HYDROGEOLOGIC MODELING IN SUPPORT OF LANDFILL LINER VARIANCE, DRY CREEK LANDFILL, EAGLE POINT, OREGON

JBR conducted groundwater modeling to evaluate a landfill design that was a variation of the standard Department of Environmental Quality (DEQ) “western Oregon” liner design system. The design called for using an alternative primary composite liner system consisting of a geosynthetic clay liner (GCL) under the primary geomembrane. Use of the GCL would eliminate the need for two feet of low permeability soil during the construction of the new landfill cell. JBR used both the Hydrogeologic Evaluation of Landfill Performance (HELP) and Multimedia Exposure and Assessment (MULTIMED) models to conduct the modeling. The HELP model was used to generate estimates of leachate production using local weather information and other parameters and MULTIMED used the leachate values generated from HELP as input into the flow component. The system was modeled for both open and closed conditions. The geometric mean of hydraulic conductivity was initially used in both models for a basalt layer at the base of the proposed landfill cell.  However, the DEQ requested that the arithmetic mean of the measurements be used instead. The modeling was redone using this arithmetic mean for hydraulic conductivity. The HELP model predicted peak-daily leakage rate for the open condition of 0.2 ft3/day and for closed conditions of 0.00047 ft3/day. The MULTIMED model was used to calculate dilution equivalent factors for both open and closed conditions. Using these factors it was demonstrated that there would be no measurable leachate impacts to groundwater at the compliance point for any of the leachate constituents.

• Jonathan Williams - Division Manager -