Hydroelectric Pump-Back Project, Sierra Nevada Mountains California

Situation

High groundwater inflows were encountered during tunnel construction through granitic rocks, part of a large underground hydroelectric project. These flows, emanating from discrete fractures in the bedrock, required expensive dewatering far in excess of that anticipated during project planning. Hydro Geo Chem was retained to evaluate the hydrogeologic conditions at the site, to determine potential sources of the groundwater, and to estimate the duration of the flows. Our investigation had to be conducted rapidly and in the presence of on-going construction activities. Additional exploratory drilling was not possible.

Hydro Geo Chem's Approach

Hydro Geo Chem used geochemical and isotopic tracers to determine that groundwater discharges originated from regional groundwater reservoir rather than local sources. Analysis of stable isotope ratios of oxygen and hydrogen in the groundwater verified that the reservoir was recharged high in the Sierra Nevada mountains rather than locally. We then conducted hydraulic analyses of the flow depletion rate to estimate flow rates into the future. Based on these analyses we concluded that, if left uncontrolled, sustained high flows would persist indefinitely.

Results

Our evaluation provided both the client and construction contractor the information needed to scale their tunnel dewatering efforts and to reach an equitable agreement regarding the costs of the additional work and construction delays.

Figure: Conceptual model of flow system contributing seepage to tunnel complex.