The effectiveness of zero-valent iron permeable reactive barriers (ZVI PRB) for removal of explosives from groundwater has been recently demonstrated. As an added benefit, ZVI can treat a variety of contaminants that may co-occur in groundwater at RDX- and TNT-impacted sites (e.g., chlorinated solvents, chromate). ZVI PRBs, however, are generally applicable in unconsolidated media and at depths of less than 70 feet below ground surface (bgs). To expand the applicability of ZVI to a broader range of settings, the injection of fine-grained ZVI into the subsurface has been proposed. An alternate approach, evaluated at the Cornhusker Army Ammunition Plant (CAAP) site near Grand Island, Nebraska, involved installation of a pair of dual-screened wells in which the conventional filter pack around the well screens has been replaced with coarse granular iron. Water moving into or out of the well screens passes through the ZVI and explosives are removed. The rate of explosives reaction with ZVI is sufficiently fast that good capture of groundwater by the wells can be achieved.
The objectives of this technology demonstration were:
- Demonstrate that TNT and RDX can be degraded in situ to acceptable levels (i.e., the method detection limit) using a zero-valent iron in situ treatment well (ZVI ISTW)
- Evaluate ISTW well pair hydraulics
- Identify design and operational factors that influence successful implementation and continued operation of the ZVI ISTW approach.
In this demonstration, granular iron was placed outside of the well screens in a pair of dual-screened wells. The wells were installed inside a large-diameter temporary casing such that the iron adjacent to the upper and lower screens could be isolated from one another in a manner that would promote groundwater circulation between the pair of dual-screened wells.
The groundwater hydrology of the well pair was evaluated and met design expectations. A primary concern in the methodology was that water entering the treatment zone would contain materials (e.g., sulfate) that would plug the treatment zone over time. Based on calculations for the site, this was not expected to be a problem, which turned out to be the case. Unfortunately, an unanticipated problem arose in which water moving out of the upper screen (which was located near the water table) became oxygenated and plugged the treatment zone. An additional problem with oxygen arose because of regionally decreasing water table elevations in the area of the demonstration. This exposed a portion of the iron adjacent to the upper screen and, together with drawdown during pumping, led to a loss of permeability in the well in which water was extracted from the upper screen. These problems could have been avoided if the screened interval was below the water table and if a packer had been placed in the well casing above the screen.
Tracer test data indicate that water recirculated between the two ISTW wells relatively quickly. Measurement of explosives concentrations in groundwater also showed that the performance of the ISTWs met design expectations. A year after installation of the ISTWs, reactivity of the iron was still sufficiently high to reduce explosives concentrations to below detection limits.
An inherent disadvantage of the design used at CAAP was that the iron could not be readily replaced. In retrospect, if the reactive material would have been emplaced as a removable “cartridge” within a large dual-screen well, it would have provided an opportunity to remedy the plugging issue. However, plugging of injection screens is an inherent problem with circulation wells, and it is difficult to say with confidence if the well design improvements discussed would represent a long-term solution.
The cost of each of the two ISTW wells was approximately $40,000. It is likely that modifications to the design would increase the cost per well. However, if the technology was implemented at a full scale in a similar setting, it is believed that the cost per well would be similar to well costs for this demonstration.