CONTINUOUS ELECTRICAL CONDUCTIVITY MONITORING TO DIFFERENTIATE DEICING CHEMICAL SOURCES

Electrical conductivity serves as a useful proxy for ionic content of natural waters. In areas where deicing chemicals are the dominant time-varying factor, point measurements of conductivity can help locate sources in both surface and ground waters. Four data logging probes were placed in streambeds in the Old Pond Meadows aquifer area of Norwell, Massachusetts and recorded temperature, pressure, and specific conductivity at fifteen-minute intervals during the winter, spring and early summer of 2012-13.

The main stream had three sensors placed in the streambed. The most upstream sensor placed near a two-lane state highway shows conductivity pulses approximately 1 hour long (full width at half-height) while the next sensor showed the same pulse passing over 10 hours just 2 km downstream. The final sensor 4.6 km farther downstream showed barely perceptible peaks.

One subbasin sensor has very high (up to 10,700 micro-Siemens/cm), frequent, and sharp (2 hour) conductivity excursions, presumably influenced by deicing chemical application on a four-lane road 170 meters from the stream and 1 km from the sensor. The pulses in conductivity allow quantification of the frequency of salt application, and if per-lane-mile application rates are known, a mass-balance can be estimated.

Averaged values for the conductivity measured over a six-month period along one stream show 328, 347, and 305 micro-Siemens/cm in the downstream direction above a municipal aquifer. A water production well 86 meters from this stream has a conductivity of 660 micro-Siemens/cm in late April, indicating a much higher salt content in the groundwater being pumped and a salt source weakly connected to local surface water in this discharge zone.