Chloride Concentrations in Wetlands along a Rural to Urban Land Use Gradient

Relationships between anthropogenic sources of Cl- and wetland surface water Cl- concentrations have received little attention. Spatial and temporal patterns of Cl- concentration were investigated in 11 wetlands in the mid to late 1990s and in 2012-2013 along a rural to urban gradient. Chloride concentrations in wetlands located in forest watersheds were <3 mg/L. Small spring-fed swamps in relatively undeveloped rural areas had elevated Cl- concentrations ranging from 10 to 50 mg/L which originated from nearby house septic systems and salt application on secondary roads. Potassium fertilizer application on areas of intensive crop production was a major source of Cl- in riparian wetlands where surface water Cl- concentrations were 50-65 mg/L. The application of deicing salts caused the highest Cl- concentrations (100-730 mg/L) which were observed in depressional wetlands in a rapidly urbanizing landscape with a denser road network. Chloride contamination in the study wetlands occurred throughout the year and diffuse surface flow paths produced a widespread pattern of elevated surface water Cl concentrations which have increased considerably in the past 10-15 years in some of the wetlands. Wetland Cl- levels in the most developed landscapes reached or exceeded chronic water quality thresholds established in the USA and Canada for negative effects on aquatic biota.