Bed Sediment Metal Concentrations as Indicators of Ecosystem Recovery in the Gallinas Creek Watershed

Measures of post-wildfire effects and recovery often include constituent-based approaches that identify natural and/or anthropogenic disturbance. Water quality and bed sediment studies are two such examples in assessing trends associated with mobilization and transport. In 2022, the Calf Canyon/Hermits Peak Wildfire burned 95% of the Gallinas Creek watershed. Because the El Porvenir Mining District (Quartz-Pegmatite vein) lies within the watershed, metal concentrations were used to establish a baseline of “post-fire condition” with a goal of evaluating recovery over space and time. Bed sediment samples from Gallinas Creek and several tributaries were collected 5 months post fire. Samples were sieved to <63 µm to reduce grainsize bias and allow for spatial comparisons relative to legacy mining activities, and analyzed for arsenic, cadmium, and copper concentrations. In Gallinas Creek, arsenic concentrations were lowest at the most upstream station (Gallinas Creek at Oak Flat, 4.1 ± 1.1 µg/g) and greatest at the most downstream station (Gallinas Creek near Montezuma, 5.3 ± 0.3 µg/g). Arsenic concentrations in streams in the Beaver Creek sub-watershed were up to twofold higher than in the Gallinas, ranging from 6-10 µg/g. Cadmium concentrations followed a similar pattern: concentrations were lowest in the most upstream station on the Gallinas (0.2 ± 0.1 µg/g) and three-fold higher at the downstream station (0.6 ± 0.2 µg/g). Concentrations of cadmium in Beaver Creek, and intermittent streams, were variable, ranging from 0.4-0.7 µg/g. Unlike arsenic and cadmium, there was no unidirectional downstream decline of copper in the Gallinas, and concentrations were low (14-25 µg/g) at all sites apart from a sediment deposit near the mouth of the Hermits Peak Mine adit (100-443 µg/g), likely due to the proximity to legacy mining activities. Although abiotic indicators are key in identifying metal sources, biological effects associated with elevated metal concentrations can only be inferred. Tissue residue studies using resident aquatic organisms measure bioavailability and provide a direct link between chemical exposure and biological effects. Benthic macroinvertebrates were not present at any of the sediment monitoring stations, likely due to the physical disturbance (e.g., high turbidity, scour, debris flows) caused by the wildfire. However, as habitat conditions improve, aquatic insects will recolonize, and, as part of this study, allow for tissue metal analysis and biological assessment of ecosystem recovery.