IDENTIFICATION OF SOLUTE INFLOWS TO A MINE DRAINAGE STREAM WITH HYPORHEIC MIXING AND VARYING DISCHARGE

Challenges for the study of reactive solute transport in streams include recognition of hyporheic exchange flows (HEFs) as components of highly variable mixing pathways potentially altering water chemistry and contaminant concentrations at the surface-water / ground-water interface. When characterizing in-stream solute loading, there is a need to quantitatively define the character and source of solutes entering streams from subsurface pathways. Synoptic sampling and tracer-injection approaches provide tools for making operational estimates of solute loading within a stream segment. At the USGS Toxic Substances Hydrology Program research site in the Mineral Creek catchment (Silverton, Colorado), subsurface sampling within meters of the stream for major ions (SO₄^2-, Cl^-, Na⁺, Ca²⁺, Mg²⁺) shows that near-stream, subsurface solute concentrations vary spatially by an order-of-magnitude in part from HEF mixing of multiple, dispersed sources of subsurface and surface waters. The strong concentration ‘tailing' of bromide observed during a tracer injection occurred, at least in part, from the HEFs. Along a 64 m study reach, water levels in the stream, in the near-stream area, and in shallow drive-points provide hydrometric indications that although the stream is nominally ‘a gaining stream', flow losses occur approximately concurrent with gains. The HEF mixing is super-posed within more extensive down-valley subsurface flow paths. Diel variation of the concentrations of the injected tracer bromide and the ambient sulfate are indications of varying discharge, further complicating the identification of inflows. Identifying near-stream, sub-surface solute sources remains a challenge to be addressed with further study of the dynamic hydrology of stream-catchment connections in addition to detailed determination of solute concentrations.