Comparison of dissolved and total metals concentrations from acute tests with saltwater organisms

Aquatic life criteria (ALC) have traditionally been expressed for metals in terms of total-recoverable or acid-soluble concentrations. Recent U.S. Env ironmental Protection Agency policy recommended use of dissolved metal conc entrations for setting water quality standards. Criteria derived from previ ous tests could be expressed in terms of dissolved metals if ratios of diss olved-to-total concentrations in those tests were consistent. Using those m etals with insufficient dissolved metals data to directly derive criteria ( arsenic (III), cadmium, chromium (VI), lead, nickel, selenium (IV), and zin c), we measured both total and dissolved metal concentrations in acute salt water static and how-through tests. Exposure conditions simulated those of original tests used to derive ALC. Partitioning of metals between dissolved and particulate forms was very consistent. Dissolved metal concentrations were greater than 90% of total concentrations in all tests, exceeding 95% i n 10 of 13 tests. Dissolved-to-total metal ratios did not vary significantl y with concentration, time, or type of test. Biological responses were cons istent with historical data. Results implied that in acute saltwater toxici ty tests used to establish ALC, metals were primarily dissolved. Thus crite ria developed for metals based on total concentrations should be equally va lid when expressed in terms of dissolved concentrations.