Marine deposits of clays which originate in sulfide-rich brackish environments are known to develop acidic conditions under oxidizing conditions in the presence of certain species of bacteria. The two study sites in this investigation were being used for municipal landfill operations and were found to have clays as either part of the deposits encountered or as imported material for liners or covers. The clays were considered highly desirable for landfill use due to their fine-grained texture and high plasticity. The study was initiated to address problems with the generation of acidic runoff from the stockpiled clay materials. Other concerns were also found to exist in the longevity of the monitoring wells, particularly in terms of the stability of the grout sealing materials in the wellbores. Based on the depositional environment of the clay and the geochemical nature of the runoff, it was hypothesized that the source of the acidity was the presence of reduced sulfur which susequently oxidized after the clays were exposed to oxygen. Such exposure occurred during excavation and stockpiling of the clays for liner and cover material, and in the introduction of oxygen during the well drilling, purging and sampling activities. A four-prong approach was taken to confirm that this hypothesis adequately explained the existing conditions. A culturing of well water samples and colorimetric testing of the culture was conducted to determine the presence of pyrite-oxidizing bacteria. The chemistry of the ground water (particularly sulfates, pH and dissolved salts) was compared to the nature of the strata encountered during drilling and intersecting the screened intervals in the well. Observations were made of the surface, oxidized clays for the appearance of secondary surfate mineral crusts and the presence of sulfur odors. Samples of these clays were were also collected and tested for pH and lime requirements by the TEA and peroxide methods. Results of the study confirmed that the acid-sulfate clays were the source of the deterioration of the monitoring wells and the generation of acidic runoff from the clays exposed to oxygen. A screening protocol was developed to identify potential future problems. This paper describes the testing methods, results of the testing and the nature of the screening protocol.