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This paper presents a methodology for predicting the quantity of leakage through a defect in a geomembrane bottom liner beneath a phosphogypsum disposal facility. The theoretical basis for the prediction methodology is introduced and then laboratory verification tests are presented and evaluated. The results indicate that fine gypsum particles can readily enter and plug defects in the underlying geomembrane liner, causing a reduction in the leakage nµe through any missed defects. Upper bound leakage predictions, assuming that gypsum does not plug the defect, overestimate the leakage rate by a factor of 2 to 6. Best estimate predictions, assuming gypsum plugs the defect, are approximately the same as or only slightly higher than measured leakage rates. Results of the analytical and laboratory work presented in this paper assisted the Florida Department of Environmental Protection to enact minimum liner design standards for phosphogypsum disposal facilities that are at the same time economically viable and adequately protective of groundwater resources.