Vertical Mixing of Desalination Reject Brine that Accumulates in Local Depressions in a Tidal Environment

Environmental impacts from coastal desalination discharges are usually studied in the near field (region within ~100 m of the outfall where dilution is caused mainly by discharge momentum) and/or the far field (region 10s of km or more from the outfall where mixing is affected mainly by large scale circulation). Here we consider an “intermediate field” scenario in which brine accumulates in local depressions within ~ 1 km of the outfall, forming “puddles” on an otherwise flat bathymetry. Such puddles are hard to resolve with numerical far field models, but can impact benthic marine life, e.g., by allowing contaminants to accumulate or respiration of organic matter in the sediments to deplete dissolved oxygen.

We model the puddle and the overlying water as a two layer system, with a stationary, dense, bottom layer containing diluted brine, and a top layer comprising ambient seawater that moves with the tidal current. Mass exchange between the two layers is modeled using formulations from the literature expressing the rate of entrainment from the bottom to the top layer as a function of Richardson number.

Using idealized bathymetry, we investigate the evolution of the brine puddle as a function of the discharge flow rate and density difference relative to ambient, the near field dilution, and periodic tidal velocities. We study the periodic variation of the puddle volume and area, and determine the criteria for which the puddle might completely erode during a portion of the tidal cycle. Additionally we bookkeep the fate of passive tracers (salt, metals, dissolved oxygen) within the puddle, assuming they obey simple (zero or first order) kinetics.

Finally we discuss the implications on benthic organisms living in the puddle and their exposures to prolonged periods of excess salinity, heavy metals, and lowered dissolved oxygen levels.