PUMPING INDUCED SUBSIDENCE AND EARTH FISSURES IN CEDAR VALLEY, IRON COUNTY, UTAH

The Utah Geological Survey conducted a detailed investigation of an earth fissure damaging infrastructure near Enoch, in Cedar Valley, Utah, and determined if land subsidence and earth fissures were affecting other areas in Cedar Valley.

Work performed for this study included (1) mapping existing earth fissures in the Cedar Valley area, (2) documenting infrastructure and other development affected or potentially affected by earth fissures, (3) making a survey of benchmark elevations in Cedar Valley and preparing a land subsidence map, (4) defining the current potentiometric surface in Cedar Valley and comparing it with historical water-level data, (5) creating cross sections of basin-fill sediments based on well-log information, and (6) preparing a map of the potentially compressible sediment in Cedar Valley.

Results of this study indicate (1) since 1939, the potentiometric surface has declined by a maximum of 120 feet, (2) Cedar Valley aquifer sediments contain a high percentage of fine-grained material susceptible to compaction, (3) lowering the potentiometric surface has caused permanent compaction of the sediments in Cedar Valley, creating up to 4 feet of land subsidence, (4) in response to subsidence, nearly 4 miles (total length) of earth fissures have formed in the western and northeastern parts of Cedar Valley, (5) damage in Cedar Valley is currently limited to a partially developed subdivision in Enoch City, and (6) one fissure has displaced the ground surface in a heavily used livestock pasture/feeding area, creating a potential for ground-water contamination.

Based on the results, we conclude the following: (1) ground-water pumping in excess of recharge is the cause of the land subsidence and earth fissures in Cedar Valley, (2) if ground-water levels in Cedar Valley continue to decline 3 feet per year, average basin-wide subsidence will likely continue at up to 0.2 feet per year, (3) the inventory of earth fissures in Cedar Valley is incomplete because fissures without offset or erosion typically exist as hairline cracks, (4) future fissure formation is likely if ground-water overdraft continues, (5) enlargement of existing, currently unrecognized, or new earth fissures may damage existing infrastructure in Cedar Valley.