(ZOGORSKI et al., 2006)
http://pubs.usgs.gov/circ/circ1292/pdf/circular1292.pdf
Ambient Groundwater Monitoring and Assessment Program – California Aquifer Susceptibility (CAS) Assessment, State Water Resources Control Board [Groundwater Monitoring and Assessment (GAMA) Program; February 21, 2001]
http://www.waterboards.ca.gov/gama/docs/info_summary_2_21_01.pdf
N. B.: Excerpt from CAS Assessment Report, “Low Level VOC Analyses”, page 3.
“Currently, most VOC action levels are in the parts per billion (ppb) range. Thus, typical VOC analytical detection limits are also at the ppb level. The CAS assessment will analyze VOCs in groundwater in the parts per trillion (ppt) range. This level of analysis will identify groundwater that is already impacted by potentially contaminating activities, but which still remain below action levels.”