Mosier Valley Ground-Water Sustainability Study

In Cooperation with the Wasco County Soil and Water Conservation District
and the Mosier Watershed Council

BACKGROUND

There are serious concerns over the sustainability of ground-water supplies from the basalt aquifers in the Mosier Creek Basin, where groundwater level declines have occurred since significant irrigation pumping began in the 1960s. The Mosier Creek Basin is located in the eastern foothills of the Cascade Range in northwest Wasco County, Oregon. The drainage area of the principal streams, Mosier, Rock and Rowena Creeks, is nearly 80 square miles. The area of interest for this study, however, is confined primarily to the lower half of the drainage basin adjacent to the Columbia River.

Water-level declines of up to 120 ft have been documented in several wells monitored by the Oregon Water Resources Department since the 1970s. These declines have forced some well owners to deepen or construct new wells and have contributed to increased energy costs to pump ground water for many other well owners. The largest water-level declines are occurring in the Priest Rapids and Frenchman Springs basalt aquifers. Irrigation, municipal, and domestic pumping, and improperly completed wells that allow well bore leakage probably all contribute to the declines. With the possible exception of municipal pumping, none of these stresses on the aquifer have been well quantified.

Another consequence of water-level declines in the basalt aquifers has been a decrease in ground-water discharge (baseflow) to Mosier Creek. Additionally, if ground-water levels have fallen below the bed of the creek, there may now be losses from Mosier Creek to the ground-water system (Lite and Grondin, 1988). Reductions in ground-water discharge could negatively impact flow and temperature conditions in Mosier Creek, particularly during the summer and fall low-flow period when typical flows are less than 3 cfs. Mosier Creek and Rock Creek are on the Oregon 303(d) list (http://www.deq.state.or.us/wq/303dlist/303dpage.htm) for temperature because they provide rearing habitat and passage for cutthroat trout, coho salmon and steelhead. Cutthroat are listed as a sensitive species and coho and steelhead are listed as threatened species in the mid-Columbia basin (http://oregonstate.edu/ornhic/data.html). The TMDL (total maximum daily load) for temperature is scheduled to be set for Mosier and Rock Creek in 2006.

Following a hydrogeologic assessment by OWRD (Lite and Grondin, 1988) the Pomona and Priest Rapids aquifers were withdrawn from further appropriations for any use other than domestic in the orchard tract area, where most pumping is concentrated. At the time of the OWRD study (1985), nearly 600 acres received irrigation from ground water and depending on the method used to estimate withdrawals, they ranged from 600 to 1,500 acre-ft per year. Today (2004), OWRD lists over 900 acres where ground water provides some or all of irrigation needs (WRIS data from OWRD web site, April 2004). By extrapolation, this 50-percent increase since 1985 may have resulted in an additional 300-800 acre-ft/yr of withdrawals. Note that the increased acreage may not have resulted in a proportional increase in withdrawals because irrigation methods have become more efficient as farms have shifted from impact sprinkler systems to drip and micro-sprinkler irrigation. Increased irrigation efficiency has no doubt partially offset the increases in withdrawals that might be expected to accompany increases in irrigated acreage since 1985.

Another factor that contributes to an unknown degree to water level declines in the Priest Rapids aquifer is discharge to the overlying Pomona aquifer and underlying Frenchman Springs aquifer via well bores. Ground water from the Frenchman Springs can also leak upward into the overlying Priest Rapids in the lower basin, where hydraulic head gradients are upward. Many of the irrigation and public supply wells in the area are not cased and sealed into a single aquifer. Ground water may flow vertically through the open borehole from one aquifer to another and “short-circuit” the natural flow system. This condition is called “co-mingling” by OWRD because it causes mixing of water from separate aquifers beyond that which would occur under natural flow conditions. State well-construction standards are designed specifically to prevent this condition. The effect is the same as if the co-mingling wells were pumping from the Priest Rapids aquifer and injecting into the Pomona aquifer or Frenchman Springs aquifer. The number of wells and the degree to which they co-mingle and contribute to water-level declines in the Priest Rapids aquifer is unknown. 

The Mosier Watershed Council has established three goals for the watershed: (1) to reverse or stabilize water-level declines in the principal aquifers of Mosier Valley, (2) to increase summer baseflows in Mosier Creek, and (3) to sustain productive, profitable agriculture in Mosier Valley (Jennifer Clark, Mosier Watershed Council, written communication 2004). To meet these goals, the Mosier Watershed Council must develop a strategy for achieving sustainability of the ground-water resource. Sustainable strategies are those that provide the largest quantity of ground water at the least cost with the fewest undesirable impacts. Determining the sustainable yield of ground water from the basin is a process that relies upon having a thorough scientific understanding of the complex, three-dimensional ground-water system. In addition to this understanding, a set of water management tools is needed to facilitate an evaluation of alternative strategies and their effects on water levels, streams and springs, and wetlands.

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