Method for Determining Acute Toxicity of an Acid Waste and Limiting Permissible Concentration at Boundaries of an Oceanic Mixing Zone Curt D. Rose*‘ and W. Gary Williams EG&G, Environmental Consultants, Waltham, Mass. 02154
Terry A. Hollister and Patrick Rodney Parrish EG&G, Bionomics, Pensacola, Fla. 32507
A new method is described for determining mortality of bioassay organisms attributable to acidic and nonacidic constituents of an acid waste and for using these measurements to derive a limiting permissible concentration for the whole waste a t boundaries of an oceanic mixing zone. The method reports mortality attributable to the acidic constituent in terms of its direct cause-hydrogen-ion concentration ({H+l) of bioassay water. Titration of acid waste into a representative sample of seawater from the mixing zone is then performed to obtain a limiting permissible concentration for the whole waste that recognizes the dominant role of neutralization in reducing concentration and toxicity of the acidic constituent in the zone. The method, which is demonstrated for a byproduct hydrochloric acid with the copepod Acartia tonsa as the bioassay organism, can be modified for use with alkaline wastes. The Ocean Dumping Final Regulations and Criteria ( I ) require that concentration of a waste material or chemical constituent in receiving waters shall not, after reasonable allowance for initial mixing in the mixing zone, exceed 0.01 of a concentration shown to be toxic to appropriate sensitive marine organisms in bioassays conducted in accordance with approved United States Environmental Protection Agency (USEPA) procedures. Presently approved procedures ( 2 ) specify that 96-h static acute bioassays be conducted in media of 20-30%0 salinity and that results be reported as the estimated concentration of waste material or chemical constituent that causes 0.50 mortality of test organisms (LC50). Ocean dischargers, who are responsible for conducting bioassays on their own wastes, presently report LC50 values to USEPA on a “dilution” basis, i.e., volume of whole waste per volume of bioassay water (pprn) that produces LC50. USEPA then determines limiting permissible concentration of waste at boundaries of the mixing zone by multiplying LCso by the 0.01 application factor. These procedures appear to be appropriate for such commonly discharged wastes as sewerage sludge, cellar dirt, dredging spoil, and most industrial wastes. Toxicity of these materials is directly related to the volume of whole waste that is added per volume of bioassay or mixing-zone water because dilution is the primary factor that reduces waste concentration in aquatic systems. Acid wastes, however, are unique among materials that are discharged into the ocean. Toxicity of acidic materials is only indirectly related to the volume of whole waste that is added per volume of bioassay or mixing-zone water since much of this acid is instantaneously removed by the carbonate buffer system. Toxicity, on the other hand, is directly related to the concentration of unneutralized acid ((H+])to which organisms are exposed. Because the importance of this distinction is widely recognized (3-5), bioassays of effects of acids and highly acidic wastes on organics almost invariably report results as jH+{or equivalent pH of bioassay water. Bioassays in
I
Present address, 151 Bear Hill Road, Waltham, Mass. 02154.
which mortality is reported as volume of whole waste that is added per volume of bioassay water are useful for the limited purpose of comparing toxicity of two or more acidic materials under identical laboratory conditions. These results, however, do not identify the direct cause of mortality and therefore cannot be meaningfully extrapolated to the environment-the major reason for conducting bioassays. The Ocean Dumping Final Regulations and Criteria recognize that neutralization in the marine environment must be realistically accounted for when determining limiting permissible concentration of an acid waste a t boundaries of a mixing zone. Section 227.71 of the Regulations specifies that “reasonable allowance [must be made] for initial mixing in the mixing zone” of a waste material or chemical constituent. This mandate can only be interpreted as meaning that all major processes that reduce concentration of an acid waste or its constituents during mixing must be evaluated when determining a limiting permissible concentration for the waste. Limiting permissible concentrations that are derived by multiplying dilution-based LCso values by 0.01 are incapable of realistically accounting for neutralization of the acidic constituent of an acid waste in the mixing zone. This is because mixing zone water often has a greater neutralizing capacity than bioassay water and, even if neutralizing capacities were equal, {H+]of a buffered medium does not increase proportionately with volume of added waste. The dilution method, because of these defects, is inconsistent with the provisions of the ocean dumping regulations. The method that we propose for determining acute toxicity of an acid waste and limiting permissible concentration at boundaries of an oceanic mixing zone is based on a bioassay design that identifies, in technically appropriate terms, LC io’s attributable to acidic and nonacidic constituents of the waste; calculation of limiting permissible concentrations of acidic and nonacidic constituents of the waste a t boundaries of the mixing zone; and determination of a total limiting permissible concentration for the whole waste that is derived by titrating a sample of waste into a representative sample of seawater from the mixing zone and which, therefore, realistically accounts for neutralization of the acidic constituent in the zone.
Demonstration of Method The method is demonstrated for a by-product hydrochloric acid with normality of 9.04 and pH of 0.2. Nonacidic components of the waste and their approximate concentrations were: chlorides, 290 000 ppm; fluorides, 10 000 ppm; oil and grease, 1 ppm; arsenic, 475 ppb; zinc, 140 ppb; lead, 130 ppb; vanadium, 110 ppb; nickel, 105 ppb; copper, 95 ppb; mercury, 85 ppb; chromium, 60 ppb; selenium, 27 ppb; and cadmium, 13 PPb. The copepod, Acartia tonsa, which for this particular acid waste is the most sensitive of three indigenous species selected for testing by Region 11, USEPA, was used as the bioassay organism. Adult animals were collected by plankton net from Big Lagoon, near Bionomics Marine Research Laboratory in Florida, and acclimated at 19% salinity and 21 f 1 “C for 48 h. Mortality was
