Study Reveals Wastewater-Related Contaminants Found In South Rim Springs

A study published in 2025 (attached) reveals that the U.S. Geological Survey and National Park Service identified wastewater-related contaminants of emerging concern (CECs) in several springs along the South Rim of the Grand Canyon. The springs were tested in 2021 and are located north of Bright Angel Wash, a channel that receives treated effluent from the South Rim Wastewater Treatment Plant.

The findings are concerning considering that evidence shows treated wastewater is finding a flow path back to water sources along the South Rim. The study notes that the springs, seeps, and tributaries of the Colorado River within the Grand Canyon provide most of the reliable drinking water resources to humans and animals in the region. The waters also contribute to a diverse and productive habitat in the Colorado River Basin, making the water quality highly important to Grand Canyon National Park.

Wastewater treated at the South Rim Wastewater Treatment Plant is reused in irrigation, facilities (toilets), and fire management. However, the study points out that upwards of a third of the treated wastewater is discharged back into Bright Angel Wash about a mile southwest of Grand Canyon Village, leading it to potentially mix with natural spring waters on the South Rim.

Once CECs make it into surface water and groundwater, they can have a detrimental effect on fish and other aquatic species, such as those effects associated with endocrine disruptors, including changes in behavior, reproduction, and sexual differentiation.

The CECs identified in the study included several PFAS and pharmaceutical compounds, though pharmaceuticals were only detected at Monument Spring, which sits at the headwall of a small tributary on the east side of Monument Creek. PFAS were found at Bright Angel Wash, Monument Spring, and upper Horn Bedrock Spring. At Monument Spring, the two pharmaceuticals detected at the highest concentrations were diphenhydramine (antihistamine) and carbamazepine (anticonvulsant or anti-epileptic drug), though metformin (diabetes drug), sulfamethoxazole (antibiotic), fluconazole (antifungal), and venlafaxine (antidepressant and nerve pain medication) were also found at lower concentrations.

The study notes that the CEC concentrations measured at Monument Spring did not exceed any drinking water standards or human health benchmarks, though most of the compounds detected have no regulatory standards. Studies looking at the ecological effects of CECs show that some can have endocrine and physiological effects, but generally, effects were observed at concentrations multiple orders of magnitude greater than what was measured during the study.

The study also noted that despite the low concentrations measured in the samples, some of the detected pharmaceuticals could still cause negative effects on aquatic macroinvertebrates, fish, algae, and microbial communities because of the complex interactions of environmental conditions and pharmaceutical mixtures in the environment.

The study points out that South Rim ecosystems rely on much less precipitation than the North Rim ecosystems, and spring habitat within South Rim ecosystems is more sensitive and vulnerable to changes in water quantity and quality. For the most part, Grand Canyon National Park is absent of major contaminant issues. The wastewater influence from the South Rim is a unique situation in a complex setting where wastewater is influencing at least one spring along the South Rim. In this case, species relying on these spring habitats are potentially exposed to chronic low levels of CECs, which could lead to greater exposure, biomagnification with increasing trophic levels, and possible generational effects.