The discharge of wastewaters containing ammonia has become a major environmental concern, especially in the Chesapeake Bay watershed. A tentative instream limit of 1-2 mglL for total ammonia has been proposed in Virginia to protect marine aquatic life. This limit was based upon the national chronic criterion for un-ionized ammonia (0.035 mg/L) which was calculated using both freshwater and saltwater toxicity data. Therefore, additional bioassays needed to be performed with marine organisms in order to refine this limit based solely on saltwater organisms.
Acute bioassays were conducted with one marine fish, the sheepshead minnow (Cyprinodon variegatus), and two marine invertebrates, the mysid (Mysidopsis bahia), and grass shrimp (Palaemonetes pugio). The acute LC 50 and the no-observed-effect-concentration (NOEC) were determined for each organism and compared to data in the literature. The species mean acute values (SMA V) for the sheepshead minnow (13.5% difference) and the mysid (14.7% difference) compared well with the literature, while the grass shrimp was more tolerant (81.2% difference) to ammonia than expected. The SMA V were calculated based upon the LC 50 data and used to assess both the national acute and chronic criterion for ammonia.
Seven day chronic bioassays were performed with the sheep shead minnow (Cyprinodon variegatus). The LC 50 and the NOEC for both mortality and growth were calculated. The acute-chronic ratio was determined to be 6.95 for the sheepshead minnow and 4.35 for the mysid. Based upon these data, the refined national criteria for unionized ammonia was determined to be 0.054 mg/L. The instream limit can be set based upon the water pH, temperature, and salinity.