Long-term sodium and chloride surface water exports from the Dallas/Fort Worth region

Sodium and chloride in surface water are typically related to urbanization and population density and can have a significant impact on drinking water sources and the subsequent 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 sodium and chloride concentrations has been found to occur in warmer climates. This study investigated long-term exports of sodium and chloride from watersheds with increasing urbanization in the humid subtropical Dallas–Fort Worth region. We compared exports to characteristics of urbanization: urban land cover, impervious surface area, and calculated contributions from wastewater discharges. Long-term data (1980–2008) were obtained from five USGS gages located in and around the cities. Exports were calculated by regression analysis between concentrations and discharge and normalized for time and the watershed area. Grab samples were collected from June 2009 to May 2010 and sodium and chloride concentrations quantified. Our results show a strong positive relationship between the mean annual sodium and chloride exports from each watershed and the percent urban land cover and impervious surface area. Long-term increases in sodium and chloride fluxes were found for the three watersheds with the highest percentage of urban land cover. The single largest contributor was wastewater effluent that was estimated to contribute approximately half of the total loads in the three urbanized watersheds. Atmospheric deposition and deicing salts accounted for small amounts of the total export for urbanized watersheds. The source of the remaining salt load is still unknown and may be a combination of non-point sources. Estimates of urban salt exports were similar to estimates from northern watersheds affected by deicing salts.

Research highlights
► Watershed NaCl exports were calculated for 1980 to 2009 for the Dallas/Fort Worth Region.
► Exports were of similar magnitude to northern cities and related to percent urban land cover.
► Wastewater treatment plants contributed approximately half of the salt exports.
► We hypothesize non-point sources may contribute large amounts to salt exports.