Impacts of groundwater metal loads from bedrock fractures on water quality of a mountain stream

Abstract

Acid mine drainage and metal loads from hardrock mines to surface waters is a significant problem in the western USA and many parts of the world. Mines often occur in mountain environments with fractured bedrock aquifers that serve as pathways for metals transport to streams. This study evaluates impacts from current and potential future groundwater metal (Cd, Cu, and Zn) loads from fractures underlying the Gilt Edge Mine, South Dakota, on concentrations in Strawberry Creek using existing flow and water quality data and simple mixing/dilution mass balance models. Results showed that metal loads from bedrock fractures to the creek currently contribute <1% of total loads. Even if background water quality is achieved upstream in Strawberry Creek, fracture metal loads would be <5%. Fracture loads could increase substantially and cause stream water quality standards exceedances once groundwater with elevated metals concentrations in the aquifer matrix migrates to the fractures and discharges to the stream. Potential future metal loads from an upstream fracture would contribute a small proportion of the total load relative to current loads in the stream. Cd has the highest stream concentrations relative to standards. Even if all stream water was treated to remove 90% of the Cd, the standard would still not be achieved. At a fracture farther downstream, the Cd standard can only be met if the upstream water is treated achieving a 90% reduction in Cd concentrations and the median stream flow is maintained.

Predicting the Benefits of Mine Water Treatment under Varying Hydrological Conditions using a Synoptic Mass Balance Approach

“…One possible mechanism for release of (filterable fraction) metals from the mine waste is the rewetting of evaporative salts formed on, and within, the mine wastes during dry antecedent conditions, 14 but further investigation would be needed to Groundwater discharge under high flow conditions is commonly observed in mining-impacted catchments 12,14,16 and can represent an important diffuse source of metals, particularly in areas where substantial fracturing of bedrock occurs. 18,35 As the Coledale Beck flow rate increases during rainfall events an increasing discrepancy occurs between the sum of all measurable upstream flows and the Coledale Beck downstream of the mine site (Figure S7), indicating the presence of additional, unmeasured, flows. These are likely shallow subsurface flows and surface runoff.…”

Spatial Distribution of As, Cr, Pb, Cd, Cu, and Zn in the Water and Sediment of a River Impacted by Gold Mining

“…Detailed examination of the concentration changes revealed that trace elements loadings might come from: the gold processing facilities, AMD from mine waste dumps, and other industrial activities. In addition, polluted ground water (Caruso and Dawson 2009) and release or resuspension of elements from contaminated sediment (Beltran et al 2010) as a response to early diagenesis can also influence trace element concentrations.…”

Trace Metal Contamination Due to Acid Rock Drainage and Its Impacts on the Fish-Bearing Pennask Creek Watershed, British Columbia

“…Acid rock drainage (ARD) is a major pollution problem throughout the world, adversely affecting both surface and ground waters (Gerhardt 1993, Gray 1996, Caruso and Dawson 2009.…”