06/25/2021
By Elizabeth Cole

The Department of Environmental, Earth and Atmospheric Sciences in the Kennedy College of Sciences announces the master’s thesis defense of Clara Cogswell on Wednesday, July 7, 2021 at 10 a.m. via Zoom.

Committee Members:

  • James Heiss
  • Daniel Obrist
  • Chris Russoniello

Title: "Climate and Seasonal Temperature Controls on Biogeochemical Transformations in Unconfined Coastal Aquifers"

Abstract:

Coastal unconfined aquifers are host to a range of biogeochemical reactions that alter groundwater-derived nutrient, metal, and other chemical loads to coastal ecosystems. Temperature is a strong control on microbially mediated reactions, thus chemical reactivity in coastal aquifers may vary spatially and temporally with changes to groundwater temperature. In this study, we investigated the influence of global groundwater and sea surface temperature controls and seasonal temperature variability on biogeochemical processing in coastal aquifers using variable-density groundwater flow, heat transport, and reactive transport models. The coupled models show that nitrate (NO3-) removal efficiency in coastal aquifers increases from 5% to 88% as fresh groundwater temperature increases from 5°C to 30°C, while ocean temperature has a negligible effect on overall net denitrification. Transient simulations based on monthly groundwater and ocean temperature measurements show that denitrification and ammonification hotspots migrate seaward seasonally within warm fresh groundwater masses. The reaction hotspots are separated by colder groundwater emplaced during winter months and are transported upward along the saltwater interface. The reaction hotspots and nitrate plumes oscillated vertically along horizontal flow paths due to buoyancy effects between warm and cold groundwater. Comparison between transient and temperature-equivalent steady-state models suggests that steady-state models adequately capture mean annual NO3- removal, but neglect local reactive transience and changes to plume geometry. The sensitivity analysis provides a first order estimate of the reactive potential of coastal aquifers at the global scale. The findings have implications for estimating coastal aquifer reactivity in the field and demonstrate a range of biogeochemical responses under globally diverse thermal regimes.