MODELING TOXICITY DUE TO INTERMITTENT EXPOSURE OF RAINBOW-TROUT AND COMMON SHINERS TO MONOCHLORAMINE

We evaluated the ability of three mathematical models to predict toxic ity to common shiners and rainbow trout during intermittent (pulsed) e xposures to monochloramine, based on data from continuous-exposure tox icity tests. If a power term far the exposure-water concentration was included in the models, a concentration x time (Cxt) model and the Man cini uptake-depuration model predicted pulse LC50s to within +/-50% of the observed pulse LC50s, for the first four pulses in toxicity tests with 2-h pulse/22-h recovery cycles. Beyond the fourth pulse cycle, t hough, the pulse LC50s predicted using the Cxt model appeared to diver ge considerably from the trend of the experimental pulse LC50s, partly because this model does not predict an incipient lethal lever (C-ILL) for either continuous or intermittent exposures. The Mancini model pr edicted the C-ILL moderately well in the common shiner intermittent-ex posure test but not in the rainbow trout intermittent-exposure test. T he Breck three-dimensional damage-repair model did not predict pulse L C50 or C-ILL values as well as did the other two models, probably beca use not enough partial-mortality data were available to parameterize t he model adequately. Although the underlying processes appear to be mo re complex than what these simple models assume, the models may still be adequate for use in regulating a few pulse discharges of monochlora mine.