Assessment and management of long-term nitrate pollution of ground water in agriculture-dominated watersheds

Abstract

The objectives of this paper are to document and evaluate regional long-term trends and occurrences of nitrate in the ground water of agricultural watersheds. In Whatcom County, Washington, elevated nitrate concentrations in ground water are of great concern. Whatcom County is recognized by heavy agricultural activities, especially an intensive dairy farm industry. Historical nitrate concentration data from 1990 to 2000 were compiled from different agencies and assembled into a single composite database. A geographic information system was used to assess the spatial and temporal variability of nitrogen data. The analysis was conducted for the whole area as well as for individual watersheds and for different land use classes. In addition, nitrate concentration variability with descriptive parameters such as sampling depth, ground water recharge, dissolved oxygen, and on-ground nitrogen loadings was also investigated. The analysis showed that the areas with nitrate concentrations above the maximum contaminant level are areas characterized by heavy agricultural activities. The shallow surficial aquifers of the study area were found to contain high mean nitrate concentrations when compared to non-surficial aquifers. The analysis showed that high nitrate presence corresponds to areas with both high ground water recharge and high on-ground nitrogen loadings. In addition, the nitrate concentration decreased with increasing sampling depth. In general, the trend of long-term nitrate concentration remained elevated in shallow aquifers due to the persistent on-ground nitrogen loadings produced by agriculture-related land use practices. Finally, the watersheds were prioritized for management intervention, alternatives, and data monitoring based on a number of decision variables.

Introduction

Aquifers are an important source of drinking water in the US and elsewhere, and these sources are vulnerable to contamination (Solley et al., 1993). Nitrate (NO₃) is the most common pollutant found in shallow aquifers due to both point and non-point sources (Postma et al., 1991). Many studies in the US have shown that agricultural activities are the main source of elevated nitrate concentrations in ground water (Spalding and Exner, 1993, Hallberg and Keeney, 1993, Wylie et al., 1995, Ator and Ferrari, 1997, Hudak, 2000, Harter et al., 2002). Elevated nitrate concentrations in drinking water are linked to health problems such as methemoglobinemia in infants and stomach cancer in adults (Addiscott et al., 1992, Lee et al., 1991, Hall et al., 2001, Wolfe and Patz, 2002). As such, the US Environmental Protection Agency (US EPA) has established a maximum contaminant level (MCL) of 10 mg/l as NO₃–N (US EPA, 2000). Nitrogen (N) is a vital nutrient to enhance plant growth (Delgado, 2001b, Delgado and Shaffer, 2002, Follett and Delgado, 2002). Nevertheless, when the nitrogen application exceeds the plant demand and the denitrification capacity of the soil, nitrogen may leach to ground water usually in the form of nitrate. The increase of nitrate pollution in ground water has led to the abandonment of numerous wells in agricultural areas (Lasserre et al., 1999).

Agricultural practices can result in non-point source pollution of ground water (Hall et al., 2001, Delgado and Shaffer, 2002). With non-point sources, ground water quality may be depleted over time due to the cumulative effects of several years of practice (Addiscott et al., 1992, Schilling and Wolter, 2001). Non-point sources of nitrogen from agricultural activities include fertilizers, manure application, and leguminous crops. For instance, the extensive use of fertilizers on row crops is considered as a main source of nitrate leaching to ground water particularly in sandy soils (Hubbard and Sheridan, 1994). Elevated nitrate concentrations in ground water are common around dairy and poultry operations, barnyards, and feedlots (Hii et al., 1999, Carey, 2002). In addition to agricultural practices, non-point sources of nitrogen involve precipitation, irrigation with ground water containing nitrogen, and dry deposition. Point sources of nitrogen are shown to contribute to nitrate pollution of ground water. The major point sources include septic tanks and dairy lagoons. Many studies have shown high concentrations of nitrate in areas with septic tanks (Cantor and Knox, 1984, Keeney, 1986, Arnade, 1999, MacQuarrie et al., 2001). This is of particular concern to rural homeowners who use shallow drinking water wells that can be easily contaminated with septic tank effluent including bacteria and viruses. Seepage from dairy lagoons has been found to be a source of elevated nitrate in shallow ground water (Erickson, 1992).

Regional assessment of ground water quality is complicated by the fact that nitrogen sources are spatially distributed (Tesoriero and Voss, 1997). The identification of areas that receive heavy nitrogen loadings from point and non-point sources is important for land-use planners and environmental regulators. In such areas, management alternatives can be considered to minimize the risk of nitrate leaching to ground water (Lee et al., 1991, Tesoriero and Voss, 1997). Accurate quantification of nitrate leaching is difficult. Nitrate leaching from the soil zone is a complex interaction of land use, on-ground nitrogen loading, ground water recharge, soil nitrogen cycle, soil characteristics, and the depth of soil.

In aquifers overlain by agricultural watersheds, elevated nitrate concentrations in ground water are expected. Degradation of ground water quality, mainly from nitrate, is of great concern for the residents of Whatcom County, Washington State (Blake and Peterson, 2001). Whatcom County is the second in dairy production in Washington State and produces more than 65% of the nation's red raspberries. High concentrations of nitrate have been detected in the surficial aquifers of the area since the early 1970s (Kaluarachchi et al., 2002). Many studies showed high concentrations of nitrate in ground water in the aquifers of Whatcom County. Erickson (1998) sampled 248 water supply wells for nitrate over a 10-week period in 1997. The results showed that nitrate concentrations ranged from less than 0.01 to 53 mg/l. Many studies showed that agricultural practices, dairy farming, and lagoons are the probable cause of excessive nitrate levels in the shallow ground water of the area (Liebscher et al., 1992, Erickson, 1992, Erickson, 1994, Garland and Erickson, 1994, Hulsman, 1998, Hii et al., 1999, Cox and Liebscher, 1997, Carey, 2002). In order to manage nitrogen pollution and other water-related issues, Washington State Legislature passed the Engrossed Substitute House Bill 2514 codified as RCW 90.82 in 1988. The legislation is known as the Watershed Management Act. The act outlines the general requirements that must be considered in the management of water resources and the corresponding environmental impacts in different geographical units. Whatcom County is mostly covered within the Water Resources Inventory Area 1 (WRIA 1).

WRIA 1 supports a variety of fish species important to the cultural heritage, economy, and the ecology of the area (Blake and Peterson, 2001). Since the role of nitrate in eutrophication is well-recognized (Wolfe and Patz, 2002), nitrate contamination of the surface water of WRIA 1 is a concern as it greatly affects the fish habitat. In general, the transport of nitrate to surface water occurs mainly via discharge of ground water during baseflow conditions (Hubbard and Sheridan, 1994, Devlin et al., 2000, Schilling and Wolter, 2001, Bachman et al., 2002). Therefore, the prevention of ground water contamination from nitrate also protects surface water quality. The issue of elevated nitrate concentrations in the ground water of WRIA 1 has called for urgent adoption of management alternatives. In order to successfully strategize the proposed management alternatives, a comprehensive study of ground water quantity and quality is needed. As a part of the overall management plan, a study of nitrogen pollution of ground water was undertaken and the study included the analysis of long-term data of nitrogen. The proposed future activities of this study include a development of a comprehensive fate and transport model of nitrate that can be used in assessing the effectiveness of various long-term management alternatives. The results from such a model can provide the lag time between the adoption of a management alternative and ground water restoration, natural attenuation potential of the aquifer, estimate the nitrate flux to surface water, calculate nitrate concentrations at critical receptors, and compute nitrate mass accumulation in ground water.

The objectives of this paper are to identify and document the regional, long-term trends of nitrate in the ground water of WRIA 1; to identify the probable sources for elevated nitrate concentration; and to analyze the variability of nitrate especially due to the key decision variables such as the on-ground nitrogen loading, land use distribution, and ground water recharge. The assessment is carried out using the geographic information system (GIS) tools (ESRI, 1999). This assessment is intended to provide recommendations related to future ground water monitoring, field testing, and aid in the development of the conceptual model for fate and transport of nitrate.