Submarine ground-water discharge: nutrient loading and nitrogen transformations

Abstract

Eutrophication of coastal waters due to nonpoint source land-derived nitrogen (N) loads is a worldwide phenomenon and perhaps the greatest agent of change altering coastal ecology (National Research Council, 2000; Howarth and others, 2000). Within the United States, a majority of estuaries have been determined to be moderately to severely impaired by eutrophication associated with increasing nutrient loads (Bricker and others, 1999). In coastal watersheds with soils of high hydraulic conductivity and permeable coastal sediments, ground water is a major route of transport of freshwater and its solutes from land to sea. Freshwater flowing downgradient from aquifers may either discharge from a seepage face near the intertidal zone, or flow directly into the sea as submarine ground-water discharge (SGD) (fig. 1). In the coastal aquifer, entrainment of saline pore water occurs prior to discharge, producing a gradient in ground-water salinity from land to sea, referred to as a subterranean estuary (Moore, 1999). In addition, processes including density-driven flow and tidal pumping create brackish and saline ground-water circulation. Hence, submarine ground-water discharge often consists of a substantial amount of recirculating seawater. Mixing of fresh and saline ground waters in the context of coastal sediments may alter the chemical composition of the discharging fluid. Depending on the biogeochemical setting, removal of fixed N due to processes leading to N 2 (dinitrogen gas) production in the nearshore aquifer and subterranean estuary may significantly attenuate land-derived N loads; or, processes such as ion exchange and tidal pumping in the subterranean estuary may substantially accelerate the transport of both land-derived and sediment re-mineralized N to estuarine water columns. As emphasized by Burnett and others (2001, 2002), a fundamental problem in evaluating the importance of ground-water discharge in marine geochemical budgets is the difficulty of collecting samples across the salinity gradients of coastal aquifers. In addition, locating and quantifying rates of submarine ground-water discharge remains a challenge due to the diffuse and spatially and temporally heterogeneous nature of discharge. As a result, with regard to the study of biogeochemical cycles and chemical loads to coastal waters, the seepage face and subterranean estuary are relatively new and under-studied zones in the aquatic cascade from watershed to sea. Processes occurring in those zones must be understood and considered for proper modeling and management of coastal water resources.