Water-quality impact assessment for hydropower

A methodology to assess the impact of a hydropower facility on downstream water quality is described. Negative impacts can result from the substitution of discharges aerated over a spillway with minimally aerated turbine discharges that are often withdrawn from lower reservoir levels, where dissolved oxygen (DO) is typically low. Three case studies illustrate the proposed method and problems that can be encountered. Historic data are used to establish the probability of low-dissolved-oxygen occurrences. Synoptic surveys, combined with downstream monitoring, give an overall picture of the water-quality dynamics in the river and the reservoir. Spillway aeration is determined through measurements and adjusted for temperature. Theoretical computations of selective withdrawal are sensitive to boundary conditions, such as the location of the outlet relative to the reservoir bottom, but withdrawal from the different layers is estimated from measured upstream and downstream temperatures and dissolved-oxygen profiles. Based on field measurements, the downstream water quality under hydropower operation is predicted. Improving selective withdrawal characteristics or diverting part of the flow over the spillway provided cost-effective mitigation solutions for small hydropower facilities (less than 15 MW) because of the low capital investment required.