DERIVATION OF SITE-SPECIFIC AMMONIA CRITERIA FOR AN EFFLUENT-DOMINATED HEADWATER STREAM

The present study was designed to derive appropriate ammonia standards for a freshwater, wooded stream on the Delmarva Peninsula (eastern shore of the United States) composed almost entirely of wastewater effluent for at least four months of the year. Representative species acute and chronic toxicity tests were performed at 20-degrees-C (summer) and 12-degrees-C (winter) to obtain seasonal standards. Acute toxicity tests using fathead minnow and Daphnia magna suggested that the site water had no effect on un-ionized ammonia toxicity. However, resident species appeared to be relatively insensitive to acute exposures of un-ionized ammonia. Warm-water LC50s adjusted for pH = 8.0 and 20-degrees-C ranged between 1.02 and 2.73 mg/L for those tests in which an LC50 could be calculated. If acute no-observed-effect concentrations (NOECs) (approximately LC25) were used, bluegill was most sensitive (NOEC = 0.295 mg/L), followed by Crangonyx (amphipod) and Menidia (silverside). Warm-water chronic testing indicated that Rana (leopard frog) was most sensitive, followed by bluegill and amphipod. Cold-water acute tests yielded definitive LC50s between 0.53 (bluegill) and 1.91 mg/L (amphipod). Hyla crucifer (spring peeper) and juvenile bluegill were most sensitive in chronic cold-water testing. Final acute values (FAVs) of 0.62 and 0.30 mg/L NH3 were obtained for 20 and 12-degrees-C, respectively. The geometric mean of acute-to-chronic ratios (ACR) based on unionized ammonia were 7.2 and 5.7, for 20 and 12-degrees-C, respectively. The final chronic values (FCVs) or maximum 4-d average concentrations at 20 and 12-degrees-C were 0.09 and 0.05 mg/L NH3, respectively. Un-ionized ammonia acute toxicity to vertebrate freshwater species (fish and tadpoles) was inversely related to temperature, whereas invertebrates often exhibited the opposite trend. Chronic testing, however, suggested that most species were more sensitive to NH3 at 20 than 12-degrees-C. This study suggests that the ACR developed by the U.S. Environmental Protection Agency (EPA) for warm-water streams may be overly conservative for very small streams in which species diversity is naturally low.