ALBERT

Description: Soil desiccation was recently field tested as a potential vadose zone remediation technology. Desiccation removes water from the vadose zone and significantly decreases the aqueous-phase permeability of the desiccated zone, thereby decreasing movement of moisture and contaminants. The two- and three-dimensional distribution of moisture content reduction with time provides valuable information for desiccation operations and for determining when treatment goals have been reached. This type of information can be obtained through the use of geophysical methods. Neutron moisture logging, cross-hole electrical resistivity tomography, and cross-hole ground-penetrating radar approaches were evaluated with respect to their ability to provide effective spatial and temporal monitoring of desiccation during a treatability study conducted in the vadose zone of the USDOE Hanford site in the state of Washington.

Description: Water saturation is an important indicator of contaminant distribution and plays a governing role in contaminant transport within the vadose zone. Understanding the water saturation distribution is critical for both remediation and contaminant flux monitoring in unsaturated environments. In this work, we propose and demonstrate a method of remotely determining water saturation levels using gas phase partitioning tracers and time-lapse bulk electrical conductivity measurements. The theoretical development includes the partitioning chemistry for the tracers we demonstrate (ammonia and carbon dioxide), as well as a review of the petrophysical relationship governing how these tracers influence bulk conductivity. We also investigate methods of utilizing secondary information provided by electrical conductivity breakthrough magnitudes induced by the tracers. We test the method on clean, well characterized, intermediate-scale sand columns under controlled conditions. Results demonstrate the capability to accurately monitor gas breakthrough curves along the length of the column according to the corresponding electrical conductivity response, and to adequately determine partitioning coefficients, leading to accurate water saturation estimates. This work is motivated by the need to develop effective characterization and monitoring techniques for contaminated deep vadose zone environments, and provides a proof-of-concept toward uniquely characterizing and monitoring water saturation levels at the field scale and in three-dimensions using electrical resistivity tomography.

Description: Soil aggregates are an important structural component of the soil matrix that could harbor Escherichia coli and provide an environment for its survival and water flow reentering. Knowledge of the exact pore locations within soil aggregates obtained using X-ray computed microtomography opens new opportunities for understanding microorganism movement within the soil matrix. The first objective of this study was to assess E. coli spatial distribution within soil macroaggregates and its potential for leaving the aggregates with the saturated water flow. The second objective was to study the relationships between the distribution and movement of E. coli within soil aggregates and the aggregates’ internal pore structures. We studied aggregates from the top (A) horizon of conventionally tilled (CT) and no-till (NT) corn–soybean–wheat rotations and native succession vegetation (NS) treatments at NSF Long-Term Ecological Research site, southwest Michigan. The results confirmed that E. coli movement in soil aggregates was mainly driven by water flow via capillary forces. E. coli r edistribution was most pronounced in CT aggregates, followed by NT, and was almost negligible in NS aggregates. Pore characteristics that positively contributed to E. coli redistribution through the aggregates were the maximum flow in the aggregate centers and the ratio of the maximum flow and pore tortuosity. The E. coli retention in the aggregate’s centers was positively related to porosity, percent of medium and large pores, and pore tortuosity.

Description: Contaminants in vadose zone environments pose a long-term source and threat to groundwater resources, human health, and the environment. A number of technical, regulatory, and policy challenges and opportunities are associated with contamination in vadose zone environments, particularly in remediation. In this special section, 12 papers present novel approaches to characterize, monitor, remediate, and predict the transport and fate of contaminants in vadose zone environments.

Description: Low water content sediments were treated with NH 3 gas to evaluate changes in U mobility as a potential field remediation method for vadose zone contamination. Injection of NH 3 gas created high dissolved NH 3 concentrations that followed equilibrium behavior. High NH 3 concentration led to an increase in pH from 8.0 to 11 to 13, depending on the water content and NH 3 concentration. The increase in pore water pH resulted in a large increase in pore water cations and anions from mineral-phase dissolution. Minerals showing the greatest dissolution included montmorillonite, muscovite, and kaolinite. Pore water ion concentrations then decreased with time. Simulations based on initial pore water ion concentrations indicated that quartz, chrysotile, calcite, diaspore, hematite, and Na-boltwoodite (hydrous U silicate) should precipitate. Electrical resistivity and induced polarization tomography (ERT/IP) was able to nonintrusively track these NH 3 partitioning, dissolution, and precipitations processes through changes in conductivity and chargeability. Ammonia treatment significantly decreases the amount of U present as adsorbed and aqueous species in field-contaminated sediments. In contrast, sediments containing a large fraction of U associated with carbonates generally showed little change. Uranium leaching from sediments containing high Na-boltwoodite decreased significantly by NH 3 treatment, but x-ray absorption near-edge structure/extended x-ray absorption fine structure showed no change in the Na-boltwoodite concentration. Therefore, NH 3 treatment of contaminated sediment acts to decrease the highly mobile aqueous and adsorbed U by incorporation into precipitates and appears to decrease mobility of some existing U precipitates (Na-boltwoodite) as a result of mineral coating.