abstract This chapter uses data from a few representative sampling sites in the Hudson basin to understand variations in major ion concentrations, which are used as one simple proxy of gross drinking water quality. Other water supply issues, including potential implications of dissolved organic carbon concentrations on drinking water quality, are also discussed. The major ion content of surface waters is largely determined by precipitation chemistry, dry deposition from the atmosphere, chemical weathering of rock and soil minerals, and anthropogenic loadings, and then modified by biogeochemical reactions that take place within the system. (1) Based on data reported for West Point, New York by the National Atmospheric Deposition Program (NADP), precipitation chemistry in the Hudson River basin is similar to that in much of the northeastern United States. As a result of upwind and regional fossil fuel combustion, sulfate and nitrate are the most abundant anions and hydrogen is the most abundant cation (i.e., dilute solutions of sulfuric and nitric acids). Ammonium, chloride, and sodium have lower concentrations, with the latter two derived mostly from marine aerosols. Chloride appears to have an additional, nonmarine, source accounting for at least 25 percent of wet deposition of this ion at West Point. (2) Major element chemistry of surface waters in the Hudson River basin strongly reflects bedrock geology of tributary catchments. Adirondackand Catskill Mountain and Hudson Highland streams have lowtotal dissolved solids (TDS) typical of ancient crystalline, metamorphic, or previously weathered coarse silicic sedimentary formations. In contrast, the significantly higher TDS of the Mohawk River reflect drainage from large areas of sedimentary rocks including limestones, carbonate-rich shales, and evaporite minerals. Anthropogenic influxes are an additional contributing factorto Mohawk TDS. (3) Hudson River dissolved ion concentrations downstream of the confluence with the Mohawk are similar to the global average for riverine flux to the ocean. Suspended particle concentrations are generally an order of magnitude lower than the global average for all rivers. (4) Essentially all surface waters in non-urban portions of the Hudson basin, including the tidal freshwater reach of the Hudson River, have sufficiently low TDS to serve as sources of relatively high quality raw water for municipal supplies, at least with respect to major ions. (5) Water quality in the basin following treatment by chlorination is more complex, however, due to relatively high concentrations of dissolved organic carbon (DOC) in the freshwater tidal Hudson River and the Croton River catchment of the NYC municipal supply. Chlorination of these waters can yield substantially more chlorina-tion byproducts, including trihalomethanes (THMs), than occurs for low DOC Catskill and Delaware watershed sources. As permissible levels of total THMs in treated municipal supplies have been reduced from 100 ppb to 80 ppb, management of surface waters for public supplies in the Hudson basin has become more complicated.
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