Vertical Distribution Characteristics of Fe and Mn in Subtropical Reservoirs During Summer

Abstract

Iron (Fe) and manganese (Mn) are redox-sensitive elements that are both important chemical indicators of water quality. To study their vertical distribution characteristics in south-subtropical reservoirs during summer, Fe and Mn concentrations in the water column, as well as the physical and chemical conditions of the water, were investigated in the lacustrine zones of nine reservoirs in the eastern of Guangdong Province, in July 2016. These results showed that Fe and Mn exhibited a strong concentration gradient in deep reservoirs due to thermal stratification. The total Fe (TFe), total Mn (TMn), dissolved Fe (DFe), and dissolved Mn (DMn) concentrations in the surface water are significantly lower than in these in bottom zones (TFe, F=6.708, P=0.032; TMn, F=9.720, P=0.014; DFe, F=8.129, P=0.029; DMn, F=11.125, P=0.016). The average concentrations of TFe and TMn are 0.399 mg·L-1 and 0.422 mg·L-1, respectively at the bottom five of the deep reservoirs, and the dissolved ions accounted for more than 70% of this concentration. In shallow reservoirs without thermal stratification, TFe, TMn, DFe and DMn concentrations in surface water are lower than those measured at bottom of the reservoir, no significant difference was detected (TFe, F=0.135, P=0.726; TMn, F=0, P=1; DFe, F=0.006, P=0.943; DMn, F=0.007, P=0.936). The average concentrations of TFe and TMn are 0.110 mg·L-1 and 0.089 mg·L-1, respectively at the bottom of the four shallow reservoirs, and they were present predominantly in particulate form. The concentrations of TFe and DFe correlated significantly (P<0.05) with the dissolved oxygen (DO) concentration, pH and depth, but did not significantly correlate with TN or TP concentration in the deep reservoirs. The gradient distributions of DO and pH levels is critical to the gradient distribution of Fe and Mn in deep reservoirs because of water stratification. Low DO concentrations and acidic conditions in these areas contributed to the release of Fe and Mn from sediment and result in high dissociation of these elements in the bottom of deep reservoirs. Simultaneously, high concentrations of Fe and Mn at the bottom of reservoirs may also attribute to the characteristic red soil of this region. The results from this study imply that the high concentrations of Fe and Mn common in the bottom of deep reservoirs is associated with the stable stratification of these reservoirs in the summer. The concentrations of Fe and Mn may be exceed water standards when deep water is taken for public water provision. The problem may be avoided with surface water supplying.