GROUNDWATER FLOW INTERACTIONS BETWEEN BASIN AND RANGE FAULTS AND THE MARKAGUNT GRAVITY SLIDE: IMPLICATIONS FOR THE PANGUITCH MUNICIPAL WATERSHED, SOUTHWEST UTAH

The Panguitch Municipal Watershed (PMW), located in the Dixie National Forest 10 km west of Panguitch, Utah, is underlain by part of the 5,200 km² Miocene Markagunt gravity slide (Biek and others, 2015). Panguitch relies on six springs in the PMW for the majority of its municipal water. Determining groundwater flow directions to these springs is complicated because of the combined affect of gravity slide structures and younger basin-range normal faults. Slide structures also change the properties of aquifers and confining units, most of which are volcanic rocks that predate the slide. Concern over potential groundwater contamination from livestock and forest fires prompted the Dixie National Forest to initiate a groundwater study to determine the origin and subsurface flow of the water that feeds the springs. Field mapping, 3-D fault modeling, and water quality monitoring were used to construct groundwater flow models for the watershed. A specialized model was created for ArcGIS products to create 3-D faults from new and existing 2-D geologic maps.

Concerns over groundwater contamination peaked during the course of the study, after the Brian Head Fire, the largest fire in Utah in 2017, burned more than 72,000 acres, including the PMW. Debris flows followed, caused by intense monsoon rains during July of 2017, damaging spring catchment facilities. These flows, as well as ash leachate infiltration and livestock re-admittance to the watershed, threatened to contaminate municipal water supplies.

The study showed that groundwater within the PMW is localized within alluvial aquifers, volcanic and volcanic sandstone bedrock aquifers, and fault zones. Two creeks, Delong Creek and Indian Hollow, have eroded through the upper plate of the Markagunt gravity slide, directly above the slide's primary ramp fault. The bedrock, intensely deformed by the slide, shows increased permeability and total potential water yield. The six springs in the PMW are located along Basin and Range normal faults and high angle gravity slide faults that crosscut the Markagunt Plateau, and their yields are clearly enhanced by the deformed slide rocks. Rock units south of the PMW springs dip north-northeast due to the ramp fault, allowing groundwater to be captured within the Butler Creek watershed to the south and from there to be guided northward to the PMW.