The fate and transport of nitrate in shallow groundwater in northwestern Mississippi, USA
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- Welch, H.L., Green, C.T. & Coupe, R.H. Hydrogeol J (2011) 19: 1239. doi:10.1007/s10040-011-0748-8
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Agricultural contamination of groundwater in northwestern Mississippi, USA, has not been studied extensively, and subsurface fluxes of agricultural chemicals have been presumed minimal. To determine the factors controlling transport of nitrate-N into the Mississippi River Valley alluvial aquifer, a study was conducted from 2006 to 2008 to estimate fluxes of water and solutes for a site in the Bogue Phalia basin (1,250 km2). Water-quality data were collected from a shallow water-table well, a vertical profile of temporary sampling points, and a nearby irrigation well. Nitrate was detected within 4.4 m of the water table but was absent in deeper waters with evidence of reducing conditions and denitrification. Recharge estimates from 6.2 to 10.9 cm/year were quantified using water-table fluctuations, a Cl– tracer method, and atmospheric age-tracers. A mathematical advection-reaction model predicted similar recharge to the aquifer, and also predicted that 15% of applied nitrogen is leached into the saturated zone. With current denitrification and application rates, the nitrate-N front is expected to remain in shallow groundwater, less than 6–9 m deep. Increasing application rates resulting from intensifying agricultural demands may advance the nitrate-N front to 16–23 m, within the zone of groundwater pumping.
KeywordsGroundwater ageNitrateRechargeGeochemical modelingUSA
Devenir et transport des nitrates en aquifère superficiel au Nord-Ouest du Mississippi, Etats-Unis
La contamination d’origine agricole de l’eau souterraine au Nord-Ouest du Mississippi, Etats-Unis, n’a pas été étudiée de façon extensive, et les flux de produits chimiques agricoles de subsurface ont été supposés minimum. Une étude a été menée de 2006 à 2008 pour estimer les flux d’eau et de solutés sur un site du bassin de Bogue Phalia (1 250 km²), dans le but de déterminer les facteurs contrôlant le transport de l’azote dans l’aquifère alluvial du fleuve Mississippi. Des données sur la qualité des eaux ont été récoltées dans un puits peu profond, selon des points de prélèvement temporaires répartis sur un profil vertical, et dans un forage d’irrigation voisin. Les nitrates ont été détectés jusqu’à 4.4 m sous le niveau statique, mais étaient absents dans les eaux plus profondes, avec des indices de réduction et de dénitrification. Les recharges ont été estimées entre 6.2 et 10.9 m/an en utilisant les fluctuations de la surface libre, une méthode de traçage par les chlorures, et des traceurs atmosphériques de datation. Un modèle mathématique d’advection-réaction a prédit à une recharge similaire, estimant aussi que 15% de l’azote introduit est lessivé vers la zone saturée. Avec les taux actuels de dénitrification et d’application d’intrants, le front de nitrate N devrait se maintenir en eau peu profonde à moins de 6–9 m de profondeur. L’augmentation des taux d’intrants liée à une intensification des pratiques agricoles pourrait repousser le front azoté à 16–23 m, à l’intérieur de la zone de pompage.
El transporte y destino de nitratos en agua subterránea somera en el noroeste del Mississippi, EEUU
La contaminación agrícola del agua subterránea en el noroeste de Mississippi, EEUU, no ha sido estudiada extensamente, y los flujos subsuperficiales de los agroquímicos agrícolas se han presumidos mínimos. Para determinar los factores que controlan el transporte de nitrato-N en el acuífero aluvial del valle del Río Mississippi, se llevó a cabo un estudio desde 2006 a 2008 para estimar los flujos de agua y solutos para un sitio en la cuenca Bogue Phalia (1,250 km2). Los datos de calidad del agua se recolectaron a partir de pozos freáticos someros, un perfil vertical de puntos de muestreo temporario, y un pozo de riego cercano. El nitrato se detectó dentro de los 4.4 m de la capa freática pero estaba ausente en aguas más profundas con evidencias de condiciones reductoras y desnitrificación. Se cuantificó la estimación de la recarga en 6.2 a 10.9 cm/año usando las fluctuaciones del nivel freático, el método de trazador de Cl– y trazadores de edad atmosférica. Un modelo matemático de advección – reacción predijo una recarga similar al acuífero, y también predijo que el 15% del nitrógeno aplicado es lixiviado dentro de la zona saturada. Con los ritmos de desnitrificación y aplicación actuales se espera que el frente de nitrato-N permanezca en el agua subterránea somera, a un profundidad menor a 6–9 m de profundidad. Los ritmos de aplicación crecientes provenientes de la demanda de la agricultura intensiva puede llevar el frente de nitrato-N a 16–23 m, dentro de la zona del bombeo de agua subterránea.
美国密西西比州西北部地下水的农业污染并没有进行过系统的研究,地下农用化学物的通量据推测是非常小的。为了确定控制密西西比河谷冲积扇含水层里硝酸盐-氮运移的因素,2006–2008年开展了估算Bogue Phalia盆地(1250 km2)某地点水和溶质通量的研究。水质数据来自于潜水含水层中的井孔、临时取样点的垂向剖面以及附近一个灌溉井。硝酸盐在水位以下4.4m内被检测到,而在更深的水里则没有硝酸盐存在,有证据表明后者处于还原环境并存在反硝化作用。根据水位波动,采用Cl–示踪方法以及大气中的年龄示踪剂确定出地下水补给量为6.2 – 10.9 cm/year。流动-反应数值模型预测的该含水层补给量结果与此相似,并估测了15%的人工氮淋滤到了饱和带中。按照目前的反硝化作用强度和施肥速率,硝酸盐-氮锋面将会局限于浅部地下水中,埋深小于6–9 m。不断扩大的农业规模导致的施肥量增加可能在地下水开采区使硝酸盐锋面下移至地下16–23m处。
O destino e transporte dos nitratos nas águas subterrâneas pouco profundas no noroeste do Mississippi, EUA
A contaminação agrícola das águas subterrâneas no noroeste do Mississippi, nos EUA, não tem sido estudada de forma extensiva, e tem-se presumido que o transporte subterrâneo de agroquímicos é mínimo. Para determinar os factores que controlam a entrada de nitrato-N no aquífero aluvionar do Vale do Rio Mississippi, realizou-se um estudo, entre 2006 e 2008, para estimar os fluxos de água e solutos numa zona da bacia Bogue Phalia (1,250 km2). Foram recolhidos dados de qualidade da água de um poço pouco profundo que capta o nível freático, de um perfil vertical de pontos de amostragem temporários e ainda de um furo de rega localizado próximo. Os nitratos foram detectados até 4.4 m abaixo do nível freático, mas estavam ausentes em águas mais profundas, evidenciando condições redutoras e desnitrificação. Obtiveram-se estimativas de recarga entre 6.2 e 10.9 cm/ano com base nas oscilações do nível freático, no método do traçador Cl– e nos traçadores atmosféricas usados para datação. Um modelo matemático de advecção-reacção previu uma recarga do aquífero semelhante, e também previu que 15% do azoto aplicado é lixiviado para dentro da zona saturada. Com as actuais taxas de desnitrificação e aplicação, a frente do nitrato-N deverá permanecer nas águas subterrâneas menos profundas, a menos de 6–9 m de profundidade. O aumento das taxas de aplicação que resultam da intensificação da procura agrícola pode causar o avanço da frente de nitrato-N até 16–23 m, dentro da zona de captação das águas subterrâneas.
Nitrate is the primary form of dissolved nitrogen in natural waters (Mueller and Helsel 1996) and is one of the largest contributors to groundwater and surface-water contamination in the world. High concentrations of nitrate in groundwater have potential health effects on drinking-water sources (Ward et al. 2005), can lead to eutrophication in streams where groundwater is a contributor to baseflow (Rabalais 2002), and can contribute to global warming (Galloway et al. 2003). Fertilizer use, livestock manure, soil mineralization, nitrogen fixation, and atmospheric deposition are the primary sources of N with farm fertilizer being one of the largest (Böhlke 2002). Increases in N fertilizer application have been significant since the late 1960s to 1970s as agricultural production has increased worldwide (Keeney 1986; Hallberg 1989). Between 1945 and 1985, the use of nitrogen fertilizer in the United States increased twenty-fold, from less than 1 million metric tons per year to more than 10 million metric tons (Mueller and Helsel 1996). As a result, several recent studies have noted concentrations above the drinking-water standards outlined by the World Health Organization (50 mg/L as NO3–; World Health Organization 2004) and the US Environmental Protection Agency (10 mg/L as N; USEPA 2006) in countries like India, China, Denmark, and the USA (Agrawal et al. 1999; Chen et al. 2005; Liu et al. 2005; Hansen et al. 2011; Puckett et al. 2011). Nutrients, including NO3– as N (nitrate-N), are naturally occurring in soils, rocks, and the atmosphere, and a national background concentration of 1 mg/L has been established for nitrate-N in shallow groundwater (well depths ≤30 m) of the United States (Dubrovsky et al. 2010). Concentrations higher than the national background can indicate influence from anthropogenic activities (Dubrovsky et al. 2010).
Due to the fertile soils in the Mississippi River alluvial plain, a region referred to locally as the Delta, the area is used extensively for agriculture. Although the primary land use is agricultural, there have been only low concentration detections of agricultural chemicals in water from pumping wells in the Mississippi River Valley alluvial (MRVA) aquifer, the irrigation source that underlies the region. Landreth (2008) sampled 705 aquaculture and irrigation wells screened in the MRVA aquifer. Nitrate was detected in water from 20% of the wells, with a maximum detection of 2 mg/L. Previous studies also show that water quality in the MRVA aquifer varies between two identified subunits. Twenty-five wells screened in Holocene alluvium and 29 wells screened in Pleistocene valley train deposits located in Arkansas, Louisiana, Mississippi, Missouri, and Tennessee were sampled as part of the US Geological Survey’s National Water-Quality Assessment (NAWQA) program (Gonthier 2003). Nitrate-N was more frequently detected and at higher median concentrations in the valley train deposits than in the alluvium. When iron concentrations were greater than 50 μg/L (under anoxic conditions), nitrate-N was not detected or was present at concentrations less than 0.5 mg/L (Welch et al. 2009). These previous studies tended to focus on the deeper portions of the aquifer currently in use for irrigation. Although the shallowest portion of the aquifer is likely most strongly affected by modern water and chemical uses, the water quality of this resource remains uncharacterized.
The factors that dominate nitrate-N contamination of groundwater and how those factors interact with local conditions to contribute to groundwater concentrations remain poorly understood. In some cases, low concentrations have been attributed to high reaction rates at sites with high concentrations of organic carbon or other electron donors (Korom 1992). In other cases, low rates of vertical transport have been cited as a factor controlling nitrate-N fluxes in aquifers underlying fine-grained soils (National Research Council 1993). Because fine-grained soils are also commonly associated with high concentrations of electron donors, the low concentrations of nitrate-N in areas such as the Mississippi Delta could be controlled by either effect. Quantitative comparisons of fluxes from nitrate-N transport and reaction are needed to determine factors controlling water quality in areas such as the Mississippi Delta.
Hydrogeology and study area description
The MRVA aquifer underlies an area of approximately 18,000 km2 and 19 counties in northwestern Mississippi. The aquifer is composed of Quaternary age clay, silt, sand, and gravel deposited by the Mississippi River and its tributaries (Arthur 1995). Average aquifer thickness is 43 m with the coarse gravel at the bottom fining upward into a layer of silts and clays which form an upper confining unit that ranges in thickness from less than 3 to 30 m thick (Arthur 1994). The two subunits of the MRVA aquifer differ in environmental setting and geologic age. The Pleistocene valley train deposits are geologically older and were deposited by high-energy braided streams. Sediments in the Pleistocene valley train deposits are coarser in grain size, and the sand and gravel layer in this portion of the aquifer is thicker and overlain by a thinner clay and silt surficial unit than the Holocene alluvium (Autin et al. 1991; Saucier 1994). The younger Holocene alluvium was deposited by meandering stream deposits and overlies the Pleistocene valley train deposits except in areas where the alluvium has been eroded exposing the valley train deposits.
Water-use data compiled in 2000 placed the MRVA aquifer as third largest in withdrawals of 66 large aquifers across the United States (Maupin and Barber 2005). Approximately 0.04 km3/day is being withdrawn, mainly for irrigation purposes (Maupin and Barber 2005). Regional groundwater flow prior to pumping for irrigation was toward the Mississippi River and southward; however, modern pumping has reversed flow toward the inner parts of the Delta (Renken 1998). Transmissivity values from six pumping tests conducted from 1954 to 1971 at wells screened in the coarse gravel portion of the MRVA aquifer ranged from 1,100 to 4,700 m2/day, and hydraulic conductivity values ranged from 40 to 120 m/day (Slack and Darden 1991). Precipitation likely is the primary source of recharge, but other contributors could be streams, lakes, upward movement from underlying aquifers, or downward seepage from irrigated lands, and lateral groundwater flow from the Bluff Hills which bound the aquifer on the east (Boswell et al. 1968). Krinitzsky and Wire (1964) stated that 5% of annual precipitation (approximately 6.6 cm) is recharged to the aquifer. A previous groundwater flow model by Arthur (2001) estimated that aerial recharge to the aquifer is 6.4 cm/year. A base-flow separation technique was used nationally to estimate values of natural groundwater recharge to the principal aquifers, which indicated that 12.7 to 25.4 cm is the mean annual recharge to the MRVA aquifer from precipitation and the interaction of groundwater with surface water (Reilly et al. 2008).
The Bogue Phalia basin (1,250 km2) is a watershed in the Delta that lies within the larger Yazoo River basin (34,900 km2; Fig. 1). Most of the Bogue Phalia basin is located in Bolivar County, MS. More than 90% of the county land use is for row-crop agriculture with the main crops being cotton, corn, rice, sorghum, and soybeans (Coupe 2002). Cotton and corn planting occurs on Dundee-type soils (fine-silty, mixed, active, thermic Typic Endoaqualfs) which compose 19% of the Delta land area and have better drainage than the Sharkey clay (very-fine, smectitic, thermic Chromic Epiaquerts that cover 26% of the Delta land area), which occurs in the interstream areas and is a dark, yellow waxy clay that tends to collect water for long periods of time (Fig. 1). Annual precipitation in the basin ranges from 114 to 150 cm, and about half of the precipitation returns to the stream as runoff from the fields, especially in the western part of the Delta where soils are much higher in clay content (Shaw et al. 2006). For the study period, annual precipitation ranged from a minimum of 92 cm in 2007 to a maximum of 132 cm in 2006.
In 2005, the Mississippi Embayment NAWQA study unit began collecting samples from air and rain, surface water, groundwater, and the unsaturated zone to investigate the sources, transport, and fate of agricultural chemicals in the Bogue Phalia basin of northwestern Mississippi. To assess the water quality of the MRVA aquifer at a site with relatively permeable soils, water samples were collected and analyzed for a variety of chemical constituents from a shallow water-table well, an irrigation well, and at a vertical profile of temporary sample points. The two wells were sampled nine times from 2006 to 2008 for major ion chemistry, nutrients, and field parameters (depth to water, pH, water temperature, specific conductance, dissolved O2 (DO), and turbidity). In addition, the five temporary sample points were sampled during June 2008 for SF6, CFCs, 3H, dissolved gases (CH4, N2, Ar, CO2, and O2), and stable isotope ratios of O and N in nitrate.
Summary of data collected in June 2008 at two permanent wells and the five temporary sampling points located at the study site in northwestern Mississippi. All concentrations in mg/L unless otherwise noted. Cation–anion balances all differed less than 2%
Type of well
Screen interval (m)
Sample depth (m)
Depth to water table below land surface
Depth below the water table (m)
Specific conductance (μS/cm)
CFC-12 apparent age in years
SF6 apparent age in years
Temporary sample point
Temporary sample point
Temporary sample point
Temporary sample point
Temporary sample point
Using a peristaltic-type pump at the water-table well and a portable, submersible pump at the irrigation well, sample collection began after purging three casing volumes and stabilizing field measurements according to USGS protocols (Koterba et al. 1995). Sample collection from the temporary sampling points differed in that Teflon tubing from the pump ran through the drill flights of the direct push equipment (the use of brand names in this report is for identification purposes only and does not constitute endorsement by the US Geological Survey). The same protocols for purging and field parameter stabilization were followed. All samples were shipped overnight on ice for analysis at the USGS National Water-Quality Laboratory (NWQL) in Denver, CO. Major ions were measured using atomic absorption spectrometry, and nutrient concentrations were quantified using colorimetry (Fishman and Friedman 1989).
Stable isotope samples (δ15N and δ18O of nitrate) were collected in a 125-ml amber polyethylene bottle with a conical-insert polyseal cap after the collection of the environmental sample. After field rinsing and collection of the isotope samples, the bottles were stored on ice. Once collection at all depth intervals was completed, the isotope samples were filtered through a 0.2-μm filter, filling the bottle only three-fourths full. The bottles were then frozen to prevent any biological reaction of the nitrogen-bearing species. Once nitrate-N concentrations were measured by the NWQL, two samples containing nitrate-N > 0.06 mg/L were shipped overnight on ice for analysis by the US Geological Survey Reston Stable Isotope Laboratory in Reston, VA, using mass spectrometry (Révész and Casciotti 2007).
Water samples were analyzed for dissolved gases, 3H, chlorofluorocarbons CFCl3 (CFC-11), CF2Cl2 (CFC-12), and C2F3Cl3 (CFC-113), and SF6 to estimate apparent ages of groundwater samples. Samples for 3H analysis were unfiltered and collected in 1,000-ml polyethylene bottles with a polyseal cap after rinsing the bottle with sample water. Tritium samples were analyzed using the direct liquid-scintillation counting method described by Thatcher et al. (1977) at the US Geological Survey Tritium Laboratory in Menlo Park, CA.
Age-dating tracers were collected after the collection of water for the analysis of nutrients, major ions, 3H, δ15N, and δ18O. All water samples were collected using nylon tubing with a 0.15-m length of Viton tubing in the peristaltic pumphead. CFCs were collected in 125-mL glass bottles with foil-lined caps following procedures outlined by the US Geological Survey CFC laboratory (US Geological Survey 2009) in Reston, VA. All bottles were stored and shipped upside down to the CFC laboratory for analysis (Busenberg and Plummer 1992). Methane was detected in some samples indicating conditions where CFCs have been known to degrade (Plummer et al. 1993). Concentrations of CFC-12, which is the most conservative CFC, were used for groundwater age estimation because CFC-11 and CFC-113 tend to degrade in anaerobic conditions. SF6 samples were collected in two 1-L plastic-coated safety amber glass bottles according to established protocols (US Geological Survey 2010). Bottles were shipped overnight to the Reston CFC laboratory for analysis using the method described by Busenberg and Plummer (2000). Samples for analysis of dissolved gases (CH4, N2, CO2, O2, and Ar) were collected in serum bottles with no headspace and analyzed by gas chromatography after creation of low-pressure headspace in the laboratory (US Geological Survey 2006). Results of the analyses were corrected for solubility in sample water at laboratory temperatures and have typical uncertainties of ±1–2%. Dissolved gases were analyzed to estimate excess air concentrations which affect calculated apparent ages from CFC and SF6 data (Plummer et al. 1993; Busenberg and Plummer 2000).
Dissolved gas calculations
In groundwater, dissolved gases originate from equilibrium exchange with the atmosphere at the water table, and dissolution of entrapped air bubbles. Air bubbles can become trapped in recharging water and entrained in the saturated zone. Similarly, denitrification produces N2 that remains in solution in recharging groundwater. As long as the hydrostatic pressure remains greater than the total pressure of gases in solution, degassing is unlikely (Blicher-Mathiesen et al. 1998). In this report, the term “excess air” refers to atmospheric gases in excess of atmospheric solubility, often caused by bubble entrainment during recharge (Aeschbach-Hertig et al. 2008), and “excess N2” refers to N2 originating from denitrification.
Excess air and excess N2 concentrations in groundwater were estimated using the concentration of N2 and Ar, their solubility in water (Weiss 1970), the atmospheric pressure, and the recharge temperature. Calculation of excess air and assumptions associated with the calculation are documented by Green et al. (2008b). The recharge temperature used in the calculation was based on the annual average groundwater temperature in the water-table well (18.6°C).
Mathematical flux modeling
For nitrate-N at agricultural sites, fN values (leached fraction of N) are typically between 0.1 to 0.5 (Böhlke 2002) which includes effects of runoff, loss of applied mass to the atmosphere, uptake by plants, and chemical transformations. A larger fraction of Cl− is expected to pass through the unsaturated zone because Cl− is less reactive than N in the soil. Values of fCl less than one could result, however, from runoff of water and solutes and from export of harvested crops.
Parameter values specified in the flux model and calibrated values estimated with the inverse model
Specified parameter values
Aquifer thickness, Hs
Unsaturated zone thickness, Hu
Adjustable parameter values
Recharge rate, R
8.8 cm/year (2.9–22)a
Effective porosity, ns
Fraction N leached, fN
Fraction Cl leached, fCl
Unsaturated zone mobile water content, nu
Denitrification rate, k
0.53 mg/L/year (0.33–1.02)
(Hill and Tiedeman 2007) where Φmin is the minimized objective function from Eq. 8, n is the number of estimated parameters, m is the number of observations, and F is the F-distribution. Nonlinear simultaneous 95% confidence intervals of parameters were computed by consistently raising (for upper confidence intervals) or lowering (for lower confidence intervals) the parameter of interest while adjusting all other parameters to maintain the objective function at a value of Φmin + δ. The confidence limit was set equal to the value of the parameter of interest at which the minimized objective function began to exceed Φmin + δ. For predictions of future nitrate-N profiles, nonlinear simultaneous 95% confidence intervals were estimated by adjusting all parameters to minimize or maximize the total N in the profile, while maintaining the objective function at a value of Φmin + δ. All solutions of confidence intervals for predictions and for parameters were validated by re-running multiple times using different starting values for the full set of parameters.
Recharge estimate methods
Results and discussion
Groundwater samples collected from 2006 to 2008 showed a surprising difference between the water-table and irrigation wells. Median values of DO, SC, Cl–, Ca, Mg, SO42–, Mn, nitrate-N, and NO2– are higher in the water-table well; whereas, for pH, K, Si, Fe, and NH4+, median values are higher in the irrigation well (data not shown). Water quality in the water-table well may reflect the influence of agricultural land use on shallow groundwater at this site because Cl−, Ca, Mg, SO42–, Mn, and nitrate-N are commonly applied to the land surface in fertilizer (Hamilton and Helsel 1995) and other soil amendments. The presence of these applied inorganic constituents at high concentrations near the water table suggests that there is downward infiltration through the unsaturated zone into the MRVA aquifer. Oxic conditions, high nitrate-N concentrations, and low Fe concentrations in the water-table well, and subsequent anoxic conditions, high Fe concentrations, and no nitrate-N in the irrigation well suggest reducing conditions in the deeper part of the MRVA aquifer.
Vertical profiles of geochemistry
Vertical profiles of geochemistry confirm that nitrate-N is attenuated during downward transport by denitrification. Nitrate is present at concentrations greater than 1 mg/L in the MRVA aquifer in the upper part of the saturated zone indicating anthropogenic sources, but is completely absent within 4.4 m of the water table (Fig. 6a and b). Nitrate is thermodynamically unstable in a Mn/Fe or SO42–-reducing zone and can undergo autotrophic denitrification where the electron donors are reduced inorganic species such as Mn2+, Fe2+, and HS– (Korom 1992). At this site, the highest SO42– concentrations observed are at or just below the depth where nitrate-N disappears from the system. In addition, Fe concentrations increase sharply below the depth where nitrate-N is depleted. Below the redox interface where nitrate-N has been depleted, decreases in SO42– concentrations suggest that SO42– reduction is occurring and increasing Fe concentrations indicate that Fe reduction is producing soluble Fe2+ in the system (Fig. 6a).
Further evidence that denitrification attenuates nitrate-N in the MRVA aquifer is the occurrence of reaction products and stable isotope enrichment in the shallow groundwater. In natural water systems, a complete loss of nitrate-N concurrent with an increase in excess N2 is evidence for denitrification. Concentrations of excess N2 produced by denitrification range from 2.4 to 8.6 mg/L (Fig. 6b); in the three samples collected at the shallowest depths, the ratio of [N2,denit]/[NO3–]0 increased with depth, indicating that denitrification progresses over a distance of approximately 3 m, and does not occur at a sharp interface. The depth profiles of water chemistry (Fig. 6a and b) show that most of the nitrate-N is lost from the system at 4.4 and 5.6 m below the water table, corresponding to the highest concentrations of excess N2. Below these depths, all nitrate-N has been converted to excess N2. Stable isotope data indicate that samples from 3.2 m below the water table with greater extent of denitrification ([N2,denit]/[NO3–]0) were associated with higher δ15N and δ18O values, 29.34 and 20.79‰ respectively, than samples from 2.0 m below the water table, with values of 16.18 and 14.61‰ respectively, as observed at other sites with active denitrification (Green et al. 2008b).
While the exact reactions driving reduction of O2, nitrate-N, and SO42– are not known, geochemical trends suggest involvement of organic carbon. Using a method from Postma et al. (1991), electron milliequivalents were calculated to assess the dominant electron donors and acceptors along the vertical depth profile. An increase in electron milliequivalents of total inorganic carbon (TIC) is similar to the loss of electron milliequivalents of nitrate-N and SO42– with depth below the water table (Fig. 6c), which indicates that organic carbon oxidation is an important electron donor for reactions occurring at the redoxcline. Because recharging concentrations of DOC in the shallowest samples were too low to account for the extent of O2 and nitrate-N reduction in deeper samples, denitrification below the water table likely is driven by organic C originating from solid phase material in the aquifer. Degradation of solid phase organic matter might also explain the general increase in NH4+ and DOC concentrations with depth (Fig. 6a and b).
Water and nitrogen fluxes
Summary of recharge estimates to the Mississippi River Valley alluvial aquifer at a site in northwestern Mississippi
The mathematical model (Eqs. 3–7a and 7b) of nitrate-N and Cl– transport indicated that fluxes of agricultural chemicals are low at this site due to low recharge and other factors, and that rates of denitrification are low, despite observations of strong reduction of Fe and SO42– in deeper samples. An estimated 85% of the applied nitrogen is lost to runoff, denitrification in the unsaturated zone, volatilization of ammonia, storage in the unsaturated zone, and exported N in harvested crops. Geochemistry at the study site suggests more strongly reducing conditions than those found at the other agricultural sites in California, Maryland, Nebraska, and Washington (Green et al. 2008b). The similar denitrification rate of 0.53 mg/L/year is surprising but emphasizes the importance of including hydrogeological analysis along with geochemical characterization in vulnerability studies. The wider range of rates reported in previous literature may relate to the effects of scale (field vs. laboratory; Green et al. 2010) and differing methods such as short-term injection-extraction tests, which may not be able to detect slow reactions occurring over the course of decades, and nitrate gradient analyses, which can be affected by the history of N inputs at the water table (Green et al. 2008b). Studies at this site and the sites in California, Maryland, Nebraska, and Washington were conducted at similar scales and used consistent methods. Prediction uncertainties for the one-dimensional advection model are shown in Fig. 7 and only include uncertainty associated with the model; thus, the uncertainties do not account for changes that may occur in the future such as changes in irrigation practices with changing land use or variations in denitrification rates as solutes move into different geochemical zones of the aquifer. However, considering the close match of predictions and observations (Fig. 7), as well as the similarity of inversely estimated parameters with previously documented estimates, the one-dimensional advection model appears to be a viable tool for predicting the occurrence and fate of nitrate-N in the MRVA aquifer at this site.
Summary and conclusions
Geochemical profiles and a mathematical model of vertical solute transport demonstrate that the MRVA aquifer underlying northwestern Mississippi at a site in Bolivar County is vulnerable to anthropogenic contamination. The flux of nitrate-N into the aquifer implies that other agricultural chemicals such as pesticides, could also migrate through the unsaturated zone into the shallow groundwater. Although conditions in the MRVA aquifer are reducing, the estimated rate of denitrification at this site, 0.53 mg/L/year, was surprisingly similar to rates that occur in aquifers with less reducing conditions. Oftentimes reducing conditions within an aquifer are seen as a sign of intrinsic invulnerability to nitrate-N contamination; however, the lack of nitrate-N detection in deeper portions of the MRVA aquifer may be a result of slow vertical travel time due to hydrogeological factors. This long time frame affords an opportunity to implement studies and balanced policies to mitigate loss of groundwater resources due to agricultural contamination.
The mathematical model of vertical movement of water and solutes was used to evaluate scenarios of the effects of increased N-applications as a result of intensifying agriculture. With current denitrification rates and current N-application rates, the nitrate-N front will reach an equilibrium depth of 7 m below the water table. Under scenarios of moderately increasing N-application rates, the migration of the nitrate-N front is affected by both the rate of increase of application, as well as, the maximum application rate. With a greater increase in N-applications, the nitrate-N front will advance as quickly as 0.16 m/year to an eventual maximum depth of 18 m below the water table which lies in the zone of pumpage from the alluvial aquifer. A great increase in N application rates is not unreasonable based on trends in use and intensifying agricultural demands. Policies for land-use management should consider that short-term and long-term vulnerability can differ greatly, and agricultural activities occurring today have far reaching implications on water-quality decades into the future. Additional study is needed to determine the sustainability of the electron donor pool and the effects of changing hydrology on the long-term vulnerability of deep groundwater in the MRVA aquifer to agricultural contamination.
The authors thank our colleagues in the US Geological Survey who contributed time, effort, and expertise, especially Patrick Mills who installed the water-table well and the five temporary sampling points. The authors would also like to extend deep gratitude to Mr. Curtis Hood of Perthshire Farms for allowing us access to his land and introducing our group to the finer art of production agriculture and for his extensive knowledge of the local agricultural history. Thoughtful reviews by Andrew O’Reilly and Brian Katz improved this report.