Northeastern Section - 53rd Annual Meeting - 2018

Paper No. 42-5
Presentation Time: 1:30 PM-5:30 PM


ARMANETTI, Casey L. and RHODES, Amy L., Department of Geosciences, Smith College, 1 Chapin Way, Northampton, MA 01063

Silicate mineral weathering reactions by carbonic acid can act as a sink for atmospheric CO2(g) if waters maintain a near-neutral pH. In contrast, pyrite weathering acidifies waters, thereby converting dissolved inorganic carbon species to CO2(g) and potentially returning CO2(g) to the atmosphere. This study investigates this idea by comparing the geochemistry of two, adjacent headwater catchments (~7.8 km2) located in the Rowe-Hawley metamorphic zone. One catchment (Davis Mine Brook) receives acid mine drainage (AMD) from a small, historical pyrite mine, and the other catchment (Maxwell Brook) serves as a reference watershed. Both catchments are underlain by the Hawley Formation, a fine-grained feldspathic greenstone interbedded with amphibolite and schist. Bedrock in the Davis Mine Brook watershed is locally hydrothermally altered, and tailings piles containing pyrite and altered bedrock contribute acidity.

Water samples were collected from streams and riparian groundwater seeps in glass bottles. Field pH was measured by inserting the probe through an O-ring cap on a bottle filled with sample to prevent degassing; field dissolved oxygen (DO) and specific conductance were also measured. Samples were analyzed for dissolved inorganic carbon (DIC), acid neutralizing capacity (ANC), major ions (Ca2+, Mg2+, Na+, K+, Cl-, NO3-, SO42-), AlTOTAL, FeTOTAL, silica, d18O, and d2H. PCO2 values were calculated from mass law equilibrium equations using DIC, field pH, and field temperature.

Preliminary results show that groundwater seep and surface waters affected by pyrite weathering have characteristic AMD geochemistry, with pH < 3.0, ANC < - 4.0 meq L-1 and highly elevated SO4, Fe, and Al (up to 25.1, 7.3, and 3.5 mmol L-1). In contrast, the reference watershed illustrates moderate alkalinity (mean pH and ANC are 6.54 and 0.2 meq L-1) with low sulfate (~0.04 mmol L-1). PCO2 is 10-65% greater in acidic groundwater seeps than in surface waters for both Davis Mine Brook and the reference watershed. For all samples, elevated PCO2 correlates better with low DO than with low pH, suggesting that dissolved gases in surface water exchanged with the atmosphere, releasing CO2(g). However, all PCO2 values are surprisingly undersaturated relative to the atmosphere, and further work needs to improve accuracy of the DIC measurement.