Field methods to assess pore-water geochemistry and quantify groundwater flow at the interface between groundwater and surface water have been well documented and developed. Zero-valent iron (ZVI) geochemical barriers commonly have been installed as a vertical wall within an aquifer to intercept organic and inorganic contaminant plumes in horizontally flowing groundwater.

In October 2004, a geochemical barrier was applied in the groundwater discharge zone of a kettle-hole pond where the well defined discharge of a dissolved phosphorus plume was focused in a nearshore area (Air Force Center for Environmental Excellence, 2004). Monitoring the performance of a remediation system at this interface required adapting sampling strategies similar to those used in groundwater/surface-water interaction studies.

Mapping of the near-shore discharge area of the plume in September 1999 by using drive points revealed elevated dissolved phosphorus levels (as great as 3 mg/L) within 20 m of shore along about 120 m of shoreline. In June 2004, drive-point sampling showed that the area of elevated phosphorus in groundwater near the pond bottom was about the same as was observed in 1999 (fig. 1). In August 2004, zero-valent iron (ZVI) was mixed into near-shore pond-bottom sediment (3% by weight) to a depth of about 0.6 m, extending 12.2 m offshore along 91.4 m of shoreline in the area of highest observed pond-bottom phosphorus. The sediment mixture was created by excavating the pond-bottom material while the pond was locally dewatered using a coffer dam and large pumps. An excavator mixing bucket blended the pond-bottom sediment and iron filings prior to placement of the mixture on the pond bottom (Air Force Center for Engineering and the Environment, 2004). Excavation of the dewatered pond-bottom provided a unique opportunity to install instrumentation for barrier performance monitoring. More information on the geochemical barrier can be accessed at the AFCEE website about the geochemical barrier.

The USGS made modifications to previously documented methods for data collection at the groundwater/surface-water interface to measure concentrations of phosphorus and other treated-wastewater constituents at the barrier. Four types of monitoring devices were permanently installed at locations within, outside, and below the geochemical barrier. To monitor the spatial distribution of the phosphorus plume with distance from shore, horizontal multiport samplers (HMPS) were installed at two depths along lines extending pondward from shore. Vertical multilevel samplers (VMLS) and multilevel diffusion chambers (MLDC) were installed at several locations to monitor expected changes in phosphorus concentrations over small vertical intervals below the pond bottom extending through the barrier. Permanent seepage meters were installed at several distances from shore to monitor water and phosphorus flux through the pond bottom inside and outside of the barrier area.

In August 2006, 2 years after the barrier was installed, the field and laboratory results indicate that phosphorus concentrations continue to be reduced as groundwater flows through the geochemical barrier into Ashumet Pond.