Salt is a widespread, serious, growing pollution problem that compromises water quality and harms aquatic life. In much of the country, the main source of salt is direct, intentional application of deicing compounds. At low levels, salt is relatively benign and inert, but in higher amounts it can be harmful to aquatic organisms and poses a hypertension risk in humans. Salt is also among the most difficult substances to remove by any practical conventional form of water treatment, and can be a significant impediment to water reuse for irrigation or drinking.
Salt has generally been considered to behave relatively simply, and indeed at low levels it is merely transported with water. Yet in higher concentrations it actually alters the physical characteristics of water, increasing its density, and leading to unexpected behaviors. Thus dense brines can persist in the bottoms of surface water bodies or be driven deep into ground water aquifers. Careful mass balance studies conducted on relatively large watersheds have unexpectedly found that a majority of deicing salt is unaccounted for on timescales of years.
We propose research that will begin to explore some of these surprising behaviors, exploring both salt-polluted water’s physical behavior and its biological impacts. Our study examines the landscape comprehensively, but places special emphasis on small surface waterbodies (SSWs) that we believe are very important yet are understudied and where biological impacts may be especially severe. In particular, SSWs can harbor dense concentrated layers of brine that can explain some of the “missing” salt, and the delays and pulses in delivery seen by us and others.
Our research design takes a careful mass balance approach that considers all important stocks and flows, and examines variations over time in considerable detail. Our conceptual model sees SSWs as occupying an important intermediate role between initial application to impervious surfaces, consequent transport by overland flow and through soils, and later passage via groundwaters to small tributaries that combine to form larger streams of the scale that are gaged by the USGS. Indeed, we believe the local scale, mechanistic nature of the proposed research complements well the extensive monitoring that has been done by the USGS for decades.
Our methods involve well-established analytical technologies but exploit automated measurements that -- with a holistic landscape approach -- are novel. We use an upstream-downstream methodology that will track salt from point of introduction to eventual arrival in gaged river. We will also use halide tracers to help deconvolute salt sources and pathways. Our biological impact assessment uses amphibians and benthic macroinvertebrates as the target organisms and considers both direct harm caused by salt, but also possible hypoxia in meromictic SSWs.
The Upper Mill River Watershed. The main stem, tributaries, and subbasins are indicated.
The tide gated Mill River (blue) has a small tidal range compared to full tidal flushing of the adjacent West River (green).