Gerilynn R. Moline

Preferential flow path development and its influence on long-term PRB performance: column study

The operating life of an Fe(0)-based permeable reactive barrier (PRB) is limited due to chemical reactions of Fe(0) in groundwater. The relative contributions from mineral precipitation, gas production, and microbial activity to the degradation of PRB performance have been uncertain. In this controlled field study, nitrate-rich, site groundwater was treated by Fe(0) in large-volume, flow-through columns to monitor the changes in chemical and hydraulic parameters over time. Tracer tests showed a close relationship between hydraulic residence time and pH measurements. The ionic profiles suggest that mineral precipitation and accumulation is the primary mechanism for pore clogging around the inlet of the column. Accumulated N(2) gas generated by biotic processes also affected the hydraulics although the effects were secondary to that of mineral precipitation. Quantitative estimates indicate a porosity reduction of up to 45.3% near the column inlet over 72 days of operation under accelerated flow conditions. According to this study, preferential flow through a PRB at a site with similar groundwater chemistry should be detected over approximately 1 year of operation. During the early operation of a PRB, pH is a key indicator for monitoring the change in hydraulic residence time resulting from heterogeneity development. If the surrounding native material is more conductive than the clogged Fe-media, groundwater bypass may render the PRB ineffective for treating contaminated groundwater.

Predicting the precipitation of mineral phases in permeable reactive barriers

The working lifetime of permeable reactive barriers (PRBs) using Fe0 as the reactive media is limited by precipitation of secondary minerals, due to reaction of groundwater with Fe0. Since PRBs are emplaced at sites with widely differing groundwater chemistry, the suite of minerals that precipitate, as well as the rate of their formation, can vary widely. Using plausible phases obtained from field PRBs, the study shows that chemical equilibrium modeling can correctly predict the amounts of precipitates formed, based on the thermodynamic properties of Fe0 and groundwater constituents. These predictions were compared to the results from the solid phase analysis from a field column experiment and from a field-installed PRB at Y-12 Plant, Oak Ridge, TN. Using the column chemical data molar distributions of the precipitates along the flow path were modeled. The maximum precipitation at the Fe0-sand interface at the influent end was predicted, where pore water showed high saturation index (SI) with respect to c...

Numerical simulation of unsaturated flow along preferential pathways: implications for the use of mass balance calculations for isotope storm hydrograph separation

An objective common to many watershed studies is to separate storm hydrographs into two components: water that was present in the watershed prior to a storm event (soil moisture and groundwater) and water which fell on the watershed during the storm. To use this approach, a number of assumptions must be made including that the composition of water in the soil moisture and groundwater reservoirs are constant and known. The objective of this paper is to show that in settings where flow and transport are dominated by preferential pathways for flow, steady state mass balance calculations for quantitative hydrograph separation may be in error. We present field data from a site where flow and transport are dominated by preferential pathways (relict fractures in saprolite of sedimentary rocks) which indicate that the δ18O content of the water in the unsaturated and shallow saturated zones is not constant over the course of a storm event. We use a numerical model to further explore the interactions between the fractures and surrounding matrix. Both the field data and modeling results indicate that the δ18O of the previous storm event(s) has a strong influence on water in the fractures. On the time scale of a storm event, only the water in the matrix immediately surrounding the fracture mixes with water in the fracture, while the bulk of the matrix is isolated from fracture flow. The spatial and temporal heterogeneity of the δ18O in the subsurface and the isolation of the most of the matrix water from flow in fractures make the measurement of a singular δ18O value for subsurface reservoirs problematic and the assumption of a constant value doubtful. Since most near-surface geologic materials have preferential flow paths, we suggest that quantitative hydrograph separation using mass balance techniques is not possible in most situations. Future field and modeling investigations using the approach outlined here could be designed to explore the important temporal and spatial scales of variability in watersheds, and lead to a more quantitative approach to storm hydrograph separation.

Estimating spatial distributions of heterogeneous subsurface characteristics by regionalized classification of electrofacies

Regionalized classification of electrofacies utilizes the statistical relationships between laboratory determined hydrologic properties and field-measured geophysical properties to estimate spatial distributions of porosity, permeability, and diagenetic characteristics. The method, illustrated with an application to the St. Peter Sandstone in the Michigan basin, combines techniques for multivariate analysis and spatial estimation. Core plug and borehole geophysical data are clustered into electrofacies that reflect the hydrologic properties and diagenetic characteristics of the formation. Electrofacies characteristics then are used to assign a class membership probability at locations where only geophysical data are available. Three-dimensional estimation of electrofacies occurrence is done by kriging datasets containing the probability of electrofacies membership at borehole locations. The discretization and kriging geometry allow three-dimensional estimation of hydrologic parameters for a large region that incorporates meter-scale heterogeneity. Finally, permeability and porosity are estimated at each grid location by probability-weighting. Because the electrofacies carry information about both the hydrologic and lithologic properties, the resulting spatial distributions provide an understanding of both the present-day flow characteristics and the extent of processes that control them.