APPLICATION OF CHRONIC IN-SITU TOXICITY TESTING TO SUPPORT AN INNOVATIVE SITE-SPECIFIC AMMONIA CRITERION FOR SALT CREEK, NEBRASKA

Extensive in-stream studies were conducted to determine a scientifically defensible site-specific water quality standard for ammonia for Salt Creek which is a naturally saline waterbody on southeastern Nebraska. One component of the program was to design and conduct an in situ toxicity testing program to calculate 30-day IC20 values for fathead minnows and channel catfish, and use those data in combination with the other studies to determine a weight-of-evidence-based site-specific water quality criterion. The in situ toxicity tests used caged fish continuously exposed for 30-days at nine in-stream locations to more accurately characterize the complex interactions of ammonia in Salt Creek waters. Two fish species were selected for this program: fathead minnows (Pimephales promelas) and channel catfish (Ictalurus punctatus). The selection of exposure chamber locations was based on in-stream concentrations of total ammonia, and natural concentrations of salinity. The experimental design evaluated two toxicological endpoints: mortality and growth; which were combined into a “biomass” determination. The 30-day IC20 value for the channel catfish was calculated to be 3.85 mg N/L based on total biomass. The 30-day IC20 value for the fathead minnow was >9.98 mg N/L, because there was less than a 20 percent decrease in biomass at downstream stations versus the control at the highest 30-day average in-stream exposure concentration biomass. These in situ IC20 values were then used with the other site-specific assessments using a weight of evidence approach to a determine a numeric site-specific water quality standard which will be fully protective of Salt Creek. These other site-specific criterion components included: 5 years of winter and summer bioassessment data, whole effluent toxicity testing, and extensive ammonia modeling. The results of all of these components were integrated and used to determine seasonal (summer and winter) site-specific total ammonia water quality standards for Salt Creek. The in-situ testing component proved to be a very labor intensive but useful tool when evaluating the effects of ammonia in Salt Creek.