HIGH RESOLUTION MONITORING OF SPECIFIC CONDUCTIVITY AS A PROXY FOR CHLORIDE IN A VALLEY-FILL AQUIFER IN ASHLAND, NEW HAMPSHIRE

The Town of Ashland New Hampshire obtains its drinking water from a very transmissive valley-fill aquifer that is located adjacent to interstate I-93 and US Route 3. During an average winter, the New Hampshire Department of Transportation applies approximately 81 tons of salt within the well head protection area for the Ashland Town well on I-93 and Route 3. Consequently, chloride levels in the town's production well have increased on average 4- 5 mg/l per year over the last decade. One problem characterizing the variable nature of the chloride contamination is the expense of collecting conventional groundwater samples. Recent in-situ conductivity monitoring, which can serve as a proxy for chloride sampling, using an automated datalogger installed in a monitoring well, detected groundwater with elevated chloride levels that rapidly pass by the well screen. Conductivity changed as much as 80 uS in a four-hour period as a spike of chloride-laden water passed through the aquifer. The chloride spikes often remain for a period of several days before returning to “background” levels. Automated conductivity dataloggers can be a useful tool compared to infrequent conventional groundwater sampling techniques, which are not likely to resolve the high frequency contaminant spikes and give a complete picture of the groundwater geochemistry in a transmissive unconsolidated aquifer with rapid transport times.