water due to increasingdemographic pressure has mounted enormous pressure on its judicious utilization. Nitrate(NO3-) stand for the most well groundwater contaminants globally , in the Moulares-Redayef aquifers and increase-known NO3- concentrations in groundwater in many regionshave been noted. Elevated concentrations of NO3- and increasing concentrations through timeare generally attributed to anthropogenic sources including agricultural fertilizers, septic andother wastewater sources, livestock facilities, and atmospheric deposition . Hence,several recent studies have noted concentrations above the drinking-water standards outlinedby the US Environmental Protection Agency (10 mg/l as N)  in countries like India, China,Denmark, and the USA .The Moulares-Redayef aquifers are quite modest in terms of yield and storage but they have aproven capability to sustain industrial, domestic and agricultural water supply as well asprovide water for crops that sustain the economy. The exponential increase in the use of thiswater resource, has led to widespread aquifer over-exploitation and groundwater qualitydeterioration. Therefore there is a need for in-depth understanding of flow and transportprocesses in these complex aquifer systems (e.g., quantitative evaluation of the resource,preservation of the quality, vulnerability assessment).Despite the likely persistence of elevated nitrate levels in these systems, questions remainabout the impact of irrigation and fertilizer use on groundwater resources.These aquifers are heavily utilized for mining industries and crop irrigation, largewithdrawals from wells and recharge from irrigation applications can substantially increasegroundwater velocities and vertical flow components , potentially affecting nitratetransport and degradation rates. However, the factors controlling the distribution of NO3-degradation in heterogeneous regional-alluvial-aquifer systems with large pumpingwithdrawals are not well understood.30 sampled wells screened in the Moulares-Redayef aquifer. Nitrate was detected in waterfrom the majority of wells, with a maximum detection of 103,76 mg/l. Nitrate was morefrequently detected and at higher median concentrations in the alluvium.Considering the depleting water resources and consequently the mounting problems,sustainable water resources development plans are needed (Nageswara Rao and Narendra2006). The policies that control groundwater exploitation are of crucial importance in waterresources management. Towards this, mapping and monitoring of existing groundwaterresources and forecasting the future resource-use scenarios are important.The integration of information on several environmental features results in zones ofpromising groundwater potential in a systematic way and forms an important aspect ofgroundwater-management studies. These data, in conjunction with ground truth information,provide details on geology, geomorphology, structural pattern and recharge conditions, whichultimately define the groundwater regime. Excessive infiltration of irrigation water canintroduce agricultural contaminants (e.g. nitrate) to shallow groundwater, increase rechargerates, and significantly alter groundwater residence times .The groundwater prospect/potential maps can show the range in groundwater yield atdifferent depths, besides indicating probable sites for recharging aquifers. Geographical 466
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEMEinformation systems (GIS) provide a means of introducing information and knowledge fromother data sources into the decision-making process and help in handling and management oflarge and complex data bases.GIS facilitates better data analysis and interpretation. Jha and Peiffer (2006) and Jhaet al. (2007) reported pertinent studies on the application of these techniques in theexploration and assessment of groundwater resources, selection of sites for artificial recharge,subsurface flow modeling, and assessment of pollution, natural recharge distribution and dataanalysis.The present paper describes the concepts, importance and applicability of GIS technologies ingroundwater studies, and critically reviews the works related to groundwater potentialassessment, to evaluate hydrogeologic factors as explanatory variables for the distribution ofchanges in NO3- concentrations over time in a complex regional aquifer system and to assessthe role of recharge dynamics on groundwater flow in a human-impacted sub watershedbased on detailed spatio-temporal field observations.II. GEOLOGY, SOIL AND HYDROLOGY OF STUDY AREA The Moulares-Redayef aquifers system occupies much the mining basin, one of themost economic basins in Tunisia and constitutes largest sedimentary depressions which aredrained by a gathering of wadis system. The surficial geology is characterized by the Plio-Quaternary sedimentary (alluvial fan deposits) deposits that are surrounded along basinmargins by Miocene bedrocks sands to the northeast and southeast highlighting themountains (fig. 1).This unit does exist where very ﬁne to ﬁne sands generally occur at the surface. However,shallow aquifers occur at relatively deeper (>15 m below ground level) depths in the centraland southern depression, where the unit is thick.The composition of soil in different surface geological units of Moulares-Redayef varies as afunction of proportions of sand, loam (silt), and clay.Average soil composition for individual soil classes was examined and later aggregated overa total zone setting. Soil composition in major wadis, and Tertiary deposits in eastern andterrains are predominantly sandy. In contrast, soil compositions in Plioquaternary terraces aremainly clayey. Surface geology and soil composition which generally characterize shallowaquifers in Moulares-Redayef basin largely control the timing and pathways of groundwaterrecharge to aquifers.Groundwater ﬂow through high permeability interﬂow zones within the goundwaters occursdominantly between successive ﬂow units (i.e. parallel to stratiform), while localized ﬂowmay occur along vertically oriented fractures and through faults in the center basin(connection of wadis). Horizontal hydraulic conductivities for the plioquaternary aquifers areon the order of 2.10-6 à 10–3 m/s with a median value of about 10–6 m/s, while for theMiocene Aquifer, are on the order of 2.10–4 m/s. Effective porosities for the mining basinrange from than 1% to greater than 3%.Lateral regional groundwater ﬂow in the aquifers is generally from topographically higherareas of the Moulares-Redayef Basin headed for the ensemble wadis. Locally, thisgeneralized ﬂow pattern is complicated by recharge from irrigation water applied to the landsurface, canal leakage, and by discharge of pumping wells. Amongst 1.03 and 5.26 Mm3/yearof industrial and irrigation water is supplied to the studied Basin from drainage ditchessupplied by the Miocene aquifer. The average annual surface-water application rate to the 467
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEMEstudy area is 1.22 m/year, and the average application rate of nitrogen fertilizers in the studyarea is approximately 15,700 kg N/km2/year. Fig. 1 Study area and Major surficial geological unitsIII. SAMPLING AND ANALYSIS To understand the chemical characteristics of groundwater and the fate and transportof fertilizer, groundwater samples were collected from 30 wells in Moulares-Redayef Basin.The collection of groundwater samples was carried out from December 2005. It must bepointed out that due to the lack of hydrogeological infrastructure at the site, includingpiezometers, no point measurements of groundwater quality could be taken. Nearly all thegroundwater sampling wells at the study site are open boreholes. The groundwater samplingdepths range from 4.5 to 60.0 m below ground surface. Be aware that although such a sampleis taken from a point, it may better represent quality of mixed groundwater over a verticaldistance between the water table and the bottom of the wellbore provided that water insidethe open borehole is well mixed.Water samples were analyzed using ion chromatography to measure nitrate (NO3–), chloride(Cl–) and sulfate (SO42–) concentrations. Concentrations of sodium (Na+), calcium (Ca2+) andmagnesium (Mg2+) were measured by inductively coupled plasma atomic emissionspectrometry. Bicarbonate (HCO3–) was determined by titration with hydrochloric acid (HCl). 1. Groundwater recharges estimatesAgricultural irrigation water is defined here as groundwater impacted by agriculturalactivities in irrigated regions. Excessive infiltration of irrigation water can introduceagricultural contaminants (e.g. nitrate) to shallow groundwater, increase recharge rates, andsignificantly alter groundwater residence times . The rating of net recharge factor in the 468
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEMEDRASTIC method Eq (1) was modiﬁed to adapt it to the Moulares-Redayef environment andused for the evaluation of fertilizer application in agriculture, rather than as a transporter.However, in the pastures, vulnerability did not exhibit a clear relationship with the frequencyof wells exceeding the standard (fig. 2). DI=Dr×Dw+Rr×Rw+Ar×Aw+Sr×Sw+Tr×Tw+Ir×Iw+Cr×Cw (1)where D: depth to groundwater, R: recharge rate (net), A: aquifer media, S: soil media, T:topography (slope), I: impact of the vadose zone, C: conductivity (hydraulic) of the aquifer r:rating for the area being evaluated and w: importance weight for the parameter. Fig. 2 The percentage of area that corresponds to the different vulnerability categories 2. Modified Net recharge Factor 2.1.Description of WetSpass model (Water and Energy Transfer between Soil, Plants, and Atmosphere in quasi Steady State)Wetspass is a model developed and integrated by ArcView  allows the calculation ofhydrological components such as: potential runoff, interception, infiltration, transpiration,evaporation from the ground surface and the natural recharge.This model is based on climate data are the physical parameters of ground such as soil type,slope, topography, land use, the hydraulic head of groundwater, rainfall, wind speed,temperature and evapotranspiration potential. The bases of these calculations are :Evapotranspiration: ET = av ETv +as ETs+ a0 ET0 + ai ETi (2)Water Space: S = a v Sv+ a s S s+ a i S i+ a0 S 0 (3)Percolation: R = a v R v+ a s R s +a0 R0 + a i R i (4)The coefficients av, as, a0 and ai are respectively the fractions: plant, soil, water andimpervious areas of a cell raster, and ETv, ETs,ET0, ETi, Sv, S s, S I,, S 0 , R v, R s , R0 and R Iare respectively evapotranspiration, runoff, bare soil, water surface and impervious areas.Precipitations are taken as a starting point for developing the balance of different componentidentified as a raster cell. The water balance of different components is treated as follows: 469
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME - The Vegetation area: the water balance depend on the seasonal precipitation (P),Intercepted fraction (I), Blade runoff (Sv), transpiration (Tv) and groundwater recharge (Rv)and can be calculated using Eq (5) (Batlaan et al;2004): P =I +S v+ T v+ R v (5) - The Blade runoff: It is calculated based on the amount rainfall intensity. Theinterception is considered as the soil infiltration. This term is calculated as follows (Batlaan etal; 2004): S v pot = C sv P I (6)Where; C sv is the runoff coefficient from an area.The potential runoff is updated with different precipitations’ intensities and according the soilinfiltration capacity : S = C Hor Sv v - pot (7)Where; C Hor is a coefficient from description of the seasonal rainfall contributing to runoff. - Evapotranspiration: Wetspass deduce the transpiration value from the value ofevapotranspiration potential estimated by Penman formula Eq (8): T rv = cE0 (8)Where; T rv is the transpiration from a vegetation area; E0 is the evaporation potential from a water surface, given by Penman equation; c is the vegetation coefficient that can be defined as a quotient of the transpiration,given by Penman-Monteith Eq (9): γ 1+ ൗ c= γ (9) 1+ ൘ r ቂ1+ c ቃ raWhere; is the constant of proportionality, is the first derivative of the vapor pressure in the saturated zone; γ is the psychometric constant; rc is the resistance; ra is the aerodynamic resistance. 2.2.The Recharge estimateThe methodology of the calculation results of the estimation of the spatial distribution ofrecharge is taken in the model based on the seasonal variation in the first place; it is possibleto have different levels of depth.Under natural or pre-developed groundwater-fed irrigation condition, net groundwaterrecharge to aquifers can be estimated using Eq (10): R = Sgw + Qbf + ETgw + (Qgwout - Qgwin) (10) 470
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEMEWhere R is net annual recharge, ∆Sgw is change in groundwater storage, Qbf is base flow towadis, ETgw is evapotranspiration from groundwater, and Qgwout - Qgwin is the netgroundwater flow from the study area. ∆Sgw, estimated using the WTF method over longtime intervals (seasonal or annual), is sometimes referred to as “net” recharge . InMoulres-Redayef basin, Qbf is inhibited during the period when wadis stages are higher thanthe water table and the shallow aquifer adjacent to major wadis experiences induced rechargethrough bank infiltration. Base flow is restricted to the early part of the dry season (i.e.,descending limb of the groundwater hydrograph) which does not affect annual water-tablerises. During the monsoon (ascending limb of groundwater hydrograph) soil moisturesustaining ET is predominantly supplied by rainfall and flood water, and ETgw via capillaryflow is inhibited by direct and indirect recharge fluxes to aquifers. The magnitude of ETgw viacapillary flow during the dry season is unclear. Net groundwater flow (Qgwout- Qgwin ) isassumed to be negligible throughout the study area due to the absence of substantial hydraulicgradients in the water table of the shallow aquifer  .IV. RESULTS AND DISCUSSIONS 1. Results samplingGroundwater samples collected showed a minimum difference between the water table andirrigation wells. Median values of Cl–, Ca, Mg, SO42–, pH, K, Si, Fe, and NO3- are higher inthe water-table well. Water quality in the water-table well may reflect the influence ofagricultural land use on shallow groundwater at this site because Cl−, Ca, Mg, SO42–andnitrate are commonly applied to the land surface in fertilizer  and other soil amendments.The presence of these applied inorganic constituents at high concentrations near the watertable suggests that there is downward infiltration through the unsaturated zone into the Mio-plio-quaternary aquifer.Estimates of groundwater recharge are shown in Figs.3, 4 and 5, for two time periods:monthly and annual recharge. The results show that actual (net) recharge is higher innorthwestern and western parts of Moulares-Redayef Basin than in eastern parts (Fig. 5). Themonthly average of the groundwater recharge of Moulares-Redayef (Fig. 3), for a period ofeight years, is from to 0.02 to 0.94 million m3 with a maximum of 0.94 in January and aminimum of 0.02 in July. Greater increases in the net recharge are observed in northwestern. Fig. 3 Monthly average estimated recharge . 471
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME Fig. 4 Annual change of recharges .regions and along the Tabadit wadi; changes in recharge are limited in many area of thebasin. Recent mean annual recharge (1997–2005) is greater than the long-term (2002 to 2005)mean recharge in some parts of the northwestern Basin. Fig. 5 Spatial variation map of annual recharge in (2004/2005) with WetSpass . 472
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEMEThe estimates of recharge for the period of high waters, revealed a spatial variation ofrecharge from West to East (5-3 mm), mainly due to rainfall gradient. The endorheicdepression of Garaaet Ed Douza and bare soils shows low value of recharge, which tend tozero. In urban areas, it was estimated that the blade refill is 2 to 3 mm, except for the northernpart of the city of Moularès where it is 3 to 4 mm due to the change in soil texture (Fig. 5).The recharge volume estimated for the year 2004 – 2005, is 2.1 million m3 whether 7.45% ofannual rainfall, 64.8% of this volume is added during the period of high water, and 35.2%during the period of low water. The total volume of recharge is 568.25 m3/ha/year (Fig. 4). 2. Relation and interactions between groundwater chemistry and recharge pathways of agricultural irrigation and industrial waterIt is shown that the greatest increases in groundwater recharge have occurred where thedensity of groundwater-fed irrigation and industrial is highest. Anomalous reductions (−0.5 to−1 mm/year between 1985 and 2007) in groundwater recharge have taken place in areas oflow groundwater abstraction for irrigation. To assess groundwater chemistry spatial andtemporal variability, a set of measurements of electrical conductivity (EC) of pumping wellshave been collected.Data show an extreme spatial variability in groundwater chemistry as illustrated by theaverage EC map (Fig. 6). An extent zone of higher mineralization can be identified, in thecentral part of the study area. Samples were classified in groups based on their position upgradient or down gradient of the wadis confluence. Higher concentrations in the downgradient sector of the aquifer cannot be attributed to progressive mineralization along flowpaths because the most down gradient wells are less mineralized. Attribution is given tosources of higher mineralization likely impact of city sewage, water irrigation and industrialwater. The chemical specificities of ions seem significantly influenced by the recharge asshown by concentrations observed in November and January, indicating water exchangebetween the different zones of the aquifer. The observed correlations between conservativeions are indicative of mixing of lower and higher concentration zones within each group.Groundwater chemistry temporal variations as indicated by EC measurements can besubdivided into two periods: the dry season (April–August) where either stableconcentrations, or the rainfall season (December–March) a progressive increase inconcentrations occur. In most cases dilution occurs due to less mineralized recharge water(and more especially near the tank), in some cases higher concentrations occurs in the SWand Central zone with observations in many wells.Changes in NO3–concentrations over time are primarily related to land use, stratigraphy, anddepth in the aquifer system. Most of the wells having increasing NO3– concentrations arelocated on down gradient side of Moulares-Reayef basin in or adjacent to urban land-useareas. Historical land use maps indicate these areas have been in urban land use for decades,whereas agricultural land use predominates in the surrounding areas. Reconstructions ofnitrogen fertilizer applications and nitrate concentrations in recharge for the study areaindicate fold increases during 1980–2010. Shallow groundwater beneath urban areas andagricultural areas in Moulares has higher NO3– concentrations (> 50 mg/l). These wells maybe more strongly influenced by southwestward flowing groundwater with higher NO3–impacted by agricultural land-use areas to the western of the basin. The presence ofthousands, boreholes backfilled with rock, which trail occasionally storm runoff intogroundwater , may also contribute to higher groundwater contaminants concentrations inthe Moulares urban area by increasing the amount of recharge from precipitation, in spite ofdiluting concentrations from up gradient land use. 473
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEMEFig. 6 Ions distributions (Na+, NO3-, SO42- et Cl-) and CE in the Moulares-Redayef basin .V. CONCLUSION The ions and pollutants concentrations generate wasted water, in the study area. Thesubstances released by humans include industrial wastes, domestic sewage, rubbish, organicand inorganic fertilizers, and pesticides, which include a range of substances that are harmfulto humans. These pollutants transported to surface water in various ways, leading todeterioration of water quality. The organic pollutants, including COD, BOD, NH3 and NO3followed sharply increasing trends from subsurface to groundwater. NO3- concentration ingroundwaters of Moulares-Redayef is higher than limit value admissible in some periods.The range of nitrate concentration is found to vary between 20 and 80 mg/l for 2009–2010.The detailed field study exploited by a high density of pumping wells (>40) reveals very highspatial variability in terms of hydraulic parameters (transfer, transmissivity) and groundwater 474
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEMEchemistry. This variability can be explained by geological factors and the impact of humanactivities. Water irrigation and industrial sewage prompt that most monitoring wells show aquite rapid hydraulic head increase is combined with an increase in EC in many of them. Theincrease in EC may be explained by two different processes or a combination of both: (1)dissolution by recharge water of salts that were deposited in the topsoil during the dry seasonleading to highly mineralized recharge water; (2) an upper limit effect such as twocompartments of the aquifer, the up gradient one being more mineralized, which aredisconnected below a given limit corresponding to the elevation of the deepest connectingfracture and get connected as the water table rises above this limit during recharge.Concluding from the analysis results above, most of the regions have a higher constraint ofwater environment and are unfit for industry. Water environment pressure along the TabaditWadi is comparatively higher and its water capacity is nearly saturated.This paper highlights the constraint effects of recharge factor on water environment layout byintegrated evaluation of both sensitivity and pressure of water environment, which is of someguiding significance in harmonizing the relationship between the industrial and agriculturaldevelopment and the water environment bearing capacity. Our results also show that theevaluation outcome is basically accordant with the actual situation in the study area. Yet, weshould vigorously promote the adjustment of pollution in order to stimulate a sustainablegrowth pattern with rapid augmentation, high efficiency, low pollution discharge and lowenergy consumption. As for the zoning method, the existing evaluation system still needs tobe improved due to the data access limit. In particular, indices like recharge, wells built-upand storm rainfall in the evaluation of water environment pressure and bio-diversity in theevaluation of its sensitivity can be considered in the future study.REFERENCES R.F. Spalding and M.E. Exner, Occurrence of nitrate in groundwater: a review. Journal ofEnvironment Quality, 22, 1993, 392–402 C. Kendall and J.J. Mc Donnell, Isotope tracers in catchment hydrology (New YorkElsevier, 1998). J.K. Böhlke, Groundwater recharge and agricultural contamination, HydrogeologyJournal, 10, 2002, 153–179. U.S. Environmental Protection Agency, 2006 edition of the Drinking Water Standardsand Health Advisories, USEPA, Washington, DC. 2006, EPA-822-R- 06-013. G.D. Agrawal, S.K. Lunkad and T. Malkhed, Diffuse agricultural nitrate pollution ofgroundwaters in India, Water Sci Technol, 39, 1999, 67–75. J. Chen, C. Tang, Y. Sakura, J. Yu and Y. Fukushima, Nitrate pollution from agriculturein different hydrogeological zones of the regional groundwater flow system in the NorthChina Plain, Hydrogeol Journal, 13, 2005, 481–492. G.D. Liu, W.L. Wu and J. Zhang, Regional differentiation of nonpoint source pollution ofagriculture-derived nitrate nitrogen in groundwater in north China, Agricol EcosystemEnvironment 107, 2005, 211–220. B. Hansen, L. Thorling, T. Dalgaard and M. Erlandsen, Trend reversal of nitrate in Danishgroundwater: a reflection of agricultural practices and nitrogen surpluses since 1950,Environment Science Technology 45, 2011, 228–234. L.J. Puckett, A.J. Tesoriero and N.M. Dubrovsky, Nitrogen contamination of surficialaquifers: a growing legacy, Environment Science Technology 45, 2011, 839–844. 475
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