Current Projects and Collaborations

Submarine Groundwater Discharge

Submarine groundwater discharge is a flow of fresh groundwater and recirculated seawater from land into the coastal zone and is recognized as a pathway of dissolved components from land to the oceans. The dissolved components often include pollutants from anthropogenic sources, for example nutrients from sewer systems or agricultural activities on land. The distribution of these sources along the coastline is not uniform, and the same is true about the magnitude of groundwater discharge. Consequently we are confronted with variable non-point source pollution on the land-ocean interface.

Radon/Nitrate/Methane Mapping in Waquoit Bay and Boston Harbor, MA.

In order to evaluate submarine groundwater discharge and its role in non-point source pollution on a large scale (many kilometers of shoreline) we constructed a radon-methane-nitrate surveying system. The surveying system is based on measurements of natural constituents of groundwater: radon and methane in coastal waters. These occur in groundwater in elevated concentrations in comparison to sea water therefore can be used as tracers of submarine groundwater discharge. Areas with high concentrations of radon and methane coupled with high nitrate values in coastal waters can then be investigated in more detail to estimate groundwater and nitrate fluxes. The promise of this technique is that we can map many kilometers of shoreline in a short time and locate areas with significant submarine groundwater discharge and non-point source pollution. The projects are funded by the Coastal Ocean Institute at Woods Hole Oceanographic Institution and the MIT Sea Grant.

A Multi-Tracer Approach for Estimating Submarine Groundwater Discharge and Associated Nutrients in Salt Marsh Estuaries.

We explore two different pathways of nutrient delivery via groundwater discharge in two contrasting New England salt marsh estuaries: in the first, freshwater is delivered primarily by overland flow; in the second freshwater input is dominated by groundwater discharge. We used geochemical tracers, radon- and radium isotopes, to determine large-scale fresh groundwater discharge. We also quantified tidally induced pore water circulation through the marsh bank sediments. A multi-tracer mass balance model was used to evaluate pore water residence time, which in turn was used to estimate carbon remineralization rates derived from pore water nitrogen increases over time. We also examined the role of the spring-neap tidal cycle in pore water exchange dynamics. The project is funded by the Woods Hole Sea Grant.

Assessment of Groundwater Derived Nutrient Inputs and Their Implication on the Occurrence of Algal Blooms in Maui.

This interdisciplinary project is lead by scientists from the University of Hawaii. I use radon and radium isotopes as tracers to assess the dynamics and distribution of submarine groundwater discharge in the coastal zone off Kihei and Waipuliani parks. The Kihei area is prone to frequent occurrences of algal blooms. The geochemical tracers revealed significant groundwater discharge in the area and currently, we are looking into links between algal blooms and groundwater discharge dynamics.

Tracing shelf-derived component transport offshore

Natural Iron Fertiliztaion in the Southern Ocean: Investigating Horizontal Iron Transport and Vertical Carbon Flux Using Radium Isotopes and ²³⁴Th.

There are regions of the high nutrient-low chlorophyll (HNLC) Southern Ocean that appear to be naturally fertilized by iron sources other than atmospheric deposition. One such area is the Southern Scotia Sea, downstream of the Antarctic Peninsula and the Shackleton Fracture Zone (SFZ). Here, we used radium isotopes as tracers of water transport to investigate the possibility that shelf-derived Fe is fueling phytoplankton blooms. We used ²²⁴Ra to calculate the rate of shelf derived iron transport into the HNLC offshore waters where we calculated the corresponding vertical carbon flux using ²³⁴Th.

Sources of Iron to the Eastern Tropical Atlantic: Does the Continental Margin Supplement Saharan Dust?

On a cross-shelf transect near Mauritania we will closely examine the potential lateral source of Fe from the continental margin, and evaluate it against atmospheric sources of Fe. We will combine measurements of specific tracers to uniquely determine the importance of lateral transport vs. dust inputs and subsurface remineralization as Fe sources to the surface ocean. We will use synchrotron x-ray analysis of particulate iron “hotspots” and distinguish dust-derived vs. continental margin iron. At the same time we will derive horizontal mass transfer rates form ²²⁸Ra and vertical transport of particles and remineralization rates form ²³⁴Th profiles.

Development of New Radiochemical Methods

Determination of ²²⁷Ac in Young Basaltic Samples.

²²⁷Ac can be used as a radiometric dating tool and tracer of melt transport rates and magma storage times. For this application I developed an extraction chromatographic method to separate ²²⁷Ac from the rock matrix. The actinium values will be used by Ken Sims (WHOI G&G) in combination with ²³¹Pa as a chronometer for dating of young mid-ocean ridge basalts in the time range of 10-100 years.

High-Sensitivity Method for ²²⁸Ra Determination in Water Samples.

Open ocean ²²⁸Ra values are very low and even pre-concentration of radium on Mn-fibers from 500 Liter samples does not provide enough activity for gamma-spectrometric determination. We are working on developing an extraction chromatographic method to separate radium from the sample matrix and its consequent measurement directly by beta-counting or via alpha-counting its grand-daughter ²²⁸Th.