Ecological Implications of Climate Change Enhanced Acid Rock Drainage in the Colorado Rocky Mountains

Preserving the ecological integrity of Colorado’s headwater streams is a challenge well-recognized for the pressures acting against it. At the forefront of issues threatening alpine aquatic ecosystems are degradation to water quality associated with acid rock drainage and shifting hydrologic regimes due to climate change. While concerns surrounding natural and anthropogenic acid rock drainage have been established across the region for decades, the progression of climate change is revealing its role in accelerating the geochemical processes behind acid rock drainage and its impairment to mountain streams. The vital intersection between these two threats is emergent to freshwater ecosystems in localized areas across the mineralized landscapes of the Rocky Mountains, where warmer temperatures, prolonged drought, and changing snowpack trends are critical factors enhancing such vulnerability.

Acid Rock Drainage

Acid rock drainage (ARD) is a natural process across mountain landscapes where sulfide-bearing minerals are present. Pyrite (FeS2) and several other metal-sulfide minerals (e.g., ZnS, FeAsS, CuFeS2) are the most common geologic sources that contribute to ARD in mountain regions, where environmental conditions are conducive to the oxidation of such minerals (Zarroca et al., 2021). Oxidation is a form of geologic weathering in which the exposure of minerals to oxygen and water results in their chemical decomposition. At the earth’s surface, oxidative conditions are abundant in the presence of atmospheric oxygen and surface water flows. However, even at depth, subsurface flows of oxygen and water throughout the vadose zone and geologic bedrock fractures can create an environment where oxidation drives the chemical breakdown of belowground minerals. For ARD, the interactions between oxygen, water, and sulfide-bearing rocks lead to a chemical reaction with one critical by-product: sulfuric acid (H2SO4). Sulfuric acid concentrates within the hydrologic flows that catalyze this oxidation reaction, which in turn acidifies the surficial and subsurface drainage flows that recharge nearby streams. Simultaneously, acidic conditions within runoff, baseflows, and shallow groundwater accelerate the dissolution of metals bound to their mineralogical structure, mobilizing metals into their ionic forms within solution (Figure 1). Any metal-bearing minerals exposed to this process are susceptible to enhanced mobilization, while trace metals (e.g., Fe, Zn, Cu, Cd, Al, Mn, Pb), metalloids (e.g., As), and rare earth elements (e.g., lanthanides) are particularly concerning (Rue & McKnight, 2020). Elevated concentrations of such metals impose lethal consequences to aquatic biota—some having more sinister impacts than others. Combining such toxicity with the deleterious effects of acidity on aquatic communities, ARD has frequently demonstrated its capacity to decimate ecosystem health.

Within high elevation streams of the Colorado Rocky Mountains, mineral rich geology is a source of water quality degradation given its role in ARD. Active ARD and its adverse impacts have been identified among eleven headwater regions across the state (Figure 2), including the unmined portions of the Upper Snake River basin near Silverthorne, CO and the Animas River near Silverton, CO. Given Colorado’s mining history, it is important to emphasize that ARD occurs naturally where sulfide-bearing minerals undergo oxidation and produce metal-mobilizing sulfuric acid at the earth’s surface and below it (Sares et al., 2006). This natural process has been expedited by anthropogenic activities since the Colorado Mineral Belt’s prosperous reputation fueled the state’s mining history throughout the 19th and early 20th centuries. As an artifact of unregulated industry across the landscape, past mining practices have left behind thousands of abandoned mines and exposed sulfide-bearing minerals as the waste from extracted ores. These legacy mines and their remaining wastes have proliferated the risk of ARD to water quality across the Rocky Mountains with what is referred to as acid mine drainage (AMD). AMD is an anthropogenically accelerated form of ARD in which mining activities have extracted sulfide-bearing minerals that were once contained within deep, underground mineral deposits – an environment where oxygen is unlikely to be present, and thus, oxidation is unlikely to occur. Given that such sulfide-minerals have been left exposed to water and oxygen at the earth’s surface, the formation of sulfuric acid via oxidation results in the acidification of surficial flows and the leaching of metals into surface waters. Today, approximately 23,000 abandoned mining-related sites remain across Colorado with 1,800 miles of streams impaired by their impacts (CDPHE, 2017). Unfortunately, as the threat of climate change progresses, the impact of natural and anthropogenic ARD is anticipated to worsen.

The Enhancement of Climate Change on Acid Rock Drainage

Climate’s role in governing the geochemical processes of ARD has gained attention in recent decades. Temperatures and snowmelt patterns are dynamic components within the hydrology of alpine watersheds, and thereby regulate the interaction between water, oxygen, and sulfide-bearing minerals. In particular, warming temperatures and shifting snowmelt regimes associated with climate change have shown to enhance the oxidization of sulfide-bearing minerals resulting in ARD. One factor involves the accelerated rate at which oxidation reactions can occur as warmer air and ground temperatures enhance the kinetics of chemical weathering processes (Crawford et al., 2019). Magnifying this effect, several studies have also documented that reduced moisture availability throughout the summer season now leads to increased oxidative exposure among the reserve of subsurface sulfide-bearing minerals naturally present within mineralized watersheds (Todd et al., 2012; Nordstrom, 2009; Zarroca et al., 2021; Rue & McKnight, 2021).

Higher mean summer air temperatures effectively increase the rate of evapotranspiration from soils and vegetation communities such that moisture replenishment within soils (i.e., recharge) is expected to decline, contributing to a lowered water table (Todd et al., 2012). Moreover, prolonged dry seasons occurring due to climate change-intensified drought and earlier annual snowmelt exacerbate subsurface dryness. Less accumulated snowpack in the winter combined with the advancement of earlier snowmelt during the spring is lengthening the Colorado dry season, where sufficient soil moisture, shallow groundwater, and streamflow are maintained to a lesser extent throughout the summer by snowmelt. As such, extended and intensified dry periods encouraging soil desiccation and a consequential drop in water table elevation are increasing the amount of subsurface sulfide minerals exposed to oxygen, rather than being protected from atmosphere within saturated subsurface conditions where oxygen is limited (Todd et al., 2012; Nordstrom, 2009; Zarroca et al., 2021; Rue & McKnight, 2021; Figure 3). When intermittent exposure to water occurs, sulfuric acid is produced from the oxidation of sulfide-bearing minerals. Acidified subsurface flows then dissolve metal-bearing minerals naturally present within soils and bedrock, and metal ions are transported and discharged into adjacent streams in elevated concentrations. Moreover, when streamflow is maintained by subsurface drainage throughout the dry season, decreased dilution rates from snowmelt further magnifies the acidification and metal enrichment caused by contaminated recharge.

Within the Rocky Mountains, long-term studies of natural ARD and anthropogenic AMD across the Upper Snake River Basin (USRB) have been paramount to discovering climate change’s enhancement of ARD. Across the last 40-years, the rate of increase in dissolved Zn, rare earth elements (REE’s), and metal-hydroxide deposition has aligned with climate change enhancing factors, such as warming temperatures, reduced soil moisture recharge, and reduced streamflow (Todd et al., 2012; Rue & McKnight, 2021). Data presented within Figure 4b demonstrate the continued increase of mean summer air temperatures and gradual decline of streamflow during baseflow dominant conditions, both of which coincide with a distinct rate of increase for Zn and Neodymium (Nb)—a rare earth metal. Additionally, daily loading rates for other heavy metals and REE’s within the USRB demonstrate corresponding rates of bioaccumulation within benthic macroinvertebrates which tend to be more tolerant of metal-enriched waters (Table 1; Rue & McKnight, 2021).

Another climate change trend in semi-arid environments is the decreased frequency, yet increased intensity of precipitation events. Nordstrom (2009) identified the propensity of intense rainstorms to weather minerals and flush greater quantities of acidic drainage and metal concentrations from ARD sites into nearby waters, including those contaminated by western mountain mining operations. As well, melting permafrost within mineralized watersheds is an emergent concern, given that the pores and rock fractures of thawed soils present new pathways for water and oxygen to reach freshly exposed, reactive surfaces of sulfide mineral sources. In the Southern Pyrenees, Zarroca et al. (2021) has observed magnification and expansion of ARD to higher elevations in the last several decades which has aligned precisely with the retreat of the periglacial zone and degradation of permafrost.

Risks of intensifying or broadening ARD are present within mineralized alpine catchments alike, such as those in the Rocky Mountains experiencing permafrost loss. In 1971, Ives & Fahey identified that permafrost within the Colorado Rocky Mountains begins at 10,500 ft. Recent evidence has signified that permafrost is thawing within this alpine region, as Knowles et al. (2019) identified the net production of carbon dioxide due to heterotrophic respiration (i.e., organic matter decay) within an alpine tundra zone along the Continental Divide. For the USRB, although remaining permafrost locations are unknown, Todd et al. (2012) reasons that thawed permafrost likely contributed to increased ARD during their study period within the 1990’s. Since then, Zn and REE’s have continued to accumulate within ARD-affected streams across the watershed (Figure 4; Rue & McKnight, 2021). Determining the intersection of thawing permafrost within mineralized watersheds will be critical to understand the growing impacts of ARD on aquatic ecosystems, whether within the eleven headwater zones where ARD is currently known and may expand, or those in which it has yet to be identified.

Ecological Impacts of Acid Rock Drainage on Aquatic Ecosystems

Maintaining the quality of Colorado’s headwaters is vital to sustaining the health of aquatic ecosystems within them. The physical and chemical stressors of ARD on aquatic biota are well-documented for their impairments to organism physiology and mortality, as well as the function and structure of freshwater communities. Among such aquatic stressors, acidification (e.g., pH~3.0-6.0), elevated dissolved metals concentrations, and deposition of precipitated hydroxides in stream beds stand out as the greatest threats. In assessing the effects of ARD on ecosystems, one must understand that different organisms and their respective variations of species have developed varying tolerances to ARD impacts. However, the general trend consistently found among degraded ARD ecosystems is their inability to support valuable biota (Figure 5), where species diversity is replaced with few acid and metal-tolerant species throughout extended stretches of streams.

With acidification, studies have demonstrated that the general trend among fish significantly correlates increasing mortality rates with decreasing pH levels due to the failure of internal salt regulation and gill function, while acid sensitivity of fish eggs often helps diminish populations (Leivestad, 1982). Two of Colorado’s most prized fish—the native cutthroat trout and the brook trout—have both demonstrated their sensitivity to pH decreasing below 5.0 with adverse effects on biochemical functions, growth, and reduced survival rates (Cleveland et al., 1986; Woodward et al., 1989). Similarly, macroinvertebrate communities common to Colorado Rocky Mountain streams exhibit stress-induced behavioral responses with downstream drift when exposed to pH ~ 4.0 conditions, along with significant mortality rates (Courtney & Clements, 1998). In streams where ARD conditions are chronic, the elimination of fish and most macroinvertebrates often results in severe simplification of ecosystem structure and function, including within food webs. One consequence of such occurring with the loss of macroinvertebrate decomposers (i.e., grazers, shredders) is the reduced capacity of decomposition, ultimately impeding nutrient cycling processes as an ecosystem service in ARD streams (Hogsden & Harding, 2012).

While acidity dissipates downstream, the continuance of many metals downstream (e.g., Zn, Cu, Cd, Pb) remains a threat to aquatic life. Returning to the Snake River basin where all fish are artificially stocked, in-situ monitoring of stocked rainbow trout demonstrated the adverse effects of natural and anthropogenic ARD sourced from the upstream headwaters of the catchment. Heavy and trace metal contamination was deduced as the main cause of trout mortality given the direct observance of Zn, Mn, Cu, and Cd accumulation within gills, while reduced feeding rates as a metals-related stress response were accounted for by weight differences in trout of the ARD site in comparison to a non-ARD affected stream (Todd et al., 2006). While mortality rates declined with distance from upstream ARD pollution, similar impacts upon reduced brook trout abundance and “fitness” due to elevated Zn concentrations within the main stretch of the Snake River have likewise revealed the impact of ARD metals in their continuance downstream (Todd et al., 2012). Implications suggest that even artificially stocked fish populations cannot be sustained, particularly when summer low flows are concentrated in ARD sourced metals.

Additional ecological consequences include the bioaccumulation of metals and rare earth elements within the food web (Figure 6). In the ARD affected stretches of the upper Animas River basin, bioavailable metals (i.e., Zn, Cu, Cd) within periphyton communities are transferred to higher trophic levels via consumption by macroinvertebrates and brook trout, within which toxic concentrations may accumulate (Besser et al., 2001). Likewise, bioaccumulation of rare earth elements within macroinvertebrates, such as those found within the Snake River, are an emergent threat given their persistence and transport into distant downstream reaches beyond ARD sites (Rue & McKnight, 2021). The ultimate concern is whether such elements are eventually transferred into terrestrial portions of the food chain, along with potentially accumulating within drinking water supplies such as where the Snake River drains into Lake Dillion—one of Denver’s main drinking water sources. As well, when moving downstream the neutralization of acidic flows by diluted inflows results in the chemical formation and deposition of iron and aluminum hydroxides, exacerbating challenges for benthic communities. The coating of channel substrate by such precipitates is frequently linked to habitat degradation for periphyton and invertebrate communities, reducing ecosystem primary productivity and biodiversity, respectively (McKnight & Feder, 1984; Herbst et al., 2018).