The value of hexavalent chromium as a reference toxicant was investigated by comparing the precision of two laboratories in preparing test solutions, examining the consistency of chromium exposure in the test chambers during experiments and determining the effect of nominal versus measured test concentrations on the calculated toxicity (EC50 or LC50). The sensitivities of the test animals to chromium were also determined.
The coefficients of variation associated with preparing chromium test concentrations were 51 and 63.8% in salt and fresh water, respectively, at one laboratory, and 136 and 14.8%, respectively, at the other laboratory. Hexavalent chromium remained stable in the hexavalent (as prepared) form during the toxicity test, with recoveries ranging from 77 to 114%. The precision of the analytical laboratory in measuring chromium-spiked fresh and salt waters ranged from 0.2 to 9.1%. The impact on the calculated EC50s or LC50s of using analytically measured versus nominal concentrations in the analyses was negligible. A comparison of seven species in 27 tests showed organism sensitivity (mean EC50 or LC50 as mg/L Cr6+) to chromium to be, in decreasing order of sensitivity: Ceriodaphnia sp. (0.031), Daphnia pulex (0.086), Mysidopsis almyra (5.1), Mysidopsis bahia (6.03), Cyprinodon variegatus (21.4), Pimephales promelas (26.1) and Lepomis macrochirus (182.9).
The 48-h LC50s for M. bahia were not significantly different (p < 0.05) either within or between laboratories during a 3-week study; values ranged from 5.49 to 7.72 mg/L Cr6+ in both laboratories (nominally) and from 4.21 to 7.23 mg/L Cr6+ when analytically verified by an independent laboratory. The acute toxicities of chromium to D. pulex differed significantly between laboratories, by almost one order of magnitude. Interlaboratory variability observed in D. pulex tests was attributed to differences in the test organisms' food. As reference toxicant tests, chromium tests are valuable benchmark indices of the relative health of test organisms over time or among laboratories. When toxicity test information is used in critical decision making, such as compliance with effluent permit limitations, a reference toxicant such as chromium contributes to quality control and assurance.