LAND SUBSIDENCE AND EARTH FISSURES IN CEDAR VALLEY, IRON COUNTY, UTAH

In May 2009, Enoch City contacted the Utah Geological Survey (UGS) to investigate a possible “fault” that was damaging infrastructure in a new subdivision in north Enoch. During a reconnaissance investigation, the UGS found a 2.4-mile-long earth fissure that had formed in response to land subsidence likely caused by ground-water overdraft (ground-water mining) of the local aquifer. In the affected subdivision, vertical displacement across the fissure has damaged streets and sidewalks, and has reversed the flow direction of a sewer line. The Central Iron County Water Conservancy District (CICWCD) subsequently funded the UGS to conduct a detailed investigation of the Enoch earth fissure, and to determine if land subsidence and earth fissures were affecting other areas in Cedar Valley.

A comparison of the current potentiometric surface with historical water-level data showed that ground-water discharge in excess of recharge since 1939 has lowered the potentiometric surface in Cedar Valley by as much as 114 feet (64 feet average). Using nearly 300 well drillers’ logs, we produced a map and simplified cross sections of Cedar Valley’s basin-fill sediments that show a high percentage of fine-grained material that is particularly prone to compaction upon dewatering. Survey-grade GPS surveying of benchmark elevations indicates that approximately 116 square miles of the ground surface in Cedar Valley has subsided as much as 4.5 feet since 1950. In response to the land subsidence, at least 3.9 miles (total length) of earth fissures have formed in the western and northeastern parts of Cedar Valley.

Our investigation documented that the areas of maximum land subsidence and earth fissure formation in Cedar Valley coincide with areas of significant ground-water-level decline and the presence of compressible fine-grained sediment in the subsurface. If ground-water levels in Cedar Valley continue to decline 3 feet per year (current average rate of decline), average basin-wide subsidence will likely continue at a rate of 0.04 to 2.4 inches per year. Continued ground-water mining and resultant subsidence will likely cause existing fissures to lengthen and new fissures to form that may eventually impact other developed areas in Cedar Valley.