SEASONAL AND LONG-TERM PATTERNS OF DISSOLVED CHLORIDES IN STREAM WATERS FROM DEICING CHEMICALS, MASSACHUSETTS

The practice of applying deicing chemicals to maintain road surfaces during the winter seasons begun during the 1940’s, then steadily increased until the 70’s, stayed leveled into the early 90’s, and again moderately increased in the past decade and half. In this study we are interested in the mechanisms of chloride return flows by investigating chloride dissolved loads, chloride concentrations in stream waters, seasonal patterns, and changes over a course of several years.

Most common deicers are sodium chloride, followed by calcium chloride and by magnesium chloride. Using dissolved chloride has an advantage of tracking all chloride deicers at the same time and, since chloride ions are conservative tracers in soils, they are also unaffected by ionic exchange interferences. Chloride tracking technique is based on calibrated chloride concentrations obtained from specific conductance signals recorded every 15 minutes by automatic preprogrammed sensor-recorder systems at several locations in central and eastern Massachusetts. These systems are maintained by the USGS who also kindly provided the 15-min interval datasets including calibrated stream flows. Chloride calibrations were done by simultaneous measurements of specific conductance and IC analysis of water samples collected at the monitoring sites. Calibrations are linear curves closely similar to the curves observed when NaCl is added to the same water samples. Dissolved load is calculated for each 15 min interval by multiplying volume of water for each interval with the corresponding value of chloride concentration.

Hydrograph traces during the winter months yield records of dissolved loads by event direct runoffs whereas baseflow traces provide data on dissolved loads by discharging groundwater. Well defined hydrograph baseflow supported intervals appear to have very similar chloride concentrations independent of the season when measured. Comparison of direct runoff dissolved chloride loads with the total annual dissolved loads suggests that only small fraction of the deicers is removed during the overland runoff events and that perhaps as high as 60% to 80% are carried by groundwater. Long-term patterns yield evidence for salt accumulation within the aquifers and a substantial retention of dissolved deicers in ground water storage volumes.