NACL CONTRIBUTIONS FROM WINTER DEICING SALT AND SODIUM RETENTION IN A SALTED WATERSHED

The anthropogenic contribution of NaCl to the Delaware River Watershed (DRW), which supplies potable water to approximately 15 million people across New York, Pennsylvania, and New Jersey, is estimated to be approximately 78 percent of the total input from all sources. This input estimation is based on the difference between the average amount of NaCl carried by DRW water as measured at the Trenton USGS gage during 2004-2014, as compared to 1944-1954. The assumption made here is that sodium and chloride levels in the DRW from the pre-salting period, 1944-1954, were strictly due to the weathering of feldspars and the dissolution of natural salts. The 78 percent anthropogenic NaCl contribution in the DRW is comparable to the 75 percent anthropogenic chloride contribution in stream water obtained from a small tributary watershed of the DRW in Mercer County, New Jersey. The latter value was calculated from the product of stream flow partition (approximately evenly split between baseflow and direct surface runoff), and the measured average sodium and chloride ratios (0.52 and 0.42, respectively) of soil solution to stream water over a two-year period in the tributary watershed. In addition, previous results have shown that annual rates of sodium retention can be as high as 48 percent in the DRW, and as high as 30 percent for chloride. These estimations of the annual sodium and chloride retention rates are based on the progressive reduction of Na/Cl molar ratios in the DRW over the entire 1944-2014 period. Due to decreasing agriculture acreage in the DRW, our conclusion is that almost all of the anthropogenic NaCl in the DRW is from the increasing use of winter deicing salt. Details of the DRW historical data and the methods for calculating the above values can be found in Sun et al. (2014, Applied Geochemistry, 48, p.141-154; and 2012, Journal of Contaminant Hydrology, 136-137, p. 96-105). Understanding the amount of sodium retention and anthropogenic sources is a critical first step for recognizing the seriousness of the salt issues in the DRW and other similar northern watersheds where sodium concentrations in recent years have been reported above the 20 mg/l limit for potable water recommended by the USEPA. Future work will focus on the translocation of the deicing salt from the soil down into the groundwater.