Effects of mineral weathering and acid mine drainage on Pco2 in Davis Mine Brook and Maxwell Brook Watersheds, Rowe, Ma.

• Main Author: Armanetti, Casey L.
• Format: Text
• Language: English
• Published: Smith ScholarWorks 2018
• Subjects: Acid mine drainage; Pyrite; Mineral weathering; Carbon flux; Watershed comparison; Weathering; Watersheds-Massachusetts-Rowe; Water-Pollution; Geology; Rowe; Carbonic acid
• Online Access: https://scholarworks.smith.edu/theses/1994; https://scholarworks.smith.edu/cgi/viewcontent.cgi?article=3071&context=theses

Acid mine drainage (AMD), an acidic iron-rich leachate that is characterized by low pH and high concentrations of sulfate and dissolved metals, is the most important mining-related water pollution problem in the world. AMD has the potential to affect the global carbon cycle by releasing CO2 to the atmosphere because the high acidity converts dissolved inorganic carbon species to CO2 gas, increasing the PCO2 of the water. Often, investigators studying PCO2 generation associated with AMD have focused on regions where acid mine waters interact with carbonate rocks because carbonate minerals chemically weather quickly and have the potential to generate significant CO2(g) if pH remains low. In contrast, this study focuses on PCO2 production related to AMD in a silicate rock setting at the historic Davis Pyrite Mine, which is situated in the Hawley-Rowe metamorphic rock belt in Rowe, Massachusetts. For the past century, pyrite rich tailings have been left in the riparian zone of Davis Mine Brook, exposed to weathering as groundwater flows through them and into the stream. Adjacent to the Davis Mine Brook watershed is the Maxwell Brook watershed. While both streams are underlain by the same bedrock, there is no apparent influence of the mining activities on the Maxwell Brook watershed, which serves as a neutral water reference for the acidic conditions of Davis Mine Brook. Comparing these two watersheds provides insight on how mineral weathering in different levels of acidity may contribute to the water chemistry observed in each setting. Water and rock samples from Davis Mine Brook and Maxwell Brook watersheds were collected in August and November, 2017. Surface water samples were collected in both watersheds, and seep and drainage samples were collected from tailings piles along Davis Mine Brook. Thin sections were analyzed with petrographic light and scanning electron microscopy. Water samples were analyzed directly for carbon, dissolved major ion, silica, iron, and aluminum concentrations, as well as d18O and d2H isotopes. Special focus is given to PCO2 concentrations, and an in-depth exploration of methodology for calculating PCO2 from field data and inorganic carbon analyses is outlined. Differences in water chemistry and PCO2 concentrations in Davis Mine Brook surface and seep water samples compared to the references samples are traced back to mineral weathering reactions based on rock sample observations. PCO2 is supersaturated in all samples relative to the atmospheric level, but the seep and drainage samples are extremely elevated, by up to six orders of magnitude (logPCO2atm = -3.4; mean reference surface waters = logPCO2 = -0.9; mean DMB surface waters = logPCO2 = 1.1; mean DMB groundwater seeps and mine drainage waters = logPCO2 = 3). Graphite is identified as a mineral source for the elevated PCO2 in acidic mine water. Lower PCO2 in Davis Mine Brook stream water located downstream of the mine appears to result from a combination of degassing and conversion of CO2(g) to carbonic acid and bicarbonate ion. These results suggest that minor amounts of graphite in metamorphic rocks weathered by AMD can cause a noteworthy CO2 flux to the atmosphere.