The origin of Onondaga Valley brines: A geochemical investigation (New York)

The origin of saline groundwater in Onondaga Valley of New York is unknown, despite over a hundred years of speculation and study. Halite beds within the Syracuse Formation or Appalachian Basin brines are the two possible sources of the salinity. To resolve the source of the brine, detailed groundwater samples from cores and piezometers drilled and installed in sediments along the axis of Onondaga Valley were analyzed for dissolved major metals, chloride, bromide, iron, and stable isotopic composition of water. Historic and other regional relevant groundwater chemical datasets were collected and reviewed for comparison purposes. Chloride concentrations range from 10 meq/L in the shallow zone of a test hole drilled at LaFayette, NY to more than 5000 meq/L at 200 ft deep at Spencer Street (Syracuse) and at the bedrock-sediment interface at the Onondaga Lake Outlet. The major source of salinity in the Onondaga Valley is dissolving halite, and not Appalachian Basin brine. Higher calcium concentrations (>100 meq/L) at the bottom of test holes at Spencer Street and at the Onondaga Lake Outlet are from gypsum dissolution coupled to sulfate reduction and organic oxidation. Geochemical cross-sections for chloride and calcium and heuristic models of solute transport suggest that groundwater in the northern Onondaga Valley is effectively stagnant and hydrogeologically separated from the southern part of the valley. Upward motion of brine from bedrock into the surficial sediments at the northern part of the valley is extremely slow, and controlled by molecular chemical diffusion and weak advection (∼0.02 ft/yr).