We assumed a conservative linear rate of increase, although it may be possible that salinity is increasing at an exponential or logarithmic rate. Streams entering the Baltimore drinking water reservoir and streams and rivers of the Hudson River Valley also showed significant increases in concentrations of chloride over the past several decades ( P < 0.05). In the White Mountains, chloride concentrations in some rural streams now exceed 100 mg/liter on a seasonal basis, which is similar to the salt front of the Hudson River estuary. Despite temporal fluctuations in precipitation in the northeastern United States throughout the study ( 11), we observed strong increases in the baseline concentration of chloride in rural watersheds with low density of roadways in Maryland, New York, and New Hampshire over the past 30 years ( Fig. Municipal records indicate that >82,000 metric tons of NaCl were applied to roadways in the city of Baltimore (not including private property and interstate highways) as deicing material during the study period ( 14). Baltimore LTER sites were not downstream of any wastewater treatment plants, which could release chloride. Detailed site descriptions and sampling protocols are described in ref. base flow), filtered in the field (47-μm glass microfiber and 0.45-μm-pore-size nylon filters), and analyzed for chloride by using a Dionex LC20 series ion chromatograph. Samples were collected weekly from 1998 to 2003 without regard to flow conditions (no bias toward storm flow vs. Streams draining forest, agricultural, suburban, and urban watersheds were sampled as part of the National Science Foundation-supported Baltimore Long Term Ecological Research (LTER) project. In this region, coverage by impervious surface increased by ≈39% from 1986 to 2000 ( 12). The Baltimore metropolitan watersheds drain into the Chesapeake Bay and represent one of the most rapidly developing areas of the northeastern United States. Within the Baltimore metropolitan area, we explored long-term changes in chloride concentrations across a broader gradient of land use to determine an empirical relationship between salinization and increasing coverage by impervious surface. Regulation of road salt was recently considered by the Canadian government after much controversy ( 6).īaltimore Metropolitan Area. Road salt is currently not regulated as a primary contaminant to fresh waters of the United States, although a recommended limit exists ( 8). Water with chloride concentrations >250 mg/liter can impart a salty taste and also contain elevated concentrations of sodium and toxic impurities from road salt ( 9), which are of concern to human health. Increases in salinity up to 1,000 mg/liter can have lethal and sublethal effects on aquatic plants and invertebrates ( 7), and chronic concentrations of chloride as low as 250 mg/liter have been recognized as harmful to freshwater life and not potable for human consumption ( 6, 8). Increased chloride concentrations in surface waters, however, can be propagated a substantial distance from roadways, leading to more widespread effects on water quality. Concentrations of chloride in soils as low as 30 mg/liter have been found to damage land plants, which typically occur in close proximity to roads ( 6). In the northeastern United States, chloride derived from salt is commonly associated with runoff from roads at latitudes above ≈39°N, particularly during winter. Salinization refers to an increase in the concentration of total dissolved solids in water and can often be detected by an increase in chloride, an important anion of many salts. Our analysis shows that if salinity were to continue to increase at its present rate due to changes in impervious surface coverage and current management practices, many surface waters in the northeastern United States would not be potable for human consumption and would become toxic to freshwater life within the next century. Mean annual chloride concentration increased as a function of impervious surface and exceeded tolerance for freshwater life in suburban and urban watersheds. We observed chloride concentrations of up to 25% of the concentration of seawater in streams of Maryland, New York, and New Hampshire during winters, and chloride concentrations remaining up to 100 times greater than unimpacted forest streams during summers. Increases in roadways and deicer use are now salinizing fresh waters, degrading habitat for aquatic organisms, and impacting large supplies of drinking water for humans throughout the region. Chloride concentrations are increasing at a rate that threatens the availability of fresh water in the northeastern United States.
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