Mineralogical and anthropogenic controls of stream water chemistry in salted watersheds

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The relatively stable concentrations of calcium (42.2–122.3mg/l) and magnesium (48.9–88.1mg/l) between 2012 and 2013 and their possible weathering paths identified by mass balance models for both soil solutions and stream water from a small salted (regular applications of winter road deicing salt) watershed in New Jersey, USA indicate that the weathering of feldspars and dissolution of carbonates are the primary sources for these cations. However, the relatively stable and lower concentrations of sodium and chloride in soil solutions (19.6–46.1mg/l for Na and 12.7–88.3mg/l for Cl) and their fluctuating and higher concentrations in stream water (14.6–103.1mg/l for Na and 15.2–260.4mg/l)) from the same watershed during the same period also indicate that road deicing salt is the primary source for sodium and chloride in stream water. Furthermore, positive correlations between calcium and sulfur concentrations (correlation coefficient r=0.77) and magnesium and sulfur concentrations (r=0.73) in stream water between 2009 and 2013, as well as positive correlations between sulfur and iron concentrations in soil compositions (r=0.27), indicate that both the dissolution of gypsum and the oxidation of pyrite into hematite might be the primary sources of sulfate in the watershed. Analyses of water chemistry from the related and much larger Delaware River Watershed (DRW) show that sodium and chloride concentrations have increased steadily (2.7 times for Na and 4.56 times for Cl for 10-year average) due to the regular application of winter deicing salt from 1944 to 2011 for which data are available. The greater increase of stream water chloride concentrations compared with sodium concentrations also results in the steady decline of Na+/Cl− molar ratios from 1.51 to 0.92 for the 10-year average during that time in the DRW and approximately 78% of the chloride in the DRW now being anthropogenic. In addition, the decline of sulfate concentration from 22.08 to 14.59mg/l (∼34%) for the 10-year average from 1980 to 2011 in the 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 activities. 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 stream temperatures combined with an increase of impervious surface area in the region. Warmer stream water might cause the temporary bloom of biota requiring silica, particularly plankton, increasing its uptake from stream water, while an expanded impervious surface area increases the contribution of low-silica runoff water to total stream discharge, thereby lowering the silica concentration in stream water. The combined results of this study illustrate the possible changing anthropogenic factors that can control stream water chemistry in salted watersheds and that these factors need to be taken into account when future water quality regulations and policy are considered.