Prediction of concentrations of toxic compounds with a water quality simulation program : a case study in the Pong River

Abstract

A fish-kill problem has been in the Pong River, Thailand for over a decade. As the river was exposed to different pollution sources such as remnants of the past untreated wastewater’s spill, aquacultural waste, and agricultural runoff, all possible fish-killing agents from these sources must be studied. From the 1999 and 2000 monitoring data, it was found that fish kills and low DO occurred at the same time. Moreover, a week before the fish kills in 1999, there were high BODs and DO at all aquaculture sites, suggesting that there might be an algal bloom, which died off and subsequently caused low DO and fish kills. The presence of the algal bloom was proved indirectly with the use of GC/MS in conjunction with the experimental aquaculture, trophic state analysis with algal enumeration, and water quality modeling. A dynamic water quality model was developed with a new method for estimating the unavailable runoff data and calibrating the flow, using a combination of lignin and tannin as conservative trace. Results of correlation coefficients (R2) from the runoff calibration were reasonably high. Root mean square error (RMSE) from the calibration of flow was comparable with the literature values, using salinity as conservative trace. RMSEs from the model calibration and validation of conventional nutrients were found to be comparable to literature values. The model predicted the bloom die-off which lowered DO and possibly caused fish kills on the same day in 1999, suggesting that the accuracy of the dynamic model was on a time scale of days, and that the use of lignin/tannin to calibrate the flow and runoff was justified. The predictive capability of the model for chlorophyll a was tested and the bloom was still predicted, suggesting that the flow and runoff were enough to cause the bloom. The analysis of the trophic state indicated that the Pong River and Chot lagoon, particularly the fish pond, were under eutrophication, and risk from the algal bloom. The monitoring of phenols also proved that remnants of untreated wastewater in the Chot lagoon did not contain significant amounts of phenols which could cause the fish kills.