Fluctuations in discharged water quality during the drying operation of agricultural reservoirs and estimation of the pond bottom environment and benthic organisms before the drying operation.

Abstract

Pond drying, which has continued as a management method to maintain reservoir functions, improves reductive conditions of bottom sediment, and maintains the reservoir water quality thereby conserving and improving the aquatic habitats. Nevertheless, water discharged from reservoirs contains bottom sediments that cause turbidity, contributing to the downstream pollution load. Here, we monitored the runoff water quality in a reservoir that was pond dried after eight years. During the final stages of pond draining, turbidity increased, with COD temporarily exceeding 1,000 mg L^－1, TN of 200 mg L^－1, and TP of 7 mg L^－1. The total COD, TN, and TP loads were, 284.4 kg, 44.4 kg, and 1.5 kg, respectively, for the 12-hr period of increased turbidity. The NO₃-N concentration increased, before increase in turbidity, and remained high after the operation. This may be attributed to reduced reductive conditions at the pond bottom resulting in suppressed denitrification. Correspondingly, previously inhabited bivalves were not observed, possibly due to the progression of reductive conditions at the pond bottom caused due to the absence of pond drying for eight years.