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Abstract A 3.3-meter deep fresh water drilling fluid reserve pit was instrumented to determine migration of contaminates from the pit during drilling and after pit closure. The ability to predict impacts from earthen drilling pits has become important, due to the recognized potential for an adverse impact on soil and water resources. The results of the field study allow operators to control chemical composition of pit wastes to prevent contaminants from migrating from an active or closed pit. To the best of our knowledge, no other reserve pit has been evaluated by instrumentation installed a priori to evaluate the environmental impact of reserve pits. The pit was equipped with neutron probe access tubes, barrel size lysimeters, vacuum extraction moisture cups and gypsum resistively blocks at 4 stations within the pit perimeter. Ground water monitoring wells, and post closure soil borings were used to assess the long-term affects of the earthen pit use and onsite disposal of the pit waste solids on soil and ground water resources. The soil moisture and chemical data generated during the 2-year field study was evaluated statistically utilizing analyses of variance (ANOVA). Soil water monitoring during pit use showed the soil profile below the pit remained unsaturated under the influence of a 3.3-m head of pit fluid. This demonstrated the clay gels [spud mud] serve as a pit surface sealant and retards percolation of fluids from the pit. Chlorides were retained in the pit by an anion exclusion mechanism. This mechanism remained stable and the exclusion volume functional in pit solids having Saturated Paste Electrical Conductivity [SPEC] of ? 24 mmhos/cm. Post closure data show clean closure of the study pit and no alteration of intended land use (improved pasture) or adverse impact to the natural environment.