Abstract
High concentrations of trace metal(loid)s exported from abandoned mine wastes and acid rock drainage pose a risk to the health of aquatic ecosystems. To determine if and when the hyporheic zone mediates metal(loid) export, we investigated the relationship between streamflow, groundwater–stream connectivity, and subsurface metal(loid) concentrations in two ~1-km stream reaches within the Bonita Peak Mining District, a US Environmental Protection Agency Superfund site located near Silverton, Colorado, USA. The hyporheic zones of reaches in two streams—Mineral Creek and Cement Creek—were characterized using a combination of salt-tracer injection tests, transient-storage modeling, and geochemical sampling of the shallow streambed (<0.7 m). Based on these data, we present two conceptual models for subsurface metal(loid) behavior in the hyporheic zones, including (1) well-connected systems characterized by strong hyporheic mixing of infiltrating stream water and upwelling groundwater and (2) poorly connected systems delineated by physical barriers that limit hyporheic mixing. The comparatively large hyporheic zone and high hydraulic conductivities of Mineral Creek created a connected stream–groundwater system, where mixing of oxygen-rich stream water and metal-rich groundwater facilitated the precipitation of metal colloids in the shallow subsurface. In Cement Creek, the precipitation of iron oxides at depth (~0.4 m) created a low-hydraulic-conductivity barrier between surface water and groundwater. Cemented iron oxides were an important regulator of metal(loid) concentrations in this poorly connected stream–groundwater system due to the formation of strong redox gradients induced by a relatively small hyporheic zone and high fluid residence times. A comparison of conceptual models to stream concentration–discharge relationships exhibited a clear link between geochemical processes occurring within the hyporheic zone of the well-connected system and export of particulate Al, Cu, Fe, and Mn, while the poorly connected system did not have a notable influence on metal concentration–discharge trends. Mineral Creek is an example of a hyporheic system that serves as a natural dissolved metal(loid) sink, whereas poorly connected systems such as Cement Creek may require a combination of subsurface remediation of sediments and mitigation of upstream, iron-rich mine drainages to reduce metal export.
Highlights
Over 64,000 inactive metal mines persist in the United States and contribute high metal loads to streams and groundwater, damaging aquatic ecosystems (Nordstrom, 2011; HudsonEdwards, 2016; Horton and San Juan, 2020)
Model estimates of mass transfer parameters using parameter estimation (PEST) with one-dimensional transport with inflow and storage (OTIS) and the tracer data (Supplementary Figure 2) measured at the hyporheic zone well clusters (e.g., 200 m downstream of the injection site for MC-Fen and 700 m downstream of the injection site for CC-PG) indicated that hyporheic storage (As) and the proportion of solute storage in the hyporheic zone compared to the stream (As / A) were greater at high flow and low flow in MCFen compared to CC-PG (Table 1, Supplementary Figures 3, 4)
Even though Fe is not considered as toxic a metal as As, Al, Cd, or Zn, which are the primary foci of remediation efforts in the Bonita Peak Mining District, our study suggests that treatment of Fe is important given the effect it has on the physical structure of the hyporheic zone at Cement Creek
Summary
Over 64,000 inactive metal mines persist in the United States and contribute high metal loads to streams and groundwater, damaging aquatic ecosystems (Nordstrom, 2011; HudsonEdwards, 2016; Horton and San Juan, 2020). In watersheds impacted by historic mining activity, chemical weathering of minerals containing high metal content can occur at rates three times as fast as natural weathering rates (Alpers et al, 2007). Some of these metal(loid)s, such as arsenic (As), copper (Cu), and manganese (Mn), are commonly found in high concentrations downstream of hard-rock mines. They pose a well-documented risk to human and aquatic health (Smedley and Kinniburgh, 2002), and their toxicity and concentration are highly sensitive to changes in pH and redox conditions of streams and groundwater (Smedley and Kinniburgh, 2002; Borch et al, 2010). The role of groundwater–surface water connectivity in mediating the toxicity and mobility of redox-sensitive metal(loid)s is not well-quantified and could have important implications for our estimates of metal fluxes from mine-impacted watersheds (Gandy et al, 2007)
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