BASALT AQUIFER RECHARGE, AGE, AND WATER LEVEL IN THE COLUMBIA BASIN GROUND WATER MANAGMENT AREA 

An Introduction to the Columbia Basin GWMA Subsurface Geologic Mapping and Hydrogeologic Assessment Project

INTRODUCTION

The Columbia Basin Ground Water Management Area of Adams, Franklin, Grant, and Lincoln Counties (the GWMA) currently encompasses approximately 8,300 square miles in south-central Washington (Figure 1).  When first designated in February 1998, in response to concerns about elevated nitrate-N concentrations in groundwater, the GWMA included all of Franklin, Grant, and Adams Counties.  In 2005 Lincoln County was added to the GWMA, and the GWMA charter was expanded to include all groundwater quality issues in the four counties. The photos in this article when clicked on will open in a larger format.

Scientific, mitigation, planning, and outreach activities dealing with groundwater quality within the GWMA largely are guided by the GWMA Plan.  Based on conclusions presented in the GWMA plan and Ecology review comments on the GWMA plan, GWMA stakeholders decided that there was a need to identify the aquifers contributing groundwater to GWMA sampling wells, the extent of those aquifers, and the recharge sources (if any) for these aquifers.  To support this, the GWMA Subsurface Geologic (or Hydrostratigraphic) Mapping Project was started in 2000. At this time, the subsurface geologic mapping effort has mapped the distribution of the major geologic features and units that influence ground water occurrence and movement beneath the GWMA. 

GWMA’s subsurface geologic mapping efforts have been done in phases tied to changing GWMA priorities and funding availability. 

These phases were as follws:

1. During 2000, 2001, and 2002 the mapping project focused on the shallower parts of the basalt aquifer system in Adams, Franklin, and Grant Counties.  This work was done to better understand the occurrence of nitrate in the shallower portions of the basalt aquifer system.
2. Between 2002 and 2004 the sediments overlying the basalt aquifer system in the 3 original GWMA counties were mapped.  The sediments were mapped to better assess the impacts of nitrate bearing shallow ground water on private, single family domestic exempt wells.
3. Beginning in 2006 and continuing through the current effort which began earnest late in 2007, the mapping efforts have extended into Lincoln County (flowing the addition of Lincoln County to the GWMA) and downwards into deeper parts of the basalt aquifer system.  This emphasis on the deeper basalt aquifer system, especially within the Grande Ronde Basalt, has been driven by the growing concern for sustaining deep aquifer production wells for both agriculture and municipalities. 

As a part of the recent subsurface mapping work, GWMA’s efforts have begun to evolve, focusing more and more on understanding ground water sources.  This change in emphasis is a direct consequence of the legislative mandate which funded the GWMA mapping efforts in 2007, 2008, and 2009.  Specifically, this work was funded by a direct appropriation from the Washington State Legislature, which was directed to the GWMA via the Washington Department of Ecology (Ecology). A key objective of this work, as mandated by the Washington State Legislature, was to:

“…submit a report to the appropriate committees of the legislature describing the dynamic relationship between groundwater and surface water in the region.”

To that end, GWMA hydrogeologists currently are working on a report that refines and upgrades the GWMA geologic database and geographic information coverage (GIS) by essentially completing the subsurface geologic mapping portions of GWMA’s subsurface mapping project and using this information in conjunction with water quality, water level, and well testing data (both existing and new) to test and refine the conceptual model describing ground water flow in the aquifers underlying the GWMA and the relationships of this groundwater to surface waters, including recharge. The basic project approach used to accomplish this goal is described in a work plan produced in mid 2007.  The remainder of this progress report focuses on key aspects of this larger effort, especially as it relates to the legislative request that accompanied the funding authorization.This progress report is subdivided into several sections which address the basic legislative request, including:

1. A discussion of the potential recharge, or lack of recharge, of the Columbia River basalt aquifer system beneath the GWMA that comes from the Lake Roosevelt pool.  Our work suggests there is little or no recharge of the basalt aquifer system underlying GWMA from Lake Roosevelt.
2. The potential for recharge of this aquifer system from areas lying east of the GWMA boundary.  This may be occurring, but the impact of this recharge on the regional aquifer system appears to be minimal.
3. A review of groundwater geochemistry data collected by GWMA scientists to investigate the age and potential for modern recharge of the aquifer system.  The deep ground water system is dominated by waters 10,000 or more years old.
4. An evaluation of water level data in wells within the GWMA.  This data was examined to assess the degree of connection between the different parts of the CRB aquifer system.  Ground water in the GWMA appears to occur in sub-basins displaying limited connections.
5. A review of GWMA’s conceptual model of how ground water recharges CRBG aquifers, moves through those aquifers and potential discharges from those aquifer systems.

It is not the intent of this report to describe all of the work and results of the current GWMA subsurface mapping and aquifer assessment effort.  That will be done later in the spring of 2009 in one or more technical reports currently being prepared by GWMA hydrogeologists.