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.
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