Nitrogen biogeochemistry of submarine groundwater discharge

To investigate the role of the seepage zone in transport, chemical speciation, and attenuation of nitrogen loads carried by submarine groundwater discharge, we collected nearshore groundwater samples (n 5 328) and examined the distribution and isotopic signature (d15N) of nitrate and ammonium. In addition, we estimated nutrient fluxes from terrestrial and marine groundwater sources. We discuss our results in the context of three aquifer zones: a fresh groundwater zone, a shallow salinity transition zone (STZ), and a deep STZ. Groundwater plumes containing nitrate and ammonium occurred in the freshwater zone, whereas the deep STZ carried almost exclusively ammonium. The distributions of redox-cycled elements were consistent with theoretical thermodynamic stability of chemical species, with sharp interfaces between water masses of distinct oxidation : reduction potential, suggesting that microbial transformations of nitrogen were rapid relative to dispersive mixing. In limited locations in which overlap occurs between distribution of nitrate with that of ammonium and dissolved Fe2+, changes in concentration and in d15N suggest loss of all species. Concurrent removal of NO 3 and NH z 4 , both in freshwater and the deep STZ, might occur through a range of mechanisms, including heterotrophic or autotrophic denitrification, coupled nitrfication : denitrification, anammox, or Mn oxidation of NH z4 . Loss of nitrogen was not apparent in the shallow STZ, perhaps because of short water residence time. Despite organic Cpoor conditions, the nearshore aquifer and subterranean estuary are biogeochemically active zones, where attenuation of N loads can occur. Extent of attenuation is controlled by the degree of mixing of biogeochemically dissimilar water masses, highlighting the critical role of hydrogeology in N biogeochemistry. Mixing is related in part to thinning of the freshwater lens before discharge and to dispersion at the fresh : saline groundwater interface, features common to all submarine groundwater discharge zones. Eutrophication of coastal waters from nonpoint source land-derived nitrogen (N) loads is a worldwide phenomenon (Howarth et al. 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 et al. 1999). In coastal watersheds with soils of high hydraulic conductivity and permeable coastal sediments, groundwater is commonly a major route of transport from land to sea for freshwater and associated land-derived nutrient loads (Valiela et al. 2000). A portion of the freshwater flowing down-gradient from coastal aquifers discharges directly to coastal waters through a seepage face that can be located near the intertidal zone or farther offshore (Giblin and Gaines 1990; Bokuniewicz 1992). Such direct discharge of groundwater into estuaries or the sea is referred to as submarine groundwater discharge (SGD). In addition to the terrestrially derived fresh groundwater and solutes, saline and brackish groundwater are often important components of SGD (Taniguchi et al. 2002; Michael et al. 2005). Processes including tidal pumping, wave set-up, and dispersion along the boundary between discharging fresh groundwater and the saline groundwater wedge beneath result in entrainment of saline groundwater, producing a gradient in groundwater salinity and resulting in discharge of brackish and saline groundwater. Hence, SGD often consists of a substantial amount of recirculating seawater, and thus results in fluxes of sediment-regenerated nutrients and other pore-water materials to coastal waters. Comparisons at a range of scales indicate that chemical loads to coastal waters from SGD commonly rival loads from riverine transport (Taniguchi et al. 2002; Slomp and Van Cappellin 2004; Kroeger et al. 2007). Chemical loads carried by SGD are typically calculated as the product of groundwater discharge rate and average concentration of the element or compound of interest in coastal groundwater. Inherent in those calculations is the assumption that chemical transport through the coastal aquifer and sediments is conservative. The salinity transition zone in nearshore aquifers, referred to as a subterranean estuary (Moore 1999), has been shown to be a critical geochemical zone where the chemical composition of the discharging fluid is altered because of the mixing of fresh and saline groundwater in the context of coastal sediments (e.g., Charette and Sholkovitz 2006) and has been proposed as a site where nutrient transformations might occur (Slomp and Van Cappellen 2004). However, with regard 1 Present address: United States Geological Survey, Woods Hole Science Center, Woods Hole, Massachusetts 02543