Ambient Toxicity Monitoring Program

About one-fourth of the stations sampled on multiple dates have exhibited recurrent lethal or sublethal toxic effects on at least one test species . This percentage is high, because investigators have targeted waterbodies with known or suspected water quality problems. Waterbodies that have exhibited recurrent toxicity are candidates for more intensive assessments, aimed at confirming the occurrence of toxic conditions or aquatic life use impairment. Priority for investigation should be based on the frequency and degree of observed toxicity, as well as other important factors, such as waterbody designated uses, actual uses, economic importance, endangered species, and known or suspected causes of impairment.

Ceriodaphnia dubia and Pimephales promelas (fathead minnow) have been used routinely for freshwater testing. The Daphnia pulex acute test was in use when the program was initiated. Cyprinodon variegatus (sheepshead minnow) has been used for saline or estuarine waters. Only three records exist for Selenastrum capricornutum. Figure 2 depicts the relative breakdown of about 2,000 tests for the primary test species.

The database offers an opportunity to examine the relative sensitivity of selected toxicity tests. For Ceriodaphnia dubia and the Pimephales promelas embryo-larval test, 453 pairs of side-by-side tests have been conducted on ambient water samples. Two hundred and one pairs of side-by-side tests were conducted on sediment samples using these species. Twenty-eight (28%) percent of the water samples, and twenty-one (21%) percent of the sediment elutriates, caused significant toxic effects to at least one test species. Figure 3 depicts results from 126 water samples and 42 sediment samples that caused toxic effects to either one or both of the species.

Ceriodaphnia dubia is generally recognized as the more sensitive of the two test species, and these results appear to provide confirmation. Both tests are used, because of differences in their sensitivity to specific pollutants. Evaluation of results from both tests may reveal potential causes of toxicity for specific waterbodies. For example, Pimephales promelas is known to be more sensitive to ammonia toxicity than Ceriodaphnia dubia.

The toxicity screening procedures, as described, have important limitations. The collection of single grab samples allows the possibility that intermittent toxicity may not be detected. The sediment elutriation procedure will extract only those contaminants that are water-soluble or loosely bound to the sediment (Nriagu and Lakshminarayana, 1989). The fact that any given station has not tested positively, does not rule out the possibility that water quality and/or sediment quality problems exist. However, uncertainty is reduced by repeated sampling over time.

During an assessment of the Guadalupe River near Victoria, Texas, Trebatoski (1991) used both laboratory and in situ toxicity tests. The study was designed to investigate the potential impact of a domestic wastewater treatment facility on the river and to examine the utility of ambient toxicity testing. Caged adult fathead minnows were exposed to the effluent and the river, at an upstream site and two downstream sites, for a 48-hour period. Both the in situ and laboratory tests demonstrated effluent toxicity. The test results differed in that acute effects were observed in the field (100% mortality for adults after six hours), whereas, chronic effects were observed in the laboratory (55% mortality for embryolarval fish after eight days). Results of in situ and laboratory tests were consistent regarding instream conditions, as neither test demonstrated significant toxicity at upstream nor downstream sites.