Long-term Sodium and Chloride Surface Water Exports from a Humid Subtropical Urban Gradient

Increasing concentrations of sodium and chloride in surface water are strongly related to urbanization and population density and can have a significant impact on drinking water and salinity of aquatic ecosystems. While the majority of research has focused on the impact of deicing salts on urban surface waters in colder climates, the effect of urbanization on these exports has been found to occur in warmer climates as well. The purpose of this study is to investigate long term exports of sodium and chloride from watersheds with increasing urbanization in the Dallas-Fort Worth metroplex in a humid subtropical climate and compare exports to common characteristics of urbanization: urban land use, impervious surface area, and wastewater discharges. Long term data (1980 to 2008) was obtained from five USGS gauges located in and around the cities. Exports were calculated by regression analysis between concentrations and discharge and normalizing for time and the watershed area. In addition, grab samples were collected from the period of June 2009 to May 2010 and analyzed for Na and Cl. Results show a very strong positive relationship between urban land use, water fluxes, and impervious surface and the fluxes of both sodium and chloride from each watershed for the decades available. Long-term increases in fluxes were found for the three watersheds with the highest percent urban land use. Fluxes were not related to the estimates of wastewater discharge, nor could atmospheric deposition or deicing salts account for significant amounts of the total export for urbanized watersheds. Based on the results the most likely source of Na and Cl is impervious surface deposition and erosion from within the watershed. Estimates of urban fluxes were lower than but similar to estimates from northern watersheds affected by deicing salts and suggest that other mechanisms may contribute to the climbing salinity in northern states.