Mammoth Spring outflow

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Mammoth Spring Hydrology

Project Chief: Lawrence Spangler, USGS Salt Lake City, Utah
Cooperator: U.S. Forest Service, Dixie National Forest
Period of Project: October 2006 to September 2010

PROBLEM

Sinkhole developed in basalt in the Red Desert area of the
Markagunt Plateau in southwestern Utah. This sinkhole and many others on the plateau are as much as 100 feet deep and formed from subsidence of the basalt into cavities dissolved in the underlying Claron limestone.

Mammoth Spring is one of the largest springs in Utah and discharges from the Claron Formation, a marly limestone which underlies volcanic (basalt) rock that caps the Markagunt Plateau in southern Utah. Dissolution of the Claron has resulted in a karstic terrain on the basalt that is characterized by extensive sinkhole development. Because of the close relation between land surface and the underlying groundwater flow system in this area, natural and anthropogenic activities on the surface such as logging, recreational vehicle use, and forest fires can potentially affect water quality and discharge of the spring. Therefore, understanding groundwater/surface-water relations in the Mammoth Creek watershed is important for effective resource management in the recharge area and the long-term protection of the spring and its ecosystem.

OBJECTIVES

1. Identify point sources such as losing streams and sinkholes, where surface water can rapidly recharge and potentially impact the aquifer directly,
2. Determine the relation between the groundwater flow system and surface-water drainage,
3. Determine groundwater travel times through the aquifer to Mammoth Spring, and
4. Determine baseline water quality and evaluate susceptibility of the spring to surface activities.

RELEVANCE AND BENEFITS

Groundwater movement in fractured volcanic rock and limestone is poorly understood when compared with that in homogenous media and these aquifers can be particularly vulnerable to human-induced impacts. Understanding groundwater/surface-water relations in this area will increase our knowledge of these types of flow systems, which then can be applied to similar hydrologic terrains elsewhere. Results of the study will be used by the U.S. Forest Service in their forest planning to assess aquifer vulnerability. The proposal also addresses USGS activities as described in the USGS Strategic Plan, 1996-2005, to meet the needs for earth science information critical for developing strategies for protection of groundwater resources.

APPROACH

Injection of fluorescein dye into swallow hole in the Claron Formation. This trace was recovered 8.5 miles away and 1,500 feet lower at Mammoth Spring.

A reconnaissance of the watershed upgradient of Mammoth Spring initially was done to locate surface-water inputs to the aquifer such as losing streams and sinkholes and to inventory significant recharge and discharge features. Fluorescent dye tracing was used to establish hydrologic connections between inputs in the recharge area and Mammoth Spring. This approach was used to determine groundwater time of travel, generalized flow paths between surface-water inputs and the spring, and the relation between the groundwater basin of the spring and the surface watershed. Activated charcoal detectors were used to detect the injected dye, which was then analyzed by either visual or fluorometric methods.

Water-quality samples were collected during base (low) flow and spring runoff, and analyzed for major ions to further evaluate potential sources of the spring water and to characterize water quality at different flow regimes. Selected samples also were analyzed for tritium, sulfur-35, and oxygen/deuterium isotopes, and dissolved gases (chlorofluorocarbons (CFCs)), to help determine sources, mixtures, and relative ages of the groundwater. Continuous measurements of water temperature and specific conductance were made to establish relations between snowmelt runoff and changes in spring flow and chemistry. Discharge (stage) of Mammoth Spring also was recorded continuously during the 3-year study period, along with current meter measurements, to determine the range in variability of the spring in response to precipitation events and to compare with changes in water-quality parameters.

PRODUCTS

Results from this 3-year study will be published as a USGS Scientific Investigations Report in 2011. Interim results of the study also were presented at the Geological Society of America Rocky Mountain Section (Orem, Utah) and National (Denver, Co.) meetings in 2009 and 2010, respectively, and were also published in the USGS Karst Interest Group meeting proceedings (2008) and the Utah Geological Association guidebook to south-central Utah in 2010.