PNNL

Abstract

Freshwater salinization is a common yet underappreciated environmental problem in rivers, yet how this process impactsthe transport of dissolved oxygen (DO) in the hyporheic zone remains unknown. Using flume experiments and numerical simulations, we have demonstrated that salinity fluctuations can alter hyporheic flow and accelerate DO transport in the streambed. The morphological dynamics of the DO transition zone are controlled by the dimensionless number Da·Ra-2, a measure of the rate of aerobic respiration to free convection. At low Da·Ra-2, the oxic front breaks up into fingers which subsequently grow, and leads to shrinking of the anaerobic zone in the underlying sediment. As Da·Ra-2 increases, the growth rate of the instability decreases, and the DO plume fingers are suppressed and delayed compared with the saltwater-freshwater interface. These results indicate that the freshwater salinization syndrome can have significant impacts on the functioning of ecosystems unless regulated and managed effectively.