Denitrification in Aquifer Soil and Nearshore Marine Sediments Influenced by Groundwater Nitrate

Abstract

We estimated rates of denitrification at various depths in sediments known to be affected by submarine discharge of groundwater, and also in the parent aquifer. Surface denitrification was only measured in the autumn; at 40-cm depth, where groundwater-imported nitrate has been measured, denitrification occurred consistently throughout the year, at rates from 0.14 to 2.8 ng-atom of N g-1 day-'. Denitrification consistently occurred below the zone of sulfate reduction and was sometimes comparable to it in magnitude. Denitrification occurred deep (14 to 40 cm) in the sediments along 30 km of shoreline, with highest rates occurring where groundwater input was greatest. Denitrification rates decreased with distance offshore, as does groundwater influx. Added glucose greatly stimulated denitrification at depth, but added nitrate did not. High rates of denitrification were measured in the aquifer (17 ng-atom of N g-1 day-'), and added nitrate did stimulate denitrification there. The denitrification measured was enough to remove 46% of the nitrate decrease observed between 40and 14-cm depth in the sediment. Groundwater discharge is known to be a source of nutrients to lakes in many parts of the world (3, 8, 34) and recently has been implicated as a source of nitrate to coastal marine sediments also (5, 14, 26). This groundwater influx may have an effect on phytoplankton productivity in the water column, which is often assumed to be nitrogen limited in marine systems (21, 25). However, groundwater-derived nitrate may also affect microbial activities in the sediments through which the groundwater effuses. In particular, this nitrate could promote denitrification, which would mitigate any effect on water-column productivity. The groundwater on Long Island, New York is known to be heavily contaminated with nitrate (18). Submarine groundwater discharge through the sediments of Great South Bay, on the south shore of Long Island, has been well characterized (2), and its influence on sediment nitrate concentrations has been documented (5). We describe here our study of denitrification in these groundwater-influenced sediments and in the parent aquifer.

Natural denitrification in the saturated zone: A review

“…The rate increases with temperature varied considerably in Lind's three sites. However, the rates at in situ temperatures are all within an order of magnitude and compare well with the reported rates at similar temperatures given by Slater and Capone [ 1987], Bengtsson and Annadotter [1989], and Smith et al [1991], that is, ---10 -1 mg N kg -1 dry sediment per day. As discussed later, the denitdhfication reported by Lind may be autotrophic.…”

The discharge of nitrate‐contaminated groundwater from developed shoreline to marsh‐fringed estuary

“…Shoreline salinity measurements and thermal imagery indicated that though variable along the shore, most discharge of contaminated groundwater occurred through sandy sediments seaward of the Spartina marsh fringe. Thus the direct release of nitrate-contaminated groundwater from high-velocity seeps into Nauset's estuarine waters circumvented the high denitrification potential of low-redox, highly organic marsh and subtidal sediments [Slater and Capone, 1987]. In fact, seepage chamber monitoring suggested little nitrate removal in these sandy surficial sediments.…”

“…Models of the effect of shoreline development on coastal water quality have usually assumed that all nitrogen applied to the land surface as fertilizer and runoff or leached from on-site wastewater disposal systems reaches surface waters; however, recent work on Cape Cod [Valiela et al, 1997] has shown that as little as 20% of nitrogen added to a coastal watershed may reach the downgradient estuary. Denitrification, the microbial conversion of nitrate to dinitrogen gas, can be an important removal process in soils along a groundwater flow path [Slater and Capone, 1987;Smith and Duff, 1988]. Efficiency of nitrogen removal depends on suitable soil conditions, i.e., high organic content and low redox potential, which promote denitrification [Capone and Slater, 1990;Korom, 1992;Starr and Gillham, 1993].…”

Model Parameters for Simulating Fate and Transport of On‐Site Wastewater Nutrients

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