Fresh ground water on the North Fork of Long Island, N.Y. is contained within a series of hydraulically isolated lenses that decline in thickness eastward. In the recent past, several Suffolk County Water Authority (SCWA) public supply wells on the North Fork and parts of the South Fork have been impacted by elevated chloride concentrations. In an effort to determine the source of the high chlorides, a 3 year study was conducted to evaluate the impacts of road salting and seawater contamination at select public supply wellfields. Two well fields in Peconic experiencing significant chloride impacts- Mill Lane and Ackerly Pond Road- were analyzed using samples obtained from vertical profile wells and focused electromagnetic induction logs. Since the relative concentration of chloride is quite high in road salt, the bromide-chloride (Br/Cl) molar ratio in groundwater affected by road salting is much lower than the Br/Cl molar ratio in groundwater impacted by lateral or vertical saltwater intrusion. The Br/Cl molar ratios of selected well samples were analyzed to determine if the chloride increases observed in the public supply wells were due to upconing from overpumping, or from road salting. An aquifer test was conducted at Ackerly Pond to determine site-specific parameters. Contributing Point source contaminants to all wellfields were identified using the Long Island Source Water Assessment Reports.

Results indicate that on the North Fork the water quality at several sites have been significantly impacted by road runoff and agricultural land use. In the case of Mill Lane and Ackerly Pond Road, road salting has impacted both well fields. This is especially true at the Ackerly Pond Road well field, since the site is located in close proximity to a recharge basin, which retains stormwater from a section of a major highway. Vertical profiles at Mill lane detected similarly abnormal Br/Cl molar ratios opposite the screen setting of the public supply wells, also indicating impacts from road salting. While the South Fork experiences seasonal fluctuations in chloride levels, attributed to saltwater upconing, the Br/Cl molar ratios are relatively unaffected. It is hoped that future changes in the operation of well fields, as well as closer monitoring of water quality trends, will allow the SCWA to continue to use the well fields, despite these water quality impacts.