Gruenenfelder, Charles R., CH2M Hill, 9 S. Washington, Suite 400, Spokane, WA 99204
North of the City of Spokane, Washington, groundwater in the Spokane Aquifer flows in a northward direction toward the Little Spokane River, following the ancestral channel of the Spokane River. Unconsolidated sediments have filled this channel (referred to as the Hillyard Trough) to a depth of over 700 feet. Several large capacity springs are dispersed over an approximate two mile wide area along the interface between the Hillyard Trough and the Little Spokane River Valley. This area has been recognized as a primary regional discharge zone for the Spokane Aquifer. The springs typically discharge 30 to 100 feet above the Little Spokane River, and are fed by groundwater from the Spokane Aquifer. Stream flow in the Little Spokane River increases approximately 250 cfs over this reach of the river, in response to the groundwater discharge.
The Spokane Aquifer within the Hillyard Trough historically has been described as a single, largely unconfined unit, developed within unconsolidated Quaternary flood deposits. Hydrogeologic investigations and literature review conducted in support of two Spokane area wellhead protection projects have provided evidence of a deep, areally extensive, confined aquifer that extends from the north Hillyard Trough into the lower reaches of the Little Spokane River Valley. This secondary, deeper aquifer is present within the north Hillyard Trough at depths of 300 to 500 feet below ground, and is overlain by 150 to 200 feet of low permeability sediments. These low permeability sediments are typically encountered between elevation 1450 and 1650 FAMSL beneath both the Hillyard Trough and Little Spokane River Valley, and may represent remnants of a glaciolacustrine (lakebed) deposit. Within the Little Spokane River Valley, the deep confined aquifer system is encountered at depths of approximately 100 to 150 feet below the valley floor. Flowing artesian conditions have been observed in some valley-bottom wells that tap the confined aquifer. The hydraulic and chemical characteristics of this confined aquifer system are not well known due to limited use of this groundwater resource.
Identification of this deeper aquifer unit may have important implications for future groundwater withdrawals, water rights decisions and groundwater development within portions of the North Hillyard Trough and Little Spokane River Valley that are part of the Department of Ecology's Water Resources Inventory Area (WRIA) 55.
Porcello, John J., CH2M Hill, 825 NE Multnomah, Suite 1300, Portland, OR 97224; (503) 235-5022, ext. 4407; jporcell@ch2m.com
A regional (100 sq mile) groundwater flow model of the Spokane Aquifer has been constructed for use in delineating capture zones for public water supply wells owned by the City of Spokane and the water utilities that comprise the Spokane Aquifer Joint Board (SAJB). This work was conducted to support wellhead protection planning efforts by the City and the SAJB. The model simulates groundwater flow in the unconsolidated sediments that comprise the Spokane Aquifer. The model boundaries extend from the Washington-Idaho state line to the confluence of the Spokane River and the Little Spokane River and include the portions of the aquifer lying west and east of Five Mile Prairie. The model is a three-layer flow model that is constructed with finite-element modeling software (Micro-Fem). The model simulates groundwater flow across the state line; pumping from wells owned by the City, the SAJB, and private owners; groundwater/surface water exchanges with the Spokane River; recharge from precipitation and anthropogenic sources; recharge that occurs from tributary valleys that drain into the Spokane Valley; and groundwater discharge to the Little Spokane River and the lower reaches of the Spokane River. The regional model simulates steady-state flow during the early fall of 1994 (the calibration period) and the spring of 1995 (which was simulated to check the model calibration). The model incorporates recent advancements in the definition of the aquifer's boundaries and hydraulic properties, and the model's construction and calibration are supported by extensive data collection activities performed as part of the City of Spokane's wellhead protection planning program.
Capture zones were delineated using three-dimensional particle-tracking methods in which the model tracked particles backwards in time from each well. For each well, the delineation process began with the identification of Special Wellhead Protection Areas (SWHPAs), which are the areas contributing groundwater to each well during periods of one year or less. The SWHPAs cover a relatively small proportion of the City, but a larger proportion of the Spokane Valley. After the SWHPAs were identified, additional delineations were performed for longer periods of time. The wellhead protection areas for these longer periods of time cover a greater proportion of the City and the entire Spokane Valley.
Buchanan, John P., Department of Geology, Eastern Washington University, Cheney, WA 99004, jbuchanan@ewu.edu; and McMillan, Kent, K-prime Geoscience, Inc., Bellevue, WA 98004
The wellhead protection plan for Fairchild Air Force Base (R&A Technical Services, 1997) suggests that the 10-year capture zone for the Ft. Wright wells extends southward, about six miles upgradient, and into Hangman valley. Similarly, the capture zone delineated for the Baxter well suggests recharge from inflow from Hangman valley (CH2M Hill, in preparation). The aquifer in Hangman valley, then, seems to be an important tributary to the "lower", or western part, of the main Spokane aquifer.
Geologic mapping by Joseph (1990) and Stoffel and others (1991) shows that Quaternary age unconsolidated deposits exist in Hangman valley, and these deposits are in contact with similar ones in the lower Spokane valley. The sedimentary unit mapped in Hangman valley is described as consisting of glaciolacustrine (lake) and flood deposits containing silt and clay interbedded with coarser sand to gravel material (Joseph, 1990). Along Hangman Creek cyclic bedding between the coarse and fine sediments can be observed, and this pattern is speculated to exist in the subsurface but few well logs describe the stratigraphy in any detail. This cyclic bedding is believed to be the product of periodic outburst floods from Glacial Lake Missoula entering the quiet waters of Glacial Lake Columbia that existed in the Spokane and Hangman valleys at the same time (Atwater, 1986; Molenaar, 1988).
Six seismic reflection profiles defining four transects (total of three miles linear distance) were surveyed in December 1996 across the axis of Hangman valley and Marshall valley in order to fix the third-dimension of the aquifer geometry. Both valleys appear to be trough-shaped and filled with 300 feet or more of sedimentary deposits sitting on top of competent bedrock at depth; it shows that the bedrock bottom of the alluvial aquifer in Hangman valley to generally lie at an elevation of about 1,400 feet above mean sea level. The average saturated thickness of the aquifer in Hangman valley is about 330 feet. No physical barriers to groundwater flow have been discovered in the subsurface that would preclude the movement of groundwater from the Hangman valley aquifer and into the lower Spokane aquifer. Very few well logs exist for the numerous wells that occur in Hangman valley, so these cannot be used as a tool to constrain or verify the seismic reflection survey work.
A transect across West 15th and West 14th Avenues shows a saturated area of about 313,000 square feet through which groundwater must travel on its way to the north toward the lower Spokane aquifer. Flow calculations show that approximately 13 cfs of groundwater flow is moving from Hangman valley and into the "lower" Spokane aquifer. This compares with Bolke and Vaccaro's (1981) estimated inflow from the Hangman valley that lies somewhere between 6 and 15 cfs on average. Waquar (1994) estimated that the flow from the Marshall Creek tributary to Hangman Creek to be from 3 to 5 cfs. Direct precipitation to the surface, and inflow from adjacent basalt aquifers, also contribute some recharge to the "lower" Spokane aquifer.
Miller, Stanley A., Water Quality Management Program, Spokane County Public Works Department, Utilities Division, 1026 W. Broadway Avenue, Spokane, WA 99260
The interaction of ground water and surface water is often considered a given in many systems. However, the quantification of the interchange both spatially and volumetrically is difficult. The standard procedure of estimating interchange through stream gaging is expensive and time consuming. Problems are especially severe when the stream shifts from a gaining system to a losing system frequently.
During summer low flow periods the calcium concentration of the Spokane River increases three fold in a 20 mile reach. The differences in the calcium concentration of the hydraulically connected Spokane Valley Aquifer and reaches of the Spokane River provide an alternative to stream measurement as a means of determining both the length of gaining and losing reaches and the volume of surface water and ground water interchange.
Using direct flow measurement at two stream locations and water quality measurements at nearly 20 stream sites, four gaining and three losing reaches of the stream were identified. A simple mass balance equation that uses a known flow volume at the upstream end of a river reach and concentrations of a chemical parameter at the upstream and downstream ends of the river reach and the ground water adjacent to the river is used to calculate changes in flow within the reach. The flow change model is capsulized in the equation:
Qo = (Ci Qi + Cr Qr - Cl Ql) / Co
where:
Qo = flow at the downstream end of a river reach
Qi = flow at the upstream end of a river reach
Qr = aquifer recharge to the river within the reach
Ql = river loss to the aquifer within the reach.
The volume of interchange calculated from chemical changes in the river system agree well with historic stream gaging records. When adjustments are made to account for differences in the location of gaining and losing reaches, the results also agree with those of three separate ground water models.
One benefit of using the chemical changes to calculate interchange is the simplicity with which gaining reaches can be broken into segments; adding a new sampling station gives a new set of flows. However, downstream from the first gaining reach of the river the difference between river and ground water calcium is small; the method does not produce good results for losing reaches of the river below the first gaining reach. This problem has led to the exploration of the use of lead isotope ratios to characterize flows.
Calcium concentrations also indicated that as much as 95% of summer low flow in the Spokane River was derived from the aquifer during the study period. Proportionally lower percentages of ground water were indicated as river flow increased. Comparing the estimated loss from the Spokane River at various river stages indicates that there is an upper limit to the rate of ground water recharge that occurs at about 3000 cubic feet per second.