The Retsof salt mine in the Genesee River Valley, Livingston County, New York, flooded after mine-roof collapses in 1994 created two rubble chimneys in overlying bedrock that intersected a confined aquifer in the basal glacial-drift deposits and continued collapse to the land surface. Groundwater in the lower confined aquifer flowed downward through the bedrock rubble chimneys causing widespread drawdown in the lower confined aquifer until the mine was completely flooded in 1996 (Yager and others, 2001). By 2005, water levels in the lower confined aquifer had nearly recovered to pre-collapse conditions but the hydraulic connection through the rubble chimneys between the brine-filled mine cavity, saline-water bedrock fracture zones, and the lower confined aquifer remained. The mine contains billions of liters of saturated brine that is slowly being displaced upward as the weight of overlying bedrock and unconsolidated sediments causes the mine cavity to very slowly close (Yager and others, 2009). This data release provides chloride-concentration data from water samples collected from wells in the groundwater-quality well network established in the Genesee River Valley by consultants for the former salt mine. The well network includes wells that were actively pumped and monitoring wells in and around the collapse area. In the nine wells monitored for chloride concentration, some wells only monitored one zone, while others monitored up to eight zones. The lower confined aquifer was monitored in six wells, while other wells monitored the underlying bedrock at various depths. Chloride data in "GeneseeValley_ChlorideData_OtherMeasurements.csv" were from monitoring wells further from the collapse area or from domestic wells north of the collapse area near Fowlerville, New York. References Cited: Yager, R.M., Miller, T.S., and Kappel, W.M., 2001, Simulated effects of 1994 salt-mine collapse on groundwater flow and land subsidence in a glacial aquifer system, Livingston County, New York: U.S. Geological Survey Professional Paper 1611, 85 p., https://doi.org/10.3133/pp1611. Yager, R.M., Misut, P.E., Langevin, C.D., and Parkhurst, D.L., 2009, Brine migration from a flooded salt mine in the Genesee Valley, Livingston County, New York—Geochemical modeling and simulation of variable-density flow: U.S. Geological Survey Professional Paper 1767, 59 p., https://doi.org/10.3133/pp1767.