Sediment toxicity testing provides an additional tool in assessing environmental impacts on streams. Toxicity testing is performed in conjunction with the Ambient Biomonitoring Network (AMNET) program, which assesses stream impairment based on analysis of macroinvertebrate communities. Testing is usually performed on AMNET sites that show severe impairment. Positive test results would implicate water sediment quality, or other physiochemical factors as possible causes of the impairment, versus physical habitat degradation. This testing further pinpoints areas of impact within a watershed, thereby reducing the amount of costly chemical analysis needed to explore sources of contamination.
Toxicity testing is performed in the laboratory using the test organism, Hyalella azteca. This measures the effects of ambient surface water, sediment, or wastewater effluent on aquatic organisms. H. azteca is an amphipod crustacean, a member of the group commonly known as scuds or sideswimmers. The species is widely distributed in North America, inhabiting unpolluted lakes, ponds, and streams here and also in South America. This organism is considered a good indicator species for this test because it lives and feeds in close contact with the sediments, often burrowing in, and it may digest bacteria and algae from ingested sediment particles. Also, this organism can be cultured in the laboratory with relative ease.
When testing ambient conditions, several of the organisms are placed in the test solution or sediment, while others are placed in a reference solution(s) of good water/sediment quality. The effects of each solution on the organisms are then compared. When testing effluents, the organisms are placed in a series of dilutions of the effluent. The lethal concentration at which 50% of the organisms exhibit mortalities (known as the LC50), or non-lethal effects such as growth inhibition (known as the EC50)are calculated at the end of the test. Toxicity testing measures both acute and chronic (sub-lethal) effects. Acute tests generally measure survival. Chronic tests are longer in duration and measure survival, growth, and fecundity (reproductivity).