Examining Chloride in an Agricultural Watershed Using a Chloride Balance Model

Elevated chloride levels in lakes, rivers, streams, and groundwater are a concern in many snowbelt states, particularly in urban areas where deicing and anti-icing salt are major sources of chloride pollution. In agricultural and mixed land use watersheds, other chloride sources such as fertilizer, water softening, manure, wastewater discharge, and industry may be more substantial sources of chloride pollution. A chloride budget was conducted for Sand Creek watershed, a chloride-impaired, agricultural watershed in southern Minnesota, with the purpose of developing a model for watersheds with limited data and characterizing important chloride sources in less urban settings. Annual chloride mass contributions were estimated for major point and nonpoint sources from June 2013 to May 2019, including wastewater treatment plants, industry, deicing salt, potash fertilizer, livestock, and septic systems using various permitting data, monitoring data, land use data, census data, and other records. The estimated annual chloride budget was used in conjunction with a water balance model to estimate monthly and annual loads and flow-weighted mean monthly concentrations. Results from the chloride balance model show good agreement with more detailed models for the watershed. Road salt, fertilizer use, wastewater treatment plant discharge, and manure application were estimated to be the largest chloride sources that impacted a number of creeks in the Sand Creek watershed. Results from the chloride balance model for the watershed suggest significant chloride retention in the watershed across seasons and that chloride loading in the stream is more sensitive to major runoff events than timing of application from individual sources.