Rates of drainage-water evaporite salt dissolution in water

Abstract

Solar evaporators are an important component of systems for managing salts in agricultural drainage water. In California, solar evaporators have been integrated into on-farm drainage management (IFDM) systems as the final stage concentration process following sequential use of drainage water on increasingly salt tolerant crops. In most cases, salts are accumulated in the evaporator basins over a number of years. During months of high precipitation, dry salts redissolve in collected rainwater. Rates of dissolution are important to the design and management of the evaporator to avoid excessive salt concentrations in water displaced from the basin during storm events. The rate at which a solid, dehydrated evaporite salt redissolves in water was simulated and also tested experimentally to validate model predictions. Both model and experimental results show that rain water collecting in an evaporator basin can become saturated within 60 min at 24°C and within 90 min at 10°C with a 0.01 m water depth over solid salt. The thickness of the diffusion layer above the solid surface was calculated in the range of 8.3 × 10 − 5 and 1.25 × 10 − 4 m. For a mostly sodium sulfate evaporite salt, pH increased up to 9 from 5.5 in 5 min, and then decreased slowly converging to about 8.7 due to likely absorption of CO 2 from the atmosphere.