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A Field Study of Water Flow and Virus Transport in Weathered Granitic Bedrock

^a Jones & Stokes, 2600 V Street, Sacramento, CA 95818
^b Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521-0424
^c U.S. Salinity Laboratory, Riverside, CA 92521

View larger version (33K): [in a new window]   Fig. 1. The distribution of granitic bedrock in California (black polygons) and the location of the four counties containing the highest number of individual sewage systems in the state (CSWRCB, 1994). (San Bernardino—124 684 units, Riverside—96 738 units, Los Angeles—77 839, San Diego—61 603 units).

View larger version (42K): [in a new window]   Fig. 2. Schematic diagram of a typical bedrock joint fracture at the study site, showing the five morphologic zones that emanate laterally from the joint fracture sidewalls (Frazier and Graham, 2000).

View larger version (60K): [in a new window]   Fig. 3. Diagram of the wire grid used to sample the weathered bedrock matrix. Grid cells that were sampled for MS-2 and Br content are denoted by black dots. All 285 cells were sampled for gravimetric water content. The three large rectangles delineated by bold lines represent the areas used for mass recovery calculations.

View larger version (23K): [in a new window]   Fig. 4. Infiltration rate profile for the tracer suspension during the 8.75-h application period. The jagged line represents measured infiltration rates, while the smooth line represents an equation fit to the data (equation shown).

View larger version (128K): [in a new window]   Fig. 5. Photograph of weathered bedrock trench exposure showing the wetting front of the tracer suspension and the distribution of the strongly adsorbing blue dye that was added to tag preferential flow pathways. The overlying soil horizons (combined thickness of 7 cm) were removed and the tracer was applied on the exposed soil–weathered bedrock interface. The wire grid used for sampling is in place against the face of the trench. Vertical scale is approximate.

View larger version (109K): [in a new window]   Fig. 6. Spatial distribution of (a) MS-2 bacteriophage and (b) bromide on the weathered bedrock trench exposure. Gray shading represents areas stained with dye. Bacteriophage concentrations expressed in log10PFU g-1 with a contour interval of 0.5 log10PFU g-1. Bromide concentrations expressed in milligrams per kilogram, with a contour interval of 15.0 mg kg-1.

View larger version (28K): [in a new window]   Fig. 7. Mean (a) bromide and (b) MS-2 concentrations as a function of depth. Error bars represent one standard deviation about the mean.

View larger version (22K): [in a new window]   Fig. 8. Distributions of (a) bromide and (b) MS-2 with increasing lateral distance from the dyed fractures shown in Fig. 5. Tracer distributions are shown for the Cr1 and Cr2 horizons only because tracer distributions in the BCrt horizon are similar to those in the Cr1 horizon, and tracer distributions in the Cr3 horizon are similar to those in the Cr2 horizon.

View larger version (13K): [in a new window]   Fig. 9. Maximum lateral distance (MLD) from the dyed fractures in Fig. 5 at which applied water, bromide, and MS-2 were detected in the bedrock matrix. Applied water content was calculated by subtracting background water content from total water content measured at the time of sampling.