DISAPPEARANCE OF REDUCED MANGANESE IN RESERVOIR TAILWATERS

Results of field studies conducted downstream of five reservoirs are used to develop an improved understanding of the disappearance of reduced manganese in tailwaters. A first-order removal formulation accurately describes the removal of dissolved manganese versus travel time downstream. Results support the theory that adsorption to the stream bottom is the primary removal mechanism. The first-order removal rate, which varied by an order of magnitude from 0.30 to 4.45 day(-1), is found to be highly correlated to the stream slope, which affects shear velocity. Using observed removal rates, a formulation based on the ratio of the stream-bottom mass-transfer velocity, which depends on bottom-shear velocity, and stream depth is developed for computing the removal-rate coefficient. The formulation also depends on substrate type (i.e., cobble or fine-grain sediments). Dissolved manganese concentrations computed with the developed mathematical model show good agreement with observations. Some questions still remain, such as whether there is an annual acclimation time for manganese removal.