The Rialto-Colton Fault in California forms the southwestern boundary of the Rialto-Colton Basin. Questions persisted concerning groundwater movement near the fault and the degree to which the fault acts as a barrier to groundwater movement from the Rialto-Colton to the Chino and North Riverside Basins. The Rialto-Colton and Chino Basins have been found to have high perchlorate concentrations. The objective of this project was to gain a better understanding of the direction of groundwater movement within the Rialto-Colton and adjacent basins in the vicinity of the Rialto-Colton Fault based on water-level data. Additionally, this project aimed to identify possible locations along the fault where it may not be an effective barrier to groundwater flow and perchlorate transport.

The Rialto-Colton Fault is a fault zone about 1 mile wide along two-thirds of its northwestern extent. The entire southeastern part of the Rialto-Colton Basin, near the Santa Ana River and Warm Creek, is the fault zone. Data indicates that the fault zone is a series of discontinuous step faults at about the midpoint along its extent. Although the gravity and aeromagnetic data are used to determine faulting in the bedrock, the data also can suggest the location of the fault in overlying sediments. Seismic data collected across the Rialto-Colton Fault indicates that the extent of the fault zone in the overlying alluvium is similar to that of the gravity and aeromagnetic data. Gaps in the fault may act as conduits through which groundwater can flow.

This project’s study area was bounded by Interstates 10, 15, and 215 and included most of the Rialto-Colton Basin, the eastern Chino Basin, and the western part of the Bunker Hill Basin. The approach involved compilation of available water-level data, evaluation of the available data and identification of data gaps, collection of additional water-level data, and construction of water-level contour maps.

Water-level data collected within the perched aquifer suggest that some water may flow from the Rialto-Colton to the Chino subbasin across the Rialto-Colton Fault through the perched aquifer, and well-bore flow data suggest that some water may flow across the Rialto Colton Fault at depth through partly consolidated Tertiary-age deposits. In both cases, the volume of water movement is likely small, and additional data collection and analyses would be required to quantify the rate of water movement. The hydrogeological data collected provided the context needed to support interpretation of chlorine and oxygen isotopes within perchlorate sampled as part of this study. The results were published in Hydrogeology Journal (Izbicki, J.A., N.F. Teague, P.B. Hatzinger, J. K. Böhlke & N.C. Sturchio, 2015, Hydrogeol. J., Vol: 23(3):467-491).

A detailed groundwater contour map in the Rialto-Colton, Chino, and North Riverside Basins in the vicinity of the Rialto-Colton Fault provides a better understanding of groundwater movement across the fault and sheds light on the effectiveness of the fault as a barrier to groundwater flow and the possible transport of perchlorate.