HYDROGEOLOGICAL MODELING OF A LIVESTOCK WATER EXTRACTION SCENARIO AT ISOLATED FENS IN SOUTH CENTRAL OREGON

Small, isolated, groundwater-dependent wetlands form an important component of the ecology in south central Oregon, about 55 kilometers NE of Crater Lake. As well as supplying water for wildlife, these fens are also used as a source of supply for cattle. In order to assess the effect of pumping water at rates of about 0.1 to 0.2 l/s on fen health, analytical and finite difference modeling were used to determine if water use was likely to have an appreciable effect, and if more complex modeling was warranted.

These fens typically have a surface area on the order of 2500m². In general they consist of an approximately 1 m thick peat layer overlying a thin clay layer, which in turn overlies a 2m thick pumice layer. Measured hydraulic conductivity for the peat layer is 0.13 to 0.39 m/d (0.23 m/d average) and 0.82 to 126 m/d (21 m/d average) for the pumice. Hydraulic characteristics for the fractured volcanics beneath the pumice are unknown, though they are separated from the pumice by another clay layer. Mildly artesian conditions occur in the pumice where peat has accumulated. Heads were measured up to 0.17 m above ground surface.

An analytical model was used to determine the shape and extent of the cone of depression resulting from pumping from the peat or the pumice layer on that layer only. The model is that of an infinite sheet. The Neuman (1972) solution was used for the peat which resulted in a cone of depression less than 40 m wide when 1 m of drawdown was reached. The Theis (1935) solution for a confined aquifer was used for the pumice layer. The peat pumping resulted in dry conditions at the 1 m deep well before the cone of depression had expanded more than 20 m from the pumping well. The pumice pumping resulted in a large cone of depression (1 km wide) but only 0.3 m deep at the pumping well.

A numerical superposition model was developed in order to determine the effects of pumping in the pumice on all three layers (peat, clay, and pumice from top to bottom). No recharge was applied in this one-year simulation. Preliminary results show that, at a pumping rate of 0.1 l/s a maximum head loss of 0.1 m at the pumping well in the pumice and 0.03 m at the same location in the peat occurs. Future versions will incorporate topography, recharge, and evapotranspiration.

Results to date show that impacts to fen hydrology would be minimal to none at the low rates of pumping investigated.