Columbia Plateau Groundwater Availability Study Completed

Problem: The Columbia River Basin in Washington, Oregon, and Idaho supports a $5 billion/year agricultural industry, leading the nation in production of apples and nine other commodities. Groundwater availability in the Columbia River basalts and the basin-fill sediments of the Columbia Plateau Regional Aquifer System (CPRAS) is a critical water-resource management issue in the Basin where the water demand for agriculture, economic development, and ecological needs is high. Groundwater availability is a primary consideration for balancing the conjunctive use of surface-water and groundwater supplies throughout the CPRAS.

Groundwater pumping from the Columbia River basalts has resulted in more than 300 ft of water-level declines in some areas of the aquifer system, placing important agricultural sectors at risk. Groundwater depletion also has contributed to adverse environmental impacts. Declining groundwater levels have significantly reduced stream flows and have contributed to loss of wetlands and degradation of aquatic habitat. Current streamflow in many subbasins is inadequate for certain fish listed under the Endangered Species Act. Conversely, in some areas of the CPRAS excess recharge from surface-water irrigation over the past century has resulted in additional ground water in both the sediments and basalts that is potentially available for use. The excess 'artificial' recharge has created wetlands now dependent on continued irrigation, and has contributed to landslides and excess sediment loads in the Columbia River at the Hanford Reach National Monument.

Approach: The Columbia Plateau Groundwater Availability study began in October 2007 and ended in 2015. The major elements of the approach include:

1. Documenting changes in the status of the system. We compiled data from existing groundwater level monitoring networks maintained by a variety of agencies in the study area. We collected additional data in areas not covered by existing networks. The goal was to characterize the current status of groundwater levels and compare with the status in 1984 when the last regional scale compilation was made as part of the USGS Regional Aquifer System Analysis (RASA).
2. Quantifying the hydrologic budget for the system. We used a variety of hydrologic data and models to quantify the major components of the hydrologic budget for the aquifer system. The primary components of recharge include infiltration of precipitation and snowmelt and irrigation return flow. The primary components of discharge include groundwater pumping and discharge to streams.
3. Updating the regional geologic framework for the basalt aquifers. The RASA study mapped the extent and thickness of the major basalt formations and basin-fill sediments. We compiled and evaluated new geologic mapping and subsurface information and used it to refine the regional framework. The refined framework was also compiled in a three-dimensional spatial database
4. Developing a groundwater flow simulation model for the system. A conceptual model of the system based on the results of elements 1-3 was used to construct a three-dimensional numerical simulation model. The simulation model was used to evaluate and test the conceptual model and later to evaluate groundwater availability under alternative development and climate scenarios.