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Geologic Controls on Groundwater Flows and Associated Nutrient Biogeochemistry in the Subterranean Estuary and Groundwater Seepage Face: Kevin D. Kroeger

Project Title: Geologic Controls on Groundwater Flows and Associated Nutrient Biogeochemistry in the Subterranean Estuary and Groundwater Seepage Face
Mendenhall Fellow: Kevin D. Kroeger, (727) 803-8747, Kevin D. Kroeger, 727-803-8747, kkroeger@usgs.gov
Duty Station: Saint Petersburg, FL
Start Date: November 28, 2004
Education: M.S., Oceanography, University of Connecticut (1997), Ph.D. Marine Biogoechemistry, Boston University (2003)
Research Advisors: Peter Swarzenski, (727) 803-8747, ext. 3072, pswarzen@usgs.gov; Mark Stewart (University of South Florida), (813) 974-8749, mark@chuma1.cas.usf.edu; Dann Yobbi, (813) 884-9336 ext. 164, dyobbi@usgs.gov		    
 

Project Description: Along much of the east and Gulf coasts of the United States, groundwater is a major route of transport of freshwater and its solutes from land to sea. Sources, transformations and losses of nitrogen and phosphorous within watersheds have been intensively studied, and although uncertainties remain, those processes are understood well enough to allow development of models to predict groundwater nutrient loads to receiving waters. However, groundwater flows and associated nutrient cycling within the near shore aquifer has received relatively little study and is poorly known. Prior to discharge to estuaries or to the sea, groundwater discharging from aquifers passes through a narrow zone of sediment referred to as the seepage face and ranging in location from just above the intertidal zone to the shallow subtidal. In many locations, entrainment of saline porewater occurs prior to discharge producing a gradient in groundwater salinity from land to sea, referred to as a subterranean estuary. With regard to the study of groundwater flows and nutrient cycling, the seepage face and subterranean estuary are relatively new and under-studied biogeochemical zones in the aquatic cascade from watershed to sea.

The objectives of this project are to examine groundwater flow, nutrient transport and biogeochemistry in nearshore portions of two coastal aquifers of contrasting geological composition. To allow direct comparison of geologic control of coastal groundwater flow and geochemistry, the research will be carried out both at a study site in Florida with a karstic limestone aquifer and carbonate estuarine sediments and at a study site on Cape Cod, Massachusetts with an aquifer composed of quartz and feldspar sands and siliciclastic estuarine sediments.

The proposed research will:

  1. Estimate regional (on the scale of a small embayment) rates of fresh and saline (recirculated) submarine groundwater discharge at each study site. This will be accomplished by analysis of naturally occurring radiochemical tracers of groundwater discharge.
  2. Quantify groundwater transport (spatial and temporal variation in rate and direction of flow) at the land/sea margin and analyze behavior of the freshwater/saltwater interface. This will be accomplished through a series of conservative tracer injections into the coastal aquifers, and results will be used to test or corroborate estimates based on the radiochemical tracers.
  3. Quantify transport and biogeochemical transformations of nitrogen and phosphorous in the coastal aquifers and sediment. This will be accomplished by quantifying nutrient concentrations, species composition, and nitrogen stable isotopic ratios within the fresh, brackish and saline groundwater in the coastal aquifers. Fluxes of nutrients through the aquifer will be calculated based on estimates of groundwater discharge and on average nutrient concentrations in the fresh, brackish and saline portions of the aquifers. Natural abundance stable isotopic ratios in nitrate, ammonium and dissolved organic nitrogen in groundwater samples, in addition to measurements of dinitrogen gas concentrations, will be used to estimate rates of nitrogen transformation.
Nitrate and ammonium concentrations and nitrogen stable isotopic ratios in a depth profile through freshwater aquifer and into subterranean estuary. For a more detailed explanation. contact Kevin D. Kroeger at kkroeger@usgs.gov.
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Last modified: 16:08:29 Thu 13 Dec 2012