TP12 Toxicity of Complex Mixtures
Tuesday, 15 November 2005: 8:00 AM - 6:30 PM in Exhibit Hall

(RUD-1117-866851) Toxicity Identification Evaluation (TIE) case study of a geothermal electric plant located in southern California.

Rudolph, J¹, Stransky, B¹, Bailey, H¹, ¹ Nautilus Environmental, San Diego, CA, USA

ABSTRACT- A geothermal electric plant located in Southern California has consistently exhibited toxicity in their monthly NPDES monitoring to the Cladoceran Ceriodaphnia dubia, whereas the fathead minnow, Pimephales promelas, has not generally shown toxicity. Upon performing acute testing with C. dubia, it was determined that samples collected within days of each other were extremely variable in their toxicity. Routine Phase I TIE treatments were generally ineffective at removing toxicity; however, it was determined that the toxicity of the samples was inversely related to pH. In addition, the presence of ammonia complicated interpretation of the TIE results because there was sufficient ammonia present in at least some of the samples to result in toxicity if the pH was increased to 9. TIE testing on several samples collected during a site visit to the plant demonstrated that sulfides and ammonia could be major contributors to the toxicity. However, further investigations into plant operations revealed that frequent chlorination events could be oxidizing ammonia to nitrite. Nitrite analysis of all archived samples indicated that, with few exceptions, a tight relationship existed between nitrite levels and toxicity. Phase II TIE investigations have identified nitrite as the primary toxicant of concern, with ammonia being a secondary toxicant (depending on the pH of the sample). Elevated TDS remains an issue with respect to chronic toxicity to C. dubia, and hydrogen sulfide could become a concern depending on seasonal changes in influent streams. Routine monthly testing is continuing in order to confirm these relationships.

Key words: nitrite, ammonia, Ceriodaphnia dubia, geothermal