Practical Stratigraphy for Groundwater Modeling of Lignite Overburden

By

BREYER, ROXANN. H., and BREYER JOHN A.

Texas Christian University, Fort Worth, TX

A surface mine on the Sabine uplift in DeSoto Parish, Louisiana, takes lignite from strata of the Wilcox Group (Paleogene) to fuel a mine-mouth power plant. Highwall and spoil instability due in part to groundwater remaining in the overburden after dewatering present operational problems at the mine and increase the cost of lignite production. A site-specific groundwater flow model could provide the data needed to design a cost-effective and efficient dewatering system for the mine if the complex geological layering in the lignite-bearing strata could be simulated in the model. The beguiling simplicity of the formal lithostratigraphic nomenclature of the Wilcox Group on the Sabine uplift conceals a complicated arrangement of intricately interbedded muddy, silty and sandy facies that come and go over horizontal distances of a few hundred feet or less.

The Chemard Lake lignite at the top of the Naborton Formation is the principal lignite seam on the property. Like the Naborton, the overlying Logansport Formation, which comprises most of the overburden, consists of a varied assemblage of muddy, silty and sandy facies. The complex geological layering in the two formations reduces to five hydrogeologic units for modeling purposes. Simulations based on a fine-grid covering 1,053 acres and containing 22,940 cells were used to evaluate specific dewatering schemes. Numeric values for 15 spatial, geologic 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.