Nitrate Transport In Groundwater Research Articles

Monte Carlo Simulations as a Decision Support to Interpret δ15 N Values of Nitrate in Groundwater.

Intense farming is often associated with the excessive use of manure or fertilizers and the subsequent deterioration of the groundwater quality in many aquifers worldwide. Stable isotopes of dissolved nitrate (δ15 N and δ18 O) are widely used to determine sources of nitrate contamination and denitrification processes in groundwater but are often difficult to interpret. Thus, Monte Carlo simulations were carried out for a site in lower Bavaria, Germany, in order to explain δ15 N observations in a porous groundwater system with two aquifers, the main aquifer (MA) and several smaller perched aquifers (PA). For evaluating potential contributions, frequency distributions of δ15 N were simulated deriving from (I) the mixing of different nitrate sources, related to land use, as input to groundwater, combined with (II) transport of nitrate in groundwater and (III) microbial denitrification. Simulation results indicate a source-driven isotopic shift to heavier δ15 N values of nitrate in groundwater, which may be explained by land use changes toward a more intensified agriculture releasing high amounts of manure. Microbial denitrification may play a role in the PA, with simulated δ15 N distributions close to the observations. Denitrification processes are however unlikely for the MA, as reasonable simulation curve fits for such a scenario were obtained predominantly for unrealistic portions of nitrate sources and related land use. The applied approach can be used to qualitatively and quantitatively evaluate the influence of different potential contributions, which might mask each other due to overlapping δ15 N ranges, and it can support the estimation of nitrate input related to land use.

Non-point source evaluation of groundwater nitrate contamination from agriculture under geologic uncertainty

The long-term effect of non-point source pollution on groundwater from agricultural practices is a major concern globally. Non-point source pollutants such as nitrate that occurs through fertilizers and animal waste eventually make their way into the aquifer by infiltrating soil. The goal of this study is to develop an approach for characterization of nitrate concentrations at potential source locations under conditions of geologic uncertainty. A Bayesian framework using the Markov Chain Monte Carlo (MCMC) approach is developed to estimate posterior probability distributions of non-point sources by incorporating nitrate concentration data as well as geologic uncertainties. The proposed approach is tested using hypothetical contamination scenarios and then validated using an application case study in North Carolina. Uncertainty existing in geologic formation (i.e., heterogeneous hydraulic conductivity field) is treated as prior and used in evaluating the likelihood function that measures the match between observed and simulated concentrations. The likelihood function computation involves a numerical model that simulates nitrate transport in groundwater from non-point agricultural sources and predicts nitrate concentrations at observation wells. Effectiveness of the MCMC approach is evaluated through a convergence analysis. Comparison among different sampling algorithms is carried out with respect to MCMC convergence diagnostics and making inference. The Bayesian inference analysis methodology developed in this research will help decision makers and water managers to identify potential areas for source containment and decide if further sampling is required.

Estimating ammonium and nitrate load from septic systems to surface water bodies within ArcGIS environments

Summary This paper presents a recently developed software, ArcGIS-based Nitrogen Load Estimation Toolkit (ArcNLET), for estimating nitrogen loading from septic systems to surface water bodies. The load estimation is important for managing nitrogen pollution, a world-wide challenge to water resources and environmental management. ArcNLET simulates coupled transport of ammonium and nitrate in both vadose zone and groundwater. This is a unique feature that cannot be found in other ArcGIS-based software for nitrogen modeling. ArcNLET is designed to be flexible for the following four simulating scenarios: (1) nitrate transport alone in groundwater; (2) ammonium and nitrate transport in groundwater; (3) ammonium and nitrate transport in vadose zone; and (4) ammonium and nitrate transport in both vadose zone and groundwater. With this flexibility, ArcNLET can be used as an efficient screening tool in a wide range of management projects related to nitrogen pollution. From the modeling perspective, this paper shows that in areas with high water table (e.g. river and lake shores), it may not be correct to assume a completed nitrification process that converts all ammonium to nitrate in the vadose zone, because observation data can indicate that substantial amount of ammonium enters groundwater. Therefore, in areas with high water table, simulating ammonium transport and estimating ammonium loading, in addition to nitrate transport and loading, are important for avoiding underestimation of nitrogen loading. This is demonstrated in the Eggleston Heights neighborhood in the City of Jacksonville, FL, USA, where monitoring well observations included a well with predominant ammonium concentrations. The ammonium loading given by the calibrated ArcNLET model can be 10–18% of the total nitrogen load, depending on various factors discussed in the paper.

Predicting Redox Conditions in Groundwater at a Regional Scale.

Defining the oxic-suboxic interface is often critical for determining pathways for nitrate transport in groundwater and to streams at the local scale. Defining this interface on a regional scale is complicated by the spatial variability of reaction rates. The probability of oxic groundwater in the Chesapeake Bay watershed was predicted by relating dissolved O2 concentrations in groundwater samples to indicators of residence time and/or electron donor availability using logistic regression. Variables that describe surficial geology, position in the flow system, and soil drainage were important predictors of oxic water. The probability of encountering oxic groundwater at a 30 m depth and the depth to the bottom of the oxic layer were predicted for the Chesapeake Bay watershed. The influence of depth to the bottom of the oxic layer on stream nitrate concentrations and time lags (i.e., time period between land application of nitrogen and its effect on streams) are illustrated using model simulations for hypothetical basins. Regional maps of the probability of oxic groundwater should prove useful as indicators of groundwater susceptibility and stream susceptibility to contaminant sources derived from groundwater.

Simulation of Nitrate Transport in Groundwater: Lenjanat Plain, Isfahan

Simulation of Nitrate Transport in Groundwater: Lenjanat Plain, Isfahan

Preliminary Evaluation of a Permeable Reactive Barrier for Reducing Groundwater Nitrate Transport from a Large Onsite Wastewater System

A permeable reactive barrier was installed between a large Onsite Wastewater System (OWS) and a monitoring well located down-gradient from the OWS. Groundwater samples from the well had shown elevated and increasing concentrations of NO3--N (>20 mg L-1 for 2 years). The barrier was constructed using woodchips from various tree species that were placed in a trench excavated to approximately the same depth as the well which was experiencing elevated NO3--N concentrations. Groundwater samples (5) were collected from the well between two weeks and four months after the barrier was installed. Groundwater NO3--N concentrations dropped by an average of 5 mg L-1, while Dissolved Organic Carbon (DOC) concentrations increased by 11 mg L-1 and Cl concentrations stayed relatively stable (<2 mg L-1 increase). The data suggest that denitrification fueled by the DOC from the barrier may be contributing to the lower NO3--N concentrations. Monitoring will continue at the site to determine the longer-term water quality trends. More research should be conducted to evaluate the applicability of permeable barriers as a best management practice for non-point sources of pollution in nutrient sensitive areas.

A new ArcGIS-based software of uncertainty analysis for nitrate load estimation.

Domestic waste water treatment using Onsite Sewage Treatment and Disposal Systems (OSTDS) (also known as septic systems) is a source of nitrogen contamination in groundwater and surface water. Estimating nutrient load from septic systems as a storm water source to surface water bodies is important to reduce nutrient load in support of Total Maximum Daily Load (TMDL) programs. Software packages of nitrate load estimation using geographic information systems (GIS) have gained popularity, not only because a GIS is efficient to process and integrate spatial data but also because skills required for applying GIS-based software are widely available. Recently, Rios et al. (2013a) developed an ArcGIS-Based Nitrate Load Estimation Toolkit (ArcNLET) to simulate nitrate transport in groundwater and to estimate nitrate load from septic systems to surface water bodies. However, the load estimates are inherently uncertain, and impacts of the uncertainty on TMDLrelated decision making have not been well studied. Uncertainty analysis for GIS-simulated results is of particular importance, because GIS is frequently used as a decision support system. Conducting uncertainty analyses within GIS environments has become popular, owing to recent development of GIS tools and technologies for coupling GIS with methods and tools of uncertainty analysis. This paper presents a new software package, ArcNLET-MC (MC stands for Monte Carlo), developed by Rios et al. (2013b) to analyze uncertainty in nitrate

ArcNLET: A GIS-based software to simulate groundwater nitrate load from septic systems to surface water bodies

Onsite wastewater treatment systems (OWTS), or septic systems, can be a significant source of nitrates in groundwater and surface water. The adverse effects that nitrates have on human and environmental health have given rise to the need to estimate the actual or potential level of nitrate contamination. With the goal of reducing data collection and preparation costs, and decreasing the time required to produce an estimate compared to complex nitrate modeling tools, we developed the ArcGIS-based Nitrate Load Estimation Toolkit (ArcNLET) software. Leveraging the power of geographic information systems (GIS), ArcNLET is an easy-to-use software capable of simulating nitrate transport in groundwater and estimating long-term nitrate loads from groundwater to surface water bodies. Data requirements are reduced by using simplified models of groundwater flow and nitrate transport which consider nitrate attenuation mechanisms (subsurface dispersion and denitrification) as well as spatial variability in the hydraulic parameters and septic tank distribution. ArcNLET provides a spatial distribution of nitrate plumes from multiple septic systems and a load estimate to water bodies.ArcNLET's conceptual model is divided into three sub-models: a groundwater flow model, a nitrate transport and fate model, and a load estimation model which are implemented as an extension to ArcGIS. The groundwater flow model uses a map of topography in order to generate a steady-state approximation of the water table. In a validation study, this approximation was found to correlate well with a water table produced by a calibrated numerical model although it was found that the degree to which the water table resembles the topography can vary greatly across the modeling domain. The transport model uses a semi-analytical solution to estimate the distribution of nitrate within groundwater, which is then used to estimate a nitrate load using a mass balance argument. The estimates given by ArcNLET are suitable for a screening-level analysis.

A Pollution Offset System for Trading Non-Point Source Water Pollution Permits

Water pollution from non-point sources is a global environmental concern. Economists propose tradable permit systems as a solution, but they are difficult to implement due to the nature of non-point sources. We present a pollution offset system for trading non-point source water pollution permits. Conventional pollution offset systems suffer from thin markets and transaction costs. In this paper, we show how to overcome these problems with a centrally managed common-pool market. We define permits as allowable nitrate loading to a groundwater aquifer. This trading system utilizes estimates of potential nitrate leaching from land uses, a set of transport coefficients generated from a simulation of nitrate transport in groundwater, an online trading system, and a linear program to clear the market. We illustrate the concept using a hypothetical case study.

Modelling nitrate transport and turnover in a lowland catchment system

Summary Nitrate transport in groundwater dominated lowland catchment systems is influenced by complex and spatially distributed physical and chemical interactions. A modelling approach was developed combining a distributed soil nitrogen model with a three-dimensional groundwater model and a reactive transport model linking nitrate turnover and availability of reaction partners such as pyrite and organic matter. The modelling approach was applied to a hypothetical case study based on data from the pleistocene lowland catchment “Schaugraben” (20 km2) in the North of Saxony-Anhalt, with focus on the investigation of interactions of spatially distributed transport and chemical processes. The modelling approach could successfully simulate transport and turnover of nitrate in a groundwater dominated catchment. The advancement of the nitrate front and the corresponding depletion of pyrite as well as the distribution of seepage fluxes and nitrate concentrations in seepage water in the channel system showed distinct spatial variation. Surface water nitrate concentrations corresponding to the average soil leachate concentrations were not completely reached after a simulation period of 200 years for a conservative transport simulation. Under reactive conditions, about 80% of the nitrate was lost due to denitrification. Given a uniformly distributed input of nitrate, drain loads developed in a sigmoidal curve defined only by travel time distribution. The average travel time was 93 years. A distributed input of nitrate resulted in reduction of travel time to 80 years due to the different arrangement of source areas and flow. The modelling approach is a step towards bridging the gap between simple large scale models and detailed small scale studies, maintaining process orientation while allowing to consider landscape heterogeneity.

Modelling flow and nitrate transport in groundwater for the prediction of water travel times and of consequences of land use evolution on water quality

AbstractA two‐dimensional model representing flow and nitrate transport in groundwater was developed and applied to a hillslope of the Kervidy catchment. The objective of the modelling was: (1) to characterize better the flow and nitrate transport in the groundwater and to determine the characteristic times of the system; (2) to explore the consequences of changes in nitrate leaching to groundwater on nitrate concentration in stream water. The finite‐difference code MODFLOW was used to simulate the distribution of hydraulic head within the groundwater. Nitrate transport was described by the convection equation solved using MT3D. MODPATH was also used to analyse flow paths and travel times in the groundwater. Four units were considered in the model: (i) the plough layer, (ii) the soil, (iii) the weathered shale and (iv) the fissured shale. Autotrophic denitrification in the shale and partly in the weathered shale was represented, as well as heterotrophic denitrification in the upper horizon of bottom land. A steady‐state average flow was assumed with a spatially uniform groundwater recharge of 2 mm day−1, corresponding to the winter mean daily recharge observed in the Kervidy catchment. Nitrate recharge rate was fixed at 100 mg l−1, which is equivalent to a nitrogen flux of 165 kg ha−1 year−1. Six scenarios of nitrate leaching changes were analysed with the model. The first two correspond to spatially uniform decreases of the nitrate recharge rate to 80 and 60 mg l−1, respectively. In the other four scenarios, nitrate recharge rate was spatially distributed along the hillslope so that the average nitrogen flux remained equal to 165 kg ha−1 year−1. Simulated hydraulic heads were similar to observed values along the hillslope, except for the summit. The transport model reproduced the spatial pattern of nitrate concentrations observed in the weathered shale groundwater, a deviation appearing only in the nearest piezometer to the stream. Travel times within the groundwater appeared to be highly variable, from a few days up to 3 years. Scenario analysis showed that a significant decrease of stream nitrate concentration can be expected following a global decrease in nitrate leaching along the hillslope. However, the fall could be very gradual in time. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Simulation of nitrate transport in groundwater with a three-dimensional groundwater model run as a subroutine in an agroecosystem model

The agroecosystem model, developed by the system research group of the University of Osnabrück (1990), describes the impact of intensive land use on the groundwater. The main substance in this model is nitrogen: the nitrogen flux through the soil down into the groundwater in the form of nitrate is simulated as a function of agriculture policy over farm management and field processes. This complex model is split into several submodels. One of these submodels describes the nitrate distribution and transport in the saturated zone of groundwater and is presented in this paper. A model is used in which a transport program is added to a groundwater flow program. It is assumed that the density of the transporting ground water is not significantly changed by the dissolved nitrate, which allows the calculation of the ground water flow and the calculation of the nitrate concentration to be separated, as proposed for the ideal tracer problem by Bear (1979). The groundwater flow is calculated from the three-dimensional groundwater flow model of McDonald and Harbaugh (1984). That model has been enlarged by a transport module for substances carried in dissolved form by groundwater. This requires the modelling of the mechanisms of convection and hydrodynamic dispersion. For the latter the standard description in porous media was used. The applicability of the model was tested for an area, about 200 km 2 in size, surrounding the village of Holdorf in South Oldenburg. The consistency of the model was tested, and a comparison with an analytical solution is presented.