Hexavalent Chromium | Vacaville, CA. This is one-fifth the current total chromium standard of 50 ppb, which includes both trivalent chromium (Cr+3) and hexavalent chromium (Cr+6). The federal MCL for total chromium is 100 ppb. Cr+3 is less toxic than Cr+6 and actually an essential nutrient at low dosages, while Cr+6 may pose a …