Surface and groundwaters in contact with mining-exposed pyritic materials have the capacity to generate acid mine drainage (AMD), an acidic, sulfate-rich, metals-laden effluent. The Davis Mine located in northwestern Massachusetts offers a model site to study the processes of natural attenuation of acid mine drainage. These include physico-chemical processes such as dilution and sorption, geochemical processes such as aluminosilicate weathering and biological processes such as transformation and cycling of sulfate, iron and acidity by bacterial metabolism. A focus of recent research undertaken at the site has been characterizing the presence and activity of these bacteria with an aim to stimulate their capacity to attenuate the severity of acidic conditions. To further this investigation, a pilot-scale treatment system was installed, composed of a modified permeable reactive barrier containing organic carbon and limestone. Down-gradient groundwater was sampled over a sixteen-month period for concentrations of dissolved metals, major cations and sulfate, along with pH and redox measurements. The results showed a decrease in dissolved metals, a possible increase in calcium and decrease in sulfate, and measurable increase in pH and corresponding decrease in oxidation-reduction potential. Major decreases in dissolved iron and aluminum were observed, a change which is not entirely consistent with metals removal by combination with biogenic sulfide alone. The additional influence of hydrolysis was proposed and the anticipated action of this alternate process found to bear resemblance to the observed changes. Groundwater composition from the experimental period was compared to previous measurements and significant changes described in pH, iron, aluminum, copper and zinc and to a lesser extent in calcium and sulfate. Comparisons were also made with concurrent surface water compositions and findings of analogous studies. Conclusions that can be drawn include: the pH and redox environment into which a treatment system is placed can greatly influence the reactions which take place, side-reactions which occur in reducing and alkalinity-generating amendments may also have an attenuating effect, and variable processes influence groundwater composition in these biogeochemically complex environments.