Local Action for a Sustainable Tomorrow

Community Forum: Water for People, Farms, and Fish
The levels of all the local(Dungeness) aquifers have dropped about 10' in the last 10 years.
The number of wells in the area has increased significantly since the 1970s.
Irrigation water that is currently drawn from the Dungeness is about half of what it was in 1979.
A large portion of the ditches are now in pipes.

Are the lowered levels of the aquifers more attributable to the decreased recharging of the aquifers from less water being distributed to the aquifers from the ditches/piping than from well usage?

Thank you for your question. Your observations regarding factors that could affect water levels in the aquifer are correct. Ground water levels fluctuate over time in response to temporal changes in recharge to and discharge from the ground water system. So changes in recharge, such as reducing leakage because irrigation ditches are converted to pipe will lower water levels in the aquifer over time until a new equilibrium is reached. Changes in discharge, such as increasing ground water withdrawals will also be reflected as lowering water levels over time. Many other factors play into recharge and discharge including climatic factors and landuse changes and estimates have been made for many of these factors for the Sequim-Dungeness area.

These estimates are used as input for a ground water model that has been developed in an attempt to better quantify and understand the hydrogeology of the Sequim-Dungeness area. A ground water model is essentially a water balance calculation where input (ground water recharge) to the model must equal output (ground water discharge) from the model. Estimates are made of each, as well as for aquifer parameters that control how water is stored and moves through the model. The model is run and model results, such as water levels in aquifers, are compared to actual field measurements. The process is repeated until model results adequately match actual measurements. The model can also be used to estimate quantities of other things such as how much ground water would flow into the river under different scenarios.

Values for many of the model parameters have been estimated from several studies over many years. A good reference for such information as well as a good description of the hydrogeology of the area is:

Thomas, B.E., Goodman, L. A., and Olsen, T. D., 1999, Hydrogeologic Assessment of The Sequim-Dungeness Area, Clallam County, Washington, USGS Water-Resources Investigations Report 99-4048, 165 pp.

This report includes several tables and figures, some of which include estimates of the recharge and discharge factors you are interested in. For example,
the estimate for domestic ground water withdrawals from self-serving wells in 1996 is 2.2 cubic feet per second (cfs). This value represents an annual average and is found in Table 11 on page 81 of the USGS report. For homes on septic systems, the majority of in-house water use is estimated to be returned to the ground via the septic system drainfield. Thus, only a portion of the domestic water withdrawn from wells is consumed and most of the consumption is related to out-door water use unless the home is on a sewer system.

Since 1996 there has been roughly 2200 additional water wells drilled in this area, mostly self-serving domestic wells. Assuming an annual average withdrawal rate of 350 gallons per day, this adds approximately 1.2 cfs to the 2.2 cfs estimated for 1996 for a current total estimate of roughly 3.4 cfs.

By comparison, the amount of water leaking from irrigation ditches in 1996 was estimated to be approximately 30 cfs, a rate roughly 15 times larger than the total amount of water withdrawn by domestic wells in 1996. This value is from Table 7 on page 72 of the USGS report. This leakage would only occur when the irrigation ditches are being operated between mid-May through mid-September, or about 35 % of the time. Thus, the annual average ditch leakage would be approximately 10.5 cfs or only about 5 times larger than the total amount of water withdrawn by domestic wells. The total diversion rate out of the river in 1996 was reported to be 74.4 cfs so approximately 40% total irrigation diversions in 1996 leaked back into the ground under the irrigation ditches.

As shown on the attached graph, the Dungeness Water Users Association has significantly reduced their diversions from the Dungeness River, primarily related to eliminating the loss of diverted water through ditch leakage. Although I don't have a reliable estimate, the Clallam Conservation District has indicated that the amount of irrigated acreage has not changed significantly in the past 5-10 years. Piping ditches has proceeded through this time period so the reduction is diversions is closely related to reductions in ditch leakage during this time period. Since more water is left in the Dungeness River, leakage from the river into the ground water has increased during this time, although likely not as significantly as the amount of ditch leakage has been reduced.

To summarize and respond to your question directly, the amount of ditch leakage reduction may be several times greater than the amount of ground water withdrawn and consumed by domestic wells over the timeframe discussed above. Because of this, one might conclude that reductions in ditch leakage may be a bigger factor resulting in ground water level declines compared to the increase in water withdrawn by domestic wells over the same time period.

Should you have additional questions please do not hesitate to contact me.

Dave Nazy
Hydrogeologist
Water Resources Policy Section
360-407-6038

Peak Moment: An Innovative Program Helps Residents Protect Their Water
http://globalpublicmedia.com/peak_moment_an_innovative_program_helps_res...
06 Nov 2007
Water is a precious resource we can't live without. Pat Pearson educates Olympic Peninsula citizens with a "Water Matters" campaign and "Shore Stewards" program for shoreline residents. New homeowners receive a "Welcome to Your Watershed" packet, while volunteers build rainbarrels to catch roof rainwater for gardens. Episode 80.