Groundwater Modeling and Settlement Analysis for Closure of the Little Blue Run CCP Disposal Area

Presented By: Douglas M. Clark, P.E. and Brianne S. Jacoby, P.G, Civil & Environmental Consultants, Inc.

The Little Blue Run (LBR) disposal area is a 900+ acre impoundment that has been used since 1975 for the disposal of coal combustion products (CCP) from FirstEnergy’s Bruce Mansfield Generating Station. The CCP materials in LBR consist of calcium sulfite scrubber material and fly ash that are hydraulically placed. As part of a long-term closure program, 952 acres of CCP will be capped with a geosynthetic liner and soil cover, resulting in a significant drop in the water level and subsequent settlement within the CCP. Recognizing that significant settlement could impact the surface drainage and final cover system following closure, a groundwater modeling and settlement analysis was undertaken as part of the closure plan.

A MODFLOW-2000 groundwater flow model was developed for the aquifer systems within and surrounding LBR. The model was constructed and calibrated to match observed groundwater levels. A long-term simulation was run to predict the water table draw-down 50 years after the final cover system is in place. Settlement in the CCP was predicted using primary consolidation calculations at 2,647 discrete points within the CCP disposal area. Consolidation soil parameters were selected based on a compilation of 77 consolidation tests on CCP samples, and historical settlement data from the site.

The post-closure water level is predicted to drop more than 100 feet with a maximum settlement magnitude of 31.3 feet occurring at the point of thickest in-situ CCP. Based on this analysis, the draw-down of the water table is predicted to significantly change the post-settlement grades, requiring major modifications to the post-closure surface water drainage system and the final cover geosynthetic materials.