Earth fissuring related to ground-water extraction and land subsidence has occurred in the Chino basin of southern California. A study of earth fissuring at the California Institution for Men-Chino (CIM) during 1993 and 1994 revealed a north-south-trending zone of fissures and ground distress approximately 150 m (490 ft) wide and greater than 1800 m (5900 ft) long through the western portion of CIM. This zone coincides with the area of maximum convex-upward curvature of subsidence profiles in an area of pumping-induced land subsidence. The fissures are expressed at land surface as prominent grabens, aligned sinkholes,open cracks, and irregularly shaped areas of subsidence. Deeper than 2.4-3 m (8-10 ft) below land surface, they are vertical, relatively uniform in width (less than 12-37 mm, [0.5-1.5 in]), filled predominantly with fine sand and silt, and, to a depth of at least 8 m (26 ft), they do not appear to narrow. The fissures at CIM are believed to have propagated upward from depth. The occurrence and appearance of fissures at CIM are similar to those reported from other alluvial basins in the western United States that have experienced extensive ground-water withdrawals. Since 1992, studies of general hydrogeologic conditions and ground-water quality at CIM have been in response to detection of volatile organic compounds in water produced from a former water-supply well. On at least one occasion, during heavy rains in January 1995, surface runoff flowed directly into fissures, indicating that they act as conduits for filtration of water into the subsurface. Results of these studies also suggest that fissures may provide preferential pathways for migration of water into or through the regional aquifer system. Indirect evidence of flow through fissures includes the seasonal occurrence of a perched body of ground water and mounding of the regional water table beneath the zone of fissures and ground distress. Ground-water quality studies have not conclusively shown that contaminated ground water moves through the fissures, However, current and planned studies should provide rr the influence of earth fissures on ground-water flow and tial effects of earth fissures on the ground-water-flow system in the Chino basin will be considered during planning for future ground-water extraction and ground-water-quality management at CIM.
