BACKGROUND METALS IN SOIL AT THE STANFORD LINEAR ACCELERATOR CENTER, MENLO PARK, CALIFORNIA

Ninety soil samples collected from undisturbed areas at the Stanford Linear Accelerator Center (SLAC) were analyzed for California Title 22 metals and Al, Fe, Mg and Mn using low detection limit ICP-MS and INAA analytical methods. Core samples were collected from depths ranging from 0 to 10 feet below ground surface from the three lithologic units underlying SLAC: the Eocene Whiskey Hill Formation, the Miocene Ladera Sandstone and the Plio-Pleistocene Santa Clara Formation.

Sample populations were analyzed statistically and modeled using a frequency distribution decomposition technique used in mineral exploration. Results of analysis show that the Whiskey Hill Formation is geochemically distinct from the other lithologic units on the basis of mercury, zinc, copper, lead and other metal concentrations. Magnesium and iron concentrations were found to be effective in discriminating geochemical populations within the Ladera Sandstone and the Santa Clara Formation.

Analysis of ten independent verification samples showed that the initial set of 90 primary samples did not capture the full range of naturally occurring metals concentrations at SLAC. The increased range of concentrations of certain metals may be due to 1) lateral changes in the stratigraphy of the Ladera Sandstone, 2) lack of weathering effects in 50% of the verification samples or 3) effects of local hydrothermal alteration.

The study shows that it is difficult to accurately characterize background metals with a small number of samples. Major elements, such as aluminum, iron, and magnesium can make an important contribution to discriminating among geochemical populations.