Nitrogen transformation and transport modeling in groundwater aquifers
Introduction
Nitrogen, originating from agricultural sites, animal feedlots, septic tanks and other waste disposal sites, is one of the most common contaminants in groundwater. In soil-groundwater systems, nitrogen species consist of ammonium–nitrogen (NH4–N), nitrite–nitrogen (NO2–N), nitrate–nitrogen (NO3–N), organic nitrogen and nitrogen gas (N2). The predominant form present is determined by the environmental conditions of the water body particularly pH, temperature, oxygen and microorganism activity coupled with the mineralization rates of labile organic nitrogen. Seasonal changes can also be a key control of the speciation balance regardless of the total nitrogen concentration of the water body (Burt et al., 1993). Excessive nitrate concentration in groundwater is a significant public health problem. The threat of methemoglobinemia causing severe oxygen deprivation, especially in children, is well documented and is also reflected in the U.S. drinking water standard of 10 mg/l as nitrate–nitrogen (EPA, 1980). There have been many studies on the adverse health effects of nitrate and nitrite in drinking water (Bosch et al., 1950, Shuval and Gruener, 1977, NAS, 1978, White, 1983, Dorsch et al., 1984). There are other environmental consequences that arise due to excess nitrogen being leached into the groundwater. Excess nitrate can contribute to eutrophication and can also be toxic to some aquatic organisms.
In many cases, the study of nitrogen transport is complicated by the presence of various nitrogen species and the transformations that can occur in the saturated zone due to ambient microbial processes. The objectives of this paper are to develop a nitrogen transport and transformation model for saturated groundwater systems and assess its performance by applying it to a field site contaminated by nitrogen. Details of the mathematical model that describes the nitrification–denitrification coupled processes are presented. The model is coded as a reaction module within the public-domain reactive transport code reactive transport in three-dimensions (RT3D; Clement et al., 1998).
In order to verify the proposed model, data regarding concentrations of ammonia, nitrate and dissolved oxygen (DO) transported in saturated porous media coupled with nitrification and denitrification processes are taken from the literature (Widdowson et al., 1988, Kindred and Celia, 1989) and are used in the validation step. Further, model calibration and validations are performed to reproduce the contamination distribution patterns observed at a field site in Australia.
Section snippets
Conceptual model: nitrogen transformations
Fig. 1 outlines the major concepts of nitrogen transformations associated with the soil-groundwater environment in the vicinity of a feedlot. The bulk of the nitrogen in wastewater at the feedlot is in the form of aqueous ammonium (NH4+). Most sediment and soil colloid surfaces are negatively charged, giving them the ability to act as cation exchangers. The ammonium anion can therefore be immobilized geochemically by adsorption to aquifer sediments. Otherwise, ammonia may be rapidly oxidized to
Model development
In this section, a mathematical model for predicting the fate and transport of nitrogen species, dissolved organic carbon (DOC), DO and biomass in the groundwater environment systems is presented. This study uses the RT3D code to analyze the nitrogen transport in saturated groundwater systems. The RT3D code solves the coupled partial differential equations that describe reactive transport of multiple mobile and/or immobile species in three-dimensional saturated groundwater systems (Clement, 1997
Model verification
To verify that the proposed model adequately describes the fate and transport behavior of the nitrogen compounds, dissolved oxygen, dissolved organic carbon and biomass, as it undergoes reaction, one-dimensional verification examples are presented.
The verification simulations are compared with the modeling results of Widdowson et al. (1988) where multiple electron acceptor respiration (oxygen-based and nitrate-based) limited decay of a carbonaceous substrate is considered. The test case assumes
Field site description
The Vasse Research Station is operated by the Western Australian Department of Agriculture and is located southwest of Western Australia (Fig. 4). The Vasse Research Station is located within a coastal plain that is flat and gently undulating. A superficial aquifer is developed in the Geographe Bay Catchment that contains the Vasse Research Station. The superficial aquifer comprises a clay and silt layer overlain by a veneer of a sand layer. The water table in the unconfined sand aquifer
Summary and conclusions
In this study, a mathematical model was developed to describe transformations and transport of nitrogen compounds in a saturated groundwater aquifer. The model was coded as a reaction module within the RT3D framework. The model was verified by comparing simulation results obtained using the code against problems previously published in the literature. Several sets of one-dimensional simulations have been run to check the self-consistency, feasibility and physical reasonableness of the model
Acknowledgments
This study was financially supported by a grant (code: 3-4-2) from the Sustainable Water Resources Research Center of 21st Century Frontier Research Program, Korean Ministry of Science and Technology. AEBRC at POSTECH partially supported this study. The field data for this research was taken from the honors thesis of Ms. Sabina Fahrner (Fahrner, 2002). We would like to thank Mr. Brett Jago and Mr. Ben O’Grady of Perth Water Corporation for their support with the honors field project.
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