INNOVATIVE USE OF BIOASSESSMENT TO DERIVE SITE-SPECIFIC AMMONIA CRITERIA FOR SALT CREEK, NEBRASKA

At present, chronic water quality criteria for the protection of aquatic life are based on partial life cycle toxicity tests conducted in laboratories and mesocosm studies, and the laboratory tests are not accurate predictors of toxicity in all cases. Bioassessment can be used to develop accurate and site-specific criteria for the protection of aquatic life.Six bioassessments were conducted during the summer from 1994 through 1999 using the USEPA rapid bioassessment procedures (Plafkin et al., 1989) in Salt Creek Nebraska. The toxicity of ammonia to the biological community in Salt Creek was measured by observing decreases in the numbers of (1) all fish and (2) cyprinid species, (3) benthic macroinvertebrate taxa, and (4) chironomid taxa at sampling sites located downstream from the Theresa Street and Northeast WWTPs. The relative difference (RD) between upstream and downstream biological metric values was used to determine in-stream impact from WWTP discharges where RD was calculated as:(upstream metric score (MS) – downstream MS) / (upstream MS + downstream MS)A continuous simulation model of total ammonia concentrations in salt creek was used to estimate the highest 30-day average total ammonia concentrations for 180-day periods prior to each bio-assessment. Total ammonia values were analyzed as an independent variable, and Rd values were analyzed as a dependent variable in linear regressions with the bio-assessment data. The coefficients of determination (R2) for the regressions of the number of fish and cyprinid species and the ammonia concentrations over a 180-day exposure period downstream from the Theresa Street WWTP were 0.83 and 0.95, respectively. The R2 values for these metrics and ammonia concentrations downstream from the northeast WWTP were 0.69 and 0.71. The strong correlations between the reductions in taxa richness and highest ammonia concentrations for 180 days prior to the bioassessment is significant because the 180-day timeframe coincides with fish spawning activity in salt creek and exposure to sensitive early life stages (USEPA, 1999). The maximum safe total ammonia concentration (RD = 0.15) predicted for the sampling location downstream from the Theresa street data was 2.12 mg-N/L. the safe concentration predicted for the site downstream from the northeast WWTP was 2.25 mg-N/L. 2.1 mg-N/L ammonia has been recommended as a site-specific criterion for segment LP2-20000 based on the bioassessment results. This value is more than twice the existing chronic ammonia water quality criteria, 0.91 mg-N/L, for the segment.