VZJ Download to Citation Manager
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Frazier, C. S.
Right arrow Articles by Anderson, M. A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Frazier, C. S.
Right arrow Articles by Anderson, M. A.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Frazier, C. S.
Right arrow Articles by Anderson, M. A.
Related Collections
Right arrow Flow
Right arrow Soil Hydrology
Vadose Zone Journal 1:113-124 (2002)
© 2002 Soil Science Society of America

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

C. S. Fraziera, R. C. Graham*,b, P. J. Shousec, M. V. Yatesb and M. A. Andersonb

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
* Corresponding author (graham{at}citrus.ucr.edu)

Received 16 November 2001.



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Where soils are shallow, effluents from on-site wastewater disposal systems (OSWDS) can be dispensed onto underlying weathered bedrock. Viruses contained in the effluents pose a threat to groundwater quality if the weathered bedrock materials do not possess the properties necessary to treat the effluents before they reach groundwater. The extent and pathways of water flow and virus transport in fractured, weathered granitic bedrock were investigated at a field site in southern California. A suspension containing MS-2 bacteriophage, sodium bromide, and blue dye was ponded at the soil–weathered bedrock interface and allowed to infiltrate for 9 h. A trench was excavated, and bedrock samples were collected and assayed for water, bromide, and MS-2 content. Distributions of dye, bromide, and MS-2 indicate that joint fractures facilitated the channeling of water and of viruses to depths >105 cm. Infiltration data suggest that fracture channeling occurred primarily during the first 50 min, after which time vertical convective flow through the bedrock matrix was the primary infiltration and transport process. The transition from fracture to matrix flow was the result of a decrease in fracture aperture caused by the swelling of pedogenic clay in the weathered bedrock matrix. Bromide and MS-2 concentration profiles suggest that the lower extent of matrix flow was 45 cm, below which water flow and virus transport occurred solely via fracture channeling. These results indicate that caution should be used when operating OSWDS on weathered granitic bedrock in California, and emphasize the need to collect morphologic and hydraulic data prior to their installation.

Abbreviations: COLE, coefficient of linear extensibility • Fed, citrate-bicarbonate-dithionite extractable Fe • MLD, maximum lateral distance • OSWDS, on-site wastewater disposal systems • PFU, plaque-forming units


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ON-SITE WASTEWATER DISPOSAL SYSTEMS are the primary means of domestic waste disposal in sparsely populated rural and suburban areas throughout the USA (Reneau et al., 1989). The most commonly employed OSWDS are septic tank–soil adsorption systems in which septic tank effluents are percolated through soil to remove potential contaminants before they reach groundwater (Kaplan, 1987). Many of the dwellings that utilize these systems also depend on local groundwater for drinking water (Bitton and Gerba, 1984). Domestic wells are often located close to septic tank leach fields and frequently draw from the same aquifers that receive septic tank effluents. Not surprisingly, contamination of well water from septic tank leachate has historically been one of the most frequently reported causes of waterborne disease outbreaks in the USA (Yates, 1985).

The potentially infective biological contaminants in septic tank effluents include helminths, protozoa, bacteria, and viruses (Perkins, 1984). Proper functioning of a soil adsorption system depends on its ability to remove these contaminants by physical filtration, surface adsorption, sedimentation, inactivation, and natural die-off (Gerba, 1984). Because of their small size, viruses (20–200 nm) generally are not removed from effluents by physical filtration or sedimentation and, consequently, are the most mobile of these contaminants.

Regardless of the type of virus involved, properties of the leach field media can significantly affect overall virus mobility by influencing the type of transport processes involved, the extent of virus–soil/regolith contact, and the nature and strength of adsorption reactions (e.g., Moore et al., 1981; Lance et al., 1982; Gerba, 1984; Lance and Gerba, 1984; Bales et al., 1989). In California, many of the dwellings that rely on OSWDS are in upland areas where soils are shallow and underlain by granitic bedrock (Fig. 1). In some cases, the bedrock has been weathered to depths >30 m (Wahrhaftig, 1965), while overlying soils can be <0.1 m thick (Frazier and Graham, 2000). Because of the shallow nature of these soils, most or all of the contaminant removal requirements of a leach field may be placed on the weathered bedrock. In these areas, the potential for groundwater contamination from OSWDS is of concern, yet relatively little information exists on the properties of weathered granitic bedrock relevant to contaminant transport and removal.



View larger version (33K):
[in this window]
[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).

 
Weathered granitic bedrock in California and much of the western USA retains many of the lithogenic structural features of the parent rock, yet often possesses the friable consistence, extensive porosity, and secondary mineral components characteristic of soil (Clayton et al., 1979; Jones and Graham, 1993; Johnson-Maynard et al., 1994; Graham et al., 1997; Frazier and Graham, 2000). However, it is generally not as weathered as granitic saprolite in the humid regions of the southeastern USA (e.g., Schoeneberger and Amoozegar, 1990; Stolt et al., 1992; Schoeneberger et al., 1995). Lithogenic structural features, namely joint fractures, may provide preferential flow pathways and decrease weathered bedrock–contaminant contact, and thereby enhance the transport of dissolved and suspended contaminants to groundwater. Properties imparted by pedogenic alteration of the bedrock may enhance or retard the transport potential of contaminants.

The objectives of this study were to investigate the extent and pathways of water flow and virus transport in weathered granitic bedrock at a field site in southern California, and to determine how these processes are influenced by the lithogenic and pedogenic features of the bedrock.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Area and Site Characterization
The study area is at an elevation of 1150 m in the foothills of the San Jacinto Mountains in Riverside County, southern California (Fig. 1). The climate is Mediterranean, with precipitation occurring mostly as rain between the months of November and April. Soils in the area consist of shallow to moderately deep Alfisols, Mollisols, and Entisols on gently rolling to steep hillslopes (Knecht, 1971; Graham et al., 1997). The site chosen for investigation is located on a nearly level ridge top (slope < 2%). Soils are loamy, mixed, mesic, shallow Typic Haploxeralfs underlain by >1.20 m of weathered granitic bedrock that shows distinct horizonation with depth (Table 1). The structure and fabric of the weathered bedrock horizons are "rock-controlled", except in the BCrt where a weak subangular blocky structure has developed (Frazier and Graham, 2000).


View this table:
[in this window]
[in a new window]
  		Table 1. Morphology of soil and weathered bedrock at the study site (Frazier and Graham, 2000).

 
Weathered bedrock at the study site and throughout the study area is extensively fractured. The largest fractures, hereafter referred to as joint fractures, are usually vertically trending and continuous from the soil–weathered bedrock interface to the lowermost depth studied (1.25 m). Preliminary dye studies showed that joint fractures are often interconnected via smaller fractures, and represent the most extensive and continuous form of macroporosity for the channeling of water in the weathered bedrock system. In the immediate study area, the fractures are on average 63 cm apart and have apertures that range from 1.0 to 1.5 cm at the soil bedrock interface, to 0.1 to 0.4 cm at a depth of 125 cm (Table 1) (Frazier and Graham, 2000).

Pedologic processes acting inwards from fracture sidewalls have resulted in the formation of a series of morphologic zones emanating laterally from the fractures (Fig. 2). A total of five lateral morphologic zones were identified in each weathered bedrock horizon. All five morphologic zones are described in detail by Frazier and Graham (2000), but only two are described here—the matrix and fracture rind.



View larger version (42K):
[in this window]
[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).

 
The matrix consists of material located >2 cm from fracture sidewalls, and the rind consists of the more highly altered material located between the matrix and fracture void space. In the matrix, clay, organic C, and citrate-bicarbonate-dithionite extractable Fe (Fed) contents are highest in the BCrt horizon, and decrease with increasing depth (Table 2). Total porosity and macroporosity follow similar trends, being highest in the BCrt and decreasing with increasing depth. Coefficient of linear extensibility (COLE) values for the weathered bedrock matrix range from 0.29 in the BCrt horizon to 0.017 in the Cr2 horizon, and generally decrease with increasing depth.


View this table:
[in this window]
[in a new window]
  		Table 2. Selected properties of the soil and weathered bedrock at the study site (Frazier and Graham, 2000).

 
The thickness of the fracture rind decreases with increasing depth (Table 1) and, at most depths (7–88 cm), exhibits a compacted, vertically trending fabric that differs considerably from the rock-controlled fabric in the matrix (Frazier and Graham, 2000). Clay, organic C, and Fed contents are higher in the rind than in the adjacent matrix (Table 2). In the upper profile, total porosity and macroporosity in the rind are lower than in the adjacent matrix (Table 2). The opposite trend exists in the lower profile.

The primary mineral composition of relatively unweathered bedrock at a depth of 1.25 m is 40% plagioclase feldspar, 21% quartz, 20% mica (mostly biotite), 13% K feldspar, and 5% hornblende. The fine sand (100–250 µm) and medium silt (5–20 µm) fractions of the weathered bedrock matrix and fracture rind contain quartz, plagioclase, hornblende, and biotite, and the secondary minerals kaolin, vermiculite, and regularly and randomly interstratified mica–vermiculite. The clay fractions (<2 µm) of the weathered bedrock matrix and fracture rind, and overlying soil horizons contain kaolin, vermiculite, regularly and randomly interstratified mica–vermiculite, and trace quantities of goethite (Frazier, 1997).

Field Tracer Experiment
An aqueous suspension containing bacteriophage MS-2 (15597-B1; American Type Culture Collection, Manassas, VA), NaBr, and a strongly adsorbing blue dye (food coloring with active ingredient FD&C Blue #1) was applied at the weathered bedrock–soil interface (i.e., top of BCrt horizon). The MS-2 phage has a diameter of 26.0 to 26.6 nm, a pHiep of 3.9, and a surface that contains both hydrophilic and hydrophobic regions (Bales et al., 1991; McKay et al., 1993). It was used because it exhibits structural features, size dimensions, and adsorptive behavior similar to that of many important human enteric viruses (Powelson et al., 1991; Goyal and Gerba, 1979; Snowdon and Cliver, 1989), and because it has relatively low inactivation rates in field environments (McKay et al., 2000; Powelson et al., 1993). Bromide was used as a tracer for water movement (McCoy et al., 1994) and for comparison with the MS-2 phage. Dye was added to tag preferential flow pathways.

To apply the suspension, the A and AB soil horizons were removed from a 2 by 2 m area to expose the soil–weathered bedrock interface. A section of 0.6 m (i.d.) PVC pipe fitted with a polypropylene float valve was sealed to the weathered bedrock surface with a silicone gel sealant. The float valve was connected via polypropylene tubing to a carboy containing the tracer suspension, and was used to maintain approximately 5 cm of hydraulic head in the PVC ring throughout the application period. A digital balance (accuracy: ±23 g) was placed under the carboy to monitor the infiltration rate of the suspension. A total of 77.83 L of suspension was applied through the PVC ring during 8.75 h. A volume of 58.96 L was applied via the float valve–carboy apparatus, and the remaining 18.87 L was applied manually at the beginning of the application period to establish the initial 5 cm of hydraulic head in the PVC ring. The tracer suspension had a pH of 4.23, an electrical conductivity at 25°C of 2.45 dS m-1, a mean temperature of 16.4°C, and contained 20 mmol L-1 NaBr and a 1:4 (dye/water) volumetric dilution of food coloring. The average concentration of MS-2 in the influent was 5.0 x 1011 PFU L-1.

Approximately 8 h after application was terminated, a trench 1.15 m deep, 1.50 m long, and 0.75 m wide was excavated beneath the center of the influent ring over a 12-h period. A square wire grid composed of 285 7.5 by 7.5 cm cells was then secured against the trench face (Fig. 3). One weathered rock sample (100 g) was collected from each cell located within the boundaries of the wetting front, and from one to two cells outside of the wetting front, in each grid row. An additional 47 randomly located grab samples were collected from fractures stained with dye, hereafter referred to as fracture samples. All samples were sealed in sterile polyethylene bags, and stored at -4°C until assayed 1 to 22 d later. A second set of 100-gram samples was collected from each of the grid cells and from fractures stained with dye for determination of gravimetric water content. Background levels of moisture, bromide, and MS-2 were determined from samples collected from unaffected areas adjacent to the trench. Total time taken for sampling was about 14 h.



View larger version (60K):
[in this window]
[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.

 
Tracer Assays
Each 100-g field sample was thawed to 10°C and split into two 50-g subsamples. One 50-g subsample was mixed with 100 mL of an aqueous 3.5% beef extract solution (pH 9.5) and centrifuged at 1250 g for 5 min. The supernatant was serially diluted, plated on lawns of log-phase Escherichia coli (ATCC 15597), and assayed for MS-2 content using the double agar overlay plaque-forming-unit method (Adams, 1959) (mean coefficient of variation was 48% = 0.31 log10 PFU g-1). Bromide was extracted from the remaining 50-g subsample via vacuum filtration, using enough water to obtain a final extract volume of 15 to 20 mL. Extracts were assayed for bromide content colorimetrically using an Alpkem (O.I. Corp., College Station, TX) automated wet chemistry system. Gravimetric water content was determined from the second set of field samples by measuring weight loss on heating to 105°C for 24 h (Gardner, 1986).

Analysis of Spatial Data
SURFER (Golden Software, 1994) was used to generate figures illustrating the two-dimensional spatial distribution of water and tracers on the trench exposure. SURFER requires several input parameters in addition to the raw data set for accurate spatial interpolation using ordinary kriging, the most important of these being the variogram model. Rather than use the default model provided by SURFER, geostatistical software (Yates and Yates, 1990) was used to generate semivariogram plots for the water and tracer data sets. These plots allowed us to determine if data were spatially correlated and, subsequently, to determine the appropriate variogram model for input into SURFER for each data set.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Infiltration of Tracer Suspension
Infiltration rates were extremely rapid during the first 40 min of application (Fig. 4). During this period, 51.3 L, or 66% of the total volume of tracer suspension applied, had infiltrated. The 18.87 L of suspension applied manually at the beginning of the application period infiltrated through fractures visible at the weathered bedrock surface so rapidly that a small vortex formed in the influent ring. Because the infiltration capacity of the bedrock was so high during the first 40 min, the float valve–carboy apparatus was not able to supply tracer suspension to the influent ring as rapidly as it was infiltrating. Thus, infiltration rates during this period were controlled by the physical limitations of the equipment, and not by surface infiltration phenomena. The desired 5 cm of hydraulic head was established in the influent ring at 50 min, after which time infiltration rates began to decrease considerably and asymptotically approach a final rate of 0.82 cm h-1 (Fig. 4).



View larger version (23K):
[in this window]
[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).

 
Tracer Distributions
Dye adsorbed strongly to the top 4 cm of weathered bedrock matrix, and to materials along joint fractures infiltrated by the tracer suspension (Fig. 5 and 6). Most of the dyed fractures were visibly continuous with the bedrock surface. Lateral movement of the dye from fractures into the surrounding matrix was minimal. Consequently, the distribution of dye closely approximates the relative location, spacing, size, and orientation of the joint fractures (Fig. 5).



View larger version (128K):
[in this window]
[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 this window]
[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.

 
The highest levels of MS-2 were found in the uniformly dyed layer of bedrock matrix directly beneath the influent ring (0–4 cm), and in dyed fracture material (Fig. 6a). MS-2 concentrations averaged 6.0 log10 PFU g-1 in the fracture samples, and 4.1 log10 PFU g-1 in the 4 cm of bedrock matrix below the influent ring. In the bedrock matrix below 4 cm, MS-2 concentrations were highest in areas immediately adjacent to dyed fractures, and decreased with increasing lateral distance from the dyed fractures. Lateral concentration gradients were especially steep at depths greater than about 45 cm. Concentrations as high as 4.1 log10 PFU g-1 in fracture samples, and 3.2 log10 PFU g-1 in matrix samples adjacent to fractures, were detected at depths as great as 105 cm.

With few exceptions, the distribution pattern of bromide closely resembled that of the MS-2 phage (Fig. 6b). The highest levels of bromide were detected in the bedrock matrix directly beneath the influent ring (0–15 cm), and in regions of the bedrock matrix adjacent to dyed joint fractures. Bromide concentrations decreased with increasing lateral distance from the dyed fractures, but the gradient was generally less steep for bromide than for MS-2. The greater width of the bromide plume suggests that the lateral movement of bromide was more extensive than for the MS-2 phage.

Tracer Concentration Profiles
Mean MS-2 and bromide concentrations in the bedrock matrix were plotted as a function of depth (Fig. 7). Instead of averaging tracer concentrations in all samples collected at a given depth, only samples within the dimensions of the influent ring were averaged to generate this figure (i.e., from 38 to 98 cm on the x axis in Fig. 6). This approach was used to minimize effects of the varying dimensions of the wetting front with depth and differences in the number of samples collected at each depth. Despite relatively high variability, mean bromide and MS-2 concentrations generally decreased with increasing depth. However, the concentration gradients of both tracers had different characteristics in the 0- to 45-cm depth range (BCrt and Cr1 horizons) than in the 45- to 105-cm depth range (Cr2 and Cr3 horizons). Above 45 cm, mean bromide and MS-2 concentrations decreased along relatively continuous, pronounced gradients with increasing depth. Below 45 cm, mean bromide and especially mean MS-2 concentrations fluctuated more substantially between individual depths, and decreased only slightly or with no discernable trend with increasing depth.



View larger version (28K):
[in this window]
[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.

 
Bromide and MS-2 concentrations in the bedrock matrix were also plotted as a function of lateral distance from the closest of the dyed fractures shown in Figure 6 (Fig. 8). At depths <45 cm (BCrt and Cr1 horizons), bromide and MS-2 concentrations decreased only slightly and remained at relatively high levels with increasing lateral distance from the dyed fractures. Conversely, at depths >45 cm (Cr2 and Cr3 horizons), bromide and MS-2 concentrations decreased along relatively steep gradients with increasing lateral distance. The gradient for MS-2 was especially steep, with little MS-2 being detected at distances >7 cm.



View larger version (22K):
[in this window]
[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.

 
The greatest lateral distance from the dyed fractures at which bromide and MS-2 were detected in each bedrock horizon, termed the maximum lateral distance (MLD), is shown in Fig. 9. In the BCrt and Cr1 horizons, MLD values for bromide and MS-2 were the same. In the underlying Cr2 and Cr3 horizons, MLD values for MS-2 were from 6 to 15 cm less than those for bromide.



View larger version (13K):
[in this window]
[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.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Water Flow and Virus Transport Pathways
Infiltration and Tracer Distribution
Figure 6 shows clearly that the distributions of bromide and MS-2 are directly related to the location and orientation of the dyed fractures. The highest concentrations of bromide and MS-2 below a depth of 15 cm were in dyed fracture material and in the matrix adjacent to dyed fractures. Furthermore, concentrations of both tracers decreased with increasing distance from the dyed fractures, especially in the lower half of the profile (Cr2 and Cr3 horizons). These data indicate that joint fractures effectively facilitate the transport of water, solutes, and environmentally significant numbers of virus colloids to substantial depths in weathered granitic bedrock.

Infiltration rates during the first 40 to 50 min of application were extremely rapid. They were many times faster than rates measured during the remainder of the application period, even though they are only conservative estimates of the actual infiltration capacity of the weathered bedrock because of limitations of the application equipment. We interpret that fracture channeling occurred primarily during this period, and that most of the 51.3 L of tracer suspension applied during this time (66% of total) infiltrated the bedrock through fractures. As a result, viruses traveled distances greater than would be expected if the suspension had infiltrated solely via the weathered bedrock matrix.

After the first 50 min, the infiltration profile is less indicative of fracture channeling and more closely resembles infiltration profiles typical of soils, in which the infiltration rate gradually approaches a value approximately equal to its saturated hydraulic conductivity (Hillel, 1982). The infiltration rate slowed considerably and asymptotically approaches a final value of 0.82 cm h-1 (Fig. 4), which is nearly equal to the in situ saturated hydraulic conductivity of a Bt horizon located 0.5 km from the study site (Graham et al., 1997). The Bt horizon of Graham et al. (1997) was derived from granitic bedrock and had a texture similar to the BCrt horizon in this study. The Bt horizon differed from the BCrt horizon only in that it had slightly higher porosity, and soil rather than rock-controlled fabric and structure.

Apparently, a transition from fracture channeling to convective matrix flow began after about 50 min of infiltration. Several mechanisms could cause this transition. It is possible that the fracture system has discrete limits, causing a shift to matrix flow once the fractures had been filled to capacity with tracer suspension. This seems unlikely, as dye was found in fractures at lateral distances greater than 3 m from the trench exposure. It is also possible that the fractures became clogged with soil or weathered bedrock material washed into them during application. However, the apertures of dyed joint fractures decreased, to the point of total closure at some depths, when wetted with the tracer suspension. When the same fractures were exposed on the trench face for sampling, their apertures increased gradually as the bedrock dried. This behavior is consistent with the moisture-dependent shrink–swell activity of clay minerals, and suggests that the observed decrease in fracture aperture was the result of hydration-induced swelling of clay minerals in the weathered bedrock materials surrounding the joint fractures. The resulting decrease in fracture aperture appears to have caused a decrease in infiltration rate and a transition from fracture channeling to matrix flow as the primary infiltration and transport process. The COLE values measured for the weathered bedrock matrix (0.017–0.029), as well as other physical and micromorphological characteristics of the fracture rind, support this interpretation (Frazier and Graham, 2000).

Tracer Concentration Profiles
When fracture channeling is the main flow process occurring in fractured, porous media, water and solutes move into the matrix adjacent to fractures almost exclusively by capillary action and diffusion-like processes (Freeze and Cherry, 1979; Grisak and Pickens, 1980; Grisak et al., 1980; Skagius and Neretnieks, 1986). As a result, lateral concentration gradients develop around fractures, facilitating the channeling. In the case of relatively unreactive substances such as water and bromide, gradients reflect differences between water and dissolved solute concentrations at the leading edge of the wetting front and those at the fracture margin (Grisak and Pickens, 1980; Germann et al., 1984; Jabro et al., 1991). In the case of reactive solutes and colloids such as viruses, lateral gradients are likely to develop as a result of adsorption reactions and/or by size and charge exclusion from matrix porosity (Bales et al., 1989; McKay et al., 1993). Distinct lateral gradients are less likely to develop and more likely to be masked by vertical gradients when vertical convective flow processes predominate in the matrix surrounding the fractures. Vertical concentration gradients (i.e., decreasing concentration with increasing depth), on the other hand, can develop as a result of both fracture channeling (Germann et al., 1984; White et al., 1986; Jabro et al., 1991) and convective matrix flow (Lance et al., 1982; Lance et al., 1984; Bales et al., 1991; Poletika et al., 1995). Using this reasoning, trends in the bromide and MS-2 concentration gradients (Fig. 7 and 8) with depth and with increasing lateral distance from the dyed fractures were used to evaluate the nature of flow processes and the transport pathways by which tracers interacted with the weathered bedrock matrix at different depths in the profile.

In the upper profile (0–45 cm, BCrt and Cr1 horizons), mean bromide and MS-2 concentrations generally decreased with increasing depth, but remained high and decreased only slightly with increasing lateral distance from the dyed fractures. The presence of distinct vertical concentration gradients and the absence of distinct lateral concentration gradients suggest that water flow and virus transport through this region of the profile occurred primarily via vertical convective flow through the matrix. Apparently, lateral movement of tracers from the fractures into the surrounding matrix was minimal in this region of the profile and/or lateral concentration gradients introduced during periods of fracture channeling were concealed by vertical gradients introduced during periods of convective matrix flow.

In the lower profile (45–105 cm, Cr2 and Cr3 horizons), bromide and MS-2 concentration gradient trends are the opposite of those in the upper profile. Bromide and MS-2 concentrations decreased along relatively steep lateral gradients with increasing distance from the dyed fractures, but decreased only slightly or with no discernable trend with increasing depth. These trends suggest that water flow and virus transport through the lower profile occurred primarily via fracture channeling. The fact that little MS-2 was detected at lateral distances >7 cm from the dyed fractures (Fig. 8) suggests that the lateral movement of viruses from fractures was minimal, and consequently, interaction between viruses and the bedrock matrix was considerably less than in the upper profile. During their study of particle transport in fractured shale saprolite, Cumbie and McKay (1999) found that the diffusion of bacteriophage-sized latex microspheres from fractures into the saprolite matrix was minimal (on the order of 3–4 mm), suggesting that the apparent lateral flux of MS-2 observed in this study was probably largely due to transport processes other than diffusion.

Maximum Lateral Distance Plots
The MLD plots in Fig. 9 provide a means to evaluate and compare the effectiveness of bromide and MS-2 as tracers for water movement and virus transport. The separation between bromide and MS-2 in the lower profile (Fig. 9) suggests that the MS-2 phage was retarded relative to bromide as the tracer suspension moved laterally from the fractures into the surrounding matrix. This interpretation is consistent with other studies that have demonstrated that the MS-2 phage is excluded from porous matrices to a greater extent than bromide during periods of fracture channeling because of its larger size and its lesser tendency to diffuse into surrounding matrices compared with solutes (Bales, et al., 1989; McKay et al., 1993; McKay et al., 2000). Clearly, bromide is not a suitable tracer for virus transport when fracture channeling is involved. Since the MLD values for bromide and applied water content are the same in all but one horizon, it appears that bromide was a suitable tracer of water movement.

Figure 9 also provides information about water flow and virus transport pathways. The lateral separation between bromide and MS-2 in the Cr2 and Cr3 horizons appears to be a direct result of the tracer suspension moving laterally from joint fractures and generally supports our interpretation of the concentration gradient data presented in Fig. 7 and 8. That is, the tracer separation apparent in Fig. 9 indicates that fracture channeling was the primary transport pathway through the lower regions of the profile, and that lateral flux from the fractures was the main process by which the tracer suspension interacted with the surrounding matrix. Likewise, the lack of tracer separation in the upper profile is consistent with the conclusion that water flow and virus transport through this region of the profile occurred primarily via vertical convective flow through the matrix.

Influence of Pedogenic Features on Virus Transport and Retention
This study has demonstrated that lithogenic joint fractures in granitic bedrock facilitate the rapid channeling of water, solutes, and suspended virus colloids. Although the influence of pedogenic features on virus transport was not evaluated directly, reasonable conclusions as to their influence can be made using observations from this study and detailed physical and morphologic data from related studies (Frazier and Graham, 2000).

An important pedogenic feature of the weathered bedrock described in this study is its tendency to swell when wetted. As mentioned above, COLE values for the weathered bedrock matrix at the study site range from 0.017 to 0.029, indicating that the weathered bedrock undergoes linear expansion of 2 to 3% when brought from oven-dryness to field capacity. Visual observations made during this study and micromorphologic characteristics noted during related studies (Frazier and Graham, 2000) indicate that the swelling is adequate to cause a substantial decrease in joint fracture aperture, to the point of total closure at some depths. Smaller fracture apertures reduce infiltration rates and increase matrix/fracture flow ratios, while fracture closure directly inhibits channeling and promotes matrix flow. Both conditions result in greater interaction between virus colloids and the weathered bedrock matrix and increase virus residence times, thereby reducing the likelihood of viruses reaching groundwater (Freeze and Cherry, 1979; Grisak and Pickens, 1980; Bales et al., 1989).

The convergence of joint fracture sidewalls during episodes of swelling has resulted in the compaction of materials in the fracture rind zone of some bedrock horizons (Frazier and Graham, 2000). The compaction has decreased the abundance and continuity of laterally orientated macropores in the rind, and formed pressure faces on fracture surfaces to depths of 88 cm. Thoma et al. (1992) demonstrated that "mineralized" coatings on the surfaces of fractures in weathered tuff could significantly affect the migration of water between fractures and the surrounding porous matrix. In a similar manner, the compacted rind described by Frazier and Graham (2000) could potentially decrease the migration of water and virus colloids into the surrounding weathered bedrock matrix. The end result would be a reduction in the amount of interaction between viruses and bedrock matrix, and an increase in the probability of viruses reaching groundwater.

The features and processes described above are characteristic of the more mafic granitic bedrocks (~5% hornblende, ~20% biotite) in the study area. Weathering of less mafic granitic bedrocks (<1% hornblende, ~10% biotite) in the study area apparently does not generate enough clay to make shrink–swell activity such an influential process (Graham et al., 1997).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Implications for Onsite Wastewater Disposal in Granitic Terrain
This study demonstrates that joint fractures in weathered granitic bedrock in southern California are capable of facilitating the rapid transport of viruses and solutes to environmentally significant depths, and that interaction between virus colloids and the surrounding bedrock matrix is minimal when fracture channeling is the primary transport process. In general, the weathered bedrock described in this study appears to present different problems for onsite wastewater disposal than the more intensively weathered saprolites of the southeastern USA. In those saprolites, lithogenic structural features initially suspected of controlling bulk hydraulic conductivity and facilitating the rapid transport of raw sewage to groundwater have been shown to be relatively ineffective preferential flow pathways at some depths because of plugging by illuvial clay and Fe- or Mn-oxides (Schoeneberger and Amoozegar, 1990; Vepraskas et al., 1991; Amoozegar et al., 1993; Williams et al., 1994; Driese et al., 2001). The results from this study also emphasize the need to require the collection of detailed morphologic and hydraulic data when designing OSWDS, a sentiment voiced by other researchers and regulators (Amoozegar et al., 1993; Brown et al., 1994; Williams and Vepraskas, 1994).


   ACKNOWLEDGMENTS
 
This research was funded in part by the Kearney Foundation of Soil Science. We thank Frank Villegas and Paul Sternberg for their assistance with field sampling and sample analysis.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




This article has been cited by other articles:


Home page
 Vadose Zone JHome page
L. Pang, M. McLeod, J. Aislabie, J. Simunek, M. Close, and R. Hector
Modeling Transport of Microbes in Ten Undisturbed Soils under Effluent Irrigation
Vadose Zone J., January 23, 2008; 7(1): 97 - 111.
[Abstract] [Full Text] [PDF]

Home page
 Vadose Zone JHome page
S. Katsura, K. Kosugi, N. Yamamoto, and T. Mizuyama
Saturated and Unsaturated Hydraulic Conductivities and Water Retention Characteristics of Weathered Granitic Bedrock
Vadose Zone J., December 16, 2005; 5(1): 35 - 47.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Frazier, C. S.
Right arrow Articles by Anderson, M. A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Frazier, C. S.
Right arrow Articles by Anderson, M. A.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Frazier, C. S.
Right arrow Articles by Anderson, M. A.
Related Collections
Right arrow Flow
Right arrow Soil Hydrology

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome