南亚热带地区水库夏季铁、锰垂直分布特征

铁(Fe)和锰(Mn)均为氧化还原敏感性元素,是水质评价的重要指标之一.为了解南亚热带地区供水水库夏季Fe、Mn的垂直分布特征,于2016年7月调查了粤东地区9座水库敞水区湖沼学变量和Fe、Mn浓度的垂直分布特征.在有明显温度分层的深水水库中Fe、Mn存在显著的梯度分布,表层水体中总Fe(TFe)、总Mn(TMn)以及溶解性Fe、Mn(DFe、DMn)的浓度显著(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)低于底层.5座深水水库底层水体中TFe、TMn平均浓度分别为0.399 mg·L~(-1)和0.422 mg·L~(-1),其中溶解态离子占70%以上,而在没有明显温度分层的浅水水库中,表层水体TFe、TMn以及DFe、DMn浓度略低于底层但无显著性差异(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),4座浅水水库底层水体中TFe、TMn平均浓度分别为0.110 mg·L~(-1)和0.089 mg·L~(-1),且主要以非溶解态存在.水体中TFe和DFe浓度与溶解氧(DO)浓度、p H值和深度呈显著相关关系(P0.05),与总磷(TP)、总氮(TN)浓度没有显著相关性.水体热分层及高的水体稳定性导致DO和p H值呈现明显的梯度分布,是导致深水水库Fe、Mn在水层间形成梯度分布的关键原因.深水水库底层水体低DO浓度和酸性条件有利于沉积物中Fe、Mn的释放,是导致Fe、Mn以溶解态存在的主要原因.同时,以红壤为主的土壤类型可能是该地区水体中Fe、Mn普遍较高的重要原因.以上结果表明,南亚热带地区夏季分层稳定的深水水库底层水体普遍存在Fe、Mn较高的现象,以深层取水的供水方式易出现Fe、Mn浓度超标,湖上层取水则可避免该问题.

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

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.