Hydrodynamic Effects on the Transport and Retention of Biocolloids in Single, Saturated Fractures
Abstract
Approximately 30% of Canadians and 50% of Americans rely on groundwater for their domestic water supplies. A significant portion of this groundwater originates from fractured rock aquifers, as they are ubiquitous throughout North America. In comparison to unconsolidated porous media aquifers, relatively little is understood about biocolloid transport in fractures. A mechanistic understanding of the transport and retention of biocolloids in fractures is important towards determining the risk of biocolloid contamination to these sources, which can have a significant impact on human health. It has been well documented in the interfacial science literature that micro-scale hydrodynamics play a significant role in the transport of particles greater than approximately one micron in diameter, but do not significantly affect the transport of smaller particles. This phenomenon, however, has never been investigated in fractures, where the larger-scale hydrodynamics are complex, and must also be considered. To bridge this knowledge gap, this research was conducted to elucidate the effects of hydrodynamics on the transport and retention of E. coli RS2-GFP and MS2 in single, saturated, fractures at the laboratory scale. To achieve this goal, dolomitic limestone samples were acquired from a quarry in Guelph, Ontario, and were fractured under a uniaxial force. The hydrologic properties of each fracture sample were characterized using hydraulic and solute tracer tests. E. coli RS2-GFP and MS2 were chosen as the study microorganisms to isolate the micro-scale hydrodynamic effects. It is well established that micro-scale hydrodynamics do not affect transport for particles in the size range of MS2, while they do affect particles in the size range of E. coli. Using a factorial design approach, a known number of either E. coli RS2-GFP or MS2 was released into the fracture under a range of specific discharges (30, 10 and 5 m/day). The resulting effluent concentration profiles were compared to isolate the effects of hydrodynamics on particle transport. The dolomitic limestone fractures were also cast with a transparent epoxy, and the cast aperture fields were measured directly. Therefore, the semi-quantitative aperture-field measurement will also be considered in these analyses. This presentation will compare the and contrast the effluent concentration profiles from these experiments and present the dominant transport and retention mechanisms within the fracture plane (e.g., settling, straining, pore exclusion) in the presence and absence of micro-scale hydrodynamics under various flow conditions. The retention mechanisms will also be linked to the semi-quantitative aperture field measurement.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H53B1408S
- Keywords:
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- 1829 HYDROLOGY / Groundwater hydrology;
- 1831 HYDROLOGY / Groundwater quality;
- 1832 HYDROLOGY / Groundwater transport