ANALYTIC ELEMENT MODELING OF THE OREGON COASTAL DUNES AQUIFER, NORTH OF COOS BAY, OREGON

A coastal dune field aquifer on the Oregon coast is an important component of the water supply for Coos Bay, Oregon. This dune field also hosts several water table lakes and wetlands, which are important both for recreational purposes and for wetland-dependent wildlife and plants. The lakes rise and fall with the water table with a near-instantaneous response to input and extraction, both from annual increases and decreases in precipitation, and to local groundwater pumping. The coastal aquifer has 20 wells for water supply extraction. Pumping these wells has affected water levels in some of the lakes. Some of the pumped water has been returned to lakes in order to maintain their level.

A model of the coastal dune aquifer was constructed with the analytic element model (AEM) GFLOW. This model is useful for rapid development of two-dimensional, steady-state, reconnaissance level simulations. No time consuming setup of finite difference or finite element grid is necessary. The AEM model was set up and calibrated to the average water levels in the five lakes in the vicinity of the pumping wells: Beale, Snag, Sandpoint, Horsfell-Spirit, and Bluebill lakes. These are water table lakes with no inlet or outlet. Hydraulic conductivity was kept constant, since the dune sands are relatively consistent in their hydraulic characteristics. The thickness of the north, central, and southern thirds of the aquifer simulate the general thickening of the aquifer from north to south, about 75 feet to 150 feet. Recharge was varied as a proportion of the 63 inches of precipitation the region receives on average.

After calibrating the non-pumping steady-state model to the five lakes, the average pumping record from 1984 was applied. This lowered the stage in all of the five lakes, with the largest decline in Spirit-Horsfell. An injection well was added to Horsfell-Spirit lake, the largest of the five. An injection rate of 0.75 mgd, or 20% of the total pumping of 3.7 mgd, was sufficient to keep the lake at its pre-pumping level of 21 feet amsl.

The AEM environment allows relatively quick and easy evaluation of changes in pumping, recharge, hydraulic parameters; and has particle tracking features, both forward and backward. The AEM enables land management agencies a powerful tool to aid in informed decision-making when groundwater is an issue.