acteristic should explain this difference in the leaching behavior. Therefore, metals concentrations, nutrients, pH, organic content, grain size, and Atterberg Limits were measured at anomalous and neighboring, otherwise similar, residential properties with low surficial arsenic concentrations. As shown in the example in Table 2, the physical and geochemical properties of the soils were nearly identical. Further, no correlation was found between phosphorous, iron, or nitrogen and arsenic concentrations. Therefore, none of the data supported the hypothesis of historic, widespread arsenic contamination (e.g., by airborne emissions) that was subsequently leached at all properties but those with anomalously high concentrations today.

Correlation with Pesticide

Because of the evidence suggesting the anomalous arsenic had been applied to specific residential properties in the 1950s or later, as discussed above, the literature was reviewed for information on arsenical products applied to turf. Historians at PHR Environmental Consultants identified a lawncare product called PAX, which contained arsenic trioxide (25.11%) and lead arsenate (8.25%), and was used for crabgrass control in Denver and other cities in the 1950s and 1960s (U.S. Patent 3057709, 1962; Stadtherr, 1963; Frost et al., 1973). PAX was a solid, granular product that was typically applied by a drop spreader at rates of about 10 to 12 kg/100 m2 of lawn, depending on the version of the product.

A single application at a rate of 12 kg/100 m2 would theoretically increase the arsenic concentration in the upper 5 cm of soil by approximately 350 mg/kg (assuming a typical loamy soil dry density of about 1400 kg/m3), while ten applications over a decade would theoretically increase arsenic concentrations by 3500 mg/kg. Dissolution and leaching of arsenic, however, would reduce concentrations in the upper 0-5 cm over time. Measured concentration profiles with depth (e.g., Fig. 12) indicate that approximately 30% of the arsenic was retained in the upper 5 cm at the time of testing, with the remainder typically distributed over the upper 30 to 60 cm (EnviroGroup, 1997). Therefore, modern-day arsenic concentrations of 100 to 1000 mg/kg are consistent with one to ten applications of PAX in the 1950s and 1960s, assuming that about 30% of the arsenic remained in the upper 0-5 cm. This range is consistent with the average observed range of anomalous arsenic concentrations on most properties. The occasional higher, individual concentrations are explained by overlap of drop spreader paths and deliberate over-application in areas with thick crabgrass. We can also imagine that some home owners applied PAX at rates that were higher than recommended due to miscalculation, incorrect spreader settings, a desire (perhaps misguided) for better performance, or to completely use up a bag of PAX (19 kg).

The arsenic to lead ratio in PAX would theoretically result in the following relationship between the total lead (Pb) and total arsenic (As) concentration in soil:

Total Pb = Background Pb + (Total - Background As)/4.2 (1)

Fig. 13. Correlation with PAX lead/arsenic ratio.

The background lead concentration is the lead concentration prior to the application of PAX, due to natural soil levels and anthropogenic contributions (e.g., leaded gas, lead paint). If the background lead concentration was the same at all points on the property, the lead and arsenic concentrations after the application of PAX would fall along a straight line defined by equation 1. In reality, the background lead concentration will vary from point to point across a yard, resulting in departure from a straight line. Nevertheless, the lead and arsenic ratios in samples at several properties with anomalous arsenic concentrations were similar to the slope of this line, or the PAX lead to arsenic ratio, as shown in Fig. 13.

On other properties, we observed a greater scatter of the data, but always to the left of a bounding line described by the PAX lead to arsenic ratio. This behavior is consistent with increased leaching of the product, and preferential leaching of the more soluble arsenic trioxide component which would cause data points to shift to the left of the PAX line in Fig. 13. Therefore, the lead to arsenic ratios on the anomalous properties are consistent with historic applications of the PAX product, and are inconsistent with applications of pure arsenic trioxide or other metal-phase arsenic (which would tend to plot below the PAX line, or randomly throughout the plot space, respectively).

A strong correlation was found between anomalous arsenic samples and the presence of perlite, one of the inactive ingredients of PAX (U.S. Patent 3057709, 1962). All residential lawn soil samples with anomalous arsenic concentrations that were examined by stereo-microscope were found to contain perlite, while no perlite was found in any of the samples with low arsenic concentrations. While perlite is also found in other products, such as potting soil, the chances of this correlation occurring by random chance are negligible.

Testing in other Areas

If the anomalous arsenic concentrations found in Globeville are due to historic applications of PAX, it is reasonable to expect anomalously high arsenic concentrations in other parts of Denver where the product was likely used, i.e., older neighborhoods that would have lawns sufficiently aged to have crabgrass infestation in the 1950s or 1960s. While random testing of other neighborhoods was difficult for Asarco to justify, a property was discovered in south Denver, approximately 8 miles south of the Globe Plant, where a former resident recalled their family using PAX on the lawn. Fortunately, the property was still in the possession of the family and sampling access was granted. Elevated arsenic concentrations (maximum 210 mg/kg), lead/arsenic ratios that fit the PAX ratio, the presence of arsenic trioxide crystals, a decreasing arsenic concentration profile with depth, and the presence of perlite, all matched the PAX finger-print that was observed in Globeville soils.

Subsequently, the U.S. EPA listed the Vasquez Boulevard - I70 Superfund Site on the National Priorities List, which included several square miles of neighborhoods to the east of Globeville where similar high arsenic concentrations were detected. While investigations were not complete at the time of this paper, the data indicate that high arsenic concentrations occur on random properties across the neighborhoods, without any apparent spatial relationship to the Globe Plant or any other point source (ISSI, 2000). Intensive sampling of selected properties shows that the arsenic concentrations are confined to property boundaries, as observed in Globeville (ISSI, 2000).

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