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Historical Ground Water Level Data

Ecology has measured groundwater levels for up to 40 years in numerous wells distributed throughout the Columbia Plateau and the GWMA.  As part of this effort, Ecology and the U.S. Geological Survey (USGS) constructed a number of multi-level observation well clusters around the GWMA in the 1970s and early 1980s to evaluate the state of the groundwater resource and the response of the basalt aquifers to development away from the direct influence (“noise”) of a pumping well.  Figure 10 shows a schematic representation of one of the multi-level well clusters.

Groundwater elevation measurements obtained during the same discrete time period can be used as an indicator of possible connection between groundwater and surface water and how well connected the basalt interflow zones are laterally and vertically.  Regular measurements taken over a number of years, particularly from non-pumping wells, provide a record of pumping and recharge to the interflow zones monitored by the well.  The water level measured in each well represents the water pressure in the interflow zone(s) open to the well.  Similarities and differences in the levels and the trends between each well of a multi-well cluster define the relationship with interflow zones monitored each wells in the cluster.  If good vertical connection exists between interflow zones, we would expect the water levels in different zones to be similar, and to react similarly over time.  Conversely, differences in levels and/or trends indicate vertical separation between the monitored zones.

 

Groundwater elevation measurements obtained during the same discrete time period can be used as an indicator of possible connection between groundwater and surface water and how well connected the basalt interflow zones are laterally and vertically.  Regular measurements taken over a number of years, particularly from non-pumping wells, provide a record of pumping and recharge to the interflow zones monitored by the well.  The water level measured in each well represents the water pressure in the interflow zone(s) open to the well.  Similarities and differences in the levels and the trends between each well of a multi-well cluster define the relationship with interflow zones monitored each wells in the cluster.  If good vertical connection exists between interflow zones, we would expect the water levels in different zones to be similar, and to react similarly over time.  Conversely, differences in levels and/or trends indicate vertical separation between the monitored zones.  

Observations from Multi-level Well Clusters

Review of historical groundwater levels from multi-level well clusters demonstrate that there is little vertical flow between the basalt interflow zones, and the aquifers comprised of these zones are separate and distinct.  The water levels from multi-level well clusters located throughout the GWMA corroborate this observation.  Figures 11 and 12 show a vertical profile of historical water level trends in two different parts of the GWMA.

Figure 11, which depicts water levels from a multi-well cluster west of Odessa, shows that each zone has a distinctly different water level.  While all three zones show declining water levels from the beginning of monitoring in 1973, each zone shows a different trend, and those differences are maintained over the entire record.  The shallowest zone (M02) experiences the most declines between 1973 and 1990, whereas the deepest zone (M04), exhibits a lesser water level decline over the same period, followed by a drastic increase in the rate of decline after 2000.  This increase in the rate of decline in the deeper zone reflects the shift in pumping from shallower to deeper aquifers in the Odessa area. The relatively high water level in the deepest zone illustrates a high degree of confinement and isolation from the shallower zones. While the drilling into the deep zone would restore the water level in a well that was originally completed in one of the shallower zones, the sharp rate of decline also shows that the deepest zone is isolated from the others, and that the higher water level condition realized by drilling into deeper zones is temporary.

The water levels in the multi-level well cluster shown in Figure 12 also show that each zone is vertically distinct and separate.  The water levels in wells completed in Wanapum interflow zones are significantly higher than the well completed in the Grande Ronde and show recovery from pumping in the 1970s.  Water levels in the Grande Ronde show relatively heavy pumping pressures and decline throughout the period of record. 

Water level elevations and historical trends in multi-level well clusters in the GWMA were evaluated with respect to the geologic framework of the Columbia River Basalt.  The water level data clearly show that basalt interflow zones form separate and distinct aquifers, In addition, the data corroborate anecdotal evidence that groundwater levels in the monitored aquifers hosted by the Grande Ronde Basalt are declining.  Declines in the aquifers in the Grande Ronde in areas where deep well irrigation is prominent commonly exceed 100 to 150 feet.  These observations have several implications:  First, the data show that natural recharge to the aquifers in the Grande Ronde Basalt is minimal, and at a slow rate where it occurs.  Recharge to the aquifers in the Grande Ronde Basalt is minimal because of the lack of vertical connection between shallow and deep aquifer zones, and a sparcity of both possible recharge locations and recharge water.  The potential for recharge decreases with depth in the Grande Ronde.  Specific reasons that recharge to the aquifer zones in the Grande Ronde Basalt is minimal include:  
  • There are few places where interflow zones that can receive recharge are exposed to surface water or precipitation, and the amount of exposure diminishes to zero with depth in the Grande Ronde Basalt.  In other words, the deeper aquifers in the Grande Ronde are not connected to surface waters anywhere in the GWMA.
  • The interflow zones that are exposed are relatively thin and do not provide much area for precipitation or surface water to infiltrate,
  • Leakage from shallower basalt interflow zones to deeper zones is very small.
  • Precipitation and winter/spring runoff is small in magnitude compared to pumping. 

A second implication of the ground water level record in GWMA wells is that existing groundwater supplies in the deeper basalt units are not reliable or sustainable in the long-term, and thus continuing to drill deeper into progressively lower aquifers in the Grande Ronde Basalt is only a temporary solution for declining water levels and pumping rates. 



Last Updated ( Thursday, 19 March 2009 )
 
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