CHARACTERISTICS, DETERMINANTS AND INTERPRETATIONS OF ACUTE LETHALITY IN DAPHNIDS EXPOSED TO COMPLEX WASTE LEACHATES

Acute lethality of the water flea Daphnia magna exposed to leachate ru noff samples from 35 Finnish mixed waste disposal sites was measured w ith 24 h tests without dilutions, followed by standard 48 h tests, and accompanied by chemical analyses of water quality. Toxicity was obser ved in 1/3 of the 343 screening test samples (including dilute ones), and at 3/4 of the sites. The average 48h-EC(I)50 value of the toxic sa mples, as determined by a power function of linear estimates of Median Survival Time, was 50% dilution (0.5 C0). The average 24 h toxicity w as slightly less (0.61 C0). There was a strong correlation between let hality in screening test (LETH}24h) and 48h-EC(I)50 (r = 0.85**, S = 0.77**). Screening test response was concentrated in areas of total o r no lethality. Very significant (p < 0.1%) linear and rank correlatio ns were found between lethality and general water quality variables, e .g. concentrations of Cl-, NH4-N and light metals. However, sometimes lethality was not observed even though the concentrations of toxic sub stances exceeded those reported to be acutely lethal in pure solutions , suggesting suppression of toxicity or method differences, and consti tuting false negatives. The inclusion of Cl- as a variable explaining variation in standard test lethality increased r2 from 0.73 in the sim ple linear regression model 48h-EC(I)50 = 110 - 0.755 LETH}24h to 0.77 in the model 48 h EC(I)50 = 102 - 0.00507 (LETH}24h)2 - 0.396 (log Cl -)2. The applicability of standard and simplified acute Daphnia tests in the characterization and assessment of toxicological risks from was te leachates was discussed.