STEPHENSON BENNETT MINE
ORGAN, DONA ANA COUNTY, NEW MEXICO
Soil and Dust Contamination
Of the contaminants detected in soil samples from residential areas, lead poses the greatest health concern. Lead was detected at concentrations as high as 32,200 ppm (in a play area), and it was detected in 170 of 208 (82%) soil samples at concentrations greater than 500 ppm.
The presence of elevated concentrations of lead in residential soil and house dust poses a threat to public health. Young children are at particular risk, since they are known to be susceptible to the neurotoxic effects of lead . The placenta is permeable to lead, and the ratio of lead in umbilical cord blood to maternal blood is about 0.9 . Therefore, maternal exposure to lead could pose a risk to the developing nervous system of a fetus.
In June 1997, the New Mexico Department of Health and the New Mexico Environment Department, with the assistance of ATSDR, conducted blood lead testing in residents of the area . Blood lead levels were determined in 69 children and 24 adults (>18 years old). Eight children (12%) had a blood lead level between 10 and 14 micrograms per deciliter (µg/dl); the rest of the children had blood lead levels of less than 10 µg/dl. All of the children with blood lead levels of 10 µg/dl or more were 6 years of age or less. None of the adults had a blood lead level of 10 µg/dl or higher.
The community has a public water system, which has been tested and shown to be free of lead contamination. Limited testing by the NMDOH did not detect lead paint in the homes. Therefore, lead contamination in soil and dust may be a significant source of lead exposure for children with elevated blood lead levels.
Arsenic was detected in soil from residential lots at concentrations as high as 860 ppm and in dust at concentrations as high as 208 ppm. Lead and arsenic co-occurred in the samples: i.e., whenever an elevated arsenic concentration was detected, lead was also present in the sample at a very high concentration.
Cadmium concentrations in soil and dust were relatively low. Although a few slightly elevated cadmium concentrations (> 50 ppm) were detected, in general, cadmium was not present in soil and dust at concentrations of health concern.
High concentrations of zinc were detected in soil and dust samples. However, zinc toxicity is low, and small amounts of zinc are required for optimal health. Therefore, zinc contamination in the soil and dust does not pose a significant health hazard.
Lead contamination was detected in about 10 percent of the ambient air particulate samples. However, the maximum concentration of lead detected (0.421 µg/m3) was below the National Ambient Air Quality Standard of 1.5 µg/m3 (90-day average). Therefore, the air concentrations of lead that were measured do not pose a health hazard.
Arsenic was detected in ambient air particulates at a maximum concentration of 7.57 µg/m3 and was detected in 31 percent of the samples at a concentration in excess of 1 µg/m3. By comparison, mean concentrations of arsenic in ambient air in the United States range from <0.001 to 0.003 µg/m3 in remote areas to 0.020 to 0.030 µg/m3 in urban areas .
In the ambient air particulate samples, the concentration of arsenic almost always exceeded the concentration of lead. This is in contrast to the soil samples, in which the lead concentration always exceed the arsenic concentration. In some of the air particulate samples, arsenic constituted 27 percent of the total particulate mass. The reason for the arsenic enrichment of suspended air particulates is not known.
The Occupational Safety and Health Administration (OSHA) has recommended that air levels of arsenic should not exceed 10 µg/m3 during an 8-hour period in an occupational setting. The National Institute of Occupational Safety and Health (NIOSH) has classified arsenic as a carcinogen and recommended that air levels of arsenic should not exceed 2 µg/m3 during any part of the workday. OSHA and NIOSH standards are derived for healthy adults who are exposed in the workplace for limited periods of time. These standards may not be protective of the general population, which can contain sensitive populations such as the young, the old, people with pre-existing disease, and people with longer-term exposures.
The maximum ambient air level of arsenic detected at the site
approaches the OSHA Permissible Exposure Level, and the arsenic
level in seven of the samples exceed the NIOSH Ceiling Level.
Therefore, arsenic was detected in ambient air particulate
samples from the site at concentrations that may pose a health
Is there a synergistic toxic interaction between zinc and lead that is of concern here?
ATSDR is not aware of information in the literature to document such an interaction. It has been reported that when the body is deficient in zinc, lead absorption from the gastrointestinal tract is increased . Therefore, under some circumstances, zinc would actually provide a protective effect from lead toxicity.
Does ATSDR have any suggestions for interior dust action levels?
ATSDR suggests that the action levels used for residential soil could also be used for interior dust. Soil constitutes a significant fraction of house dust. Furthermore, the EPA's Integrated Exposure Uptake Biokinetic model for lead assumes that the ratio of soil to dust ingestion is 45:55. Therefore, the same action level could apply to both media.
Would an action level of 20 ppm arsenic in soil be protective of public health?
An action level of 20 ppm arsenic in soil would be protective of public health for ingestion of soil. A higher action level may be justifiable depending on (1) the frequency, length, and extent of exposure and (2) the bioavailability of arsenic from soil (The EPA - Region 8 determined that the bioavailability of arsenic from mine tailings was about 50%).
What levels of soil contaminants would be protective of domestic animals?
Some residents of the area raise horses and other domestic animals. Grazing animals ingest substantial amounts of soil while grazing; therefore, such animals would be at greater risk for toxicity from soil contaminants than humans. Therefore, soil action levels to protect animals may have to be even lower than what is needed to protect human health.
The EPA is proposing to use HEPA vacuum cleaners to remediate interior contamination when dust levels of lead and arsenic equal or exceed outdoor soil levels. Would this be protect public health?
The use of HEPA vacuum cleaners in conjunction with wet mopping and wiping inside the house can reduce indoor contamination. However, unless the source of lead contamination is removed (i.e., mine wastes, residential soils), recontamination of house dust will occur.