Metals—including lead (Pb), copper (Cu), cadmium (Cd), uranium (U), and zinc (Zn)—are the most prevalent contaminants at Superfund sites across the country. Lead alone triggers more Superfund actions than any other industrial chemical or waste product. The amount of soil and groundwater from the more than 12,000 military and non-military small arms firing ranges (SAFR) that requires remediation is too vast to remove and dispose in landfills, yet poses a long-term threat to human health and the environment, particularly children. Metals are found both as discrete particles and as metal complexes in the soil matrix, complicating treatment. Phosphate-induced metal stabilization (PIMS) using Apatite II is a technology developed to meet this challenge by treating the contamination in place. At the U.S. Army Camp Stanley Storage Activity (CSSA), a sub-installation of Red River Army Depot in Boerne, Texas, a demonstration of an in-situ soil mixing process using Apatite II for remediation of Pb-contaminated soil was conducted.
PIMS is a technology that stabilizes metals using a natural and benign additive, Apatite II derived from processed fish bones, which chemically binds metals into stable, insoluble minerals. Treatment can occur either by mixing the Apatite II amendment directly into the soil or by emplacing the amendment within a permeable reactive barrier for passive treatment of groundwater. Apatite II is suitable for most types of soil and groundwater and for contaminant concentrations from parts per billion (ppb) to weight percent levels and is not affected by environmental conditions such as low pH, moisture content, water infiltration rate, the presence of organic compounds, or a thriving ecosystem. It reduces the bioavailability of the metals if the treated soils are ingested, particularly important for public health concerns and wildlife. Apatite II can hold up to 20% of its weight in Pb, U, or other metals, which are stable under a wide range of environmental conditions for geologically long time periods. Apatite II works to sequester metals by four general, non-mutually exclusive processes depending on the metal, the concentration of the metal, and the aqueous chemistry of the system: by heterogeneous precipitation on the surface of the Apatite II, by buffering the pH, by surface chemi-adsorption, and by biological stimulation, which remediates metals as well as sulfate, nitrate, TNT, RDX, and perchlorate. Apatite II has been tested successfully at bench- to field-scale on soil leachates and on water from mining and industrial sites.
The demonstration at CSSA represented the first full field-scale application of this technology to SAFR soils. Remediation involved treating soil that had greater than 3/8-inch fragments and unexploded ordnance removed by a sieving operation, followed by soil mixing using Apatite II. The soil mixing of 2.5% by weight of Apatite II into the 3,000 cubic yards of contaminated soil was accomplished in less than 2 weeks using a backhoe and front end loader and two staff. Lead in the leachates collected from shallow lysimeter monitoring wells was reduced from over 280 ppb in untreated soils to less than 4 ppb in the Apatite II-treated soils, below the U.S. Environmental Protection Agency maximum contaminant level of 15 ppb. Apatite II treats the dissolved metals and the residual particulate metal fragments in the soil as they dissolve over time. The cost at the time of application in 2000 was $22 per cubic yard of treated soil. The costs today are from $30 to $40 per cubic yard of treated soil and about $60 per million gallons of treated water.
The benefits of this technology are the ability to treat the metal-contaminated materials in place without removal and disposition at a landfill or other treatment facility. Apatite II is available in large quantities at relatively low cost. It can be mixed directly with the contaminated soil, used as a liner, or mixed with grout, clay, and other reactive media. In addition, Apatite II can be used in combination with other technologies and amendments. The technology should not harm existing ecosystems and, therefore, is ideal for re-vegetation efforts and wetlands development. (Project Completed - 2006)