Status and trends of water quality at Zion National Park: Water years 2006–2016

Weissinger, Rebecca; Sharrow, Dave

The National Park Service collects water-quality samples approximately monthly at three fixed water-quality stations in Zion National Park: La Verkin Creek, North Creek, and the North Fork Virgin River. Data collection began at La Verkin Creek and North Creek in November 2005, and at the North Fork Virgin River in July 2008. This report analyzes data from the beginning of the period of record for each station through water year 2016 to test for trends over time. Concentrations are also compared to relevant water-quality standards for the State of Utah. Key findings include: • Continued exceedances of state water-quality standards for fecal indicator bacteria in the North Fork Virgin River indicate a need for improved management upstream of the park to protect public health and safety and the need for continued monitoring as management actions are implemented. • Water quality in North Creek appears to be stabilizing after a wildfire burned much of the watershed in 2006. • Changes in water quality at La Verkin Creek are likely to be natural in origin and are not of a magnitude that warrants management concern at this time. We used a seasonal Kendall test to test for trends in median concentrations over time. Few trends were detected in any water-quality parameters at any of the stations evaluated for Zion National Park, indicating that seasonal median concentrations have remained relatively stable over time. Only specific conductance, sulfate, and turbidity at La Verkin Creek showed statistically significant declines in seasonal median concentrations. The cause of these declines is not known, but likely is natural because this watershed has few developments and no changes in land uses or management practices. North Creek and the North Fork Virgin River had no statistically significant trends in seasonal median concentrations. A portion of the North Creek watershed burned in 2006, followed by a catastrophic flood in 2007. While seasonal median concentrations did not show statistically significant trends after the fire, high and low concentrations in the dataset appeared to change over time. To evaluate changes in high and low concentrations, we used quantile regression to test for trends at the 0.1 and 0.9 quantiles, corresponding to the lowest 10% and highest 90% of data values. Alkalinity, nitrogen, and specific conductance decreased in the 0.9 quantile for post-fire concentrations at North Creek, and pH increased in the 0.1 quantile over time. Trends in the low and high concentrations of water-quality parameters suggest that post-fire variability is stabilizing in North Creek. Nitrogen species may have fallen below a critical threshold for eutrophication. This observation is supported by a reduction in observed algal blooms at the site since 2006. Although not statistically significant, there have also been fewer exceedances of the pH water-quality standard in North Creek in recent years. Comparisons to water-quality standards reiterated previously recognized management issues, including pH at North Creek and fecal-indicator bacteria (E. coli) and water temperature at the North Fork Virgin River just upstream from the park boundary. The State of Utah has included the North Fork Virgin River on a list of impaired waters, known as a 303(d) list, for these parameters. The Utah Division of Water Quality is undertaking a Total Maximum Daily Load analysis for the North Fork that will include a plan to bring surface-water quality into compliance. Given its location in a warm desert environment, the state is investigating whether the North Fork Virgin River in Zion National Park should be reclassified from a cold-water aquatic system to a warm-water aquatic system. Extensive sampling and analysis indicate that the greatest amount of E. coli loading is from pastures adjacent to the river that are flood-irrigated and grazed. Other possible sources include human waste.

Status and trends of water quality at Zion National Park: Water years 2006–2016

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