Modeling The Impact Of A Phosphogypsum Stack On The Groundwater Aquifer

Abstract

The phosphate industry in the state of Florida is the largest producer of phosphate in the country, producing about one-third of the world's phosphate. By-product gypsum slurry from this industry is allowed to precipitate in large ponds where a topographical "stack" of waste material is created. Environmental concern for this practice is two-fold, involving the hydraulic impact of the topographic mound on the surficial aquifer, and the contaminants (radionuclides and hydrofluoric acid) contained within the highly acidic (pH = 1.0-3.0) process water (most stacks are unlined). This study has characterized hydrologic parameters (i.e. transmissivity and storativity) of this gypsum stack, and has mathematically modeled volumetric fluxes of water in the system. Results can be used to determine the chemical impact of contaminants through further geochemical speciation and transport modeling, although none has been attempted here. Although the water budget analysis of the stack indicates a large flux of process water from its base (1591 m^3/day), only a small portion (~1%) of this total is modeled as having an impact on regional flow. Most of the process water drains into ditches from which it evaporates. When other potential water losses are considered (i.e. drain dewatering from the stack interior and flank evaporation) the actual impact of stack waters may be even less. Sensitivity of the modeled environment shows that leakage rates from the gypsum stack are the most critical values determined in this research. Assumption of steady state water volumes in stack ponds provides a direct calculation of the leakage rate (3.0 x 10^-4 day^-1) from evaporative losses; essentially, output rates equal input rates. Findings in this research are in support of the topographical mounding theory, and provide a model upon which volumetric water flux from above-ground tailings piles can be determined. This groundwater model has been tailored to fit the site-specific characteristics of the situation considered, but can be applied to virtually any medium of groundwater flow in the same configuration