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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5044

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Model Limitations and Suggestions for Future Work

Although the SVRP aquifer model presented in this report provides a relatively good fit between simulated and measured quantities, indicating that the overall simulated ground-water flow is a reasonable representation of ground-water flow in the SVRP aquifer, the model is subject to limitations. These limitations, discussed in the following paragraphs, should be taken into consideration when using the model.

In general, the scale of the model and the level of detail are intended for analysis of aquifer-wide water-supply issues. Although the model might be useful for providing boundary conditions for smaller scale investigations, the model lacks sufficient details for direct application to small-scale investigations such as the analysis of capture zone for an individual well. Additionally, the model is not intended for application to contaminant-transport issues such as the prediction of contaminant traveltimes or flow paths. A contaminant-transport model would require a substantially greater amount of hydrogeologic detail for the contamination site.

There is insufficient hydrologic information to determine ground-water inflow from Spirit and Hoodoo Valleys to the SVRP aquifer. The calibrated model in this report assumes no inflow from Spirit and Hoodoo Valleys. However, alternative model D indicates that the amount of inflow from the Spirit and Hoodoo Valleys cannot be determined reliably from model calibration with the present calibration data. Monitoring water levels in and near Spirit and Hoodoo Valleys could provide important data to better evaluate inflows from those valleys to the SVRP aquifer.

In Hillyard Trough and the Little Spokane River Arm, ground-water flow in the lower unit is not well understood. Water levels in the lower unit are monitored in only two wells, and the horizontal and vertical extents of the clay layer separating the upper and lower units are not well known. The model assumes that the clay layer completely isolates hydraulic contact between the lower and upper units, but this assumption requires critical evaluation. Monitoring water levels and conducting aquifer tests in both the upper and lower units might lead to better understanding of the hydrogeology in Hillyard Trough and Little Spokane River Arm.

There is significant uncertainty in the simulated seepages from Lake Pend Oreille and Coeur d’Alene Lake. In addition, the water-level fluctuations in wells near Coeur d’Alene Lake are not well understood. A detailed study of ground-water flow in the Coeur d’Alene area, including aquifer tests to estimate hydraulic conductivity in the vicinity of Coeur d’Alene Lake, might provide data that can be used to constrain the simulated seepage from Coeur d’Alene Lake.

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