Pathogenic Interference in Short-Term Whole Effluent Toxicity Tests: A Case Study
Pathogen interference in Whole Effluent Toxicity (WET) testing results in unnecessary costs to National Pollutant Discharge Elimination System (NPDES) permit holders. This is a condition identified explicitly in the EPA methods by which these tests are conducted. This kind of interference can result in false positive test results which, if not identified properly, can result in wasted testing and permit violations. There are methods that can be employed to identify pathogen interference.
The discharge from Florida Power and Light’s Sanford Plant, Volusia County, Florida provides a case study for this problem. This facility has whole effluent biomonitoring requirements in their NPDES permit. 7 day chronic definitive bioassay tests are conducted on the fathead minnow, Pimephales promelas. Some of the bioassay tests conducted on the discharge from the Sanford Plant have resulted in test endpoint reductions requiring further actions on the part of the permit holder.
The patterns of these reductions have been consistent with pathogen interferences as outlined in the EPA method. In the case presented here, UV treatment was employed in order to demonstrate that this interference was occurring. Discharge samples were split into two treatment groups: untreated and UV treated samples. Parallel tests were conducted. The test results are consistent with pathogen interference.
| % Effluent | Untreated Effluent | UV Treated Effluent | ||||
| Final Survival (%) | Average Dry Weight (mg/fish) | Coefficient of Variance (%) | Final Survival (%) | Average Dry Weight (mg/fish) | Coefficient of Variance (%) | |
| 0 | 100 | 0.684 | 5.13 | 100 | 0.670 | 4.45 |
| 6.25 | 100 | 0.638 | 4.66 | 100 | 0.640 | 7.77 |
| 12.5 | 80 | 0.520 | 42.10 | 100 | 0.701 | 6.56 |
| 25 | 100 | 0.612 | 5.17 | 100 | 0.665 | 5.21 |
| 50 | 77.5 | 0.522 | 12.39 | 100 | 0.751 | 6.30 |
| 100 | 32.5 | 0.247 | 109.6 | 100 | 0.702 | 2.14 |
| IC25 | – | 51.58% | – | – | >100% | – |
A demonstration of test method interference from pathogens would provide the permit holder with evidence that a given test event was a false positive response as opposed to a permit violation.The survival and dry weight between replicates in the test dilutions are very uneven (e.g. CV 109.60%) and the coefficient of variance of the control group is relatively small (5.13%). In the UV treated effluent, the mortality is completely eliminated. Furthermore, the dry weight between replicates in the test dilutions is now very even (e.g. CV 2.14%).