One hundred and eighty-six soil samples from Northwest Oregon were tested for arsenic content. The highest values measured were 13.9 ppm in the A horizon (site C4) and 20.4 ppm in the B horizon (Site P4). Arsenic was not detected in 28 A horizon samples and 23 B horizon samples. Data are grouped based on the age and rock type of underlying bedrock. Lithologic groups with six or more data points were compared statistically to ascertain if groups are distinct. Analysis of Variance (ANOVA) multiple comparison tests indicate that the arsenic content of the Marine Sediments and Sedimentary Rocks group samples is distinguishable from the Quaternary Basalts group in the A horizon and all other groups in the B horizon. Kruskal-Wallis multiple comparison tests indicate that the arsenic content of the Marine Sediments and Sedimentary Rocks group is distinguishable from the Quaternary Basalts, Quaternary/ Tertiary Sediments and Sedimentary Rocks and Volcanic Sediments groups in both the A and B soil horizons. The ANOVA and Kruskal-Wallis tests compared A and B horizon data by lithologic group. The ANOVA shows the Marine Sediments and Sedimentary Rocks group in the A horizon is distinct from the Quaternary Basalts in the A and B horizon. The Kruskal-Wallis test yielded the same result. Per the ANOVA, the Marine Sediments and Sedimentary Rocks in the B horizon are distinct from all other tested groups. The Kruskal-Wallis test shows the Marine Sediments and Sedimentary Rocks group in the B horizon as distinct from the Quaternary Basalts, Quaternary/ Tertiary Sediments, and Volcanic Sediments groups in the A and B horizon. A K-means cluster analysis was used to group all available data independent of underlying bedrock. Three, four, and five group analyses were conducted, and the results of these tests were compared to the data grouped by underlying rock type. No correlation between the groups resulting from the K-means cluster analysis and groups based on underlying lithology was found. This analysis supports the creation of a map distinguishing arsenic content in the soils above Marine Sediments and Sedimentary Rocks group units from arsenic content in all other tested lithologic groups. The mean and standard deviations of these groups (in ppm) are: A horizon: Marine Sediments (6.09 ±2.66); other groups (3.10 ±3.19); B horizon: Marine Sediments (10.26 ±4.65); other groups (3.13, ±2.52). This analysis indicates that geologic context must be taken into account when determining background levels of naturally occurring arsenic in soils.

This study examines the natural background concentrations of arsenic in the soils of southwest Oregon, using new samples in addition to data collected from previous theses (Khandoker, 1997 and Douglas, 1999). The original 213 samples were run by ICP-AES with a reporting limit of 20 ppm, and only three samples had detected values. The original samples were tested again (2013) at a lower reporting limit of 0.2 ppm by ICP-MS, as were 42 new samples (2013), to better ascertain the natural levels of arsenic in undisturbed soils. The aim is to add to the existing DEQ data set, which has been used to establish new regulatory levels based on natural levels in the environment that are both safer and more economically viable than the former risk-based remediation levels (DEQ, 2013). The maximum and mean concentrations, respectively, for each province (with high formation map unit) are 85.4 and 21.99 ppm for South Willamette Valley (Tfee), 45.4 and 5.42 ppm for the Klamath Mountains (Jub), 11.9 and 2.76 ppm for the Cascade Range (Tbaa), 10.6 and 5.15 ppm for the Coast Range (Ty), 2.32 and 1.29 ppm for the Basin and Range (Qba) and 1.5 and 1.20 ppm for the High Lava Plains (Tmv). In addition, the distribution and variance of arsenic in the A and B soil horizons is assessed in this study by comparing deviation at a single site, and also by comparing A and B horizons of 119 PSU sites. One of 18 new sites sampled for this study (distinguished with the HH prefix), site HH11, was randomly chosen to evaluate differences at a single location. Site HH11 is an Inceptisol soil above volcanic rock (KJdv map unit) located at 275 meters elevation in Douglas County within the Klamath province. Five samples were taken from the A and from the B horizons at site HH11. The means and standard deviations were 3.74 ± 0.44 for the A horizon and 4.53 ± 0.39 for the B horizon. The consistency and low deviation within each horizon indicate that a single sample within a horizon is a good representative of that horizon and supports the field methodology used in this study of taking only one sample in the A horizon and one sample in the B horizon. Wilcoxon Rank-Sum test determined that A and B horizons for the 119 sites that had data for both the A and B horizons were not statistically different (p-value 0.76). Arsenic concentration is not associated with a particular horizon for these sites. However, differentiation between soil horizons increases with age (Birkeland, 1999), as does accumulation of the iron oxides and sulfide minerals on clay surfaces (McLaren et al., 2006) which concentrate in the B horizon. These associations warrant further study to see how they relate to arsenic level, soil development and age in Oregon soils. Lastly, this study statistically examines six potentially important environmental predictors of naturally occurring arsenic in southwestern Oregon: site elevation, geomorphic province, mapped rock type and age, and sample soil order and color (redness). A Classification and Regression Tree Model (CART) determined soil order, elevation and rock type to be of significant importance in determining arsenic concentrations in the natural environment. According to the regression tree, arsenic concentrations are greater within Alfisol and Ultisol/Alfisol and Vertisol soil orders, at lower elevations below 1,207 meters, and within soils from sedimentary, mixed volcanic/sedimentary and unconsolidated rock types.