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Water-quality conditions in the Central Columbia Plateau were evaluated as part of the National Water Quality Assessment Program on the basis of analysis of nutrient, suspended-sediment, and pesticide data collected from 1959 to 1991 at 105 surface-water sampling locations. Three subunits were delineated based on differences in surface-water hydrology and water-quality conditions in the study unit. The movement of surface water through the Quincy-Pasco subunit in the southwestern part of the study unit is artificially controlled by the Columbia Basin Irrigation Project. The highly variable streamflows in the Palouse subunit in the eastern part of the study unit are more naturally influenced by storm runoff and snowmelt. The North-Central subunit in the central and northern parts of the study unit has few perennial streams and an extensive network of ephemeral streams. Non-point sources account for at least 97 percent of the estimated inputs of nitrogen and phosphorus to the study unit. Locally, however, point sources may dominate: most of the nutrient loading to the South Fork Palouse River during low streamflows comes from the Moscow and Pullman sewage treatment plants (STPs). The Quincy-Pasco subunit receives the highest rates of fertilizer and pesticide applications (approximately 132 pounds of nitrogen, 17 pounds of phosphorus, and 2.3 pounds of pesticides per acre per year) in the study unit. The combined annual loads of total nitrogen and phosphorus from four large drainages from the QuincyPasco subunit (which together represent approximately 0.8 percent of the drainage area of the Columbia River below Priest Rapids Dam) are about 3.4 percent and 0.7 percent, respectively, of the corresponding annual load: in the Columbia River below Priest Rapids Dam. Nutrient concentrations in the Quincy-Pasco subunit vary seasonally and spatially as a result of the operations of the irrigation project and of changing proportions of irrigation return flows, unused irrigation water, and ground-water seepage in the surface drains and wasteways. Higher phosphorus concentrations occur during the irrigation season, when suspended-sediment concentrations also are highest. Detections of pesticide compounds throughout the year in the Quincy-Pasco subunit indicate that pesticides may be transported to surface waters by agricultural and storm runoff and influx from the groundwater system. Variations in nitrogen-species concentrations in the Quincy-Pasco subunit during and after the irrigation season indicate interactions between ground and surface waters: constituents are carried to the groundwater table by artificial recharge, shallow ground water then discharges to surface drains and wasteways, and concentrations are diluted by unused excess water during the irrigation season. Concentrations generally increase downstream as a result of contributions from agricultural runoff and ground-water seepage, except when mitigated by dilution and (or) biological assimilation of nutrients. Variations in water quality in the Palouse subunit relate to runoff from agricultural lands during storms and to dilution of STP discharges. There is more precipitation in this part of the study unit, and the loess soils are particularly subject to erosion during storms. Runoff transports nutrients, sediment, and possibly pesticides to the streams. Nutrient and suspended-sediment concentrations are highest from November through April in most of this subunit. Several STPs operate in the Palouse subunit, and nutrient concentrations in tributaries affected by these STPs are highest during low streamflows. The chronic water-quality criterion for the protection of aquatic life was exceeded at 9 of the 10 sites in the study unit where water samples collected during the 1970's were analyzed for DOT and its metabolites; the overall frequency of detection was low, however. DOT was detected in 97 percent of the whole-fish samples collected in the irrigation project. U.S. Environmental Protection Agency and Washington State surface-waterquality chronic standards for dieldrin were exceeded at four of nine sites; dieldrin was detected in 43 percent of the whole-fish samples collected in the irrigation project, and the human health criterion for edible fish fillets was exceeded in four fish. Better information is needed about the timing, amounts, and new types of pesticides applied in the study unit. Many of the pesticides investigated in previous studies currently are not used, or are not used as much as other compounds. Some of the compounds currently in use are less stable and more soluble than pesticides used in the past and may result in a larger flux of pesticides to surface waters from the shallow ground-water system. More data are needed to further improve understanding of water quality in the Central Columbia Plateau. Little is known about surface-water quality outside of the irrigation project and the Palouse River Basin or about water-quality conditions in barren and range lands and ground-water-irrigated farm lands. Because streamflows in the irrigation project are relatively uniform from year to year, it is possible to estimate constituent loads in this part of the study unit with fewer samples than are required for the rest of the study unit. INTRODUCTION The Central Columbia Plateau is 1 of 60 study units targeted for study by the National Water Quality Assessment Program (NAWQA), which is designed to describe the status and trends in the quality of the Nation's groundand surface-water resources and to link the status and trends with an understanding of the natural and human factors that affect the quality of these resources. The program integrates information about water quality at a wide range of spatial scales from local to national, and focuses on water-quality conditions that affect large geographic areas of the Nation or that occur frequently within numerous smaller areas. The building blocks of NAWQA are the study unit investigations, which will be conducted in 60 of the major hydrologic basins in the United States. The main components of the study unit investigations are (1) analysis of existing data to aid in the study design, (2) occurrence and distribution assessment to characterize the broad-scale geographic and seasonal distributions of water-quality conditions in relation to major contaminant sources and background conditions, (3) long-term monitoring to assess long-term changes in selected aspects of water-quality conditions, and (4) source, transport, and effect studies to address specific questions about the characteristics and causes of water-quality degradation in individual areas. Information about the relations between water quality and upstream or upgradient characteristics is an integral part of all four components of NAWQA activity. The information from each study unit will forrr the basis for later synthesis at regional and national scales. NAWQA's national synthesis involves comparisons of water quality on a national basis and, where a given condition exists in only a few study units or where there are insufficient sites to discuss across a gradien* side-byside discussions of study unit findings. The national synthesis also will include regional survey results. These comparisons are intended to highlight consistent national patterns in concentrations and frequencies of exceedance of water-quality standards as a function of land-use groups, population density, amounts of chemical applications, season of the year, time (trends), streamflow, soil type, and other factors. Comparisons of water quality across the Nation will aid in recognizing the effects of human activity on water quality and will show how widespread are violations of water-quality standards for drinking water and for the protection of aquatic life. One of the first activities in the NAWQA study units is the analysis of existing data in order to (1) develop a conceptual model of the water-quality conditions within each basin, (2) provide an historical perspective on the water quality of the Central Columbia Plateau study unit, and (3) summarize the current understanding o f various aspects of the water-quality conditions in the study unit and of the natural and human factors that affect those aspects of water quality within the study unit. The review of existing water-quality data describes the strengths and weaknesses of available information and the implications for understanding water-quality issues, and is then used to guide new data collection activities of the NAWQA program. The analysis of existing data continues throughout the study period, and findings will be published along with the results from newly collected data (Hirsch and others, 1988). This report, one of the first products of NAWQA, is the analysis of existing water-quality data for nutrients, suspended sediment, and pesticides in surface waters of the Central Columbia Plateau study unit. A similar analysis of existing water-quality data for ground water was completed by Jones and Wagner (1995). Purpose and Scope This report provides an initial analysis of nutrient, suspended-sediment, and pesticide concentration data collected from 1959-1991 in the Central Columbia Plateau study unit. The purposes of this report are to (1) identify the spatial and temporal patterns of concentrations and loads of nutrients, suspended sediment, and pesticides within the Central Columbia Plateau study unit, (2) use existing water-quality data to guide additional data collection in the study unit, (3) contribute to a synthesized description of the status and trends in the quality of the Nation's water resources, (4) document findings for future NAWQA activities in the Central Columbia Plateau study unit, and (5) present a preliminary conceptual model of the spatial and temporal patterns of concentrations and loads of suspended sediment, nutrients, and pesticides within the study area, based on the available data a

Nutrients, suspended sediment, and pesticides in streams and irrigation systems in the Central Columbia Plateau in Washington and Idaho, 1959-1991