Type of Document Dissertation Author Santos, Isaac R. Author's Email Address firstname.lastname@example.org URN etd-11032008-144843 Title Submarine Groundwater Discharge Driving Mechanisms and Biogeochemical Aspects Degree Doctor of Philosophy Department Oceanography, Department of Advisory Committee
Advisor Name Title William C. Burnett Committee Chair Jeffrey Chanton Committee Co-Chair Markus Huettel Committee Member Philip Froelich Committee Member Thorsten Dittmar Committee Member Bill Hu Outside Committee Member Keywords
- Geochemical Tracers
- Permeable Sediments
Date of Defense 2008-10-16 Availability unrestricted AbstractSubmarine groundwater discharge (SGD) is an important pathway for trace element and nutrient cycling in the coastal ocean. I use a combination of hydrological and geochemical tracer observations to gain insights into the natural and anthropogenic mechanisms driving SGD into coastal water bodies. Nutrient measurements in a subterranean estuary (STE) were used to discuss the biogeochemical controls of SGD endmember concentrations and to derive inputs associated with fresh and saline SGD.
222Rn, CH4, and associated variables in seawater were monitored nearly continuously at a coastal site in the Gulf of Mexico for almost two years. Surprisingly, the variability of 222Rn and CH4 over short (e.g., hourly) time scales was generally comparable to or even more pronounced than fluctuations over much longer (e.g., monthly) time scales. While high tracer concentrations usually occurred during low tide and low tracer concentrations during high tide, this pattern was occasionally inverted or absent indicating that no single model can be used to describe the entire dataset. This implies that seasonal investigations of SGD tracers in the coastal ocean may be masked by short-term variability. Modeled SGD patterns were strongly associated with the neap-spring tidal cycle, at least during a period of extreme drought when minimum external disturbances were present. Multiple independent approaches used to separate the relative contribution of fresh and saline SGD indicated that transient marine forces, likely tidal pumping, dominate benthic advective exchange at this coastal plain site.
I also tested whether biogeochemical processes in a STE alter nutrient concentrations that are discharged into the ocean via SGD. Nutrient distributions were consistent with a sequence of reactions in a narrow (~2 m) surface layer where nitrate is initially exhausted (likely due to denitrification), organic nitrogen is remineralized releasing ammonium, and some DOC remains. Nutrient cycling in this STE was primarily fueled by oxygen and labile organic matter supplied by tidal pumping of seawater into the coastal aquifer. Even though fresh SGD accounted for only ~5% of total volumetric additions, the interaction between fresh SGD with components in the STE provided 22-34% of DOC and ~50% of nitrogen inputs, with the remainder associated with recirculated seawater. While SGD volumetric inputs are similar seasonally, changes in the biogeochemical conditions of this coastal plain STE led to higher summertime nutrient fluxes, suggesting a link between coastal primary productivity and nutrient production.
Finally, I applied naturally-occurring geochemical tracers (222Rn, 223Ra, 224Ra, 226Ra, CH4, 18O, and 2H) to assess anthropogenic-driven groundwater discharge in Mangueira Lagoon, Brazil. Modeling of radon inventories indicated that groundwater advection rates in the numerous irrigation canals are 2 orders of magnitude higher than along the lagoon shoreline. In spite of the relatively small area of the canals, they contributed nearly 70% of the total (~57,000 m3/d) groundwater input into the entire Mangueira Lagoon. Dredging of these canals cut through aquitards which previously restricted upward advection from the underlying permeable strata. In spite of the small volume contribution (~2% of precipitation), groundwater accounted for 50-70% of major ion inputs into the lagoon. The irrigation channels may therefore represent an important but previously overlooked source of nutrients and other dissolved chemicals derived from agricultural practices into this and other lagoons.
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