A QUANTITATIVE GROUNDWATER FLOW MODEL TO FACILITATE DEWATERING OF OVERBURDEN AT A LIGNITE MINE IN LOUISIANA

Groundwater remaining in the overburden after dewatering contributes to highwall and spoil instability at a lignite mine on the Sabine uplift in DeSoto Parish, Louisiana. A site specific, quantitative groundwater flow model provides the data needed to design an effective and cost efficient dewatering system for the mine. The Chemard Lake lignite of the Wilcox Group (Paleogene) is the principal lignite seam on the property. It is found at depths between 20 and 140 ft, and averages 6 ft thick. The complex geological layering in the strata above and below the lignite reduces to five hydrogeologic units for modeling purposes. A regional model covering 54 square miles was designed and run to supply hydraulic context for smaller local models used to evaluate specific dewatering schemes. The regional model was based on a custom-built, coarse grid with nodal spacings ranging from 200 to 1,000 feet. It contains five layers, each with 7,825 cells (39,125 cells in all). The local models were based on a fine grid built using telescopic mesh refinement. The fine grid covered 1,053 acres and contained 4,588 100 ft x 100 ft cells in each of five layers. Numeric values for 15 spatial, geological and hydrogeological parameters were entered in every cell. Repeated model runs with varying well spacings and pumping rates demonstrated that the overburden could be effectively dewatered in most cases in less than 30 days. Even allowing for significant down time and groundwater recharge, the dewatering schemes evaluated by the model runs should be able to dewater the overburden. The most cost efficient schemes have dewatering wells placed on 100-foot centers perpendicular to groundwater flow paths and 200- to 400-foot centers parallel to groundwater flow paths. More effective dewatering of the overburden will increase the stability of the highwall and spoil pile. Increased spoil and highwall stability will improve lignite recovery rates and lower operating costs.