DETERMINING THE INPUT OF ROAD SALT IN THE CASPERKILL CREEK VIA WATER CHEMISTRY AND STREAMBED MINERALOGY

To determine the influence of de-icing agents on stream water chemistry, we compared levels of major cations and anions dissolved in the Casperkill Creek and stream sediment chemistry for June 2006. Located in New York's Hudson River Valley, the Casperkill Creek watershed drains through highly developed land, with up to 60% considered to be impervious to surface runoff. Road salt, which dissolves easily into solution, is commonly used following snowfall to melt ice on streets and walkways. Elevated levels of chloride are potentially dangerous to many organisms living in the stream, and are also considered unsafe in drinking water. The bedrock geology consists of weakly metamorphosed calcareous shales and dolomites. pH measurements and alkalinity titrations reflect the acid buffering capacity of the bedrock, with pH values increasing from ~7.5 in the headwaters to > 8 toward the mouth of the stream. With the use of x-ray diffraction, we determined that the mineral composition of the stream bed is dominantly chlorite, illite, quartz, Ca-feldspars, and dolomite, none of which contain significant sodium or chloride. In a stream unimpacted by urbanization, the ions dissolved in the water column reflect the rocks and soils through which the stream flows. Using an inductively coupled plasma atomic emission spectrophotometer, we discovered elevated levels of sodium in the stream water, however. These measurements closely match concentrations of chloride, which were measured to be as high as 200 mg/L during high water flow, as determined by silver nitrate titration. The levels of both ions appear strongly correlated to the percentage of impervious surface in the watershed, implying the influence of road salt. Concentrations of chloride > 250 mg/L are considered not potable for human drinking and toxic to sensitive freshwater organisms.