Hydraulic continuity is a scientific term that describes how easily water flows between groundwater and surface water (streams, rivers, lakes, and wetlands). When hydraulic continuity is high, water flows easily between ground water and surface water, which impacts how water should be managed. This is true because anything done to the groundwater (such as pumping from wells, or pollution that seeps into the groundwater) will affect the surface water, and vice versa.
Why is hydraulic continuity an issue in the Chehalis Watershed?
In the Chehalis watershed, most of the groundwater currently being used is believed to be in close hydraulic continuity with surface water. This groundwater is drawn from the shallow, water-table aquifer; most wells are less than 100 feet deep.
While this close hydraulic continuity is important for many reasons, the topic arises most frequently related to consumptive water use. Ecology will basically not approve new applications for ground water use because of its management philosophy that the ground water and surface water should be considered as virtually one connected system, unless site-specific studies show otherwise. Concerns about too much water being allocated, about low summer stream flows, and about water quality have prompted Ecology to stop issuing new water rights that would consume water from the streams and rivers.
When hydraulic continuity is high, groundwater pumping can affect streamflows in two ways. First, if a well is close enough to the stream, it is possible to actually suck water from the stream toward the well. Second, wells may intercept groundwater that would otherwise have contributed to streamflow. This can be true even for wells that are far away from the stream.
What is known about hydraulic continuity in the Chehalis Watershed?
Previous studies have indicated that hydraulic continuity is high in most areas of the Chehalis. Data from one study suggest that the speed of groundwater flow is rapid, averaging 16 feet per day2. Another study indicates that water flows into the Chehalis and Black Rivers from the groundwater at a rate of between 1.8 and 3.1 cubic feet per second (cfs) per river mile3. At this rate of inflow, groundwater is adding up to 30 cfs to the river over a ten-mile length. This is very significant, considering that a typical August streamflow in the Chehalis River at Grand Mound is 242 cfs.
What does this mean for people who want to drill new wells and begin to use groundwater?
This means that Ecology will likely not approve any new applications for groundwater use because of its management philosophy that assumes new groundwater use will impact surface water, unless proven differently. Individuals can still drill wells for personal use, under the exempt well provision4.
It may also be possible, through site specific field studies, for people seeking approval for a new groundwater use to show that the desired use would not harm streamflows. This would require a field study and policy approval from Ecology.
Do we have enough information to understand hydraulic continuity in the Chehalis Watershed as much as we need to?
No. Currently there is enough information to know that hydraulic continuity is likely to be high throughout the watershed. There are some actual data to quantify hydraulic continuity for the Black River/Scatter Creek region of the watershed. What is needed is a determination of aquifer characteristics for the entire shallow aquifer, both along the length of the Chehalis River and across the width of the valley.
What are some possible solutions?
Some possible approaches/solutions for the hydraulic continuity issue are as follows:
Status Quo - no new actions related to hydraulic continuity .
This is the "business as usual" alternative. The results are likely to be the following:
No new groundwater rights will be approved.
Existing groundwater use will continue to impact streamflows.
Conduct a groundwater study that provides the information necessary to address the hydraulic continuity issue.
This study would provide specific information about the character of the groundwater throughout the Chehalis watershed that would allow decision-makers to better evaluate whether an individual water right application would impact streamflows. This study would also provide the information to evaluate whether a strategic groundwater pumping schedule could be developed for a particular site that would delay the impact on the river until the high flow period.
Such a study would require extensive field testing to determine aquifer properties throughout the watershed. This study would include delineations of river sections that lose water to groundwater and river sections that gain water from groundwater.
Recommend that Ecology develop a new hydraulic continuity policy (statewide or for the Chehalis) that allows water right applicants to employ more flexible strategies for meeting their water needs given that hydraulic continuity is an issue.
These strategies could include identifying areas of no hydraulic continuity (could be new groundwater sources), identifying areas where the timing of pumping groundwater could be managed to eliminate any negative impact on streamflows, or transferring surface water rights to groundwater withdrawals to lessen the impact on streamflows.
Footnotes
1 Primary author: Cynthia Carlstad, Tetra Tech/KCM
2 Garrigues, R.S., Sinclair, K., and Tooley, J. 1998. Chehalis River Watershed Surficial Aquifer Characterization. Washington State Department of Ecology Publication No. 98-335.
3 Sinclair, K.A. and Hirschey, S.J., 1992. A Hydrogeologic Investigation of the Scatter Creek/Black River Area, Southern Thurston County, Washington State: The Evergreen State College, masters thesis, 192 p. + plates.
4 There are numerous other issues related specifically to exempt wells that are discussed in the Exempt Wells Issue Paper.
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