Seasonal mixing from intermittent flow drives concentration‐discharge behaviour in a stream affected by coal mine drainage

Abstract

Abandoned mining operations continue to severely degrade many ecosystems worldwide by releasing acidic water and/or heavy metals into surface and groundwater. Contaminant concentrations in affected streams vary with discharge in patterns that reflect both geochemical reactions and variable mixing of contaminated and non‐contaminated waters. However, controls on concentration‐discharge (C‐Q) patterns remain unclear, particularly for constituents that experience changing solubility across redox and pH gradients. Understanding the C‐Q behaviour of contaminants aids in predicting both downstream transport and effects on aquatic life under variable flow. Here, we examined the C‐Q behaviours of non‐reactive (Na, K, Ca, Mg, Cl−) and reactive (Fe, Mn, Al, H+, SO42−) solutes in a stream contaminated with acid mine drainage in northeastern Ohio, USA. Concentration‐discharge patterns at the watershed outlet primarily reflected mixing of contaminated baseflow with intermittent inputs of high pH water draining from a passive limestone treatment system into the stream. The treatment system acted as an ephemeral tributary that mitigated contamination in the stream by diluting solutes, raising pH, and driving metal precipitation, but only when flow was present during wet seasons. Consequently, AMD‐derived reactive solutes (H+, Fe, Mn, Al) decreased with increasing stream discharge while relatively conservative solutes (e.g., Ca, Mg, K, Na) decreased only slightly or were chemostatic. This study highlights both the unique C‐Q patterns of reactive solutes when compared to those of non‐reactive solutes and the potential for intermittent streams to control C‐Q behaviour in headwater catchments.

Hydrogeochemical Response of a Variably Saturated Sulfide‐Bearing Mine Waste‐Rock Pile to Precipitation: A Field‐Scale Study in the Discontinuous Permafrost Region of Northern Canada

“…Concentration‐discharge ( cQ ) plots provide semi‐quantitative illustration of solute concentrations in relation to discharge flux at effluent locations from catchments (Cartwright, 2020; Godsey et al., 2009; Knapp et al., 2020; Li et al., 2017; Maher, 2011) or mine‐waste units (Shaw et al., 2020; Wellen et al., 2018). Empirical power‐law correlation between concentrations of solutes ( c ) and discharge fluxes at the discharge point ( Q ) is recommended ( c = nQ m , wherein m and n are constants).…”

“…The normalized ratio of the coefficient of variation of concentration (CV c = 100*standard deviation ( σ )/average ( μ )) to the coefficient of variation of discharge flux (CV Q ), (i.e., CV = CV c /CV Q ), was used to complementarily evaluate cQ behavior: chemostatic behavior (CV < 0.2), moderately chemodynamic behavior (CV > 0.2), and strongly chemodynamic (CV > 1) (Hoagland et al., 2017; Shaw et al., 2020). Combination of m and CV (Figure S3b in the Supporting Information ) is used to illustrate the importance of hydrological processes versus (bio) geochemical and mineralogical reactions controlling effluent‐water chemistry.…”

“…Thus, the cQ relationships provide insights into the coupling of hydrological processes with (bio)geochemical and mineralogical reactions in catchments, which can be used to delineate physicochemical processes controlling drainage‐water chemistry from mine‐waste units. Few studies assess drainage behavior focusing on local catchments using cQ relationships to inform improved management practices at mine sites (Shaw et al., 2020; Wellen et al., 2018). The use of cQ relationships to diagnose the drainage‐water quantity and quality derived from waste‐rock piles requires further assessment.…”

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

“…Additionally, aluminium and iron oxide deposits on streambeds can be toxic to aquatic organisms (Niyogi et al, 2002), including rainbow trout, (Todd et al, 2006) microbes, algae and macroinvertebrates (McKnight & Feder, 1984). Metal concentrations in ARD affected streams are tied to patterns in discharge, and can be highly temporally variable (August et al, 2002; Brooks et al, 1998; Nordstrom, 2009; Shaw et al, 2020).…”

“…Metal concentrations in ARD affected streams are tied to patterns in discharge, and can be highly temporally variable (August et al, 2002;Brooks et al, 1998;Nordstrom, 2009;Shaw et al, 2020).…”