Small arms firing ranges (SAFRs) located on Department of Defense (DoD) facilities are, in many cases, constructed next to wetland areas, including ponds, lakes, and streams. Access to wetland areas (especially forested wetlands) is typically limited due to a lack of roads. Standard environmental remedial options and monitoring techniques are expensive to implement due to the nature of the terrain and seasonal changes in water flow and salinity. Thus, there is a need for a relatively low-cost, passive, in situ treatment technology for exclusion of toxic metals and sediment in runoff water that can meet the needs of the variable terrain requirements. This project addresses the contamination of surface receiving waters by munitions residue-contaminated runoff water and sediment from training ranges.
Two factors influence the amount of lead transported off-site by surface water runoff: the mass of lead fragments left on the range and the velocity of the runoff water. The velocity of the water can successfully be controlled at outdoor ranges by using vegetative, organic, removable and/or permanent ground covers, and by implementing engineered controls which slow down surface water runoff and prevent or minimize the chances of lead migrating off-site. Current methods for treating heavy metals in runoff water include precipitation and flocculation, treatment with ion exchange resins, and phytoremediation. The costs of these technologies are driven by size and complexity of the site being treated, pre-treatment requirements, and post-treatment/disposal of contaminated treatment waste. The performance objectives of this demonstration were to validate the use of the filter sock to remove sediment bound metals as well as soluble metals (i.e., lead) from SAFR runoff water in a manner that was less expensive, time-consuming and labor-intensive to training range management. A second objective was to validate the sediment transport model developed by ERDC-EL for use on SAFRs.
This technology is a reactive filter barrier to trap both metal-contaminated sediment and soluble metals from stormwater runoff. It combines the proven use of geotextile fabric woven into a tubular shape (a “sock”) filled with sand and with the addition of amendments to adsorb both suspended sediments from surface water as well as cationic (such as lead [Pb], zinc [Zn], and copper [Cu]) and anionic (such as antimony [Sb]) metals, metalloids, and metals bound to the suspended solids. The filter sock is National Pollutant Discharge Elimination System (NPDES)-approved for use on construction sites in order to control transport of sediment in surface water.
The sand filter sock performance model was applied to the North Kinder Range at Fort Leavenworth, KS. The model was used to assess sand filter sock performance for a design storm. Performance measurements consisted of required filter sock diameter and length, and estimate of filter sock life due to sediment clogging. Other measurements included removal of suspended solids, mass of sediment trapped, and change in the filter sock removal coefficient and saturated hydraulic conductivity for the design storm.
Bench-scale treatability testing established the Kd and leachability of the sand/amendment combinations. Time Release Amendment Phosphate System™ (TRAPPS™) was selected as the amendment for the reactive filter socks.
The demonstration was performed on the North and Center Kinder Ranges at Fort Leavenworth, KS. The North Range was set out according to the runoff water model; the Center Range was anad hocdesign. On both ranges, filter socks filled with sand and metal sorption amendment were placed in the flow path of heavy metal-contaminated runoff water. Sediment that pooled upflow of each reactive barrier and the contents of each reactive barrier on both ranges were sampled prior to project demobilization. The solids were analyzed for heavy metals and Toxicity Characteristic Leaching Procedure (TCLP). On both ranges, the Pb concentration in sediment deposited upstream of the barriers was much higher than that in the barriers themselves. Reactive barrier filler material passed TCLP for Pb which would allow disposal of the barriers in a non-hazardous waste site or possible re-use on-site.
Implementation issues associated with this technology were the site soil erodibility, the concentration of sediment carried by the surface water runoff, and the annual volume of storm runoff water. Runoff water with high sediment concentrations requires more frequent change-outs of the foremost reactive barrier as the barrier will clog more rapidly, which will increase the cost of maintaining the technology. In drought years, the life of the barriers would be extended, while in rainy years, or tropical climates with high rainfall, and high sediment transport, the lifetime of the barrier could be reduced.