This technical memorandum presents a brief overview of how Hawai‘i-specific background metals concentration ranges were derived and how the data may be used when making site decisions. The Hawaiian Islands Soil Metal Background Evaluation Report evaluated naturally occurring, background concentrations of metals and related elements in the volcanic soils of Hawai‘i. The database used in the study was compiled from literature review of previous research projects, soil data requested from the Natural Resources Conservation Service database, and soil data collected by the Hawai‘i Department of Health (HDOH). A total of 180 samples representing surface soil concentrations of 29 chemicals throughout the seven main Hawaiian islands (Kaua‘i, O‘ahu, Moloka‘i, Lana‘i, Maui, Kaho‘olawe, and Hawai‘i Island) were included for the background analysis. Samples with probable anthropogenic sources were excluded from the study; therefore, the study results represent the concentration ranges expected to be associated with background conditions and not site-related contamination.
Estimated background ranges of metals and related elements were primarily generated from evaluation of cumulative probability plots and spatial and soil type distribution univariate plots (i.e., the combined plots analysis). The plots were evaluated to determine if the dataset for the target element may represent more than one distinct concentration population. If a single population was identified for the target element, then the data were evaluated to confirm that they most likely represent background conditions. If two or more populations were identified for the target element, detailed spatial analysis, elemental association, geochemical analysis and professional judgment were used to distinguish between concentrations representing background conditions and elevated concentrations that may represent contamination. Estimates made using the combined plots analysis were validated by performing correlation coefficient and geochemical regression studies to analyze the geochemical association of metals that show strong correlation with reference metals (e.g., Al, Fe) in natural soils.