Publication Summary

Currently, Washington State does not have an ambient (background) monitoring program for groundwater. To test a proposed technical approach for assessing groundwater conditions, a pilot study was undertaken in the Centralia-Chehalis area.

Primary goals of the study were to 1) refine the technical approach and 2) evaluate the resources required to conduct a project of this scope.

This report presents the results of the pilot study. During 2006, a follow-up report will be published summarizing project costs and lessons learned from this study.

The four plates associated with this publication are linked below.

This report presents the results of a 2003-05 hydrogeologic assessment of the Centralia-Chehalis area surficial aquifer. The study was undertaken by the Washington State Department of Ecology to pilot test a standardized technical approach for a new state groundwater assessment program.

The primary technical objectives of the pilot study were to characterize the hydrogeologic setting of the study area, monitor and describe local groundwater/surface water interactions, and monitor and describe current ambient groundwater quality and water level conditions. Study activities in support of these objectives included developing a well inventory and database, conducting a dry-season seepage evaluation of the Chehalis and Newaukum rivers, and assembling and monitoring area-wide well and instream piezometer networks.

The Centralia-Chehalis area surficial aquifer, which is underlain by Miocene-age continental sediments and bedrock, is comprised of a complex assemblage of unconsolidated Pleistocene-age to Holocene-age glacial and alluvial deposits. Monitoring results indicate that groundwater in the aquifer system is in close hydraulic connection to study area rivers throughout most of the valley. Changes in the aquifer matrix and grain size strongly influence the local groundwater geochemistry, resulting in a marked contrast in reducing/oxidizing (redox) condition between the northern and southern portions of the study area. Current overall groundwater quality is good, with only limited occurrences of nitrate (as nitrogen) above 5 mg/L.

This document serves as an example of the type of technical report that would be generated by a long-term state groundwater assessment program, if the proposed assessment approach were applied to other basins of interest. A separate report evaluating the overall success of this study is in progress. The follow-up report will present lessons learned and will also evaluate the costs and benefits of the proposal for Ecology management consideration.

Link to EIM data for User Study ID GAP0001