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A Hydrologic and Geochemical Model of Saltwater Intrusion in Cape May County, New Jersey
Osgood, Christian D.
Osgood, Christian D.
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1995
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Earth and Environmental Science
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http://dx.doi.org/10.34944/dspace/8645
Abstract
In this study, quantitative modeling techniques were used to examine groundwater flow and saltwater intrusion in Cape May County, New Jersey. Increased pumping of freshwater from coastal aquifers following urbanization has triggered invasion of seawater and forced many municipalities to abandon and seal formerly productive supply wells. Previous studies predicted a sharp interface between saltwater and freshwater based on simple advective models. Such models do not account for hydrodynamic dispersion and influences of geologic heterogeneity on solute transport, nor do they account for buoyancy-driven flow. To simulate saltwater intrusion in Cape May, a numerical model was used that couples equations of flow and solute transport by advection and hydrodynamic dispersion, accounts for permeability variation in space and considers flow due to lateral density gradients. In addition, unlike previous studies which examined only the physical mixing of seawater and freshwater, chemical reactions associated with saltwater intrusion were examined. The hydrologic models suggest two major flow regimes. A surficial flow system, controlled by local topographic highs and lows, predominates in the uppermost Holly Beach aquifer. In the deeper Cohansey Formation, groundwater migrates across the peninsula primarily from the northeast to the southwest, reflecting the regional slope. The extent of saltwater intrusion is controlled mainly by the vertical permeability of clay confining units between aquifers. High vertical permeabilities, to some extent, prevent saltwater intrusion by allowing surface water to discharge into confined aquifers. The results of geochemical modeling suggest that chemical processes may occur during intrusion as a result of mixing between saltwater and freshwater. Na+ exhibits conservative behavior at low salinities and non-conservative removal at high salinities. Ca2+ displays non-conservative addition over the entire range of groundwater compositions, the amount of enrichment increasing with chloride concentration. H+ exhibits non-conservative removal primarily at low chloride concentrations. At low salinities, mineral dissolution may control non-conservative behavior, which is supported by the conservative behavior of Na+ (suggesting little ion-exchange), the considerable depletion of H+, the undersaturation of calcite in Cohansey groundwater, and the relatively high ratio of Ca2+ to Na+. However, at high salinities ion-exchange predominates as suggested by 2:1 removal of Na+ relative to enrichment of Ca2+, the lower amounts of H+ depletion, and the higher saturation state with respect to calcite. In addition, ion-exchange, overall, may be more important in liberating Ca2+ than mineral dissolution.
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