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Effects of Road Salt on Bedrock Wells

Maine DOT has documented locations and times at which chloride concentrations increased unexpectedly in bedrock ground water. In particular, chloride concentrations increased during periods of low water levels before road salting commenced for the winter. Maine DOT hypothesizes that chloride from road salt accumulates in disconnected bedrock fractures by diffusion during high ground-water periods in the spring and then release the constituents later in the year as water levels recover from lows. The objective of this study is to determine the process (timing and hydraulic conditions) of chloride transport to ground water in bedrock aquifers. The setting for this two year study will be four existing wells at locations (not yet determined) along roads that receive road-salt applications and are known to be affected by chloride. (1) Characterize the well and locations and nature of fractures by use of borehole geophysical methods (caliper, acoustic televiewer, fluid resistivity, natural gamma). (2) Perform miniature hydraulic tests (borehole flowmeter, chemical analyses for salt constituents, temperature) on isolated sections of the borehole to determine hydraulic and chemical contributions of fractures and "background" concentrations. (3) Monitor continuous, long-term water levels and specific conductance at depths corresponding to transmissive fractures. (4) Use a vertical string of diffusion samplers to sample for chloride in discrete zones periodically. (5) Analyze results. The project will be completed under a cooperative effort with the Maine USGS office.