Hydrological Control on Nitrate Delivery Through the Groundwater-Surface Water Interface

Near stream groundwater surface water interfaces (GSI) are considered to provide natural remediation for groundwater nitrate before it discharges into surface water bodies, largely through denitrification, but this function is not well understood based on the existing literature. The purpose of this study was to assess the combined influences of hydrological properties on groundwater nitrate delivery from terrestrial to aquatic systems. The study site, on the Eastern Shore of Virginia, is in the mid - Atlantic coastal plain of the Delmarva Peninsula. N03' dynamics during both baseflow and bank storage (groundwater recharge by stream water) were examined in columns of undisturbed streambed sediments from the field. The laboratory experimental design incorporated two processes: groundwater discharge into the stream (forward flow) and groundwater recharge from the stream (reverse flow). The mean denitrification rate from the laboratory microcosm study was 361.3 mmol N m'2 d'1. The column experiments, combined with a numerical transport model, revealed that flow velocity and organic matter content are the most dominant factors contolling nitrate removal at the GSI. The bank storage experiment showed that N03' removal capacity persisted within the intact column during the discharge - recharge processes. The redox sequence was redeveloped through the column almost immediately after the reverse flow was reset to simulate the discharge condition. Hydrologic exchange induced by bank storage enables microbial communities to retain and utilize nitrate at the GSI that might otherwise be exported from the system. A field and numerical modeling study of riparian hydrology was conducted to investigate the N03' flux and water table response occurring during bank flood events. Aquifer head responded relatively rapidly to changing stream stage during storm events. A variably saturated flow model specifically developed for this problem provided a reasonable match to the field observations. The reduced groundwater discharge due to the formation of a groundwater ridge within the stream bank was suggested by both field observations and model simulation. The increased residence time in the GSI favor microbial reactions such as denitrification, during which the stream N03' loading is attenuated. This effect may not be important due to the limit bank storage in the field site with steep sloping banks and well - drained aquifer material.

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