Effects of hydrologic variability and remedial actions on first flush and metal loading from streams draining the Silverton caldera, 1992–2014

Abstract

This study examined water quality in the upper Animas River watershed, a mined watershed that gained notoriety following the 2015 Gold King mine release of acid mine drainage to downstream communities. Water-quality data were used to evaluate trends in metal concentrations and loads over a twodecade period. Selected sites included three sites on tributary streams and one main-stem site on the Animas River downstream from the tributary confluences. During the study period, metal concentrations and loads varied seasonally and annually because of hydrologic variability and remedial actions designed to ameliorate the effects of acid mine drainage. Water-quality data were divided into two periods based on the timing of remedial activities in the watershed. The first period includes active water treatment, surface reclamation and installation of bulkheads in adits; the second period includes the decade following these activities. Water-quality data were used to estimate annual and monthly zinc loads using the Adjusted Maximum Likelihood Method (using LOADEST software) and U.S. Geological Survey streamflow data. This study presents one of the first applications of LOADEST focused on metal loads. Monthly flow-weighted concentrations were analysed using a Mann-Kendall trend test to determine the direction, magnitude, and significance of temporal trends in zinc loading in any given month and using t-test comparisons between the two periods. Zinc loads estimated for the Animas River below the tributaries indicate decreased zinc loading during the rising limb of the hydrograph in the second period, perhaps reflecting a reduction of snowmelt-derived zinc load following surface reclamation activities. In contrast, base-flow zinc loading increased at the main-stem site, perhaps because of the cessation of water treatment in tributary streams. Flow weighting of monthly load estimates yielded increased statistical significance and enabled more nuanced differentiation between the effects of hydrologic variability and remedial activities on zinc loading.

Climate‐Driven Increases in Stream Metal Concentrations in Mineralized Watersheds Throughout the Colorado Rocky Mountains, USA

“…The second approach involved computing and evaluating trends in flow‐adjusted concentrations, a method employed in previous water quality trend studies (e.g., Hirsch et al., 1991; Mast, 2013). Concentrations of relatively non‐reactive solutes produced by mineral weathering, such as SO 4 and Zn (and likely Cu at pH < 5), generally increase with decreasing discharge in mountain streams (Heil et al., 2022; Li et al., 2022; Petach et al., 2021; Winnick et al., 2017). For each sample site, a relationship between concentration and streamflow was estimated for SO 4 , Zn, and Cu by a regression line fit to data on a C‐Q plot of the log of the measured concentration ( C m ) versus the log of the “index flow” ( Q I ).…”

Hyporheic Reaction Potential: A Framework for Predicting Reach Scale Solute Fate and Transport

“…Acid-mine drainage (AMD) is a ubiquitous problem globally, generated from the oxidation of sulfide minerals, which produces low pH, high-metal­(loid) concentration waters that affect drinking water quality, aquatic toxicity to plants and animals, negative health outcomes, and bridge corrosion . Despite the $2.9 billion spent by the U.S. Government alone to mitigate effects of AMD on recreational and municipal waters, AMD-impacted waterways and watersheds persist, in part because mine-remediation efforts often focus on capture and treatment of point sources flowing from mines, while diffuse sources and oxidation pathways continue to source AMD at the watershed scale. , Additionally, groundwater-flow pathways, geochemical composition, reaction rates, and residence times are temporally variable and shift seasonally, which alters groundwater inputs and metal­(loid) loading into streams − making any remediation strategy difficult to employ over both time and space. Hydrologic events, such as snowmelt runoff and storms, affect the hyporheic zone, the mixing zone at the surface water-groundwater (SW-GW) interface.…”

Physical–Chemical Recovery of a Montane Stream After Remediation of Acid Mine Drainage: Timing and Extent After Turning off the Tap

“…Remediation of AMD‐generating mine sites can include a variety of physical–chemical improvements (e.g., containment, liming, or removal of contaminated tailings and soils; management of water flow paths; and treatment of contaminated surface water and groundwater; see Johnson & Hallberg, 2005; Kefeni et al, 2017; Naidu et al, 2019). Chemical changes in receiving waters during and after remediation are sometimes monitored (Petach et al, 2021; Runkel et al, 2009); however, most major AMD‐remediation efforts extend over a long period of years to decades (see Mebane et al, 2015; Petach et al, 2021), thus precluding analysis of the rate of physical–chemical recovery after individual remediation efforts are completed. Knowledge of recovery time frames following individual remediation efforts would help to predict future recovery at AMD‐affected sites.…”

“…Although many waters receiving AMD inputs have been studied worldwide, we are aware of only a few similar on–off situations that have been studied: Leadville Mine Drainage Tunnel on the Arkansas River, Colorado (Nelson & Roline, 1996); Junction Creek, Sudbury, Ontario, Canada (Gunn et al, 2010); and Lake Arnoux, Québec, Canada (Mocq & Hare, 2018). None of those authors reported extensive physical–chemical results; but Petach et al (2021) reported long‐term chronological sequences of a physical–chemical parameter (Zn concentrations), after more than a decade of remedial actions in the Upper Animas River watershed, Colorado. Most other studies have involved multiple, concurrent remediation activities, often spanning multiple years, that confounded before–after interpretation of the effectiveness of water treatment.…”