Matthew Olson and Dr. Greg Carling, Department of Geological Sciences
Southern Utah is world renowned for its beautiful canyons and red rock features. The Navajo Sandstone, an Early Jurassic formation created by 2000 feet of petrified sand dunes, is one of the primary features of this unique landscape. Groundwater in the Navajo Sandstone is important for sustaining life in arid southern Utah communities and ensuring recreational usability for state and national parks. Springs that discharge from the Navajo Sandstone aquifer in Zion National Park are a primary source of water to the Virgin River, which provides water for a number of towns in arid southern Utah (Christensen et al., 2005). The town of Springdale, for example, diverts water from the Virgin River year round for agriculture and drinking water. Other cities in southern Utah rely on water from the Navajo, which is accessed through wells and springs. A better understanding of the groundwater conditions in the Navajo Sandstone is needed in order to protect this valuable resource.
Groundwater recharge is defined as the water able to infiltrate into the subsurface and replenish the underlying aquifer. In arid regions, such as southern Utah, the amount of groundwater recharged (recharge rate) is only a small portion of the annual precipitation. The water that does not infiltrate into the aquifer travels on the surface as runoff, evaporates, or is used by plants in transpiration. In order to calculate recharge rates, data from multiple sources is needed to determine the amount of water entering and leaving the aquifer in a specific area. The presence of natural springs in sandstone outcrops allowed for this study to take place. In particular, springs found in sandstone alcoves allowed for a more accurate measurement of discharge from the aquifer. The area of infiltration is calculated taking into account the topography and probable groundwater flow direction. This value is compared with the annual precipitation of the region in order to calculate the amount of water that enters into the aquifer.
A significant amount of time was spent assessing potential springs that could be used for this study. Although a number of springs were found in sandstone outcrops, a fewer number were located in alcoves. Even more difficult proved to be finding springs that met these parameters and also had sufficient discharge data. Discharge data, or the rate of flow, for many springs throughout Utah was measured by the Unites States Geological Survey (USGS) and stored as KML data on waterqualitydata.us. The data, which includes water chemistry and temperature along with flow data, was sorted on excel spreadsheets and separated by location. Springs that provided adequate flow rates were observed using Landsat Imagery on Google Earth to determine whether the spring was located in a sandstone alcove. Spatial limitations and remoteness made this process somewhat difficult, however after observing much of the data, five springs were found to meet all parameters necessary for the study.
The five springs used for this study are located in alcoves of the Navajo Sandstone and are found near two of southern Utah’s national parks. Three of the springs; Holman Spring, Cabin Spring, and Neck Spring are on the Isle in the Sky area of Canyonlands Naitonal Park (figure 1). The other two springs; Sleepy Hollow Spring and Bar M Spring are located just outside of Arches National Park (figure 2). Data obtained from the USGS provided the amount of water that was discharging from the aquifer in these areas. In order to calculate the amount of water entering into the aquifer, the area of recharge was determined using ArcGIS (figure 1). A watershed model in GIS created by Dr. Dan Aimes from the engineering department at BYU was run using topography and flow direction based on slope in order to determine the parameters of the drainage basin for each spring. A polygon was created over the basin and was used to determine the area for each spring. This value was then multiplied by the annual precipitation of the region found at prism.oregonstate.edu. The annual precipitation was found at .234 inches per year. Comparing the spring discharge to the annual infiltration as a ratio, the recharge rate was able to be calculated. Table 1 shows the variables and recharge rates calculated for each spring: Previous studies by Dr. Greg Carling have shown that recharge rates in arid climates are generally between one and five percent. The results from this study show that three out of five springs were in conjunction with these past studies. Because it is difficult to determine aquifer characteristics and recharge rates, it is important that new methods be created to provide crucial understanding to arid communities and habitats that rely on these aquifers. Although, some results from this study had margins of error, this method has proven to be accurate to some degree. It is likely that other factors not mentioned in this study would contribute to the recharge rate. For example, evaporation and the transpiration of plants in the drainage basin was ignored, as well as transpiration at the spring site. The results would certainly improve if other components of the systems were factored into the calculation. However, the values calculated show that this method can be useful for gaining a better understanding of the recharge rate in an aquifer. This study could also be repeated in certain arid towns of southern Utah in order to help the citizens understand how slowly their aquifer recharges. This would certainly promote conservation and help regulate future development for these small but important desert communities.