Research

My research focuses on understanding the hydrologic and geologic controls that drive groundwater flow, mixing, and nutrient cycling in nearshore aquifers. I am particularly interested in the influence of waves, tides, terrestrial recharge, storm surges, sea level rise and other land-sea processes on the exchange of water and chemicals between the ocean and subsurface in estuarine, beach, bay, marsh, and marine environments.

Coastal Groundwater

Tides and waves in the presence of terrestrially-driven fresh groundwater discharge in the intertidal zone sets up a cell of circulating seawater in beach aquifers. Seawater infiltrates across the upper beachface and circulates downward and seaward before discharging near the low tide mark, mixing with underlying fresh groundwater along circulating flowpaths. I use a combination of field measurements and variable-density groundwater flow and solute transport models to investigate the rates, magnitudes, residence times, and the spatial and temporal dynamics of mixing between fresh and saline groundwater within beach aquifers. My interests also focus on identifying the role of longer-term and episodic drivers of change (e.g. sea level rise, storm surge, rainfall events, climate variability) on the quantity and quality of groundwater beneath the coastline.

The paper based on our work on salinity dynamics in beach aquifers was selected for AGU's Water Resources Research Editor's Choice Award, and was featured in EOS's Research Spotlight A New Level of Understanding for Coastal Aquifers, the Environmental Monitor, and the Delaware News Journal.

  • Driving forces of freshwater flow and saline groundwater circulation in coastal aquifer. Tides in the presence of a terrestrial freshwater gradient form a cell of circulating seawater in the intertidal zone of beach aquifers.
  • Measured monthly salinity in the intertidal subsurface over a 1 year timeframe. Salinity and saline circulation was highest when the seasonal freshwater hydraulic gradient was lowest. The lowest measured salinity was observed when the seasonal freshwater gradient was highest and freshwater through-flow was greatest.
  • Model domain, boundary conditions, and aquifer parameters used to numerically simulate variable-density flow and solute transport in a coastal aquifer under tidal and seasonal forcing.
  • Simulated salinity at the top of the model domain over a tidal (top panel), spring-neap cycle (middle panel), and seasonal cycle (bottom panel).
  • Measured and simulated conservative tracer plume 7 days following injection near the high tide mark.

Groundwater-Surface Water Interactions

Groundwater-surface water exchange impacts the availability of nutrients, metals, and other chemicals in marine ecosystems. I am interested in the role of waves, tides, currents, and bedforms on impacting fluid and chemical exchange between groundwater and coastal surface water bodies. Surface water and groundwater flow is tightly coupled, thus downwelling surface water can introduce solutes into the shallow subsurface and undergo chemical transformation before discharging back into the water column, altering water quality. We have characterized the spatial distribution of infiltration, recharge, and discharge zones across a sandy beach at wave to tidal time scales, and are working to quantify wave, tidal, and current-induced benthic exchange in Indian River Bay, Delaware using differential pressure sensors and ADCPs

  • Snapshot of subsurface and surface conditions. The saturation boundary was located landward of the exit point indicating that surface observations cannot be used to identify the groundwater discharge zone at our site.
  • Schematic of beach groundwater levels beneath the swash zone.
  • Instrument transect deployed across the upper beachface (top panel), view from RGB imager used to monitor the swash edge (bottom left), view from thermal imager used to monitor the surface saturation boundary (bottom right). Swash edge and saturation boundary extracted from timestack images of pixel intensities recorded by the imagers.
  • Photo of one of the six instrument arrays.
  • Photo of main instrument transect. Cables were routed away from the transect in a trench leading up the beach to dataloggers.
  • Photo of thermal and RGB imagers mounted to aluminum tower.
  • Example visible-band RGB time stack imagery showing detected swash edge in blue (top panel). The grayscale is pixel intensity. Example thermal time stack imagery showing surface saturation boundary in blue (bottom panel). The grayscale is temperature mapped to pixel intensity.
  • Subsurface saturation beneath the upper beachface in response to swash over a tidal cycle.
  • Water content response to swash at three depths in the unsaturated zone at a high-energy beach (top panel) and low-energy beach (bottom panel).
  • Infiltration rates per swash event at three different locations across the intertidal zone on a high (Herring Point) and low (Cape Shores) wave beach.

Biogeochemistry

The hydrology of groundwater systems linked to the biogeochemical framework of coastal groundwater systems. Groundwater flow controls the residence time and thus the exposure time of solutes to microbial communities, and the supply of reactants to biogeochemical reaction zones. To understand the interactions between land-ocean driving forces, groundwater flow, and biogeochemistry, we linked a variable-density groundwater flow and solute transport model to a reactive transport model to simulate DOC degradation, aerobic respiration, denitrification, and sulfate reduction in a beach aquifer under the influence of tides. The results reveal which types of beaches are capable of transforming the largest quantity of nutrients in groundwater before discharging to the sea. The results also demonstrate the sensitivity of mixing-dependent and mixing-independent reactions to 5 physical and hydrologic factors. We have also conducted combined field, laboratory, and numerical modeling studies to investigate the interplay between hydrology, microbiology, and geochemistry in beach aquifers

  • Overlaid chemical parameters show geochemical reactions along freshwater and saltwater flow paths. Sulfate reduction fueled by DOM produces hydrogen sulfide in the intertidal mixing zone. The hydrogen sulfide reacts with Iron(III), causing iron oxide dissolution, Iron(III) reduction, and iron sulfide precipitation.
  • Pore water chemistry in the intertidal circulation cell.
  • Variable mineralogy at a beach groundwater discharge zone. Iron(II) sulfide (July; top) and Fe(III) oxyhydroxide (August; bottom)