Understanding potential futures of riverine chloride impairment in New England USA due to climate change, groundwater storage, and human activities.

Abstract:

Road salt impairment may threaten future potability of urban water resources and stress aquatic life throughout snowy temperate watersheds. We contrast scenarios to project chloride flux, storage, and impairment throughout the Merrimack R. watershed, NH/MA, USA using the river-network scale Non-point Anthropogenic Chloride Loading (NACL) model, built within the Framework for Aquatic Modeling of the Earth System (FrAMES). NACL simulates five chloride sources and represents long-term subsurface storage as mobile-immobile exchange at the catchment (grid-cell) scale. Tested scenarios that contrast major drivers include: road salt application rates (current recommendations versus recent inventories); groundwater storage uncertainty (low versus high storage effect); development (dispersed versus urban infilling) and future climate (low [B1] versus high [A1FI] carbon emission scenarios). Simulations that reduce road salt application rates to recommended levels significantly reduce threshold-dependent impaired river length from 20 to 5% within a few years, driven by flushing from headwater catchments. Concentrations downstream, however, decrease modestly and lag the change in loading because of chloride released slowly from groundwater storage. The scenarios suggest best practices and urban infill can mitigate legacy chloride contamination over a few decades. Conversely, dispersed development increases the near-term extent of threshold impaired river length, but downstream concentrations rise slowly as chloride concentrations increase in previously pristine groundwater pools. A warming climate plays a small role until late in the century when reduced snowfall from high emissions scenarios requires less road salting. Reducing road salt use is necessary to mitigate chloride impairment, but expectations and monitoring programs should acknowledge that achieving reasonable water quality goals will take years.