This 116-page report presents the results of an investigation by the Utah Geological Survey of land subsidence and earth fissures in Cedar Valley, Iron County, Utah. Basin-fill sediments of the Cedar Valley Aquifer contain a high percentage of fine-grained material susceptible to compaction upon dewatering. Groundwater discharge in excess of recharge (groundwater mining) has lowered the potentiometric surface in Cedar Valley as much as 114 feet since 1939. Groundwater mining has caused permanent compaction of fine-grained sediments of the Cedar Valley aquifer, which has caused the land surface to subside, and a minimum of 8.3 miles of earth fissures to form. Recently acquired interferometric synthetic aperture radar imagery shows that land subsidence has affected approximately 100 mi² in Cedar Valley, but a lack of accurate historical benchmark elevation data over much of the valley prevents its detailed quantification. Continued groundwater mining and resultant subsidence will likely cause existing fissures to lengthen and new fissures to form which may eventually impact developed areas in Cedar Valley. This report also includes possible aquifer management options to help mitigate subsidence and fissure formation, and recommended guidelines for conducting subsidence-related hazard investigations prior to development.

The purpose of this study is to assess the impact of septic tank soil-absorption systems on ground-water quality for three areas in Cedar Valley where septic tank soil-absorption systems are typically used for wastewater disposal. These areas have some existing development, but we anticipate that there will be additional development in the future. The Utah Geological Survey evaluated the potential impact of the projected potential development on ground-water quality based on septic-tank-system densities using a mass-balance approach similar to an analysis conducted by Hansen, Allen, and Luce for Heber and Round Valleys, Wasatch County, Utah. The selection of the evaluated areas was made in consultation with local government officials. This study may be used as a model for other evaluations of the impact of proposed subdivision site(s) on ground-water quality and allow planners to more effectively determine appropriate development densities.

This CD contains a 125-page comprehensive study of the hydrogeology of Cedar Valley, Utah County, located in north-central Utah. The report includes 72 figures; two plates, one of which is a potentiometric map of the basin-fill, bedrock, and several perched aquifers; and seven appendices of data. Field investigations included groundwater chemistry sampling, regular water-level monitoring, and multiple-well aquifer testing. The field data were incorporated into a 3D digital groundwater flow model using MODFLOW2000. Seventy percent of the recharge to the Cedar Valley aquifer system is from precipitation in the Oquirrh Mountains. Groundwater generally flows from west to east and exits the aquifer system mostly as interbasin flow through bedrock to the northeast and southeast. The groundwater model showed a 39-year (1969-2007) average recharge to the Cedar Valley groundwater system of 25,600 acre-feet per year and discharge of 25,200 acre-feet per year. A significant volume of precipitation recharge (perhaps 4300 acre-feet per year) does not interact with the basin-fill aquifer but travels within bedrock to discharge to adjacent valleys or as bedrock well discharge. 125 pages + 2 plates

This report characterizes the relationship of geology to groundwater occurrence and flow, with emphasis on determining the thickness of the valley-fill aquifer and water yielding properties of the fractured rock aquifers. Develops a water budget for the drainage basin and classifies the groundwater quality and identifies the likely sources of nitrate in groundwater.