Seasonal variations in sulfur isotopic composition of dissolved SO4 2− in the Aha Lake, Guiyang and their implications

Abstract

The Aha Lake is a seasonal anoxic water system in the southwest of Guiyang City, Guizhou Province, China. Seasonal variations in SO 4 2-concentrations and their isotopic compositions in lake water as well as in the tributaries were investigated in this study. The results showed that sulfate concentrations in river water range from 0.94 to 6.52 mmol/L and their δ 34 S values range from -14.9‰ and 0.9‰, while lake water has sulfate concentrations ranging from 1.91 to 2.79 mmol/L, and δ 34 S values from -9.8‰ to -5.9‰. It is suggested that coal mining drainage is the major source of SO 4 2-in the Aha Lake. Rainfall, sewage discharge, sulfide oxidation and gypsum dissolution have made only limited contributions. Different depth-dependent distributions of dissolved SO 4 2-and δ 34 S were developed for both DB and LJK in summer and winter. Due to water overturn, δ 34 S values display homogenous vertical distributions in winter and spring. While in summer and autumn, significant positive shifts of δ 34 S were clearly observed in epilimnion and bottom strata as a result of water stratification. High δ 34 S values in epilimnion may result from the retention of rainwater during water stratification. Dissimilatory sulfate reduction by bacteria was thought to be responsible for the increase of δ 34 S value in hypolimnion.

Sulfide oxidation affects the preservation of sulfur isotope signals

“…The isotopic composition of sulfate above 7 m depth was similar to the isotopic composition of sulfate in rainwater ( Fig. 2B; δ 34 S rainwater = -7‰; Song et al, 2011). The constant concentrations and δ 34 S value of -10.5‰ ± 2.1‰ between 7 and 20 m water depth are consistent with coal-derived sulfate, the main source of sulfate to the lake (δ 34 S coal = -12‰; Song et al, 2011).…”

“…2B; δ 34 S rainwater = -7‰; Song et al, 2011). The constant concentrations and δ 34 S value of -10.5‰ ± 2.1‰ between 7 and 20 m water depth are consistent with coal-derived sulfate, the main source of sulfate to the lake (δ 34 S coal = -12‰; Song et al, 2011). An increase in δ 34 S sulfate and the presence of sulfide below 22 m depth indicates that microbial sulfate reduction was active within the water column.…”

A Particular River-Whiting Phenomenon Caused by Discharge of Hypolimnetic Water from a Stratified Reservoir

“…Previous investigation showed that seasonal thermal stratification existed in Aha Reservoir normally from June to October [ 10 , 13 ]. As shown in Fig 3 , there is thermal stratification in September and no stratification in March.…”

“…An anoxic hypolimnion is developed as a result of a stratification regime and causes notable changes in the water quality of the hypolimnion. It is well known that water below the thermocline is normally depleted of dissolved oxygen, and contains high concentrations of SO 4 2- and hydrogen sulfide produced during the process of anaerobic decomposition, and elevated levels of metals such as iron and manganese brought into solution from the bottom sediments as a result of the strong reducing conditions [ 1 , 8 , 10 , 19 ]. The water discharged from the reservoir hypolimnion to the culvert and discharge pipe is markedly anoxic and enriched in sulfur during June-October (Tables 1 and 2 ).…”

“…Persistent input of acid mining drainage has resulted in a series of environmental accidents in Aha Reservoir since the end of the 1990s. The reservoir water turned black and fish kills occurred after sudden mixing [ 8 , 10 ]. A large number of studies have been carried out to investigate the biogeochemical cycling of heavy metals and sulfur within the reservoir [ 8 – 15 ], but less attention was paid to the released water and its effects.…”