Waste treatment operators have a good understanding of their operations and know from experience and instrumentation feedback when treatment processes are operating correctly. In this situation, many water-quality conditions that can affect the outcome of a toxicity test (e.g., hardness, conductivity, suspended solids, pH, temperature) are relatively constant and predictable. In contrast, hardness, conductivity, and concentration of suspended solids can vary greatly in ambient waters with rainfall or snow-melt; pH can vary two standard units or more in response to season or even over daily cycles due to algal photosynthesis; and temperature can increase or decrease rapidly in response to weather conditions. Water-quality conditions in receiving streams can also change rapidly due to upstream spills or intermittent releases of batch-process effluents (e.g., cooling tower operations, which typically release a large volume of ion-rich waste water over a short period of time). In short, temporal variation in water quality is an important source of background noise that can complicate quantification of low levels of ambient toxicity.
Aquatic organisms are about as good at detecting toxicants in receiving waters as they are at detecting toxicants in effluents. However, the apparent or actual sensitivity of the organisms to some toxicants can be affected by other chemicals or water-quality factors. The sensitivity of test organisms to toxicants and their vulnerability to nontoxicant interferences are particularly important in ambient testing where the signal-to-noise ratio is low. Specific examples demonstrate this point. High but nontoxic concentrations of sodium can lower the toxicity of lithium to Ceriodaphnia (Stewart and Kszos, 1996). Thus, lithium at a concentration of 5 parts per million (ppm) in a sodium-rich waste water (e.g., 140 ppm sodium) might show no evidence of toxicity, whereas lithium is distinctly toxic at a concentration of 1 ppm in low-sodium (e.g., 5-10 ppm) ambient water. Calcium or other hardness-contributing materials can also lower the toxicity of nickel (Kszos et al., 1992) and other metals.
Physical variables also can affect the apparent sensitivity of test organisms. For example, naturally occurring particulate matter (algae, bacteria, and/or sediment) can lower the apparent sensitivity of organisms in two ways. First, some particulate matter (notably, algae, bacteria, and detritus) can be used as food by freshwater microcrustaceans. The nutritional benefits of the "extra food" can be important. For ambient toxicity tests of water samples from two sites in East Fork Poplar Creek (EFPC) (a stream that receives various waste waters from the DOE's Oak Ridge Y-12 Plant), we found that filtering the water to remove naturally occurring particulate matter significantly lowered Ceriodaphnia reproduction. The mean reduction in Ceriodaphnia reproduction caused by filtering the water was slightly larger at one site (9.7 percent) than it was at the other site (7.4 percent), but the effect of filtration was statistically significant at both sites (p = 0.030 for 15 tests at km 24.1 of EFPC; p = 0.019 for 21 tests at km 23.8 of EFPC, Student's T test). (Sites in East Fork Poplar Creek are identified by distance upstream from its confluence with Poplar Creek, a tributary of the Clinch River.) In contrast to many