Risk of sulfide formation from acid sulfate soil in rice/shrimp farming systems in Vietnam

Rice/shrimp farming is practiced by farmers in the peri-coastal area of the Vietnamese Mekong Delta where saline intrusion in the dry season creates conditions suitable for shrimp production. Shrimp require ditches (approx. 1.4 m deep) to be constructed around the rice fields to create habitat suitable for shrimp survival. As acid sulfate soils (ASS) are common in the region, construction of shrimp ditches often leads to the exposure of pyritic material in the excavated soil that forms earthen banks around the rice field. Hence the products of pyrite oxidation can be washed into the ditches where organic wastes from shrimp production, known as sludge, also accumulate. The decomposition of sludge in ditches creates conditions favouring reduction and the formation of sulfide which is toxic to shrimp (Goppakumar and Kuttyamma, 1996). The magnitude of sulfide produced and the potential toxicity to shrimp may then depend on the presence of sludge and the severity of the acid sulfate soil in the ditch or bank.
An incubation study was conducted to determine the potential hydrogen sulfide production from bank soil and ditch sludge taken from two farms of contrasting severity of acid sulfate soil (high and low total potential acidity). A glasshouse column study, replicating field conditions, was conducted to determine the actual sulfide concentration produced in water over ditch soil, sludge or sludge with eroded bank soil taken from the same two farms.