Dye Tracing at Grand Canyon: Snapshot of a Complex Groundwater System

Most of the snow that falls on the rims of Grand Canyon National Park (GRCA) makes its way into the groundwater and flows from hundreds of springs across the canyon. Many aspects of the park's groundwater system are poorly understood, including the pathways water takes through thousands of feet of rock, how quickly water travels through those pathways, and the vulnerability of springs to contamination and climate change.

To better characterize the park's groundwater resources and answer these questions, park scientists and cooperators have conducted groundwater tracer studies on the North Rim (Jones et al. 2018; Tobin et al. 2021) and on the South Rim by injecting and tracing dye from sinkholes and faults on the rims to springs in the canyon. With the forecasted decrease in groundwater in the Grand Canyon region because of climate change (Tillman et al. 2020) and continued development on the rims, these studies are crucial for determining areas of vulnerability that will inform park management decisions on protection of the park's water supply.

Methods of Dye Tracing in Remote Wilderness: North Rim

The first dye trace study ever conducted in the Grand Canyon region took place from 2015 to 2017 and was the deepest and longest dye trace in the United States to date (Jones et al. 2018; Tobin et al. 2021). The goal of this project was to outline an area on the North Rim that directly contributes water to Roaring Springs, the park's only drinking water supply for developed areas.

Four food-grade, fluorescent dyes were injected into five sinkholes on the North Rim over this period, and monitoring occurred at 29 remote locations in the canyon (Fig. 1). Packets of activated charcoal (dye receptors) were installed at these locations to capture dye by adsorption or molecular bonding to the surface of the charcoal (Fig. 2c). These dye receptors were replaced every month for three years to test for dye presence, requiring thousands of hours of field work by park scientists, technicians, cooperators, and volunteers. Access to these remote sites was extremely difficult and was achieved through backpacking in off-trail terrain, by helicopter, and by rafting on the Colorado River.

Once dye receptors were collected, they were analyzed for the presence of any of the four dyes. If one or more of the dyes was detected at a location, the associated spring would be considered connected to the sinkhole with the matching dye(s). Three of the five dyes were received at 20 receptor sites across a 50 km distance (about 31 miles) during this study. Although dye could only be detected at monthly intervals, other evidence suggests that water moves through the system in a matter of days (Jones et al. 2018). However, dye detection at this large number of sites represents an incredible success considering the huge distance dye traveled through the system, the complexities of groundwater flow paths, the extreme remoteness of receptor sites, and the risk of dye degrading in the sun once discharged at springs.

The results of the North Rim dye trace revealed that springs are part of a large, interconnected groundwater system (Tobin et al. 2021) and that flow paths are more complicated than previously thought. While the sinkhole injection locations were chosen based on their theorized contribution to Roaring Springs (Jones et al. 2018), only one of the injected dyes was received there. A preliminary contributing area for Roaring Springs was outlined on the North Rim based on this result, and a water budget—an account of the water entering, leaving, and stored in a groundwater system—was calculated using this contributing area (Chambless et al. 2023).

This first pass at a water budget showed imbalanced accounts, further indicating the complexity of the system, and highlighting the need for additional dye tracing to refine this contributing area. Spring contributing areas and water budgets can then be used with climate data to forecast how springs may be affected by climate change. Dye tracing is therefore an integral part of projecting changes in the GRCA groundwater system.

Methods of Dye Tracing in Remote Wilderness: South Rim

Dye tracing is also being used to assess the influence of treated wastewater on springs below the South Rim. Understanding the vulnerability of these springs to contamination is critical, as many riparian species rely on these springs for habitat, and backcountry users rely on them for drinking water.

In 2021 and 2022, the United States Geological Survey (USGS) collected water samples from treated wastewater discharged from the South Rim Wastewater Treatment Plant into Bright Angel Wash (a channel underlain by a major fault) and from springs below the rim to test for non-natural contaminants that are indicators of wastewater influence (Beisner et al. 2023). These include per- and polyfluoroalkyl substances (harmful manufacturing chemicals, referred to as PFAS), pharmaceuticals, nitrates, caffeine, artificial sweeteners, and other chemicals that indicate human influence and are not yet regulated in wastewater by the Environmental Protection Agency. Several of the contaminants that were detected in treated wastewater were also detected at two springs below the South Rim: Monument and Horn springs.

To follow up on these contaminant detections, the USGS partnered with the GRCA Hydrology Program to conduct a major dye trace project on the South Rim of the park, funded by a USGS/NPS Grant and Grand Canyon Conservancy. The purpose of this dye trace is to better understand groundwater flow paths and rates below the South Rim. In April 2023, scientists released dye into the treated wastewater in Bright Angel Wash, which quickly sunk into the subsurface (Fig. 2b). The methods of this ongoing dye tracer are the same as those of the previous study on the North Rim, though there is a higher resolution instrument at Monument Spring to continuously measure dye concentrations there. As of August 2023, no dye has yet been detected at springs below the South Rim, but scientists will continue to monitor these sites over the next two years.

Dye Trace and Grand Canyon Drinking Water

Over the next decade, the Grand Canyon National Park water delivery infrastructure and wastewater treatment systems will be upgraded. There are also plans to change the South Rim

and most of the inner canyon water supply from Roaring Springs groundwater to Bright Angel Creek (BAC) surface water (Fig. 1).

This change in the drinking water source should increase resiliency to climate change, as water volume of Bright Angel Creek is much larger than that of Roaring Springs. There are at least 34 springs that contribute water to Bright Angel Creek, so the creek's contributing area on the North Rim is much larger than the Roaring Springs contributing area. This larger contributing area increases the vulnerability of the water supply to contamination from activities on the North Rim, especially considering that water can travel through the system in a matter of days (Jones et al. 2018).

These activities may include infrastructure development, road work, vehicular accidents, oil and chemical spills on State Highway 67, possible leaks from gas stations, construction staging areas, contamination from bison and cattle wallowing in and around sinkholes, and a wastewater treatment plant. Therefore, outlining the BAC contributing area and calculating its water budget with additional dye trace studies on the North Rim will be crucial to determine spring vulnerability to contamination and how climate change will affect the connected springs.

The GRCA Hydrology Program plans to begin additional dye injections on the North Rim and tracing in the canyon beginning in spring 2024 and will continue through spring 2026. This project will be carried out in cooperation between GRCA, GCC, Northern Arizona University, and USGS. Already, GCC 2023 donor funds are facilitating preliminary work on the North Rim to select sites for dye injection. This dye tracer project will face the same access limitations and exceptionally high field work costs as the 2015–2017 dye trace project. It will also include 20 additional dye receptor sites and a continuous dye detection instrument at selected sites to capture flow rate through the system hourly rather than monthly.

We hope to continue unraveling the complexities of this groundwater system through this cooperative project to inform park management about contamination vulnerability and how declining groundwater recharge from climate change may affect springs in the canyon. These vital pieces of data will be used by park management to best protect Grand Canyon National Park springs for the millions of annual visitors and ecosystems that depend on them.

References

Beisner, K.R., Paretti, N.V., Jasmann, J.R., Barber, L.B. (2023) Utilizing anthropogenic compounds and geochemical tracers to identify preferential structurally controlled groundwater pathways influencing springs in Grand Canyon National Park, Arizona, USA, Journal of Hydrology. 48(1010461). https://doi.org/10.1016/j.ejrh.2023.101461.

Chambless, H.E., Springer, A.E., Evans, M.A., Jones, N.A. (in press) Deep-karst aquifer trends in a water-limited system, Grand Canyon National Park. Hydrogeology Journal.

Jones, C.J.R., Springer, A.E., Tobin, B.W., Zappitello, S.J., Jones, N.A. (2018). Characterization and hydraulic behavior of the complex karst of the Kaibab Plateau and Grand Canyon National Park. Geological Society Special Publicatio

Tillman, F.D., Gangopadhyay, S., Pruitt, T. (2020). Recent and projected precipitation and temperature changes in the Grand Canyon area with implications for groundwater resources. Scientific Reports. 10 (19740). https://doi.org/10.1038/s41598-020-76743-6

Tobin, B.W., Springer, A.E., Ballensky, J., Armstrong, A. (2021). Cave and karst of the grand canyon world heritage site. Zeitschrift Fur Geomorphologie. 62, 125–144, https://doi.org/10.1127/zfg_suppl/2021/0693