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In February 1994, the construction company, under the oversight of Tohono O'odham Nation representatives and its consultant, dug test trenches on the site. Trenches were dug in several areas for the purpose of locating former burial trenches and collecting soil samples. During the trenching activities, buried asphalt and chunks of what appeared to be lime were found. In addition, samplers encountered stained subsurface soils with a diesel fuel odor. Three of the five subsurface soil samples, collected at depths of three and eight feet, showed the presence of total petroleum hydrocarbons (TPH) at levels up to 14,700 parts per million (ppm). Although not applicable to tribal lands, the State of Arizona has established a cleanup standard of 100 ppm for TPH in soils. Subsequent sampling, at five and ten foot depths, yielded five of 39 samples with TPH levels exceeding the 100 ppm standard. The highest result from the second batch of soil samples was 2120 ppm TPH [4].

In the area surrounding the arroyo, 1140 acres drain into the pit which is 30 feet deep in some places. Based on the average annual runoff and capacity of the pit, consultants estimated that the pit can hold all of the annual runoff. Infiltration through the bottom of the pit appears to be high and, as a result, all of the runoff is expected to recharge the groundwater. Depth to groundwater is about 700 feet in this area. One estimate shows that any pollution existing in the pit could reach the groundwater in as soon as five years [6]. A drinking water well is located about one mile down gradient of the Choulic Gravel Pit [3].

For about six months of the year, during the rainy seasons, there is standing water in the arroyo [3]. The concern for cattle and wildlife drinking any pooled water in the contaminated area prompted the local water utility to sample the surface water at the site. The water utility reported that methylene chloride was found at 44 ppb and 12 ppb in the surface water samples [1]. The Maximum Contaminant Level (MCL) for methylene chloride in drinking water is 5 ppb. The area around the pit is now fenced and cattle and most wildlife have no access to the standing water.

Under a cooperative agreement with the U.S. Environmental Protection Agency, the Tohono O'odham Nation plans to conduct an environmental investigation of the Choulic Gravel Pit [7]. The results of the environmental investigation will provide more information about the types and levels of contaminants associated with the site. This contaminant data can serve as the basis for a more definitive public health evaluation of the site.

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