Lawrence Berkeley National Laboratory, Earth Sciences Division, 1 Cyclotron Road, MS 90-1116, Berkeley, CA 94720, United States.
J Contam Hydrol. 2010 Mar 1;112(1-4):45-63. doi: 10.1016/j.jconhyd.2009.10.006. Epub 2009 Oct 31.
Electron donor amendment for bioremediation often results in precipitation of secondary minerals and the growth of biomass, both of which can potentially change flow paths and the efficacy of bioremediation. Quantitative estimation of precipitate and biomass distribution has remained challenging, partly due to the intrinsic heterogeneities of natural porous media and the scarcity of field data. In this work, we examine the effects of physical and geochemical heterogeneities on the spatial distributions of mineral precipitates and biomass accumulated during a biostimulation field experiment near Rifle, Colorado. Field bromide breakthrough data were used to infer a heterogeneous distribution of hydraulic conductivity through inverse transport modeling, while the solid phase Fe(III) content was determined by assuming a negative correlation with hydraulic conductivity. Validated by field aqueous geochemical data, reactive transport modeling was used to explicitly keep track of the growth of the biomass and to estimate the spatial distribution of precipitates and biomass. The results show that the maximum mineral precipitation and biomass accumulation occurs in the vicinity of the injection wells, occupying up to 5.4vol.% of the pore space, and is dominated by reaction products of sulfate reduction. Accumulation near the injection wells is not strongly affected by heterogeneities present in the system due to the ubiquitous presence of sulfate in the groundwater. However, accumulation in the down-gradient regions is dominated by the iron-reducing reaction products, whose spatial patterns are strongly controlled by both physical and geochemical heterogeneities. Heterogeneities can lead to localized large accumulation of mineral precipitates and biomass, increasing the possibility of pore clogging. Although ignoring the heterogeneities of the system can lead to adequate prediction of the average behavior of sulfate-reducing related products, it can also lead to an overestimation of the overall accumulation of iron-reducing bacteria, as well as the rate and extent of iron reduction. Surprisingly, the model predicts that the total amount of uranium being reduced in the heterogeneous 2D system was similar to that in the 1D homogeneous system, suggesting that the overall uranium bioremediation efficacy may not be significantly affected by the heterogeneities of Fe(III) content in the down-gradient regions. Rather, the characteristics close to the vicinity of the injection wells might be crucial in determining the overall efficacy of uranium bioremediation. These findings have important implications not only for uranium bioremediation at the Rifle site and for bioremediation of other redox sensitive contaminants at sites with similar characteristics, but also for the development of optimal amendment delivery strategies in other settings.
电子供体修复通常会导致次生矿物的沉淀和生物量的生长，这两者都可能改变流动路径和修复效果。沉淀和生物量分布的定量估计一直具有挑战性，部分原因是天然多孔介质的固有非均质性和现场数据的稀缺性。在这项工作中，我们研究了物理和地球化学非均质性对科罗拉多州里弗尔附近生物刺激现场实验中矿物沉淀和生物量积累的空间分布的影响。利用现场溴化物突破数据通过反向输运建模推断出渗透系数的非均质性分布，而固相 Fe(III) 含量则通过假设与渗透系数呈负相关来确定。通过现场水地球化学数据验证，采用反应性传输建模来明确跟踪生物量的生长，并估计沉淀和生物量的空间分布。结果表明，最大的矿物沉淀和生物量积累发生在注入井附近，占据了高达 5.4vol.%的孔隙空间，主要由硫酸盐还原反应产物组成。由于地下水中普遍存在硫酸盐，注入井附近的积累不受系统中存在的非均质性的强烈影响。然而，在下游区域的积累主要由铁还原反应产物控制，其空间模式受物理和地球化学非均质性的强烈控制。非均质性可能导致矿物沉淀和生物量的局部大量积累，从而增加孔隙堵塞的可能性。尽管忽略系统的非均质性可以对硫酸盐还原相关产物的平均行为进行充分预测，但也可能导致对铁还原细菌的总体积累、以及铁还原的速率和程度的高估。令人惊讶的是，模型预测在非均相 2D 系统中被还原的铀总量与在均质 1D 系统中相似，这表明铀生物修复的整体效果可能不会受到下游区域 Fe(III) 含量非均质性的显著影响。相反，靠近注入井附近的特征可能是决定铀生物修复整体效果的关键。这些发现不仅对里弗尔现场的铀生物修复以及其他具有类似特征的地点的其他氧化还原敏感污染物的生物修复具有重要意义，而且对其他地点的最佳添加剂输送策略的发展也具有重要意义。
