Identification of nitrate and sulfate sources in groundwater using dual stable isotope approaches for an agricultural area with different land use (Chuncheon, mid-eastern Korea)

Abstract

The identification of sources and behavior of agricultural contaminants is important to control and manage groundwater quality of aquifer systems in rural areas. In this study, hydrogeochemistry of major constituents and stable isotope ratios of NO₃⁻ and SO₄²⁻ in groundwater were determined to identify contamination sources and transformation processes occurring in soils and shallow groundwater of Yupori (Chuncheon district), a rural area in Korea with intense agricultural activities. The western gently sloped part of the study area, used mainly for vegetable cropping, was characterized by locally recharged Ca-Cl(NO₃ + SO₄) type groundwater with elevated NO₃⁻ concentrations (>10 mg L⁻¹ NO₃⁻-N). Low δ³⁴S values of sulfate and δ¹⁵N values of nitrate between 6 and 10‰ indicated that a mixture of chemical fertilizers and manure is responsible for groundwater contamination with agro-chemicals. In the steeper eastern part of the study area, in which fruit orchards are the predominant land use, the groundwater type was Ca-HCO₃⁻ water with lower NO₃⁻ (<3 mg L⁻¹ NO₃⁻-N) and sulfate concentrations. Elevated δ¹⁵N values of NO₃⁻ indicated that manure was the predominant NO₃⁻ source, but lower fertilizer application rates were the predominant reason for the lesser extent of groundwater contamination. This study demonstrates that a thorough evaluation of hydrodynamic and hydrochemical parameters in concert with multiple stable isotope ratios including dual isotopes of NO₃⁻ (N, O) and SO₄²⁻ (S, O) constitutes an effective approach for identifying sources and transformation processes of NO₃⁻ and SO₄²⁻ in shallow groundwater systems underneath agricultural areas with different land use.

Introduction

Due to the increase of anthropogenic nitrogen inputs to many watersheds (e.g. Howarth et al., 1996, Boyer et al., 2002), NO₃⁻ contamination constitutes an increasing problem in many shallow groundwater systems underneath agricultural areas. Elevated levels of nitrate in drinking water can cause serious health problems (Fan and Steinberg, 1996, Wolfe and Patz, 2002, US EPA, 2000). Yupori, a small (1 km²) agricultural basin in Chuncheon (Korea), is characterized by an increase in concentrations of NO₃⁻ in shallow groundwater likely caused by multiple non-point and point sources. Kaown et al. (2007) reported on nitrate contamination of groundwater in the area in 2004 and showed that more than 63% of the obtained samples had nitrate concentrations above the human affected value (3 mg L⁻¹of NO₃-N; Burkart and Kolpin, 1993, Babiker et al., 2004), while about 23% exceeded the drinking water standard of Korea (10 mg L⁻¹of NO₃-N). Agricultural activities in the vegetable fields and the storage and disposal of animal waste are suspected to be the major contributors to the rising NO₃-N concentrations in groundwater of the area (Kaown et al., 2007). Because groundwater is the major source of potable water in Yupori, identification of the contamination sources of groundwater in this basin is essential for determining the cause of increasing NO₃⁻ concentrations and for controlling groundwater quality in the aquifer system in the future.

The stable isotopes of nitrogen (δ¹⁵N-NO₃⁻) have been extensively used to identify the sources and the fate of NO₃⁻ in groundwater and stream water displaying characteristic ranges of isotopic values depending on their sources (Böhlke and Denver, 1995, Karr et al., 2003, Kellman and Hillaire-Marcel, 2003, Mayer et al., 2002, Thorburn et al., 2003, Widory et al., 2004, Moore et al., 2006). However, because δ¹⁵N-NO₃⁻ values are often modified by isotopic fractionation caused by physical, chemical and microbial process like volatilization, nitrification and denitrification, using δ¹⁵N-NO₃⁻ values alone is often not conclusive for identification of the origin of NO₃⁻ in aquatic systems.

Multi-isotopic approaches (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻, δ³⁴S-SO₄²⁻, and δ¹⁸O-SO₄²⁻) provide a useful method for identifying the origin of NO₃⁻ in contaminated water (Cravotta, 1997, Aravena and Robertson, 1998, Rock and Mayer, 2002, Marimon et al., 2007). Cravotta (1997) used stable isotopes of carbon, nitrogen, and sulfur to identify sources of nitrogen in surface waters. Aravena and Robertson (1998) used multiple isotopic tracers (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻, δ³⁴S-SO₄²⁻, δ¹⁸O-SO₄²⁻ and δ¹³C) to evaluate the processes involved in nitrate attenuation in groundwater. Rock and Mayer (2002) used isotopic compositions of NO₃⁻ and SO₄²⁻ to identify sources and processes affecting these solutes in shallow and deep groundwater and surface water. Marimon et al. (2007) elucidated the origin of high fluoride contents in groundwater of the central region of Southern Brazil using the dual-isotopic composition of NO₃⁻ (δ¹⁵N and δ¹⁸O) and SO₄²⁻ (δ³⁴S and δ¹⁸O). However, dual-isotope analyses alone do not always provide conclusive information about sources and processes that these solutes have undergone in aquifer systems. Chemical data and hydrogeological information should be simultaneously used to interpret isotopic compositions to determine the sources and biogeochemical history of NO₃⁻ and sulfate in aquatic systems (Mayer, 2005).

This study used a dual-isotopic approach in combination with chemical and hydrogeological data to determine sources and processes affecting NO₃⁻ and SO₄²⁻ in shallow groundwater of a granitic area with intense agricultural activities in the Yupori watershed in Korea. A key objective was to identify the causes for increasing NO₃⁻ contamination in the aquifer and its dependence on land use in order to inform future management decisions aimed at protecting groundwater quality.