PRELIMINARY INVESTIGATION OF THE EXTENT OF SALTWATER INTRUSION INTO THE CONFINED COHANSEY AND ESTUARINE SAND AQUIFERS IN LOWER/MIDDLE TOWNSHIPS CAPE MAY COUNTY, NEW JERSEY

Cape May County forms the southern peninsular of New Jersey with Delaware Bay to the west and Atlantic Ocean to the east. It relies wholly on ground water for its public water supply. Geologically it lies within the New Jersey Coastal Plain, a thick accumulation of Cretaceous through Recent unlithified sands and clays. The Miocene Cohansey Formation/aquifer dominated by sand with local interbedded clays is a major confined aquifer used for regional water supply. Due to proximity of saline water bodies on three sides the Cohansey water quality has been monitored for many years. Recent monitoring at one key monitor well has shown a trend of increasing chlorides with time suggesting saltwater intrusion. At the same time, a public supply well in the Cohansey has shown a rise in chloride levels with time since 2005.

In response, the New Jersey Geological and Water Survey working with one regional water supplier oversaw the construction of several observation wells to help clarify the salt-water issues near this Cohansey public supply well. The observation wells were logged with geophysical logs including gamma, SP, SPR and long and short resistivity logs. These data in conjunction with other regional wells helped define the hydrogeologic framework (aquifers, aquifer thickness, confining units). Data suggested a weakness in a thinning upper confining unit, which may contribute to the saltwater intrusion. In addition, the geophysical logs, primarily resistivity helped assess the preliminary water quality in the Cohansey. Field sampling/analysis of the development water confirmed the borehole geophysics data. Ground water samples from the observation wells were further analyzed for major ions. These data plotted on Stiff and Piper diagrams confirmed the borehole geophysics and chloride specific field analysis indicating an extension of the zone of saltwater intrusion over one mile to the north along Delaware Bay well. To date, the salt-water intrusion appears restricted a 1-mile long area close to the Cape May County Delaware Bay coastline. Monitoring for salt-water intrusion of the Cohansey and deeper aquifers in Cape May continues.

Borehole geophysical logs including gamma, SP, SPR, long and short normal are effective tools for defining saltwater intrusion in Cape May County, NJ.