Department of Geosciences, University of Wisconsin-Milwaukee, 3209 N. Maryland Ave, Milwaukee, WI 53211, United States.
RSI EnTech, LLC, Contractor to the U.S. Department of Energy Office of Legacy Management, 2597 Legacy Way, Grand Junction, CO 81503, United States.
J Contam Hydrol. 2024 Jul;265:104391. doi: 10.1016/j.jconhyd.2024.104391. Epub 2024 Jun 24.
Natural river flooding events can mobilize contaminants from the vadose zone and lead to increased concentrations in groundwater. Characterizing the mass and transport mechanisms of contaminants released from the vadose zone to groundwater during these recharge events is particularly challenging. Therefore, conducting highly-controlled in-situ experiments that simulate natural flooding events can help increase the knowledge of where contaminants can be stored and how they can move between hydrological compartments. This study specifically targets uranium pollution, which is accompanied by high sulfate levels in the vadose zone and groundwater. Two novel experimental river flooding events were conducted that utilized added non-reactive halides (bromide and iodide) and 2,6-difluorobenzoate tracers. In both experiments, about 8 m of traced water from a nearby contaminant-poor river was flooded in a 3-m diameter basin and infiltrated through the vadose zone and into a contaminant-rich unconfined aquifer for an average of 10 days. The aquifer contained 13 temporary wells that were monitored for solute concentration for up to 40 days. The groundwater analysis was conducted for changes in contaminant mass using the Theissen polygon method and for transport mechanisms using temporal moments. The results indicated an increase in uranium (21 and 24%), and sulfate (24 and 25%) contaminant mass transport to groundwater from the vadose zone during both experiments. These findings confirmed that the vadose zone can store and release substantial amounts of contaminants to groundwater during flooding events. Additionally, contaminants were detected earlier than the added tracers, along with higher concentrations. These results suggested that contaminant-rich pore water in the vadose zone was transported ahead of the traced flood waters and into groundwater. During the first flooding event, elevated concentrations of contaminants were sustained, and that chloride behaved similarly. The findings implied that contaminant- and chloride-rich evaporites in the vadose zone were dissolved during the first flooding event. For the second flooding event, the data suggested that the contaminant-rich evaporites continued to dissolve whereas chloride-rich evaporites were previously flushed. Overall, these findings indicated that contaminant-rich pore water and evaporites in the vadose zone can play a significant role in contaminant transport during flooding events.
自然河流洪水事件可以将污染物从包气带中动员起来，并导致地下水浓度增加。描述这些补给事件中包气带释放到地下水中的污染物的质量和迁移机制具有特别的挑战性。因此，进行高度控制的原位实验，模拟自然洪水事件，可以帮助增加对污染物可以储存的位置以及它们如何在水文隔室之间移动的了解。本研究特别针对铀污染，这种污染伴随着包气带和地下水中的高硫酸盐水平。进行了两次新的实验性河流洪水事件，利用添加的非反应性卤化物（溴化物和碘化物）和 2,6-二氟苯甲酸示踪剂。在这两个实验中，从附近污染较少的河流中引入了约 8 米的示踪水，在一个 3 米直径的盆地中泛滥，并渗透过包气带进入富含污染物的无压含水层，平均持续 10 天。含水层中含有 13 个临时井，监测了长达 40 天的溶质浓度。地下水分析使用 Theissen 多边形法进行污染物质量变化，使用时间矩进行迁移机制分析。结果表明，在两次实验中，铀（21%和 24%）和硫酸盐（24%和 25%）污染物从包气带向地下水的质量迁移增加。这些发现证实，在洪水事件期间，包气带可以储存并向地下水释放大量污染物。此外，在示踪洪水之前，检测到了更早的污染物，同时浓度也更高。这些结果表明，富含有污染物的包气带孔隙水在示踪洪水之前被输送到地下水。在第一次洪水事件中，污染物的浓度持续升高，氯的行为也类似。结果表明，包气带中富含有污染物和氯化物的蒸发岩在第一次洪水事件中溶解。对于第二次洪水事件，数据表明，富含有污染物的蒸发岩继续溶解，而富含有氯化物的蒸发岩之前已经被冲洗掉。总的来说，这些发现表明，包气带中富含有污染物的孔隙水和蒸发岩在洪水事件期间可以在污染物迁移中发挥重要作用。
