USGS Dive Team Assists Hydrologists with Contaminant Study in Ashumet Pond, Massachusetts
On July 21, 2009, Dann Blackwood and Rick Rendigs of the U.S. Geological Survey (USGS) Woods Hole Coastal and Marine Science Center's dive team successfully completed scientific diving operations at two long-term study sites in Ashumet Pond in Mashpee, Massachusetts. The dive work supports a continuing cooperative effort with team leader Tim McCobb of the USGS Water Resources Discipline (WRD)'s Massachusetts-Rhode Island Water Science Center.
WRD scientists are conducting an ongoing study to evaluate the spatial distribution, concentrations, and ultimate fate of various chemical pollutants that were discharged at the Massachusetts Military Reservation and have infiltrated into groundwater and local ponds in the upper Cape Cod area.
One part of this study reveals the impact of effluent from a decommissioned sewage-treatment plant that operated on the Massachusetts Military Reservation for almost 60 years, from 1936 to 1995. The effluent was dumped into infiltration beds, and the resultant sewage plume has been identified in groundwater samples approximately 6 km downgradient from the plant. (Groundwater flows "downgradient," from areas where the water table is higher to areas where it is lower.) A segment of this sewage plume currently discharges as groundwater along the shoreline of Ashumet Pond, originating from the infiltration beds approximately 500 m upgradient from the study site.
One of the components of sewage effluent is phosphorus (P), which in sufficient concentrations causes eutrophication of freshwater lakes, changing their condition from low biologic productivity and clear water to high biologic productivity and water made turbid by the accelerated growth of algae. Decomposition of the abundant algae reduces dissolved oxygen in the water, which can cause other organisms, such as fish and shellfish, to die. An estimated 516 metric tons of phosphorus was disposed of in the infiltration beds during the operation of the plant.
In order for scientists from WRD to determine the scope and concentrations of phosphorus entering the pond, USGS Woods Hole divers were enlisted over the years to implant various types of samplers (such as gel, diffusion, dialysis, and seepage meters) into the sediment of the pond to enable the collection of representative pore-water samples near the interface between the groundwater and the surface water. (For example, see related Sound Waves article "USGS Woods Hole Dive Team Deploys Samplers for Study of a Phosphorus Plume.")
At the two long-term study sites in the area, multidepth drivepoint wells have been installed to more than 30 m below the pond's sediment surface. These wells make it possible to sample groundwater from a range of depths. USGS divers recently assisted with uncapping the wells and installed tubing in order for WRD scientists to pump the wells from the surface and sample the groundwater for phosphorus.
Voluminous amounts of algae were encountered along the bottom of the pond at the nearshore well site (20 m from shore), restricting visibility and making it a challenge to locate the nearshore wells. The deeper site, about 40 m from shore, was virtually free of algae along the bottom.
Most of the dissolved phosphorus in the sewage plume is in the form of inorganic orthophosphate (PO4). In 2004, the installation of a permeable barrier of iron filings mixed with nearshore pond sediment was installed along the shoreline of the study area; the iron filings have a high affinity for and capacity to adsorb PO4 and reduce its concentration in groundwater discharging to the pond. Initial results have been encouraging regarding the ability of the barrier to sequester PO4; details and results from these studies may be found in the following publications:
in this issue:
Dive Team Assists with Contaminant Study