Role of the riparian zone in controlling the distribution and fate of agricultural nitrogen near a small stream in southern Ontario
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 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).
References (40)
- et al.
Using isotope fractionation of nitrate–nitrogen and nitrate–oxygen for evaluation of microbial denitrification in a sandy aquifer
J. Hydrol.
(1990) - et al.
Quantifying groundwater discharge to a small perennial stream in southern Ontario
Canada. J. Hydrol.
(1998) Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere: a review
Chemical Geology (Isot. Geosci. Section)
(1986)- et al.
An integrated approach to hydrogeologic investigations—a case history
J. Hydrol.
(1983) Factors affecting the export of nitrate–nitrogen from drainage basins in southern Ontario
Water Res.
(1978)- et al.
isotope biogeochemistry and natural denitrification process in groundwater: application to the chalk aquifer of northern France
Geochim. Cosmochim. Acta
(1988) - et al.
Attenuation of nitrate in aquitard sediments of southern Ontario
J. Hydrol.
(1996) - et al.
Denitrification in nitrate-contaminated groundwater: occurrence in steep vertical geochemical gradients
Geochim. Cosmochim. Acta
(1991) - et al.
Effects of sludge disposal on groundwater nitrate concentrations
J. Hydrol.
(1993) Evaluation of the origin and fate of nitrate in the Abbotsford Aquifer using the isotopes of and in NO3−
Appl. Geochem.
(1995)
Dilution of nonpoint-source nitrate in groundwater
J. Environ. Qual.
Stable isotopes of oxygen and nitrogen in source identification of nitrate from septic systems
Ground Water
Nitrate depletion in the riparian zone and stream channel of a small headwater catchment
Hydrobiologia
Riparian wetlands and water quality
J. Environ. Qual.
Groundwater nitrate dynamics in grass and poplar vegetated riparian buffer strips during the winter
J. Environ. Qual.
Factors influencing nitrate depletion in a rural stream
Hydrobiologia
Ground water flow paths in relation to nitrogen chemistry in the near-stream zone
Hydrobiologia
Cited by (258)
Sources and transformations of riverine nitrogen across a coastal-plain river network of eastern China: New insights from multiple stable isotopes
2024, Science of the Total EnvironmentSources and seasonal variations of nitrate in the coastal multiple-aquifer groundwater of Beihai, southern China
2024, Journal of Contaminant HydrologyMultiple stable isotopic approaches for tracing nitrate contamination sources: Implications for nitrogen management in complex watersheds
2024, Ecotoxicology and Environmental SafetyHydrogeochemical changes during artificial groundwater well recharge
2023, Science of the Total Environment