Modeling nitrate contamination of groundwater in agricultural watersheds

Summary

This paper presents and implements a framework for modeling the impact of land use practices and protection alternatives on nitrate pollution of groundwater in agricultural watersheds. The framework utilizes the national land cover database (NLCD) of the United State Geological Survey (USGS) grid and a geographic information system (GIS) to account for the spatial distribution of on-ground nitrogen sources and corresponding loadings. The framework employs a soil nitrogen dynamic model to estimate nitrate leaching to groundwater. These estimates were used in developing a groundwater nitrate fate and transport model. The framework considers both point and non-point sources of nitrogen across different land use classes. The methodology was applied for the Sumas–Blaine aquifer of Washington State, US, where heavy dairy industry and berry plantations are concentrated. Simulations were carried out using the developed framework to evaluate the overall impacts of current land use practices and the efficiency of proposed protection alternatives on nitrate pollution in the aquifer.

Introduction

Agricultural activities are probably the most significant anthropogenic sources of nitrate contamination in groundwater (Carey and Lloyd, 1985, DeSimone and Howes, 1998, Gusman and Mariño, 1999, Birkinshaw and Ewen, 2000, McLay et al., 2001, Ledoux et al., 2007, Oyarzun et al., 2007). Elevated nitrate concentrations in drinking water can cause methemoglobinemia in infants and stomach cancer in adults (Lee et al., 1991, Wolfe and Patz, 2002). As such, the US Environmental Protection Agency (US EPA) has established a maximum contaminant level (MCL) of 10 mg/l NO₃-N (US EPA, 1995). Nitrogen is a vital nutrient to enhance plant growth. This fact has motivated the intensive use of nitrogen-based fertilizers to boost up the productivity of crops in many regions of the world (see for instance Laftouhi et al., 2003). Nevertheless, when nitrogen-rich fertilizer application exceeds the plant demand and the denitrification capacity of the soil, nitrogen can leach to groundwater usually in the form of nitrate which is highly mobile with little sorption (Meisinger and Randall, 1991, Birkinshaw and Ewen, 2000, Shamrukh et al., 2001). Many practices result in non-point source pollution of groundwater and the effects of these practices accumulate over time (Schilling and Wolter, 2001). These sources include fertilizer and manure applications, dissolved nitrogen in precipitation, irrigation flows, and dry atmospheric deposition. Point sources of nitrogen are shown to contribute to nitrate pollution of groundwater. The major point sources include septic tanks and dairy lagoons and many studies have shown strong correlation between high concentrations of nitrate and these sources (Erickson, 1992, Arnade, 1999, MacQuarrie et al., 2001).

Many studies in the literature have developed management options for protecting groundwater quality from nitrate contamination. Yadav and Wall (1998) examined the costs of obtaining acceptable nitrate levels in the drinking water of Garvin Brook area in Winona County, Minnesota. In this area, an average of 34% of the sampled domestic wells had nitrate in excess of the MCL. The protection alternatives implemented in the area included the maintenance of selected septic systems, reducing the overall nitrogen application rate, split applications of fertilizers, and the proper crediting of nutrients available in the soil. Since fate and transport models were not utilized in their study, it was not clear if the nitrate concentration of 10 mg/l NO₃-N or less was reached. Additionally, the lag time between the adoption of the protection alternatives and groundwater restoration was unknown. Bernardo et al. (1993) developed a modeling framework for assessing the environmental and economic consequences for protecting groundwater quality. The framework consists of three stages: (i) a crop simulation and chemical transport model, (ii) an optimization model, and (iii) a groundwater flow model. The output from the framework provides optimal practices across the study area. The main shortcoming of this study is that nitrate concentration in the aquifer is controlled through constraints of nitrate leaching from the soil. Therefore the aquifer’s ability to naturally attenuate nitrate was not considered. Kim et al., 1993, Kim et al., 1996, Lee and Kim, 2002 developed a procedure to determine the optimal fertilizer use considering the occurrences of nitrate in groundwater. They assumed that a fixed proportion of the fertilizers applied will ultimately leach to groundwater and that the time period between application and entrance into the aquifer is represented by a constant time lag. In groundwater, nitrate was assumed to be decayed at a specific rate. Based on these assumptions, an equation was developed to estimate nitrate concentration in groundwater as a function of the on-ground nitrogen loading from fertilizers. They derived an optimal tax rate on nitrogen fertilizer use, which would lead to the optimal fertilizer use and the maximum net benefits subject to nitrate concentration equivalent to MCL. Since these studies did not account for nitrogen mass in the soil prior fertilization and due to the assumption that nitrate in the groundwater exclusively comes from fertilizers, optimal nitrogen fertilizer application rates may be overestimated. This overestimation verily leads to ultimate nitrate concentrations in the aquifer beyond the MCL.

Previous studies that were involved in the modeling of nitrate fate and transport in groundwater and in developing management options to minimize nitrate concentration in groundwater can be classified into the following two broad categories (according to Fig. 1 which conceptually depicts the interacting processes that govern nitrate occurrences in groundwater): (i) studies that incorporated soil transformation models to determine nitrate leaching to groundwater (Refsgaard et al., 1999, Lasserre et al., 1999, Birkinshaw and Ewen, 2000, Ledoux et al., 2007) and (ii) studies that did not encompass soil transformation models in the development of nitrate fate and transport models of groundwater (Mercado, 1976, Carey and Lloyd, 1985, Shamrukh et al., 2001).

In the first group of these studies, a wide range of soil models were used and incorporated. These models were readily available from the literature such as PRZM (Carsel et al., 1985), LEACHP (Wagenet and Huston, 1987), GLEAMS (Leonard et al., 1987) and NLEAP (Shaffer et al., 1991), or were developed specifically to cope with a site of interest and to address certain objectives with a particular format of output (see for instance Almasri and Kaluarachchi, 2004a) or simply to reduce the amount of data needed to crank up the model as proposed by Ling and El-Kadi (1998) who developed a simple lumped-parameter analytical model of soil transformations of nitrogen.

The development of a conceptual model that best accounts for the different parameters influencing nitrate fate and transport in groundwater is ultimately the key to a successful simulation of nitrate concentration in groundwater. The conceptual model of nitrate fate and transport in groundwater integrates generally the following components (Refsgaard et al., 1999, Lasserre et al., 1999, Gusman and Mariño, 1999, Birkinshaw and Ewen, 2000, Nolan et al., 2002, Almasri and Kaluarachchi, 2004a, Ledoux et al., 2007, Almasri, 2007): (i) watershed hydrology; (ii) land use cover to compute the spatial distribution of on-ground nitrogen loadings; (iii) detailed assessment of all nitrogen sources in the study area and their allocation to the appropriate land cover classes; (iv) approximate description of the nitrogen dynamics in the unsaturated zone (including both soil and vadose zones); (v) realistic estimation of nitrate leaching to groundwater; (vi) understanding the groundwater flow system; (vii) accounting for groundwater–surface water interactions with the proper characterization of N-transformations in surface water bodies; and (viii) detailed description of nitrate fate and transport processes in groundwater.

Characterization of nitrogen sources and identification of areas with heavy nitrogen loadings from point and non-point sources is important for land use planners, environmental regulators, and is essential for developing fate and transport models. Once such high-risk areas have been identified, preventative measures can be implemented to minimize the risk of nitrate leaching to groundwater (Lee, 1992, Lee et al., 1994, Tesoriero and Voss, 1997, Ramanarayanan et al., 1998). Accurate quantification of nitrate leaching to groundwater is difficult due to the complex interaction between land use practices, on-ground nitrogen loading, groundwater recharge, soil nitrogen dynamics, and soil characteristics. This complex interaction is conceptually illustrated in Fig. 1. When conducting a regional-scale analysis and modeling, it is important to understand the interaction of the aforementioned factors to account for the transient and spatially variable nitrate leaching to groundwater. It is thus essential to use a soil nitrogen model. In turn, groundwater fate and transport models are vital in simulating the impact of proposed protection alternative measures that protect groundwater quality and reduce the level of contamination.

The very objective of this paper is to develop a conceptual modeling framework that integrates the on-ground nitrogen loadings, nitrogen soil dynamics, and nitrate fate and transport in groundwater. The modeling framework accounts for point and non-point sources of nitrogen. This integration is of great importance to realistically account for the different processes that nitrogen undergoes and in order to arrive at rational estimates of nitrate concentrations in groundwater. To demonstrate the applicability of the framework, Sumas–Blaine aquifer of Whatcom County, Washington State, US was considered.