Ambient Toxicity Monitoring Program

Ambient toxicity data generated through the program have been used to meet a variety of monitoring objectives, ranging from screening for toxicity to determining whether toxicity is a potential causal factor in observed aquatic life use impairment.

Toxicity testing was integral to binational studies conducted to identify the extent and effects of toxic substances on the Rio Grande. Many toxic "hot-spots", primarily tributaries on both sides of the border, were identified that caused toxic effects to test species. Likewise, the program has provided the means to document that ambient toxicity is infrequently observed on the main stem of the Rio Grande.

The TNRCC has developed a model to integrate the results of toxicity testing with other assessment information, designated uses, and relative resource value. The model is used to rank waterbodies for corrective actions (TNRCC 1994).

Also, ambient toxicity testing has supplemented assessment methods applied for a variety of special studies. (Kolbe 1994 and 1995) describes the use of ambient toxicity testing to confirm toxicity as one of several water quality problems contributing to a depauperate fish community in Linnville Bayou. The effectiveness of corrective actions taken by the Phillips 66 Refinery and Petrochemical Complex in Brazoria County, Texas were documented by toxicity testing. Phillips 66 discharges treated wastewaters to Linnville Bayou. The discharge contributes almost all instream flow during dry weather. Phillips 66, the only permitted point source on the bayou, discharges an average of four-million gallons per day.

Historically, studies associated with the Phillips 66 discharge documented impacts such as oily sheens, elevated pH, low dissolved oxygen, and high ammonia and sulfide concentrations. Severe impacts to fish communities of Linnville Bayou were also observed during several studies. During August 1992, investigators collected nine fish species at an upstream site; however, no fish could be collected 0.4 miles downstream from the discharge (mosquitofish, Gambusia affinis, were observed in the shallows at the downstream site). Only five fish species were collected nine miles downstream, and mosquitofish comprised 97% of the individuals in this sample. Ambient toxicity testing allowed investigators to identify toxicity as a possible contributing factor to observed fish community impairment. Effluent toxicity has since been attributed to chlorides, ammonia, and one or more organic toxicants.

Phillips 66 has since completed several treatment plant upgrades to attempt to achieve compliance with permit conditions. The most significant improvements resulted from the identification and removal of ammonia sources in September 1992 and the startup of activated sludge and powdered activated carbon treament systems in April 1993.

Recent ambient toxicity test results suggest a reduction in the occurrence of ambient toxicity that appears to coincide with the removal of ammonia sources (Figures 4 and 5).

Historically, water samples collected from Linnville Bayou at Hasema Road (Station No. 1300.0726), located immediately downstream from the discharge, caused toxic effects to both Ceriodaphnia dubia and Pimephales promelas. Toxicity tests conducted since December 1992 have shown no significant effects on test organisms exposed to ambient water collected from the bayou. Although, effluent tests conducted by the company continued to indicate occasional whole effluent toxicity violations. Phillips 66 is working with EPA to resolve the issue (Phillip Jennings, EPA, personal communication).

Fish community sampling conducted by (Kolbe 1995) indicates that recovery of the fish community has occurred in the reach downstream from the discharge. During a recent study, index of biotic integrity values downstream from the discharge were comparable to an upstream reference site and sites located further downstream.