Home Archived April 24, 2019
(i)

  California Water Science Center

The Location, Extent, and Hydrologic Characteristics of the Rialto-Colton Fault, San Bernardino County, California

Project Photo
CA552

Problem: The location, extent, and hydrologic characteristics of the Rialto-Colton Fault, which has been defined as the hydrologic boundary between the Rialto-Colton and Chino basins, are not precisely known. Knowledge about the fault is important because (1) it is critical to an accurate determination of regional hydrology; (2) the location of the fault barrier may influence the performance of an individual well in the vicinity of the fault and, therefore, influence decisions on where to site new production wells or the operation of existing production wells; (3) the location of production wells relative to the fault may have legal implications; (4) the fault barrier may affect movement of ground water of different quality across the fault; and (5) subsidiary faults splaying from the Rialto-Colton Fault may affect ground-water movement in the vicinity of the splay.

Historically, the location, extent, and concepts of the hydrologic characteristics of the Rialto-Colton Fault are based on sparse historical water-level measurements made in the Rialto-Colton, Chino, and North Riverside basins. This sparseness of data has led to several assumptions concerning the fault that may or may not be true. It has been assumed by previous investigators that the fault extends from the Badlands, south of the Santa Ana River, northwestward to Barrier J (fig. 1), a fault trending perpendicular to (southwest-northeast) the mapped trace of the Rialto-Colton Fault. There currently is no information available concerning the presence of the fault south of the river and data north of Barrier J also do not exist and, therefore, it cannot be determined whether the Rialto-Colton Fault extends north of the barrier to the mountain front. It also has been assumed that the fault is a barrier to ground-water movement along its entire mapped length.

The location and extent of the Rialto-Colton Fault shown in Woolfenden and Kadhim (1997) , completed as part of the ongoing project "Effects of water banking on ground-water flow and quality in the Rialto-Colton basin, San Bernardino County California", is based on work by Morton (1974), which shows it extending from Barrier J in the northwest to the Badlands in the southeast. Barrier H, a fault splaying from the Rialto-Colton Fault, was shown to trend subparallel to it (fig. 1). It was assumed that underflow across the fault only occurred near the Santa Ana River in the southeastern part of the basin and at shallow depths. It also was assumed that the rest of the fault was a barrier to ground-water movement.

A preliminary ground-water flow model developed for the same project has led to a revision of the assumptions about the extent and concepts of the hydrologic characteristics of the Rialto-Colton Fault. The location of the fault is the same as that published in Woolfenden and Kadhim (1997); however, it has been extended to the San Gabriel Mountains in the northwestern part of the basin. Barrier H has been eliminated and replaced by a shorter fault splay trending nearly perpendicular to the Rialto-Colton Fault. Underflow occurs across a greater portion of the Rialto-Colton Fault in the southeastern part of the basin, and at greater depths. The quantity of underflow estimated by the ground-water flow model averaged about 19,000 acre-feet per year between 1945-96. The rest of the fault, including the extended portion northwest of Barrier J, is assumed to be a barrier to ground-water movement.

Changing the extent of the fault splay affected simulated water levels in the vicinity of the splay; however, changing the orientation of the splay had little or no effect. Simulated water levels do not match measured water levels in the vicinity of the fault splay. Changes in the extent of the Rialto-Colton Fault across which underflow may occur in the model, as well as changes in the depths at which it may occur, significantly affects simulated water levels. The effects of changing from a no-flow boundary along the rest of the extent of the Rialto-Colton Fault to a different boundary type are unknown. Determining a more accurate location, extent, and hydrologic characteristics of the Rialto-Colton Fault by using geophysical and hydrologic techniques may be the best approach to resolving some of the problems facing water managers in the Rialto-Colton basin and refining the hydrologic concepts of the basin in the vicinity of the fault.

Objective: The objectives of this study are to (1) more precisely define the location and extent of the fault; (2) identify locations along the fault where it may act as a barrier to ground-water movement in the alluvium; (3) define the hydrologic characteristics of the Rialto-Colton Fault at specific locations; (4) provide information on subsurface rock and soil type and their depths; (5) improve the understanding of the geohydrology of the Rialto-Colton and Chino basins in the vicinity of the fault.

Relevance and Benefits: The major benefits of this study are obtaining a more accurate location of the Rialto-Colton fault, which forms the southwestern geohydrologic boundary of the Rialto-Colton basin, and refining the hydrologic concepts of the basin in the vicinity of the fault. Distribution of water has become a critical issue in the Rialto-Colton basin, and the location of certain production wells relative to this fault and the source of water to these wells have become the focus of interjurisdictional conflicts among water managers. This study will help to provide a more sound scientific basis for the water managers to address these conflicts.

This study also will benefit an ongoing project in the Rialto-Colton basin by providing information needed to refine concepts of both the basinwide ground-water flow system and the ground-water flow system near the Rialto-Colton Fault. Data will be collected in areas of the Rialto-Colton, Chino, and North Riverside basins where few data exist. Information gained from this project also will contribute to refinement of concepts of the regional ground-water flow system in the upper Santa Ana River watershed, which will benefit the Santa Ana River NAWQA as well as any future projects in the Rialto-Colton, Chino, and North Riverside basins.

Approach:
PHASE I-GEOPHYSICAL INVESTIGATION AND EXTENSION OF GROUND-WATER FLOW MODEL

Task 1-Define the general location and extent of the Rialto-Colton Fault using gravity and aeromagnetic methods. The main methods to be used in this task are the interpretation of gravity and aeromagnetic data to derive subsurface information. Such data are effective in defining the thickness of sedimentary cover (depth to basement) in three-dimensions, the locations of faults that vertically offset the basement surface, and the locations of faults that offset the basement laterally. Any faults that vertically or laterally offset the basement surface likely also cut the alluvium and are, therefore possible sites of ground-water barriers in the alluvium. Existing geophysical data sets provide a solid foundation on which to base the proposed investigation of the Rialto-Colton Fault. A three-dimensional gravity inversion procedure ( Jachens and Moring, 1990) was developed and refined over the past few years for application to ground-water studies (Langenheim and Jachens, 1996), and is well-suited for the Rialto-Colton Fault study. Because the location and extent of the Rialto-Colton Fault is not precisely known, using microgravity surveying and aeromagnetic methods to identify the location and extent of the Rialto-Colton Fault in the basement rocks is a cost-effective first step.

Moderately densely-spaced regional gravity data will be inverted and used, in conjunction with outcrop geology, well control, and seismic profile data, to estimate the thickness of sedimentary cover over the Rialto-Colton basin and vicinity. Abrupt local variations in the depth to basement revealed by this inversion will identify locations of possible faults cutting the basement surface. Detailed profiles of closely spaced gravity observations, sited on the basis of the depth to basement model, will be interpreted in terms of the existence and precise location of faults. High-resolution aeromagnetic data will be interpreted in terms of laterally offset basement rocks and structures, thus identifying possible strike-slip basement faults.

All interpretations will be continually revised and refined during the first year as new data and interpretations become available from the other geophysical and hydrologic tasks. If the scope of the project is expanded to include determining the extent and location of faults internal to the Rialto-Colton basin, additional time will be required.

Task 2-More precisely define the location of the fault in selected areas using seismic-imaging methods. The USGS-Geologic Division high resolution imaging group uses both seismic reflection and refraction imaging. Seismic reflection imaging is the most common method of evaluating subsurface stratigraphy. Seismic reflection imaging provides an image of the subsurface stratigraphy, and has been used extensively in oil and gas exploration. The imaging done by the Geologic Division high resolution imaging group, however, is of much higher resolution with shallower target depths than that used in oil and gas exploration. Seismic refraction measurements have been used to determine physical parameters associated with the subsurface by measuring the velocity and amplitude characteristics of seismic waves as they move through the earth. For example, water-saturated sand has much higher seismic velocity than dry sand, allowing identification of depth to water.

Combined seismic reflection and refraction techniques, used in conjunction with computer inversion techniques to convert the data to a velocity model of the subsurface, are useful for locating faults and fractures; identifying the dip of the faults; identifying differences between consolidated sediments, loose alluvial sediments, and hard rock; and, mapping stratigraphy laterally from a borehole. Seismic imaging techniques have been used successfully in several locations, including at Camp Navajo, Arizona where the location of faults and impermeable layers (basalt), lateral variations of confining layers, and ground-water conduits were determined from seismic imaging data to aid in the siting of production wells; at Portrero Canyon, Los Angeles County, California where lateral variation of rock velocity corresponding to changes in degree of saturation, rock composition, and location of faults were determined to help in the evaluation of the movement of ground water (M.J. Rymer, oral commun., 1998); at the Morongo Indian Reservation in Riverside County, California the structure, stratigraphy, and depth of basin fill were defined to determine the suitability of the basin to store water; and in Cherry Valley, San Bernardino County, California where the fault geometry, basin structure, and location of gravel beds were determined to locate potential water-storage and well sites (A.H. Christensen, oral commun., 1998). It is anticipated that these techniques will be suitable for use in the Rialto-Colton basin.

As with Task 1, all interpretations will be continually revised and refined during the first year as new data and interpretations become available from the other geophysical and hydrologic tasks. If the project is expanded to include determining the extent and location of faults internal to the Rialto-Colton basin, additional time will be required.

Task 3-Compilation of existing data and extension of ground-water flow model into Chino basin. Data for production wells will be obtained and compiled, including water levels, water chemistry, and well operation and maintenance schedules in the Rialto-Colton and Chino basins. In addition, the existing ground-water flow model will be extended into Chino basin a limited distance to allow conversion of the Rialto-Colton Fault from the model boundary to a horizontal flow barrier. Calibration of the hydraulic characteristics of the Rialto-Colton Fault along various segments and in different layers will provide a first step in determining the hydrologic characteristics of the fault. The data collected and the extended ground-water flow model will be used, in conjunction with results from the geophysical investigation, to determine locations for paired wells across the Rialto-Colton Fault. Task 3 will be performed concurrently with Tasks 1 and 2.

PHASE II-QUALITATIVE HYDROLOGIC INVESTIGATION

Task 1-Install multiple-depth observation wells at selected sites along the fault trace. Information from the geophysical surveys, Tasks 1 and 2, and previous investigations will be used to select sites for detailed hydrologic study, including test drilling, borehole geophysics, water-level measuring, ground-water sampling and, possibly, aquifer testing. Paired multiple-depth well sites will be installed on opposite sides of the Rialto-Colton Fault. The number of paired well sites installed will depend on the results from Phase I. At each site on opposite sides of the Rialto-Colton Fault, two to three multiple-depth observation wells will be installed in test holes, and water levels measured to determine water-level differences across the fault. If results from the geophysical investigations conducted during Phase I indicate that the fault splay near the Barrier H and Rialto-Colton Fault junction exists, multiple-depth well sites will be constructed in the Rialto- Colton basin, in the compartment between the splay and the Rialto-Colton Fault, and in the Chino basin. Water levels will be collected at these three multiple-depth well sites to determine water-level differences across the fault splay and across the Rialto-Colton Fault.

Task 2-Ground-water sampling at multiple-depth wells. Water will be sampled from all multiple-depth wells installed during Task 2. Samples will be analyzed for major ions, nutrients, oxygen-18, deuterium, and carbon-14. These data will be useful in determining depth-dependent water chemistry and possible flow across the Rialto-Colton Fault, and across the fault splay mentioned in Task 3.

PHASE III-AQUIFER TESTING

Aquifer testing will be conducted to determine the role of the Rialto-Colton Fault and possible subsidiary faults in the movement of ground water in the Rialto-Colton and Chino basins, and to determine general aquifer properties near the Rialto-Colton Fault. Aquifer testing will depend on findings from Phases I and II, installation of paired multiple-depth well sites installed during Phase II in the vicinity of the proposed aquifer tests, and the availability of existing production wells in the vicinity of two of the proposed aquifer tests.

Task 1-Conduct an aquifer test near the fault splay. Depending on the fault geometry in the vicinity of the Rialto-Colton Fault and Barrier H junction, the availability of a suitable production well, and on results from Phase II, an aquifer test will be conducted near the fault splay. The test will be performed in the middle water-bearing unit, which is the main water-producing unit in the Rialto-Colton basin. A suitable production well does exist near the fault splay, however, it may not be available for pumping. If conducted, information on the effects of pumping near the Rialto-Colton Fault and the fault splay and qualitative aquifer properties will be obtained.

Task 2-Conduct an aquifer test near the intersection of the Rialto-Colton Fault and the Santa Ana River. If the Rialto-Colton Fault exists in the vicinity of the Santa Ana River, and water-level and water-chemistry data indicate a connection across the fault between the Rialto-Colton and Chino basins, an aquifer test will be conducted. A suitable production well may exist for this test and may be available. If possible, packers will be used in the production well to conduct separate aquifer tests of the middle and upper water-bearing units, and the river-channel deposits to determine the effects of pumping at this site and aquifer characteristics near the fault.

Task 3-Conduct an aquifer test north of Barrier J. If results from the geophysical investigation conducted during Phase I indicate the Rialto-Colton Fault exists above Barrier J, and water-level and water-chemistry data indicate a connection between the Rialto-Colton and Chino basins, a third aquifer test may be conducted. It is doubtful that a suitable production exists in this vicinity, therefore, a test hole will be drilled, and test well will be installed and perforated in the middle water-bearing unit. This test will provide similar information to that provided in Tasks 1 and 2.

Task 4-The updated ground-water flow model will be completed and will include all results from Phases I, II, and III. A report will be written to document the changes and effects on the geohydrology of the Rialto-Colton and Chino basins in the vicinity of the Rialto-Colton Fault.

Upon completion of Phases I, II, and III, the location and extent of the Rialto-Colton Fault will be known more accurately, and the understanding of the boundary conditions between the Rialto-Colton basin and the adjacent Chino and North Riverside basins will be improved. Stratigraphic, lithologic, water-level, and water-chemistry data will be collected in areas where little or no depth-dependent data exist in the Rialto-Colton, Chino, and North Riverside basins. These data will contribute to the refinement of hydrologic concepts in the Rialto-Colton basin, and will provide a foundation on which to build future investigations in the Chino and North Riverside basins. An improved understanding of the ground-water flow system in the Rialto-Colton basin will be beneficial in determining the movement of imported water under selected artificial recharge alternatives, and in determining the movement of a perchlorate plume and its potential impacts on Chino and North Riverside basins. Knowledge of the hydrologic conditions of the Rialto-Colton Fault will contribute to the understanding of regional ground-water flow in the upper Santa Ana River watershed.

Progress and Significant Results in FY 1999: Work during fiscal year 1999 focused on geophysical investigations conducted by the Geologic Division and on extension of the ground-water flow model into the Chino and North Riverside basins. Microgravity and aeromagnetic methods were used to define the general location and extent of the Rialto-Colton Fault. After completion of the gravity and magnetic work, seismic-imaging methods were used to locate the fault more precisely in the alluvium. Work on extending the ground-water flow model into the Chino and North Riverside basins and incorporating the initial updated location of the Rialto-Colton Fault was begun.

Plans for FY 2000: Work during fiscal year 2000 will focus on updating the ground-water flow model. The final updated location of the Rialto-Colton Fault will be incorporated into the model, and modifications to boundary conditions and aquifer parameters will be made. Other work will include alternative conceptual model testing and updating the Arc-Info AML's developed during the CA494 project.

Reports:

A U.S. Geological Survey Open-File report will present the results of the gravity and aeromagnetic survey. Included in this report will be a new gravity map of the Rialto-Colton basin and vicinity; a new model of thickness of aquifer sedimentary deposits (depth-to-basement) of the Rialto-Colton basin; and maps showing locations of probable faults that cut the alluvium.

A U.S. Geological Survey Open-File report will present the results of the seismic imaging work. Included in this report will be an interpreted seismic section along each seismic line. The seismic sections will show fault locations, fault dips, stratigraphic variation across the faults and extending from boreholes, correlations with existing geophysical and borehole data, estimates of depth to basement, and estimates of deeper aquifers. Stacked, migrated (if necessary) seismic reflection images along each seismic line, and velocity models along each seismic line also will be provided.

A U.S. Geological Survey Open-File report describing the well construction, lithologic and borehole geophysical logs, and water-level and water-chemistry data for all well sites completed for this study may be prepared.

A U. S. Geological Survey Water-Resources Investigations Report describing the location and hydrologic characteristics of the Rialto-Colton Fault may be prepared. Included in this report will be a summary of results from the geophysical surveys; interpretation of the water-level and water-chemistry data; and results of any aquifer tests conducted.

Additional publications such as journal articles may be completed during the course of the project. These publications may include a description of the extended model; comparison between model results from the existing ground- water flow model and from the same model with updated information on the Rialto-Colton Fault boundary; and analysis of model results pertaining to improved understanding of both regional ground-water movement and ground-water movement near the Rialto-Colton Fault.

Number: CA552
Location: San Bernardino County
Cooperating Agencies: San Bernardino Valley Municipal Water District
Project Chief: Linda R. Woolfenden
Period of Project: October 1998 through September 2003
Team: Linda Woolfenden, Eric Reichard, Megan Anderson, Bob Jachens, Mike Rymer, Rufus Catchings

Accessibility FOIA Privacy Policies and Notices

USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://ca.water.usgs.gov/projects/projects00/ca552.html
For Page Information: Send Us a Message
Page Last Modified: Thursday, 05-Jan-2012 15:16:26 EST