Environmental Geochemistry

Environmental Geochemistry
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Coastal Environmental Geochemistry research at the Woods Hole Coastal and Marine Science Center spans multiple ecosystems and topics, including coastal wetlands, aquifers, and estuaries

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Coastal aquifers are groundwater systems that cross land-ocean boundaries. These systems represent a nexus of the world's geologic, hydrologic, and marine systems. Coastal aquifers provide freshwater to more than one billion people who live along the coast and interact with coastal hazards and coastal ecosystems alike. In the twenty-first century, projected climate change and human population growth will dramatically affect coastal aquifers. For example, saltwater intrusion exacerbated by groundwater extraction and rising sea levels will degrade the quality and quantity of freshwater available in coastal aquifers. Watershed activities such as agriculture, industry, and urbanization will also affect groundwater resources. 

Image of USGS personnel collecting salt marsh sediment cores

NAGT summer intern, Kelly Sanks, prepares to collect salt marsh sediment cores with her advisor, Dr. Meagan Gonneea (Cape Cod, MA).

(Credit: Jennifer O'Keefe Suttles, Woods Hole Coastal and Marine Science Center. Public domain.)

View of muddy, eroding coastal bluffs with a visible permafrost layer and tumbling tundra on top.

Photograph of the actively eroding coastal permafrost bluff on Barter Island, located on the northern coast of Alaska.

(Credit: Shawn Harrison, USGS Pacific Coastal and Marine Science Center. Public domain.)

These same factors also contribute to changes in the rate and quality of groundwater being discharged into the coastal ocean. This coastal and submarine groundwater discharge process is nearly ubiquitous on all types of coastlines and delivers water and associated constituents to nearshore ecosystems, including estuaries and wetlands, coral reefs, rocky shorelines, and the continental shelves.

CMHRP scientists and academic collaborators are developing regional- and national-scale models of coastal groundwater changes in response to sea level rise and natural and managed coastal hydrology. The modeling effort supports local and regional management of nutrient and other chemical loads to coastal ecosystems by providing detailed information about groundwater discharge rate, contributing landscape areas, and predicted changes in the water table. CMHRP models also examine rising groundwater levels as a coastal hazard, predicting changes in flooding risk for cities and other developed land and examining the impact on wetland elevation. Predictive modeling also focuses on seawater intrusion into coastal aquifers due to rising sea level and groundwater withdrawals. These groundwater hazard assessments provide a valuable complement to the overland coastal flooding and coastal change models also being developed and applied within the CMHRP. 

The CMHRP collects site-specific data on the distribution, movement, and chemistry of groundwater to both characterize coastal aquifer conditions and better understand the mechanisms controlling these properties. The CMHRP also directly samples coastal aquifers to evaluate the role groundwater plays in delivering terrestrial nutrients, organic matter, trace elements, and pollution to submarine and coastal ecosystems. The CMHRP specializes in the use of radioisotopic tracers to determine water residence times and sources of coastal groundwater. The CMHRP also develops original instrumentation and approaches for acquiring data in coastal aquifer systems, including a buoy to acquire time series of radon data and drone-based thermal imaging to identify groundwater discharge.

 

The CMHRP also collects oceanographic data and geophysical imagery that provide critical additional information about coastal aquifer systems. In the tropical Pacific, the CMHRP has collaborated with the Department of Defense Environmental Research Program and acquired high-resolution data on waves, currents, and tides, as well as temperature and salinity, to assess the vulnerability of groundwater on the Marshall Islands. On Arctic Alaska’s Barter Island , geophysical methods image shallow subsurface structure, revealing the distribution of ice polygons, fresh pore waters, and saltwater and documenting the degradation of permafrost during summer thaw cycles. The stability of permafrost bluffs near Barter Island depends not only on such permafrost thaw, but also the patterns of groundwater discharge. 

The CMHRP also studies how interactions between coastal groundwater and intruding seawater affect nutrient dynamics and ecosystem function within caves that intersect coastal aquifers on carbonate and volcanic coastlines. Scuba divers from the USGS and collaborating partners deploy physical sensors, chemical sensors, and hydroacoustic devices to monitor the mixing of freshwater and saltwater and to track chemical constituents. Researchers also use customized devices to sample water, gas, and fauna in order to identify nutrient sources, describe the carbon cycle, and characterize the microbial communities in these low oxygen subterranean estuaries.

Image shows a cave diver in a flooded cave

A diver within a section of the flooded Ox Bel Ha cave system, Yucatán Peninsula, where USGS researchers and their collaborators are studying the subterranean estuary. The guideline seen alongside the diver is a standard safety procedure.

(Photograph © HP Hartmann)