School of Engineering, University of California Merced, Merced, CA 95343, USA; Civil and Environmental Engineering Department, Washington State University, Pullman, WA 99164, USA.
Civil and Environmental Engineering Department, Washington State University, Pullman, WA 99164, USA.
Water Res. 2017 Mar 1;110:288-296. doi: 10.1016/j.watres.2016.12.022. Epub 2016 Dec 18.
Mercury bioaccumulation in aquatic biota poses a widespread threat to human and environmental health. Methylmercury (MeHg), the toxic form of mercury, tends to build up under anaerobic conditions in the profundal zones of lakes. In this study we performed a two-year assessment of spatial and temporal patterns of dissolved oxygen, nitrate, MeHg, manganese (Mn) and iron (Fe) in Occoquan Reservoir, a large run-of-the-river drinking water reservoir in Virginia, USA. A tributary to the reservoir receives input of nitrate-rich tertiary-treated wastewater that enhances the oxidant capacity of bottom water. Multiple lines of evidence supported the hypothesis that the presences of nitrate and/or oxygen in bottom water correlated with low MeHg in bottom water. Bottom water MeHg was significantly lower in a nitrate-rich tributary (annual mean of 0.05 ng/L in both 2012 and 2013) compared to a nitrate-poor tributary (annual mean of 0.58 ng/L in 2012 and 0.21 ng/L in 2013). The presence of nitrate and oxygen in bottom water corresponded with significantly lower bottom water MeHg at an upstream station in the main reservoir (0.05 versus 0.11 ng/L in 2013). In 2012 the reservoir exhibited a longitudinal gradient with nitrate and oxygen decreasing and MeHg and Mn increasing downstream. In both study years, there was a clear threshold of oxygen equivalent (3-5 mg/L), a metric that combines the oxidant capacity of nitrate and oxygen, above which MeHg (<0.05 ng/L), Mn (<0.3 mg/L) and Fe (<0.5 mg/L) were low. Results indicated that the addition of nitrate-rich tertiary-treated wastewater to the bottom of anaerobic reservoirs can reduce MeHg concentrations, and potentially decrease mercury bioaccumulation, while increasing the safe water yield for potable use.
水体生物体内的汞生物积累对人类和环境健康构成了广泛威胁。在湖泊深部缺氧条件下，汞的有毒形式——甲基汞（MeHg）易于积累。本研究对美国弗吉尼亚州大型河流型饮用水水库 Occoquan 水库溶解氧、硝酸盐、MeHg、锰（Mn）和铁（Fe）的时空模式进行了为期两年的评估。水库的一条支流接收富含硝酸盐的三级处理废水，增强了底层水的氧化剂容量。多种证据支持这样的假设，即底层水中硝酸盐和/或氧气的存在与底层水中低 MeHg 相关。在富含硝酸盐的支流中（2012 年和 2013 年的年平均值均为 0.05 ng/L），底层水中 MeHg 的含量明显低于硝酸盐含量低的支流（2012 年的年平均值为 0.58 ng/L，2013 年为 0.21 ng/L）。在水库的主要支流上游，底层水中硝酸盐和氧气的存在与底层水中 MeHg 的含量显著降低相对应（2013 年分别为 0.05 与 0.11 ng/L）。2012 年，水库表现出硝酸盐和氧气向下游逐渐减少，MeHg 和 Mn 逐渐增加的纵向梯度。在这两个研究年份中，存在一个明确的氧当量（3-5 mg/L）阈值，该指标综合了硝酸盐和氧气的氧化剂容量，超过该阈值，MeHg（<0.05 ng/L）、Mn（<0.3 mg/L）和 Fe（<0.5 mg/L）含量就会较低。结果表明，将富含硝酸盐的三级处理废水添加到底部缺氧水库中可以降低 MeHg 浓度，从而减少汞的生物积累，同时增加可用于饮用的安全水量。
