New Mexico Geological Society Annual Spring Meeting — Abstracts

Billing smelter slag and waste piles in Park city near the southern border of the city of Socorro, New Mexico, are typical of waste piles associated with metal mining in an earlier time. Lead-zinc ore from the Magdalena Mining district about 30 miles west of Socorro is the source of the slag dumped on the site. Environmental controls during the 19th 20th century were to non-existent. What could not be sold was discarded with little regard to possible contamination.

The site was abandoned nearly 100 years ago, but the waste remains. Esimates by the New Mexico Environmental Improvement Division (EID) of the amount of slag indicate 300,000 plus tons, covering 500,000 square ft to a depth of up to 40 ft. The site is underlain by alluvial deposits of the Sierra Ladrones Formation (Pliocene/Pleistocene). Ground water of generally good quality lies approximately 200 ft below the surface and flow easterly.

The contamination potential of the site was brought to the EID's attention and they initiated a study in Junes 1989. In conjunction with an ongoing program to study potential contamination by the mineral industry, the New Mexico Bureau of Mines and Mineral Resources (NMBM&MR) began a study of the site in January 1990. Preliminary testing of soils in the immediate vicinity of the slag by both the EID and NMBM&MR indicate that the lead content is many times background (0.003%), decreasing rapidly both upstream to the west and downstream toward the Rio Grande. Lead content of the clay-size fraction ranges from 0.015 to 0.21 %, whereas that of the sand-and-larger fraction ranges from 0.003 to 2.15%. Studies show that the soil is calcite-rich, alkaline, and has a high buffering capacity.

This research demonstrates 1) that lead in the slag and waste materials is not soluble under the alkaline conditions at the site; 2) transport by runoff is probably thus restricted to particulate movement; 3) even if mobilized, lead would move toward the water table at a very slow rate based on recharge studies of similar setting elsewhere in the Rio Grande Valley; 4) airborne distribution of lead in the area deserves further study; and 5) as a measure of contamination potential, EPA's Extraction Procedure-Toxicity analyses are not relevant under Western climate/soil conditions.