Nitrate Dynamics in Riparian Forests: Groundwater Studies

This study was conducted to assess the removal of groundwater nitrate (NOa-) in different soil drainage classes within three riparian forests located in Rhode Island. A solution of NOa-and a conservative tracer [either bromide (Br-) or chloride (CI-)] was applied in growing and the dormant seasons to trenches upgradient of wetland locations with hydric soils (poorly and very poorly drained soils) and transition zone locations with somewhat poorly and moderately welldrained soils located immediately upslope of the wetlands. To assess removal, the change in groundwater concentrations of NO3relative to the concentration f the conservative tracer was observed in monitoring wells located in each soil drainage dass from June 1989 through April 1990. Removal of groundwater NO~was consistently high in the wetland locations, generally in excess of 80% in both growing and dormant seasons. In the transition zones, attenuation was less than 36% during the growing season, and ranged from 50 to 78% in the dormant season. Attenuation i the transition zones was positively correlated with water table elevations. Transition zone attenuation was high in the dormant season relative to the growing season likely because high water tables during the dormant season caused the contaminant plume to be exposed to soil with higher organic matter. The results suggest hat both wetlands and transition zones between wetlands and uplands can be important sinks for groundwater NO~. N (NO~) is a difficult pollutant to control due to its high mobility in soils and groundwater and the large inputs of this ion to groundwater from both agricultural practices (Keeney, 1986; Baker et al., 1975) and unsewered residential developments (Cain et al., 1989; Koppleman, 1978). Many studies have focused on quantifying the sources of NO ~ loading to groundwater (Gold et al., 1990; Keeney, 1986). Recently several studies have focused on landscape features that remove NO ~ after it has entered groundwater. In particular, strips of riparian forest have been determined to be important in maintaining stream water quality in areas of intensive agriculture (Karr and Schlosser, 1978; Lowrance et al., 1984; Jacobs and Gilliam, 1985; Peterjohn and Correll, 1985). Groundwater NO3 moving through riparian zones is subject to denitrification, plant uptake and microbial immobilization. Marked differences in NO~ attenuation among riparian zones, however, have been reported and attributed to spatial variation in soil type, organic matter, hydrology, and vegetation (Whigham et al., 1988; Cooper, 1990). Soil organic matter levels and microbial activity are concentrated near the surface and decline sharply with depth (Parkin and Meisinger, 1989). For NO~ removal to occur through plant uptake, groundwater levels must be elevated within the root zone during the growing season. Plant uptake may be minimal during the late winter and early spring when groundwater flow is often at its annual maximum (Bormann and Likens, 1979). Univ. of Rhode Island, Dep. of Natural Resources Science, Kingston, RI 02881. Current address of P. Groffman is: Inst. of Ecosystem Studies, Box AB, Millbrook, NY 12545-0129. Received 4 Sept. 1991. *Corresponding author. Published in J. Environ. Qual. 21:659-665 (1992). Because the fate of groundwater NO~ may be influenced by the degree of interaction with the biologically active zone of the soil, natural soil drainage classes offer a potentially useful approach to evaluate broad-scale patterns of NO~ removal in riparian forests. Soils are classified into one of seven drainage classes based on morphological characteristics that reflect the frequency, duration, and seasonal timing of saturation or partial saturation during soil formation (Brady, 1974). Drainage classes differ in a number characteristics that may affect groundwater NO~ -N removal including depth to seasonal high water table, vertical distribution and percentage organic matter in the solum, and timing and location of anaerobic conditions within the soil profile. The objectives of the research reported in this paper were to: (i) quantify the relative attenuation of groundwater NO~ in different soil drainage classes within riparian forests, (ii) compare attenuation in the dormant and growing season, and (iii) relate observed attenuation to soil and hydrologic characteristics. A companion paper describes a study conducted on the same sites that focused on NO3 removal processes in the riparian zone (Groffman et al., 1992). EXPERIMENTAL APPROACH Determining the fate of a groundwater contaminant moving through a riparian forest can be confounded by the influences of dilution as well as spatial and temporal variations of the contaminant. In this study we applied a solution of NO ~ -N along with a conservative tracer (Bror C1-) to trenches upgradient of both upland-wetland transition zones and wetland zones of a riparian forest. The change in concentration of NO~ relative to the concentration of the conservative tracer in the groundwater was then observed as the contaminants moved through the riparian zone. The dilution of the introduced contaminant plume was determined from the observed dilution of the conservative tracer. A decline in NO~ concentration in excess of that caused by dilution was attributed to attenuation (Trudell et. al., 1986).