DEOXYGENATION IN A MOBILE-BED RIVER .2. MODEL CALIBRATION AND POSTAUDIT

Benthic metabolism accounts for over 90% of oxygen uptake in the Tarawera, a mobile-bed river receiving treated pulp mill effluent. A steady-state DO model was developed in 1984 which uses zero-order kinetics for benthic metabolism. During calibration a strong linear relationship was identified between the river deoxygenation rate (benthic oxygen uptake rate) and the river BOD concentration just below the main outfall. When tested against non-steady monitoring data the model successfully predicted monthly-average DO concentrations but tended to "overshoot" when waste loads varied. This suggests that the mobile-bed communities have some ability to attenuate the effects of varying waste loads. A post-audit of the model using recent monitoring data shows that the model can predict day-to-day variations of DO under average waste loading conditions but not when there are sudden large changes of waste load. It can be inferred that there are two different time scales operating: surface microbial communities respond quickly (1-2 days) to variations in waste load, but following a major reduction in waste load, reworking of deeper sediments depresses river DO for several weeks. There is a need to develop "biofilm" type models of mobile-bed rivers in order to refine predictions of DO under non-steady waste loadings.