Assessing multiple techniques for identifying various sources of chloride to urban groundwater and streams

Chloride (Cl) concentrations in urban surface waters and groundwaters in temperate climate regions have been rising, with this primarily attributed to the application of road salt. While this may be the case in many locations, other anthropogenic sources of Cl, such as wastewater, landfill leachate, softened water, and fertilizers, may play an important role but are often overlooked. Natural Cl sources must not be overlooked either. Proper identification of the chloride source is necessary to guide potential mitigation measures. This study demonstrates and assesses a unique suite of five techniques that have been used to characterize Cl sources affecting freshwater systems, including established and more novel methods-temporal Cl concentration patterns (including continuous specific conductance and water pressure monitoring), Cl/Na ratios, Cl/Br ratios, water isotopes, and artificial sweeteners. These were applied to an urban site with a variety of groundwater and river/stream end members, with additional groundwater-surface water interactions between them. Together, these five techniques identified influences at the site of several chloride sources, including road salt, wastewater, landfill leachate, and natural deeper groundwater. Moreover, spatial and temporal patterns in their influence were revealed. No one method was successful at clearly identifying the salt sources impacting a given water or location, especially across the entire year. The methods were further assessed on the benefits and limitations of the assessment they provide, including relative cost, to aid fellow practitioners in selection of source characterization methods.