Collaborative Research: The Advance of Colloid Mobilization and Transport Fronts

小行星9418172 地下水中放射性核素、金属和非极性有机化合物的迁移受到固定含水层沉积物吸附的严重限制。 然而，在胶体的存在下，这些低溶解度的污染物迁移的距离远大于那些只考虑溶解和吸附，固定相之间的污染物的分布模型预测。 胶体的存在要求在迁移模型中包括污染物的吸附的、移动的相。 这一要求将我们的注意力转移到含水层中胶体的移动、运输和沉积上。 胶体的形成可能是由于含水层中的化学或物理扰动造成的原地沉淀或流动。 当化学扰动增加胶体和颗粒表面之间的排斥力时，胶体被动员并随地下水运输。 如果引起扰动的溶质的输运相对于地下水被延迟，则被动员的胶体将最终通过溶质前沿并遇到尚未受到溶质或胶体动员剂影响的沉积物。 在这些条件下，我们预计胶体将重新沉积在晶粒上，并将保持在那里，直到溶质“赶上”。“我们假设，由化学扰动动员的胶体的运输永远不会超过胶体动员剂的运输。 为了验证这一假设，我们建议（1）开发一个模型，同时考虑胶体和胶体动员剂的运输和（2）进行一系列小规模，中规模和现场实验模拟和测试胶体动员和运输。 该模型将由加州大学洛杉矶分校的研究人员开发和严格测试。 该模型将考虑胶体沉积和释放、胶体和颗粒表面的微观和宏观电荷异质性、保留的胶体对胶体沉积和释放的影响、溶质吸附和解吸以及“宏观”异质性（即，位于含水层沉积物中）。 它将制定模型胶体和溶质运移在一个和两个维度的实验室实验，它将扩展到三维的现场实验。 小规模和中等规模的实验将在科罗拉多大学的水资源实验室进行。 实验中使用的材料将包括赤铁矿和高岭石胶体、石英和羟基氧化铁涂层的石英多孔介质以及磷酸盐十二烷酸（表面活性剂），并从现场分离NOM作为胶体动员剂。 将进行小规模柱实验，以确定中间规模和现场实验的参数。 中等规模的实验将在一个长10米、高2米、宽5厘米的二维槽中进行，槽中填充有均匀和非均匀（分层）多孔介质。 水槽实验将直接检验胶体运输与胶体动员剂运输有关的假设。 实地实验将在南卡罗来纳州乔治敦的Barouch森林科学研究所进行。BFSI现场的表层含水层主要由石英砂、羟基氧化铁和层状非均质组成。 提出了一个现场实验，将检查沉积和动员的合成高岭石碰撞标记的稳定同位素（氘或18 O）或钛作为硅的同晶替代品。 分别注入将测试的效果，十二烷酸和NOM丰富的水从附近的池塘作为胶体动员剂在好氧和亚氧部分的含水层。

9418172 Elimelech The transport of radionuclides, metals, and non-polar organic compounds in groundwater is severely restricted by adsorption to immobile aquifer sediments. In the presence of colloids, however, these low-solubility contaminants migrate over distances much greater than those predicted by models that consider only the distribution of the contaminant between the dissolved and the adsorbed, immobile phase. The presence of colloids requires inclusion of an adsorbed, mobile phase of the contaminant in transport models. This requirement has shifted our attention to the mobilization, transport, and deposition colloids in aquifers. Colloid formation may occur by in situ precipitation or mobilization caused by chemical or physical perturbations in the aquifer. When a chemical perturbation increases the repulsive forces between the colloid and grain surfaces, colloids are mobilized and transported with the groundwater. If the transport of the solute causing that perturbation is retarded relative to the groundwater, the mobilized colloid will eventually pass the solute front and encounter sediments that have not yet been affected by the solute, or colloid-mobilizing agent. Under these conditions, we expect that the colloids will be re-deposited on grains and will remain there until the solute "catches up." We hypothesize that the transport of colloids mobilized by a chemical perturbation will never exceed the transport of the colloid-mobilizing agent. To test this hypothesis, we propose to (1) develop a model that will simultaneously account for the transport of the colloids and the colloid-mobilizing agent and (2) conduct a series of small-scale, intermediate-scale, and field experiments simulating and testing colloid mobilization and transport. The model will be developed and rigorously tested by the UCLA researchers. The model will account for colloid deposition and release, microscopic and macroscopic charge heterogeneity of colloid and grain surfaces, the effect of retained colloids on the deposition and release of colloids, solute adsorption and desorption, and "megascopic" heterogeneities (i.e., laying in aquifer sediments). It will be formulated to model colloid and solute transport in one and two dimensions for the laboratory experiments and it will be extended to three dimensions for the field experiment. The small-and intermediate-scale experiments will be conducted at the University of Colorado's Water Resources laboratory. The materials used in the experiments will include hematite and kaolinite colloids, quartz and ferric oxyhydroxide-coated quartz porous media, and phosphate dodecanoic acid (a surfactant), and isolate NOM from the field site as colloid-mobilizing agents. Small-scale column experiments will be conducted to identify parameters for the intermediate-scale and field experiments. The intermediate-scale experiments will be conducted in a two-dimensional tank of 10 m length 2 m height, and 5 cm width filled with homogeneous and heterogeneous (layered) porous media. The tank experiments will directly test the hypothesis relating colloid transport to the transport of the colloid-mobilizing agent. The field experiment will be conducted at the Barouch Forest Science Institute site in Georgetown, S.C. The surficial aquifer at the BFSI site is composed primarily of quartz sand, ferric oxyhydroxides, and layered heterogeneity. A field experiment is proposed that will examine the deposition and mobilization of synthesied kaolinite collids labeled with a stable isotope (deuterium or 18O) or titanium as an isomorphous substitute for silicon. Separate injections will test the effects of dodecanoic acid and NOM-rich water from a nearby pond as the colloid-mobilizing agent in both oxic and suboxic portions of the aquifer.