Nutrient inputs to the coastal ocean from submarine groundwater discharge in a groundwater-dominated system: Relation to land use (Kona coast, Hawaii, U.S.A.)

Knee, Karen L., Joseph H. Street, Eric G. Grossman, Alexandria B. Boehm, and Adina Paytan

Limnol. Oceanogr., 55(3), 2010, 1105-1122 | DOI: 10.4319/lo.2010.55.3.1105

ABSTRACT: We evaluated the magnitude and composition (in terms of salinity, 223Ra and 224Ra activity, and nutrient concentrations) of submarine groundwater discharge (SGD) fluxes at 11 sites on the Kona coast of Hawaii. Discharging groundwater was typically brackish to saline and had high Ra activity relative to ocean water and fresher, inland groundwater. Brackish SGD fluxes, estimated using a mass-balance approach based on excess coastal ocean 224Ra activity, ranged from 3 to 1300 L m-1 of coastline min-1. Fluxes of the freshwater component of SGD, calculated based on salinity, ranged from 2 to 310 L m-1 min-1. Nutrient fluxes into coastal waters from SGD varied by up to three orders of magnitude among sites. Nitrate + nitrite (N + N), phosphate, and silica concentrations showed strong, inverse linear correlations with salinity in coastal seawater and groundwater. These correlations were consistent with previous work that documented conservative mixing between fresh, high-nutrient groundwater and saline, low-nutrient seawater at a few coastal ocean sites. We extend this conservative relationship to more coastal ocean sites and to groundwater in the coastal aquifer. N + N concentrations in the fresh component of discharging groundwater were higher at sites near golf courses and those with a greater percentage of bare land within a 5-km radius, and silica concentrations in the fresh component of discharging groundwater were higher at sites with more bare land and lower population densities. However, neither urbanization nor agriculture was significantly correlated with groundwater nutrient concentrations on this relatively undeveloped coast.

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