The soil lead sample were collected adjacent to the heavily used urban highway. In 2004 and 2006, in situ XRF measurements were recorded and each individual spectral measurement was a 60 second exposure and four spectra were averaged at each selected location on the field site.
The field portable XRF was calibrated each day using NIST Standard Reference Materials (SRM) 2710 and 2711. The XRF typically yielded values within 10 % of the NIST SRMs, which matches the instrument specifications.
The triangular highway median appeared undisturbed in Spring 2003, however a car fire incident occurred in Summer 2003. (A USGS satellite photo is available on the linked page) Since this time, Cal-Trans has placed mulch on the site, so students must determine the soil horizon to make surface measurements. Since the 2006 sampling, the far western portion of the median was cemented so that portion is now inaccessible.