New Mexico Geological Society Annual Spring Meeting — Abstracts

Billing smelter slag and the nearby soil have been examined by the U.S. Environmental Protection Agency (EPA) and by New Mexico authorities, including the New Mexico Bureau of Mines and Mineral Resources (Bureau). Cleanup of lead-contaminated soil by the EPA
has been accomplished during 1990-91. Before the cleanup began, the Bureau sampled three of the many 100' x 100' squares laid out and evaluated by the EPA. The three squares were identified as C1100, 12000, and L2400 by the EPA. Lead contamination in the soils of the three squares as reported by EPA was 1876, 4203, and 1221 ppm lead, respectively.

Thirteen samples were taken in each square by the Bureau, from approximately the same positions as those used for portable XRF analyses by the EPA. Composite lead values for the same squares were 1800, 9900, and 1700 ppm, respectively, by wet chemistry. In addition, the Bureau analyzed the whole sample taken, including the < 2mm fraction excluded from some previous testing and probably also from in the EPA's XRF methods. The Bureau samples were broken down into various size fractions. The percentage of each fraction was measured and the lead concentration was determined by wet chemistry. Altogether 273 analyses were performed, all in duplicate and some in quadruplicate. The highest lead concentration in all three squares was in the clay-size fraction (4300, 18,600, and 4000 ppm Pb, respectively), although this fraction accounted for only 3 or 4 wt. % of the squares sampled.

The larger particles (>2mm) in 11 samples from square C1100 were analyzed for lead contamination on the particle surfaces. Before treatment the lead concentration of the 11 samples averaged 505 ppm. After washing in distilled water the value was 310 ppm, additional washing in 5% acetic acid lowered the value to 280 ppm, and additional boiling in 0.25 EDTA solution further dropped the value down to 230 ppm.

The depth of the contamination was determined in two squares (12000 and L2400) which we believe were relatively undisturbed since the smelter was abandoned about 90 years ago. In both squares the lead dropped to background levels below the depth of 3 inches. Therefore, we concluded that downward movement of lead contamination into the water table is not a problem.

Mineralogical examination of the fine fraction of leadcontaminated soils from C1100 square revealed absence of crystalline lead compounds. Glassy particles in the samples indicate that slag or flue particles carried lead to the soil. Further, the Pb/Zn ratios in high-and low-density mineral separations lead us to conclude that lead is adsorbed on mineral particles.

It is likely that weathering has been responsible for release of lead to the mineral surfaces in the soil. However, because of the alkalinity of these soils, the mobility of lead is controlled by movement of Pb-bearing particles, as opposed to chemical transport in solution.