Sediment oxygen demand enhancement at the sediment/water interface of a shallow pond induced by pressure fluctuations from raindrop impact at the water surface

Abstract

The increase of Sediment Oxygen Demand (SOD) by raindrop impact at the water surface of shallow ponds was investigated. A model was built to simulate pressure fluctuations at the sediment/water interface driven by raindrops striking the water surface of the pond continuously at constant intervals of a few seconds when the water depth h in the pond is on the order of 5 to 100 cm. Simulated pressure fluctuations at the sediment/water interface were translated into bed shear velocity describing the intensity of near – bed turbulence. Modeled shear velocity increases as the depth of water in the pond increases. Also, shear velocity becomes larger with increased terminal velocity of raindrops which was varied from 400 to 600 cms−1. The DO flux from the water column to the sediment surface, i.e. SOD, was calculated using the obtained shear velocity. Modeled SOD can be solely dependent on the microbial oxygen uptake rate (μ) in the sediment when μ < 500 mg L−1 d−1. While, SOD depends on the shear velocity, which in turn depends on the water depth (h) in the pond when h < 20 cm, the microbial oxygen uptake rate (μ) becomes solely limiting and SOD is no longer water – side controlled when h > 20 cm and μ > 1000 mg L−1 d−1. This simulation result suggests that raindrops impinging on the water surface of the pond can increase the SOD as can increase in height of bed roughness and/or the velocity of water flowing over the sediment surface as a turbulent flow. SOD induced by raindrop impact at the water surface can be equal to that for turbulent flow, or even larger depending on the water depth and microbial oxygen uptake rate.