The Seasonal Variability of Trace Metals and their Transport Mechanisms in Intermittent Streams

Abstract

Many intermittent and ephemeral washes drain metals-rich catchments, but the natural concentrations andtransport mechanisms of metals in their streamwater discharge are mostly unknown. In the 1960s, anomalously high metal content detected in southeastern Arizona streams led to the discovery of porphyry copper deposits in upstream areas of the Santa Rita Mountains. Although no large-scale mining operations have yet occurred, ground preparation activities have taken place in a catchment that is drained by the Barrel Canyon wash. Mine disturbances have the potential to not only impact water quality in Barrel Canyon, but also downstream in Davidson Canyon and in Cienega Creek Natural Preserve—sections of which are designated as “Outstanding Arizona Waters” by the State of Arizona. In addition to compiling historical records of trace metals concentrations in Davidson Canyon and CienegaCreek since 2002, this study also collected 128 new samples at four different sites between January 2021 and February 2024. Three of the four sites are connected in series via ephemeral surface washes and intermittent/perennial sections of streamflow, and could be potentially impacted by mobilized contaminants originating from the porphyry copper deposit in the Santa Rita Mountains. The fourth site is a perennial reach of Lower Cienega Creek, and is upstream of potential contamination originating from the Santa Rita Mountains. ISCO autosamplers were installed and used at each site to capture stormwater discharge brought on by heavy rains and monsoonal thunderstorms. Additionally, surface water baseflow at each site was frequently sampled throughout the study’s sample-collection period. Results from all four sites indicate that the concentrations of trace metals and major dissolved ions instormflow generally have opposite behaviors. When groundwater-fed baseflow is the primary contributor to streamflow, the concentrations of both particulate and dissolved trace metals are low, while the concentrations of major dissolved ions are high. During stormflow, when the stream is composed mostly of precipitation runoff, dissolved ion concentrations are low, while the concentrations of both particulate and dissolved trace metals are at their highest. These results suggest that the sources of trace metals in the Davidson Canyon and Cienega Creek subwatersheds are not from groundwater, but instead are being flushed out from the soil profile at relatively shallow depths during heavy rains. A seasonal cycle of dissolved arsenic concentrations increasing and decreasing in tandem withstreamwater temperature was observed at the three sites connected in series. Dissolved arsenic concentrations at the fourth site were generally higher but also more variable than at the other three sites. Possible reasons for these observed trends, such as the redox environment and the activity of iron-cycling microbes are discussed. Results from water quality analyses were compared to relevant federal and state water quality standardsspecific to trace metals. 14 of the 128 new samples exceeded at least one water quality standard, and many samples exceeded multiple different standards concurrently. 11 out of the 14 samples that exceeded thresholds were found in either Barrel Canyon or Davidson Canyon—the two sites closest to potential mine runoff. The five primary contaminants of concern were arsenic, lead, copper, iron, and zinc. This study found evidence that the concentrations of trace metals in Barrel Canyon wash, DavidsonCanyon, and Lower Cienega Creek have increased from their historic norms, beginning around late 2022. In particular, dissolved zinc concentrations in baseflow were found at unprecedented levels. It is possible that metals-rich soil from the Santa Rita Mountains was disturbed during mine preparation activities, and is now reacting with and/or being transported downstream by oxygenated rainwater, causing metals concentrations to increase along the drainage flow path. These results highlight the vital importance of continual water quality monitoring in areas that could be impacted by mining activities.