A decrease in pH downstream from the hydroelectric dam in relation to the carbon biogeochemical cycle

To better understand the decreases observed in pH levels downstream from the associated hydroelectric dam, the impounded Wujiang River in Southwest China was investigated. Study results indicate that the average pH decrease from upstream to downstream of the hydroelectric dam could be up to 0.47 units, and pH differences were particularly apparent during water thermal stratification. Notably, pH was controlled by [CO₂]/[CO₃ ²⁻] ratios in this impounded river. The decrease in [CO₂]/[CO₃ ²⁻] ratios and associated increases in pH at the surface of the reservoirs were mainly due to the prevalence of photosynthesis, while the reverse phenomenon was observed at the bottom due to respiration. The evidence from δ¹³C_DIC clearly demonstrated these processes. The increase in phytoplankton biomass enhanced this pH decrease, while dissolved organic carbon had limited impacts on the pH variation. The decrease in pH downstream from the hydroelectric dam resulted from the development of pH stratification in the water profile and the dam operations that release bottom waters for hydropower generation. Consequently, the cascade in hydropower development could increase the risk of river acidification.