These three (3) abstracts have been submitted to the organizing committee for the symposium on the Hydrogeology of Washington, sponsored by the Washington Department of Ecology and to be held in Olympia, Washington in August. Email me if you want more info about these topics and/or the conference.
The system is mostly recharged through precipitation, through areas of exposed basalt or basalt overlain by thin and/or permeable sediments. The Wanapum aquifer discharges downward to the Grande Ronde aquifer, which in turn discharges to the basement rock below and possibly to the Spokane Valley - Rathdrum Prairie aquifer. The steptoes around Medical Lake form a ground water divide that isolates the aquifer system from the eastern Columbia Plateau aquifer system, where groundwater flow is mostly toward the southwest. Ridges of basement rock also separate the deeper part of the groundwater flow system. Interestingly, a leaky wastewater system at Fairchild Air Force Base has caused an artificial mound of groundwater in the Wanapum aquifer beneath the base, resulting in radial groundwater flow.
The Wanapum and Grande Ronde aquifers are semiconfined to confined systems, with hydraulic gradients ranging from 0.006 to 0.121 and 0.011 to 0.182 respectively; the vertical gradient is downward. The interbed between the two aquifers forms an aquiclude and causes a difference in head ranging between 6-64 m, and tends to increase to the north and east. From May to October, 1994, heads were observed to decline about 1.5 m in both aquifers. Wanapum aquifer parameters are estimated as: hydraulic conductivity, 0.06 to 3.69 m/d; transmissivity, 0.4 to 1.9 m2/d; and storativity, 0.00002 to 0.0005.
The Spokane Valley - Rathdrum Prairie aquifer system, covering approximately 425 square miles, is one of the most prolific in the western United States and is designated a sole source aquifer by the Environmental Protection Agency. Numerous technical studies have been made on this important and interesting ground water flow system, yet we are unable to answer a fundamental question: nobody really knows how much water is in the ground beneath Spokane!
It is well understood that most recharge to the unconfined Spokane Valley - Rathdrum Prairie aquifer occurs in Idaho, where surface water is lost through numerous side-valley lakes and by the influent Spokane River. Ground water flow moves generally southward and westward in the Rathdrum Prairie, and then to the west across the Washington/Idaho state line where little water is added to the system in Washington.
In 1968, U.S. Geological Survey estimates based on water-balance calculations made by Pluhowski and Thomas suggested that flow from Idaho to the Spokane valley "averages about 1,000 cubic feet per second, or about 650 million gallons a day." Later USGS reports place the total Idaho recharge to the aquifer system at about 800 cubic feet per second (Drost and Seitz, 1978), and in their development of a finite-element model of the aquifer system, Bolke and Vaccaro (1981) calculated a throughflow across the state line at 457 cfs. An attempt by Painter in 1991, also utilizing a mass balance approach accounting for all recharge points in Idaho, resulted in an approximation of 753 cfs crossing the state line.
Recent modeling by this author using MODFLOW/EM to simulate ground water flow through the Spokane valley portion of the system shows 307 cfs crossing the state line. More important are the results of sensitivity analyses on the model that show a range in state line flow from 129 to 650 cfs depending on variations in hydraulic conductivity and saturated thickness.
The Chamokane valley aquifer system is located in a north-south trending valley in northeastern Washington and is filled with as much as 152 m of Pleistocene age sediments of glaciogenic origin. Excellent aquifers exist in coarse-grained glaciofluvial sediments with fine-grained glaciolacustrine deposits acting as aquicludes. An upper unconfined aquifer exists in the uppermost 10 to 30 m thick outwash deposit and is heavily utilized by irrigators and smaller domestic wells. A lower confined to semi-confined aquifer exists in the central and northern portions of the valley, and comprises the lowermost 80 m of sand and gravel in the valley fill sequence. The log of a deep hatchery production well in the southern part of Chamokane valley shows that there is a complex interfingering relationship between the gravels comprising the lower aquifer and the fine-grained sediments, and clear definition of the lower aquifer system in this area is problematic.
The upper aquifer is perched on a laterally extensive silt/clay unit of variable thickness that was deposited during two predominate stands of glacial Lake Columbia. Groundwater flow in this aquifer is from north to south, at an average gradient of 0.004. The upper aquifer is recharged primarily through infiltration of water from Chamokane Creek that is entirely influent in the central part of the valley, and to a lesser extent through direct precipitation and tributary watersheds. Discharge of ground water from the upper aquifer is primarily through five major contact springs near Ford, Washington. The lower aquifer has measured heads higher than the water levels in the upper aquifer, as revealed by a single deep well in the central part of the valley. Recharge to the deeper system is not well understood, and is presumed to occur in the northern part of the valley.
Eight water wells drilled into the upper aquifer at four sites show remarkably high transmissivity and hydraulic conductivity values. Average transmissivity of 9,164 m2/d and an average hydraulic conductivity of 0.94 cm/s, while extraordinary, are realistic when compared to other similar aquifers found in glaciogenic deposits.
The ground water model MODFLOW was used to simulate flow through the aquifer system in order to assess the effects of large ground water withdrawals on the upper aquifer and the springs. Depending on antecedent conditions in the aquifer and winter/spring recharge, cumulative spring discharge can be reduced from four percent to 11 percent by the irrigation withdrawals. In addition, a high discharge production well has been drilled into the lower aquifer at the Spokane Tribal Fish Hatchery in the southern part of the Chamokane valley. A second ground water flow model was used to assess the effect of the well on the nearby springs; certain pumping schedules showed a significant influence on the flow from Metamooteles spring, suggesting a hydraulic connection between the systems.