Jensen Allison M, Scanlon Todd M, Riscassi Ami L
Department of Environmental Sciences, University of Virginia, 291 McCormick Road, Charlottesville, VA 22904, USA.
Environ Sci Process Impacts. 2017 Dec 13;19(12):1505-1517. doi: 10.1039/c7em00419b.
Wildfires alter forested ecosystems, which include large stores of mercury (Hg) and organic carbon, two compounds that are closely linked in vegetation, soils, and streamwater. Studies have shown that wildfires release elevated levels of mercury to the atmosphere which can be locally redeposited and leave charred organic material (vegetation and litter) on the soil surface. Both can contribute to the elevated mobilization of Hg into lakes and streams. However, no studies have conducted a detailed examination of hydrological transport of Hg following a wildfire. This study investigates the coupled transport of mercury and carbon at Twomile Run, a headwater stream located in the forested mountains of Shenandoah National Park, in the year following a low-severity wildfire. Weekly baseflow samples and bi-hourly high-flow storm samples were analyzed for dissolved and particulate mercury (Hg and Hg, respectively), dissolved organic carbon (DOC), UV absorbance at 254 nm (UV, surrogate for DOC quantity and character), and total suspended solids (TSS), and were compared with identical measurements taken from a nearby unburned watershed. For all flow conditions sampled at the burned site (which did not include the 2 months following the fire), streamwater Hg and DOC concentrations, and corresponding UV, were similar to the unburned system. TSS concentrations varied between sites but overall differences were relatively small in magnitude and likely attributable to site differences rather than fire effects. Notably, the Hg per unit of TSS at the burned site was an order of magnitude higher than the unburned site (2.66 and 0.13 ng Hg per mg TSS, respectively) for 8 months following the fire, resulting in elevated Hg concentrations for the range of flow conditions, after which there was a rapid return to non-disturbed conditions. Streamwater total Hg fluxes roughly doubled (0.55 to 1.04 μg m yr) as a consequence of the fire, indicating that in addition to changing atmospheric and terrestrial Hg cycling, fires can rapidly and significantly alter the streamwater Hg which has implication for downstream ecosystems. These findings are particularly relevant as the occurrence and severity of wildfires are expected to increase in the mid-latitudes in response to climate change.
野火会改变森林生态系统，其中包括大量的汞（Hg）和有机碳储存，这两种化合物在植被、土壤和溪水中紧密相连。研究表明，野火会向大气中释放更高水平的汞，这些汞可能会在当地重新沉积，并在土壤表面留下烧焦的有机物质（植被和枯枝落叶）。这两者都会导致汞向湖泊和溪流中的迁移增加。然而，尚无研究对野火后汞的水文传输进行详细研究。本研究调查了位于谢南多厄国家公园森林山区的源头溪流Two-mile Run在轻度野火发生后的一年中汞和碳的耦合传输情况。对每周的基流样本和每两小时一次的高流量暴雨样本进行分析，检测溶解态汞和颗粒态汞（分别为Hg和Hg）、溶解有机碳（DOC）、254nm处的紫外吸光度（UV，作为DOC数量和特性的替代指标）以及总悬浮固体（TSS），并与从附近未燃烧流域采集的相同测量值进行比较。在燃烧地点采集的所有流量条件样本（不包括火灾后的两个月）中，溪流水汞和DOC浓度以及相应的UV与未燃烧系统相似。TSS浓度在不同地点有所变化，但总体差异幅度相对较小，可能归因于地点差异而非火灾影响。值得注意的是，火灾发生后的8个月内，燃烧地点每单位TSS中的汞含量比未燃烧地点高一个数量级（分别为每毫克TSS含2.66和0.13纳克汞），导致在该流量条件范围内汞浓度升高，之后迅速恢复到未受干扰的状态。由于火灾，溪流水总汞通量大约增加了一倍（从0.55微克/平方米·年增加到1.04微克/平方米·年），这表明除了改变大气和陆地汞循环外，火灾还能迅速且显著地改变溪流水汞含量，这对下游生态系统具有重要意义。随着预计中纬度地区因气候变化野火的发生频率和严重程度将会增加，这些发现尤为重要。
