Role of the riparian zone in controlling the distribution and fate of agricultural nitrogen near a small stream in southern Ontario

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Abstract

Uncultivated riparian areas can play an important role in reducing nutrient loading to streams in agricultural watersheds. Groundwater flow and geochemistry were monitored in the riparian zone of a small agricultural watershed in southern Ontario. Hydraulic and geochemical measurements were taken along a transect of monitoring wells extending across the riparian area into an agricultural field. Chloride and nitrate concentrations in groundwater samples collected from the agricultural field were much higher than in samples from the riparian area. A sharp decline in both nitrate and chloride concentrations was observed near the field–riparian zone boundary. It appears that increased recharge within the riparian zone, as compared to the artificially drained field, caused nitrate-rich groundwater from the field to be diverted downward beneath the riparian zone, thus limiting the input of agrochemicals to the riparian area and consequently protecting the stream from potential contamination. Geochemical data also indicated that nitrate was attenuated in the downward moving groundwater. Patterns of dissolved oxygen concentrations and redox potential in the subsurface coincided with the pattern defined by groundwater nitrate. These patterns indicated that conditions within the riparian zone and at depth near the field–riparian zone boundary were conducive to denitrification. A linear relation between the δ15N and δ18O values of nitrate from the monitored transect also supported denitrification as the primary nitrate removal mechanism. This study provides a new conceptual model of how riparian zones may prevent nitrate contamination of streams, and highlights the need for a complete understanding of both groundwater flow and geochemistry in riparian environments.

Introduction

Nitrate contamination of water resources is widespread in areas of intensive agricultural activity. The nitrate contamination results from the excessive use of inorganic and organic fertilizers and/or by tillage that releases nitrogen stored in the soils. In agricultural watersheds in southern Ontario, there is ample documentation of nitrate contamination of groundwater (Gillham, 1991; Ontario Farm Groundwater Quality Survey, 1993; Robertson et al., 1996) and surface water (Hill, 1978; Neilsen et al., 1982; Hill, 1988). Much of the nitrate contamination in surface water arises from direct groundwater discharge and groundwater input by tile-drainage networks. Therefore, the abundance and fate of nitrate in groundwater can have a major influence on surface-water quality.

Most streams in southern Ontario are separated from agricultural fields by uncultivated strips of land, commonly called riparian zones or buffer strips. These riparian zones consist of narrow bands of natural vegetation (trees, shrubs, and grasses) that remain uncultivated because the land is too wet, too steep, or too difficult to clear for agricultural activity. Numerous studies have shown that riparian zones can play an important role in reducing nitrate concentrations in groundwater discharging to streams (Peterjohn and Correll, 1984; Jacobs and Gilliam, 1985; Cooper, 1990; Haycock and Pinay, 1993; Jordan et al., 1993; Gilliam, 1994)

Even with the abundant evidence supporting nitrate removal in riparian areas, the role the riparian zone plays in removing groundwater nitrate remains unclear. The primary processes of subsurface nitrate removal within these riparian zones are generally considered to be denitrification (Jacobs and Gilliam, 1985; Cooper, 1990; Lowrance et al., 1995), vegetative uptake (Lowrance, 1992), or dilution (Altman and Parizek, 1995; Komor and Magner, 1996). However, in many studies the exact mechanism of nitrate removal and the role hydrology plays in nitrate attenuation have not been well established. Hydrological controls on groundwater flow patterns can have a major influence on the distribution and fate of nitrate (Hill, 1990). Still, few studies have carefully examined the link between groundwater flow paths and nitrate concentrations (Hill, 1996). To discern the actual role of riparian zones in removing nitrate from groundwater, a better understanding of the relation between groundwater flow and chemistry is required.

A variety of techniques can be used to identify the processes controlling nitrate removal. Hydrogeochemical data can be used to delineate redox conditions in the subsurface and infer the occurrence and location of denitrification zones (Postma et al., 1991; Starr and Gillham, 1993). The concentrations of conservative tracers, such as chloride or bromide, relative to nitrate, can establish the importance of dilution in decreasing nitrate concentrations. Measurement of in-situ denitrification rates using the acetylene blocking technique can provide direct evidence of denitrification (Smith et al., 1991; Starr and Gillham, 1993). Fractionation of nitrogen and oxygen isotopes, which form parts of the NO3 molecule, can provide additional evidence of denitrification and dilution (Mariotti et al., 1988; Böttcher et al., 1990). It may also be possible in the future to use 15N isotopes to quantify uptake of groundwater nitrate by plants (Komor and Magner, 1996).

This study had two major goals. The first was to examine the relationship between groundwater flow and nitrate transport from an agricultural field into a riparian zone. The second was to assess the fate of nitrate and determine the mechanisms of nitrate attenuation within the riparian zone. Our approach includes the use of geochemical and isotopic techniques in combination with detailed groundwater flow measurements near a small headwater stream in southern Ontario.

Section snippets

Study Area

This study was conducted in the spring of 1996 (March to May) within the 600 ha eastern sub-basin of the Kintore Creek watershed, located approximately 35 km east of London, Ontario (Fig. 1). The eastern sub-basin forms part of an ongoing paired-watershed study by the Upper Thames River Conservation Authority (UTRCA). UTRCA staff have continuously monitored streamflow and collected stream-water samples from both sub-basins for more than 10 years in an attempt to quantify the effect of

Monitoring wells

As part of the overall field program being conducted at the study site, extensive instrumentation was installed throughout the study field, the riparian zone, and along the stream. This instrumentation included more than 200 monitoring wells in the field, riparian zone, and stream. A brief description of the monitoring wells used in this study is presented below.

A transect of monitoring wells was installed in the northern portion of the study area. The transect begins in the western part of the

Groundwater flow

Hydraulic-head data from the monitoring-well network defined the groundwater flow patterns along the instrumented transect. Fig. 4 shows a vertical cross section of hydraulic-head contours measured on April 18, 1996. Long-term observations of water levels along the transect have shown that hydraulic-head values change seasonally, but the general groundwater flow patterns remain consistent year-round. The hydraulic-head contours show that groundwater from the field flows laterally toward the

Conclusions

The riparian zone in this study had a major influence on the distribution and fate of groundwater nitrate. Previous studies of nitrate attenuation in riparian zones have indicated that nitrate removal occurred primarily in the shallow, organic-rich sediments of the riparian zone through denitrification and plant uptake. At this study site, the hydrologic contrast between the tile-drained field and the riparian zone had a controlling influence on groundwater flowpaths. The hydraulic-head data

Acknowledgements

Excellent assistance in the field was provided by Carl Rumpf, Tim Bennet, Gwyn Graham, Paul Johnson and Bob Ingleton. The manuscript benefitted significantly from the insightful reviews of Drs. S. Komor, A.B. Cooper and D. Lerner. Sources of funds include Agriculture and Agri-Food Canada under the Canada-Ontario Green Plan, Waterloo Centre for Groundwater Research, and the Natural Sciences and Engineering Research Council of Canada (NSERC).

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