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Yakima Nitrates

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WA11E - Particle Tracking of Groundwater in the Lower Yakima Basin

Problem - Links between nitrate application to the land surface and measured nitrate concentrations in groundwater or surface water are poorly understood in the Yakima River basin due to the variety of potential nitrate sources. Fertilizer-intensive irrigated agriculture has been prevalent in the basin for decades, and since 1994, the growth of dairy operations has resulted in numerous liquid-manure processing reservoirs and a variety of associated crops farmed for nutrient-management purposes. These nitrate loadings are then attenuated at an unknown rate by biological uptake in plants and subsequent denitrification or immobilization by subsurface microbial communities. Residual nitrate concentrations have contaminated drinking-water wells, where they can have human health implications, and they have discharged to surface waters, where they may disrupt ecosystem function.

Objectives - The primary objective of this investigation is to inform nitrate transport studies in the Yakima River basin by quantifying groundwater flow directions, travel times, groundwater ages, and exchanges between aquifers and surface waters; a secondary objective is to evaluate the numerical stability of a mass transport model built on an existing groundwater flow model and to assess its appropriateness for testing specific nitrate management scenarios.

Relevance and Benefits - An important part of the U.S. Geological Survey (USGS) mission is to provide scientific information to manage the water resources of the Nation, to enhance and protect our quality of life, and to improve our understanding of the environment and natural resources. The investigation described here closely aligns with the USGS strategic science directions "The Role of Environment and Wildlife in Human Health: A System that Identifies Environmental Risks to Public Health in America" and "A Water Census of the United States: Quantifying, Forecasting, and Securing Freshwater for America's Future".

Approach - The study objectives will be met by enhancing an existing regional groundwater-flow model for the Yakima River basin, which the USGS has nearly completed. The existing model can be used to simulate groundwater heads and flow directions, exchanges between groundwater and surface water, and the effects of groundwater withdrawals on hydraulic heads and streamflow. Model enhancements, including additional boundary conditions and application of particle-tracking tools, will allow the model to be used to examine the sources, migration, and fate of nitrate in groundwater in the Yakima River basin. The existing model will also be enhanced with contaminant-transport capabilities, and the resulting model will be evaluated for numerical stability and consistency with observed nitrate concentrations to assess whether it may appropriately be used to test potential future nitrate-management scenarios.

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