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Water Quality For nearly every type of water use, whether municipal, industrial, or agricultural, water has increased concentrations of dissolved constituents or increased temperature following its use. There- fore, the water quality of the water bodies that receive the discharge or return flow are affected by that use. In addition, as the water moves down- stream, additional water use can further degrade the water quality. If irrigation return flow, or discharge from a municipal or industrial plant, moves downstream and is drawn back into an aquifer because of ground-water withdrawals, the ground-water system also will be affected by the quality of that surface water. Application of irrigation water to cropland can result in the return flow having poorer quality because evapotranspiration by plants removes some water but nof the dissolved salts. As a result, the dissolved salts can precipitate as solids, increasing the salinity of the soils. Additional application of water dissolves these salts and moves them farther down gradient in the hydro- logic system. In addition, application of fertilizers and pesticides to cropland can result in poor- quality return flows to both ground water and surface water. The transport and fate of contami- nants caused by agricultural practices and munic- ipal and industrial discharges are a widespread concern that can be addressed most effectively if ground water and surface water are managed as a single resource. Water scientists and water managers need to design data-collection programs that examine the effects of biogeochemical processes on water quality at the interface between surface water and near-surface sediments. These processes can have a profound effect on the chemistry of ground water recharging surface water and on the chem- istry of surface water recharging ground water. Repeated exchange of water between surface water and near-surface sediments can further enhance the importance of these processes. Research on the interface between ground water and surface water has increased in recent years, but only a few stream environments have been studied, and the transfer value of the research results is limited and uncertain. The tendency for chemical contaminants to move between ground water and surface water is a key consideration in managing water resources. With an increasing emphasis on watersheds as a focus for managing wafer quality, coordination between watershed-management and ground- wafer-profection programs will be essential to protect the quality of drinking water. Further- more, ground-water and surface-water interac- tions have a major role in affecting chemical and biological processes in lakes, wetlands, and streams, which in turn affect water quality throughout the hydrologic system. Improved scientific understanding of the interconnections between hydrological and biogeochemical processes will be needed to remediate contami- nated sifes, to evaluate applications for waste- discharge permits, and to protect or restore biological resources. 77