Investigation of Road Salt Constituents in Bedrock Aquifers in Maine

In 2007, the U.S. Geological Survey (USGS) and Maine Department of Transportation (MaineDOT) began investigation of the timing and mechanisms of ground-water flow and road-salt transport in bedrock aquifers adjacent to State highways in Maine. Previous data collected by MaineDOT show that road-salt constituents move slowly through bedrock aquifers or accumulate in fractures in these aquifers. To investigate these effects, we are collecting continuous water-level, specific conductance, and water temperature data in four wells, all known to be affected by road salts and reporting in real time; borehole geophysics in all four wells; periodic chloride samples; and periodic depth-integrated profiles of water-quality constituents.

 

Three of our four wells are used solely for monitoring; the fourth is used as water supply for a homeowner. Data collected from the monitoring wells show that (1) specific conductance is highly correlated with chloride concentration; (2) a distinct transition zone between salty and fresh water exists, and this zone moves vertically with time but corresponds generally to the depth at which active fractures exist;  (3) during many recharge events – even during the summer, after the salt-spreading season -- specific conductance fluctuates in proportion to water-level rise, but not always; and (4) response time of specific conductance to changes in water levels varies by well. Data collected from the water-supply well show that temperature and specific conductance are affected by pumping, but overall the response of specific conductance to recharge events is similar to that observed in the other wells. Specific conductance has increased particularly in the water-supply well during early autumn 2008, prior to the application of road salts.

 The data-collection effort will continue through the winter of 2009. USGS and MaineDOT anticipate that the data will provide a basis for conceptual models of the interactions among fractured bedrock, concentrations of road-salt constituents, and recharge events.