Nitrate Plume Research Articles

Numerical modeling of groundwater flow and nitrate transport in karst and wastewater irrigated agricultural and forest landscapes, PA, USA

AbstractUnderstanding the influence of treated wastewater reuse for irrigation on nitrate groundwater contamination in karstic landscape is critical for developing long‐term management plans and assessing the sustainability of these practices. The Pennsylvania State University has been operating a secondary treated wastewater effluent irrigation site—the “Living Filter”—since the 1960s. The objective of this study is to develop a regional groundwater flow and nitrate transport model to assess the impact of treated wastewater and disposal of large volumes of wastewater for irrigation of agricultural and forested lands on nitrate groundwater contamination in a karst landscape in the Spring Creek Watershed in central Pennsylvania. The modular three‐dimensional finite‐difference ground‐water flow model (MODFLOW) was used to construct the groundwater flow model of the local aquifer and simulate three‐dimensional regional changes in the water table over time. The simulation of nitrate fate and transport in the aquifer was conducted by developing a groundwater mass transport model using MODFLOW and a modular three‐dimensional transport multi‐species model (MT3DMS). The calibrated groundwater model yielded a root mean square error of 5.41 ft for the area of irrigation and 24.67 ft for the entire modeled area. Results of the calibrated model show the extension of a nitrate plume over most of the study area with concentrations below 10 mg·L−1 under the current 2015 setting and the long‐term 2035 scenario of increased irrigation rates. The nitrate migration was dominated by the presence of two high conductivity groundwater troughs adjacent to the study area resulting in a near steady‐state total mass since the 1990s.

Pu distribution among mixed waste components at the Hanford legacy site, USA and implications to long-term migration

Beginning in 1943, process wastes containing approximately 1.85 × 1015 Bq (200 kg) of plutonium (Pu) were released into unlined cribs, trenches, and field tiles at the Hanford Site, Washington, USA. The 216-Z-9 (Z-9) unlined trench received over 4 × 106 L of mixed Pu processing waste from the Plutonium Finishing Plant (PFP) consisting of high ionic strength (∼ 5 M nitrate, ∼ 0.6 M aluminum), acidic (pH ∼ 2.5) solutions, which also contained the organic solvents: tributyl phosphate (TBP), dibutyl butylphosphonate (DBBP), carbon tetrachloride (CCl4), and lard oil. A small fraction of Pu migrated deep into the subsurface vadose zone to depths of 37 m, but the mechanisms controlling Pu migration beneath the trench are unknown. In this study, we determined Pu partitioning behavior in a series of binary and ternary batch experiments containing aqueous, organic, and solid phases representative of the waste constituents and natural sediments of the Z-9 trench in order to develop a conceptual model for the transport of Pu in the subsurface. Our results show that Pu at equilibrium with low pH, high nitrate waste and in the presence of a TBP/organic phase can migrate as a Pu-TBP-nitrate complex in the organic phase as long as the low pH and high nitrate concentrations are maintained. Reducing the nitrate concentrations or increasing the pH will lead to Pu partitioning into the aqueous phase from the organic phase with subsequent sorption to native Hanford sediments. The results of this work suggest that Pu migration in the subsurface is likely driven by weak sorption of aqueous Pu under low pH conditions as well as the formation of Pu-TBP-nitrate complexes in the organic phase. Long-term Pu migration will be limited by the transient nature of the low pH conditions and the dispersion of the nitrate plume.

Climate and Seasonal Temperature Controls on Biogeochemical Transformations in Unconfined Coastal Aquifers

AbstractCoastal aquifers are host to a range of biogeochemical reactions that alter groundwater‐derived nutrient, metal, and other chemical loads to coastal ecosystems. Temperature is a strong control on microbially mediated reactions; thus, chemical reactivity in coastal aquifers may vary spatially and temporally with changes to groundwater temperature. In this study, we investigated the influence of global groundwater and sea surface temperature controls and seasonal temperature variability on biogeochemical processing in coastal aquifers using variable‐density groundwater flow, heat transport, and reactive transport models. The coupled models showed that nitrate removal efficiency in coastal aquifers increased from 5% to 88% as fresh groundwater temperature increased from 5°C to 35°C, while ocean temperature had a negligible effect on removal efficiency. Transient simulations based on monthly groundwater and ocean temperature measurements showed that denitrification and ammonification hotspots migrated seaward seasonally within warm fresh groundwater masses. The reaction hotspots were separated by colder groundwater emplaced during winter months. The reaction hotspots and nitrate plumes oscillated vertically along horizontal flow paths due to buoyancy effects between warm and cold groundwater. Comparison between transient and temperature‐equivalent steady‐state models suggests that steady‐state models adequately capture mean annual NO3− removal, but neglect local reactive transience and changes to plume geometry. The sensitivity analysis provides a first‐order estimate of the reactive potential of coastal aquifers considering globally diverse thermal regimes. The findings have implications for regional‐scale estimates of groundwater nutrient fluxes and for predicting coastal aquifer reactivity in a warming climate.

Benzene degradation in contaminated aquifers: Enhancing natural attenuation by injecting nitrate

Natural attenuation processes depend on the availability of suitable electron acceptors. At the megasite Zeitz, concentrations of the main contaminant benzene were observed to increase constantly in the lower aquifer to levels of more than 2.5 mM. This was accompanied by decreasing concentrations of sulphate (SO42−), which has been previously shown to be the main electron acceptor for benzene oxidation at this site, resulting in an electron acceptor-limited, sulphidic benzene plume. Therefore, a field experiment was conducted to stimulate benzene biodegradation by injecting nitrate (NO3−) into the sulphidic benzene plume aiming (i) to recycle sulphate by nitrate-dependent sulphide oxidation, and (ii) to serve as direct electron acceptor for benzene oxidation. Within 60 days, 6.74 tons sodium nitrate (NaNO3) were injected into the lower aquifer, and the resulting biogeochemical effects within the benzene plume were monitored for more than one year by chemical and microbiological analyses of groundwater samples taken from various depths of ten monitoring wells located in three observation lines downstream of nitrate injection. Nitrate was microbiologically consumed, as shown by changes in δ15N-NO3− and δ18O-NO3− values, partial nitrite accumulation, and changing ratios of Na+/NO3−. Main electron donors for nitrate reduction were reduced sulphur compounds, verified by changing δ34S-SO42− and δ18O-SO42− values, partially increasing sulphate concentrations, and strongly increasing abundances of typical sulphur-oxidizing, nitrate-reducing bacterial taxa within the nitrate plume. The general absent hydrogen isotope fractionation of benzene, also in the sulphidic, nitrate-free part of the plume, indicates that benzene was not biodegraded by sulphate-reducing consortia. However, detected small carbon isotope fractionation of benzene points to in situ benzene biodegradation processes in the plume, probably supported by nitrate. In conclusion, nitrate injection resulted in changing redox conditions and recycling of sulphate in the sulphidic, sulphate-depleted benzene plume due to microbial oxidation of reduced sulphur species, leading to presumably favored conditions for in situ benzene biodegradation.

Feasibility Assessment of Long-Term Electrical Resistivity Monitoring of a Nitrate Plume.

Long-term monitoring solutions at contaminated sites are necessary to track plume migration and evaluate the performance of remediation efforts. Electrical resistivity imaging (ERI) can potentially provide information about plume dynamics; however, the feasibility and likelihood of success are seldom evaluated before conducting a field study. Coupling flow and transport models with geoelectrical models provide a powerful way to assess the potential effectiveness of an actual ERI field campaign. We present a coupled approach for evaluating the feasibility of monitoring nitrate migration and remediation using 4D time-lapse ERI at a legacy nuclear waste facility. This kilometer-scale study focuses on depths below the water table (∼70 m). A flow and transport model is developed to perform simulations of nitrate migration and removal via a hypothetical pump-and-treat system. A tracer injection is also simulated at the leading edge of the nitrate plume to enhance the conductivity contrast between the native subsurface and the groundwater fluids. Images of absolute bulk conductivity provide limited information concerning plume migration while time-lapse difference images, which remove the static effects of geology, provide more useful information concerning plume dynamics over time. A spatial moment analysis performed on flow and transport and ERI models matches well during the tracer injection; however, inversion regularization smoothing otherwise limits the value in terms of locating the center of mass. We find that the addition of a tracer enables ERI to characterize plume dynamics during pump-and-treat operations, and late-time ERI monitoring provides a conservative estimate of nitrate plume boundaries in this synthetic study.

Adaptive observation-based subsurface conceptual site modeling framework combining interdisciplinary methodologies: a case study on advancing the understanding of a groundwater nitrate plume occurrence.

Traditional site characterization and laboratory testing methods are insufficient to quantify and conceptualize subsurface contaminant source-pathway-receptor heterogeneity issues, as they hamper groundwater risk assessment and water resource management using mathematical modeling. To address these issues, we propose an adaptive observation-based conceptual site modeling framework, which emphasizes the need for the iterative testing of hypotheses centered on specific questions with clearly defined objectives using interdisciplinary tools (including, but not limited to, geology, microbiology, hydrogeology, geophysics, and the chemistry of solute fate and transport). Under this framework, we present a case study aimed at a goal-oriented investigation of the source and occurrence of a groundwater nitrate plume previously identified using chemical concentration data from sparsely distributed, conventional, and regional groundwater monitoring wells. These investigations occurred in stages, with the first comprehensive outcome of cost-efficient, non-invasive surface geophysical surveys localizing subsurface heterogeneities laying the groundwork for collaborative, minimally invasive, direct push-based investigations followed by groundwater chemical and stable isotope analyses for source fingerprinting and bioprocess evaluation. Despite the obvious need for further refinement of the conceptual site model as new data become available, we illustrate that the step-by-step integrative framework was useful for systematic maximization of the strengths of different investigation methodologies. Such frameworks and approaches should be encouraged for successful environmental site characterization, monitoring, and modeling.

The evaluation and prediction of agriculture-related nitrate contamination in groundwater in Chengdu Plain, southwestern China

The aquifer beneath Chengdu Plain is a main water resource for drinking and industry, which has severe groundwater nitrate contamination due to the shallow water table and intensive agricultural activities. The aim of this study is to estimate nitrogen loading to the aquifer and to predict nitrate concentration in groundwater in response to changes in land-use and agricultural practices, i.e. nitrogen fertilizer application, in ‘Jingyang region’ of the Chengdu Plain, one of the most developed agriculture areas in southwestern China. The framework utilized a geographic information system to account for the spatial and temporal distribution of on-ground nitrogen sources and corresponding loadings, and employed an export coefficient model to estimate the corresponding nitrogen loss. The simulation of nitrate fate and transport in the aquifer was conducted by developing a groundwater mass transport model using MODFLOW and MT3DMS. A number of predictive simulations for year 2026 were carried out to evaluate the impact of land-use and proposed future management options on controlling groundwater nitrate contamination. The results show that a nitrate plume would extend over most of the study area with some local areas having concentrations exceeding 20 mg L−1 under the current setting and the scenario of increasing agricultural land use. Reduction in fertilizer application rates did mitigate nitrate contamination; however, this effect diminished with the expansion of agricultural land use. Land-use planning should consider the significant effects of agriculture to improve the quality of groundwater in the Chengdu Plain.

Modification to the DRASTIC framework to assess groundwater contaminant risk in rural mountainous catchments

Mountainous headwater catchments are safeguards of quality groundwater and hence require special protection against contamination by anthropogenic sources. However, methods currently handling contamination risk fail to produce reliable results in mountainous watersheds because they overlook the influence of downhill flows and contaminant transport in the validation process. To overcome this difficulty, a new model based on so-called “concentration profiles” is presented that combines the DRASTIC framework for evaluation of intrinsic vulnerability, the categorization of land uses for evaluation of specific vulnerability to nitrate and Processing Modflow graphical user interface for simulation of nitrate transport. This model was tested in a mountainous region of Northern Portugal. The risk of groundwater contamination by nitrate was generally classified as moderate. The risky areas are regions used for agriculture and livestock production. These activities have raised nitrate concentrations of spring water (15–25 mg·L−1) downstream the risky areas. The Modflow simulations linked the risky areas (contaminant sources) to actual nitrate plumes (contaminant sinks) and modeled nitrate distributions at specific groundwater travel times. Winter plumes could be simulated for the 1-year stress period, and hence are flushable in a short time span. Spring and summer plumes could only be explained by contaminant transport during 10–20 years. In these cases, even if contaminant sources were immediately neutralized, the washout of nitrate would take decades. These results may hold back the fulfillment of sustainable development goals related to water and sanitation until 2030, and hence deserve reflection by water planners and policy makers. The modeling exercise provided extra evidence that safeguarding the catchment headwater is the keystone of groundwater quality protection in mountainous catchments. Therefore, application of this modified DRASTIC to other mountainous areas may not need to resort to Processing Modflow. The study comprises the main paper (this paper) and a MethodsX companion paper.

Investigating “net” provenance, N source, transformation and fate within hydrologically isolated grassland plots

Agricultural landscapes contain many different soil types with heterogeneous nitrogen (N) attenuation capacity. Typically, a zone of contribution (ZOC) surrounding a borehole is used to interpret subsurface hydro-biogeochemical functional capacity. This presents a “net” interpretation of source and attenuation within these calculated areas. Herein, we use the concept of ZOC commonly used for borehole screen intervals but for an end-of-pipe location within four hydrologically isolated plots. Water samples from end-of-pipe and piezometer locations are examined for nitrogen (N), biogeochemical, dissolved gas and isotopic viewpoints to elucidate multi-layered “net” water provenance, N source, transformations and fate. Results showed a nitrate (NO3−-N) plume migrating in shallow groundwater (between 0.39 and 8.07 mg N/L), with low concentrations in the shallow artificial drainage system (below 3.22 mg N/L). Water provenance data showed distinct signatures of: precipitation and deep groundwater at 3–4 m below ground level (bgl) and water entering, migrating and discharging at the end of pipe location. The latter signature was caused by enrichment of δ18O-H2O during migration. This means there was disconnectivity on site with no interaction between water migrating through the drainage pipe at 1 m and deeper groundwater migrating at 3–4 m depth. The analysis of NO3−-N concentration and its isotopic signature (δ15N-NO3− and δ18O-NO3) identified further connections between screen interval depths and an up-gradient organic point source with elevated NO3−-N migrating at this depth and different transformation processes occurring at different depths. Temporally NO3−-N concentrations at this depth have decreased over time. Fenton et al. documented an average of 7.5 (±4.5) mg N/L whereas Ibrahim et al. documented an average of 6.8 (±3.7) mg N/L at this depth. The point source was removed in 2006 and NO3−-N concentration in the present study have further reduced to an average of 3.9 (±2.8) mg N/L. End-of-pipe data at 1 m bgl highlighted connectivity with the overlying plot and showed different water attenuation functionality than the deeper system. End-of-pipe locations clustered together along the denitrification line. This highlighted a consistency of signals across the four plots in terms of what occurs in the soil profile above the drain installation depth of 1 m. At 3–4 m bgl however, samples varied spatially showing inconsistency between the end-of-pipe locations and plots indicating the occurrence of different processes.A fuller characterisation of dairy farm N sustainability can be deemed using the “net” provenance, N source, N transformation and fate methodology presented. Future work should investigate how drainage design (shallow and groundwater) affects N transformation and the “net” concept developed herein should be rolled out to rank dairy farms in terms of their N attenuation capacity.

Iodate and nitrate transformation by Agrobacterium/Rhizobium related strain DVZ35 isolated from contaminated Hanford groundwater

Nitrate and radioiodine (129I) contamination is widespread in groundwater underneath the Central Plateau of the Hanford Site. 129I, a byproduct of nuclear fission, is of concern due to a 15.7 million year half-life, and toxicity. The Hanford 200 West Area contains plumes covering 4.3 km2 with average 129I concentrations of 3.5 pCi/L. Iodate accounts for 70.6% of the iodine present and organo-iodine and iodide make up 25.8% and 3.6%, respectively. Nitrate plumes encompassing the 129I plumes have a surface area of 16 km2 averaging 130 mg/L. A nitrate and iodate reducing bacterium closely related to Agrobacterium, strain DVZ35, was isolated from sediment incubated in a 129I plume. Iodate removal efficiency was 36.3% in transition cultures, and 47.8% in anaerobic cultures. Nitrate (10 mM) was also reduced in the microcosm. When nitrate was spiked into the microcosms, iodate removal efficiency was 84.0% and 69.2% in transition and anaerobic cultures, respectively. Iodate reduction was lacking when nitrate was absent from the growth medium. These data indicate there is simultaneous reduction of nitrate and iodate by DVZ35, and iodate is reduced to iodide. Results provide the scientific basis for combined nitrogen and iodine cycling throughout the Hanford Site.

An approach to validate groundwater contamination risk in rural mountainous catchments: the role of lateral groundwater flows

Computer models dedicated to the validation of groundwater contamination risk in the rural environment, namely the risk of contamination by nitrate leachates from agriculture fertilizers, are frequently based on direct comparison of risky areas (e.g., cropland, pastures used for livestock production) and spatial distributions of contaminant (nitrate) plumes. These methods are fated to fail where lateral flows dominate in the landscape (mountainous catchments) displacing the nitrate plumes downhill and from the risky spots. In these cases, there is no connection between the spatial location of risky areas and nitrate plumes, unless the two locations can be linked by a contaminant transport model. The main purpose of this paper is therefore to introduce a method whereby spatio-temporal links can be demonstrated between risky areas (contaminant sources), actual nitrate plumes (contaminant sinks) and modeled nitrate distributions at specific groundwater travel times, thereby validating the risk assessment. The method assembles a couple of well known algorithms, namely the DRASTIC model [1,2] and the Processing Modflow software (https://www.simcore.com), but their combination as risk validation method is original and proved efficient in its initial application, the companion paper of Pacheco et al. [3].

An assessment of groundwater salinization in Haryana state in India using hydrochemical tools in association with GIS

Unplanned abstraction of groundwater due to various land use land cover activities and variations in monsoonal rainfall have greatly affected the availability and quality of groundwater resources in semi-arid regions of India. In the present study, a study of the hydrogeochemical characteristics of groundwater was undertaken in the Sonipat district of Haryana in India together with the use of stable isotope (δ18O and δD) measurements and GIS analysis. A total of 53 groundwater samples were collected from seven blocks of the district, and 14 water quality parameters and stable isotopes (δ18O and δD) were analysed to infer hydrogeochemical processes taking place in the area. The integration of hydrochemistry with GIS is very helpful to understand the factors governing in the area. The majority of the samples showed Na–Cl type of hydrochemical facies. The trilinear plot for major cations and anions in groundwater indicates dominance of sodium, calcium, chloride and bicarbonate ions. Nitrate plumes in the groundwater appear to be migrating in groundwater from the central and south-western parts of the area towards the urbanized areas. A total of 64% of the samples exceed the maximum permissible limit of 1.5 mg/L given by WHO for fluoride. Besides natural sources such as silicate and carbonate weathering, ion exchange, and reverse ion exchange, the leaching of surficial salts and untreated industrial wastes along with unregulated abstraction are contributing to poor groundwater quality in the study area. An assessment of saturation indices has shown that groundwater in the area is unsaturated with respect to anhydrite, halite and gypsum suggesting significant contribution of Ca2+, Mg2+ and other ions in the groundwater. A scatter plot of δ18O versus Cl also suggests mixing of saline water with fresh groundwater.

Perturbation induced by a nitrate plume on diffusion of solutes in a large-scale clay rock sample

Mudstones are widely studied in the context of waste landfill leachates or as geological barriers for waste management. Some of the technological wastes undergo biogeochemical degradation, releasing sulfate or nitrate plumes. The concomitant increase of salts concentration and ionic strength may perturb the geological media and affect transport properties. The study investigated the effect of a nitrate plume on the diffusion of ions. The confinement of ionic solutes was quantified in Callovian-Oxfordian mudstone. A 0.5molL−1 NaNO3 solution was injected in a large-scale sample with a low water/rock (≪1.0Lkg−1) close to environmental conditions. The in-diffusion of HTO, 36Cl, NO3−, 22Na, 137Cs was monitored during the perturbation. A minor impact was observed on poral water chemistry and on ions diffusion. The anion exclusion was only reduced by 19% for chloride and effective diffusion of cations reduced by 20 to 30%. Diffusion data of NO3− was quantified and anion exclusion found to be intermediate between that of halogen anions and that of water. Values of diffusion anisotropies were adjusted to a(36Cl)=1.9 and a(22Na)=2.5, close to data in sound sample. All these results confirmed a limited perturbation by 0.5molL−1 nitrate plume on the confinement properties of the clay rock. Additional data is provided on major ions released from the rock and diffusing towards the injection solution. It may be useful for further chemistry-transport modeling including co-diffusion of ions, surface diffusion, anion exclusion and effect of ionic strength.

Nitrate-N removal using slowly released molasses barrier in a shallow aquifer: Obstacles from lab/pilot-scale results to field application

This study was conducted to test the field applicability of a well-type barrier system containing Slowly Released Molasses (SRM) as an extra carbon and energy source to promote the indigenous bacterial denitrification activity. A total of 22 wells were placed as a grid system in the 4 m × 4 m square by 1-m interval in one alluvial area. A total of 70 SRM rods were placed in the center of the grid system to consist of a field-scale SRM system. Nitrate-N plume (72 mg N L−1 of nitrate-N) was introduced into the SRM system and change in nitrate-N concentration was monitored for 90 hours. Nitrate-N concentration decreased up to only 39% while 79-85% of removal efficiency was attained from lab/pilot experiments. Such a difference probably resulted from heterogeneity/anisotropy of the aquifer, DO concentration in the shallow groundwater, and a wide SRM rods-containing well distance. Spatial heterogeneity/anisotropy induced the preferential groundwater flow paths in which nitrate plume moved so fast and met the limited quantity of molasses molecules within or near the paths. Relatively aerobic condition of the shallow aquifer had not changed during the test period. Relatively far distance (100 cm) between the wells led to the incomplete mixing of the released molasses and nitrate-N plume. A detailed characterization of a target site should proceed before applying the SRM system for more effective nitrate-N treatment in the field.

Factors affecting agricultural nitrogen removal in riparian strips: Examples from groundwater-dependent ecosystems of the Po Valley (Northern Italy)

The role of riparian ecosystems in mitigating the effects of diffuse agricultural sources is recognized in several regulatory measures and public policy initiatives in many parts of the world. This study aimed to evaluate the N buffering capacity of semi-natural riparian zones associated with spring-fed lowland streams, also known as “fontanili”, representing the most important groundwater-dependent ecosystems in Northern Italy. Monitoring parcels were set up in nine riparian sites selected to cover a range of different soil properties and hydrogeological settings, and to sustain the evaluation of the main drivers affecting their N removal efficiency. Subsurface water level, nutrient concentrations and the main hydro-chemical parameters were monitored along transects of piezometers installed from crop fields to the spring channels. On selected samples from two sites stable isotopes of the water molecule were also determined. Median NO3− input concentrations from adjacent cropland to the riparian sites ranged from 0.10 to 21mgNL−1, with maximum values exceeding the drinking water limit recorded during the summer and winter fertilization periods. Highly variable groundwater nitrate patterns were found in the riparian areas, including short nitrate plumes extending from the adjacent cropland into some riparian zones, or in others, small patches where NO3− declined at variable distance from the stream. Some chemical indicators (e.g., NO3−/Cl− ratio, O2, DOC) suggested that NO3− attenuation was mostly due to the denitrifying activity occurring in the subsurface aquifers in specific conditions (hot spots and moments), although, in some cases, physical processes such as dilution also contributed. The overall N removal efficiency was greater than 90% in four sites, 74%, 34% and 30% in three sites, and zero in the remaining two sites. Useful predictors of the nitrate removal capacity were factors linked to the water residence time, such as the hydraulic conductivity, the soil texture and the slope of the riparian profile, together with the water table depth and soil organic carbon. A combination by standardized averaging of these five factors supported a clear discrimination of sites with zero or low N removal effectiveness from those with high efficiency.

Controls on mixing‐dependent denitrification in hyporheic zones induced by riverbed dunes: A steady state modeling study

AbstractThe hyporheic zone is known to attenuate contaminants originating from surface water, yet the ability of the hyporheic zone to attenuate contaminants in upwelling groundwater plumes as they exit to surface water is less understood. We used MODFLOW and SEAM3D to simulate hyporheic flow cells induced by riverbed dunes and upwelling groundwater together with mixing‐dependent denitrification of an upwelling nitrate ( ) plume. Our base case modeled labile dissolved organic carbon (DOC) and dissolved oxygen (DO) advecting from surface water, and DO and advecting from groundwater, typical of certain agricultural areas. We conducted sensitivity analyses that showed mixing‐dependent denitrification in the hyporheic zone increased with increasing hydraulic conductivity (K), decreasing lower boundary flux, and increasing DOC in surface water or in groundwater. Surface water DOC, groundwater , and K were the most sensitive parameters affecting mixing‐dependent denitrification. Nonmixing‐dependent denitrification also occurred when there was surface water , and its magnitude was often greater than mixing‐dependent denitrification. Nevertheless, mixing‐dependent reactions provide functions that nonmixing‐dependent reactions cannot, with potential for hyporheic zones to attenuate upwelling plumes, depending on geomorphic, hydraulic, and biogeochemical conditions. Stream and river restoration efforts may be able to increase mixing‐dependent reactions by promoting natural processes that promote bedform creation and augment labile carbon sources.

Role of a groundwater–lake interface in controlling seepage of water and nitrate

Summary The effects of the hydraulic characteristics of a groundwater–lake interface on discharge and nitrate loading to a lake were investigated. The interface is defined as the zone separating the adjacent aquifer (10’s of m) and the lake bed (10’s of cm) itself. The study combines field data using several tracers (water, oxygen isotopes, and nitrate) and numerical modeling. The hydraulic head distribution, a nitrate plume and seepage rates were observed over a two-year period along a ∼100 m long transect reaching from an agricultural field into the lake. The groundwater–lake interface system was simulated with a 2D steady state flow and nitrate transport model (FEFLOW). The observations showed that discharge to the lake was doubled-peaked, with a peak discharge near the shore line followed by an almost (classical) exponential decrease, and a second peak further off-shore. The nitrate plume also extended 60–80 m off-shore. By calibrating the model to measured discharge and the outline of the nitrate plume it was demonstrated that; (1) the ratio of horizontal to vertical hydraulic conductivity (anisotropy) was very important and on the order of 50 and (2) the lake bed acted as a hydraulic barrier by having a much lower hydraulic conductivity than that of the relatively homogeneous aquifer. We suggest that the barrier is formed by an extensive plant cover that can trap finer materials and produce a surface colmation layer. The simulation results show that when a barrier is present the total groundwater discharge to the lake can be up to a factor of two lower and that approximately 50% of the nitrate bypasses the barrier. This proportion of the nitrate loading will therefore also bypass the plant cover and discharge directly to the lake off-shore potentially leading to algal blooms under N-limited conditions in the lake water column.

Slowly released molasses barrier system for controlling nitrate plumes in groundwater: A pilot-scale tank study

A well-type barrier system containing solidified molasses as a reactive medium was developed to promote the indigenous denitrifying activity and to treat nitrate plumes in groundwater. Three slowly released molasses (SRM) barrier systems harboring 60, 120, and 120 SRM rods, which were named System A, B, and C, respectively, were operated to examine nitrate removal efficiency in a pilot-scale sandy tank. These SRM systems induced a consistent removal of nitrate without pore clogging and hydraulic disturbance during the test period. The initial nitrate concentration was 142mgL−1, and the concentrations decreased by 80%, 84%, and 79% in System A, B, and C, respectively. In particular, System C was inoculated with heterotrophic denitrifiers, but the nitrate removal efficiency was not enhanced compared to System B, probably due to the prior existence of indigenous denitrifiers in the sandy tank. The presence of nitrite reductase-encoding gene (i.e. nirK) at the site was confirmed by denatured gradient gel electrophoresis analysis.

Aquifer plume delineation of mixed population nitrate data using ArcGIS

The city of Enid collects most of its water for public distribution from groundwater. This groundwater is supplied from five wellfields—Enid, Drummond, Cleo Springs, Ringwood, and Ames—covering the Enid Terrace Aquifer, the Drummond Confined Aquifer, and the Cimarron Terrace Aquifer. This research explores nitrate distribution in the aquifer by analyzing trend graphs for the wells and estimating nitrate plumes for the wellfields. The US Environmental Protection Agency (USEPA) requires that public water systems not exceed a nitrate concentration that is ≤ 10 mg/L because nitrate above this level causes Blue Baby Syndrome in infants. Information for this study was provided in the form of sampling data compiled by the city of Enid. This raw data was then analyzed and distilled to determine nitrate existence and extent using USEPA standards. This article presents a methodology to delineate the high nitrate plumes in five wellfields.

Natural Attenuation of Perchlorate in Denitrified Groundwater

Monitoring of a well-defined septic system groundwater plume and groundwater discharging to two urban streams located in southern Ontario, Canada, provided evidence of natural attenuation of background low level (ng/L) perchlorate (ClO4⁻) under denitrifying conditions in the field. The septic system site at Long Point contains ClO4⁻ from a mix of waste water, atmospheric deposition, and periodic use of fireworks, while the nitrate plume indicates active denitrification. Plume nitrate (NO3⁻ -N) concentrations of up to 103 mg/L declined with depth and downgradient of the tile bed due to denitrification and anammox activity, and the plume was almost completely denitrified beyond 35 m from the tile bed. The ClO4⁻ natural attenuation occurs at the site only when NO3⁻ -N concentrations are <0.3 mg/L, after which ClO4⁻ concentrations decline abruptly from 187 ± 202 to 11 ± 15 ng/L. A similar pattern between NO3⁻ -N and ClO4⁻ was found in groundwater discharging to the two urban streams. These findings suggest that natural attenuation (i.e., biodegradation) of ClO4⁻ may be commonplace in denitrified aquifers with appropriate electron donors present, and thus, should be considered as a remediation option for ClO4⁻ contaminated groundwater.