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

Abstract

Abstract. As residential development, on-site wastewater disposal, and groundwater contamination increase in the coastal zone, assessment of nutrient removal by soil and sedimentary processes becomes increasingly important. Nitrogen removal efficiency depends largely on the specific flow paths taken by groundwater as it discharges into nitrogen-limited estuarine waters. Shoreline salinity surveys, hydraulic studies, and thermal infrared imagery indicated that groundwater discharge into the Nauset Marsh estuary (Eastham, Massachusetts) occurred in high-velocity seeps immediately seaward of the upland-fringing salt marsh. Discharge was highly variable spatially and occurred through permeable, sandy sediments during low tide. Seepage chamber monitoring showed that dissolved inorganic nitrogen (principally nitrate) traversed nearly conservatively from the aquifer through shallow estuarine sediments to coastal waters at flux rates of 1-3 mmol m -2 h -1. A significant relationship between pore water NO3-N concentrations and NO3-N flux rates may provide a rapid method of estimating nitrogen loading from groundwater to the water column.

Groundwater Flow to the Coastal Ocean

“…Infrared imaging has been used to identify the location and spatial variability of SGD by exploiting the temperature difference between surface water and groundwater at certain times of the year (Portnoy et al, 1998;Miller and Ullman, 2004;Mulligan and Charette, 2006). While this technique is quite useful for identifying spatial discharge patterns, it has not yet been applied to estimating flow rates.…”

The Three Scales of Submarine Groundwater Flow and Discharge across Passive Continental Margins

“…Schematic diagram of the continental-shelf scale of submarine groundwater phenomena showing the variable position of the fresh-saline interface in multiple confined aquifers on the shelf, the variable widths of the mixed zone at the interface, the flow of saline water inward from the exposed edges of confined aquifers, and the upward movement of saline groundwater induced by geothermal heating at depth. that carry groundwater recharged on land into these offshore discharge zones, the water currently being discharged is often too old to have been impacted by common anthropogenic pollutants such as nitrate (Portnoy et al 1998;Kroeger and Charette 2008), pesticides, solvents, or hydrocarbons. This is not the case for nearshore discharge.…”

“…Among the issues that have motivated the development of a better understanding of this topic are balancing of ocean elemental budgets (Shaw et al 1998;Charette et al 2005), remediating eutrophication and other types of contamination of coastal water bodies (Giblin and Gaines 1990;Portnoy et al 1998), protecting coastal groundwater supplies from saltwater intrusion (Barlow 2003), and ensuring the health of groundwater-dependent coastal ecosystems (Valiela et al 1990). Given rapid growth in this field, however, hydrogeologic processes that may be similar in terms of physics but that operate at different spatial scales have not always been adequately distinguished from each other.…”

Using radium isotopes to characterize water ages and coastal mixing rates: A sensitivity analysis

“…This information is essential to put SGD-related fluxes of nutrients (e.g., Portnoy et al 1998;Krest et al 2000;Slomp and Van Capellen 2004), dissolved metals (e.g., Basu et al 2001;Montluçon and Sañudo-Wilhelmy 2001;Windom et al 2006), and other pollutants (e.g., de Sieyes et al 2008;Standley et al 2008) in context and use them to inform nutrient budgets, assessments of risk to humans and coastal ecosystems, monitoring programs and management decisions.…”