Sources of chloride to Minnesota waters.

Abstract

Chloride is an emerging environmental concern, as long-term increases in chloride levels have been observed in surface waters and groundwater resources across North America. Elevated chloride levels in lakes, rivers, streams, and groundwater are a concern in northern regions, particularly in urban areas where deicing salt can be a major source of chloride pollution. Chloride is also an issue in many municipalities where wastewater treatment plant effluent exceeds federal water quality standards for chloride and discharges to sensitive receiving waters. While use of deicing salt for winter road maintenance is known to be a major source of chloride, limited research has investigated chloride discharged from water softener use. Additionally, chloride sources such as water softeners and fertilizer can be more important sources in watersheds with agricultural and mixed land use. This thesis examined sources and implications of chloride pollution to the environment. The first chapter presents a chloride budget for the state of Minnesota to estimate the amount of chloride discharged from household water softeners as well as other domestic, environmental, and industrial sources. The second chapter presents a chloride balance model for Sand Creek Watershed, a chloride-impaired, agricultural watershed in southern Minnesota. The third chapter is a review of the costs and benefits of household water softening relative to environmental impacts and other water treatment alternatives. The fourth chapter examines commercial-scale water softening and its impacts on septic system performance. The analyses employed multiple types of data, including effluent monitoring data, sales records, and groundwater data, and utilized various statistical, spatial, and survey methods. Results from the statewide chloride budget found that road salt was the largest source of chloride to the environment, but that WWTPs and fertilizer were also substantial sources, discharging 221,300 t and 209,900 t annually. Household water softening was found to contribute substantial chloride loads to WWTPs in communities with hard drinking water, contributing 65% of the chloride discharged to all 613 municipal WWTPs statewide. Findings from the statewide and watershed chloride balances indicate that in areas with more agricultural and mixed-land use, WWTP effluent and fertilizer application are important chloride sources that together are comparable to road salt chloride contributions. Results from the watershed chloride balance model suggest significant chloride retention in the watershed across seasons. The review of softening costs and benefits found that softening water can provide economic benefits to households and communities, but there is limited evidence to characterize the economic costs from chloride discharge to the environment, particularly related to impacts on chloride-sensitive species and drinking water quality. Results from the septic system study showed that systems receiving water softener backwash had chloride levels that were between 2-20 times higher than septic systems not receiving softener backwash as well as higher biological oxygen demand (BOD) and total suspended solids (TSS), although the difference was not statistically significant for BOD and TSS. Water softeners were estimated to contribute between 83-95% of the chloride measured in septic systems receiving softener backwash. The analyses show that the relative importance of chloride sources is dependent on local characteristics, that feasible and economic solutions to mitigate chloride contamination will vary across communities, and that communities may need to employ multiple strategies to achieve meaningful and cost-effective chloride reductions.