Mineralogical and Anthropogenic Controls of Stream Water Chemistry in Salted Watersheds

Analyses of major cation and anion concentrations in stream water and soil solutions from two salted (regular applications of winter road deicing salt) watersheds located in the northeastern United States indicate that both mineralogical and anthropogenic factors are important in controlling water chemistry. The relatively stable concentrations of calcium and magnesium, as well as their possible weathering paths identified by mass-balance models, indicate that the weathering of feldspars and the dissolution of carbonates are the primary sources for these two cations in the small, salted Centennial Lake Watershed (CLW, 1.95 km ²). However, the relatively stable and lower concentrations of sodium and chloride in soil solutions, and their fluctuating and higher concentrations in stream water from the CLW, indicate that road deicing salt is the primary source for these ions in stream water. Furthermore, positive correlations between calcium and sulfur concentrations and magnesium and sulfur concentrations in soil solutions, as well as positive correlations between sulfur and iron concentrations in soil compositions, indicate that both the dissolution of gypsum and the oxidation of pyrite into hematite are the primary sources of sulfate in the CLW. Analyses of water chemistry from the related and much larger Delaware River Watershed (DRW, 17560 km ²) show that sodium and chloride concentrations have increased steadily due to the regular application of winter deicing salt over the 68 years for which data are available. The more rapid increase of stream water chloride concentrations, relative to the increase in sodium, also results in the steady decline of Na⁺/Cl^-molar ratios in the DRW over that time. In addition, the reduction of sulfate and increase of bicarbonate concentration since 1980 in DRW stream water may be attributed to the decline of sulfate levels in atmospheric deposition resulting from enhanced national and state environmental regulations and a shift in local economic activity away from heavy industry. There also are more periods of low silica stream water concentrations in the DRW than in the past, perhaps as a result of recent increases in summer water temperatures due to global climate change. The combined results of this study illustrate the many changing anthropogenic factors that can control stream water chemistry in salted watersheds and that these factors need to be taken into account when considering future water quality regulations and policy.