Characterization of Groundwater-Surface Water Interactions and Their Effect On Aquatic Ecosystems in a Mountain Stream

Groundwater-surface water interactions (GSI) can influence the mobility and bioavailability of chemical constituents, and hence exposure to aquatic organisms in streams. Upwelling groundwater can cause chronic and/or acute exposures if the groundwater contains elevated aqueous-phase constituents.  GSI investigations were conducted in the Red River that borders the Chevron Mining Inc. mine near Questa, New Mexico as part of a Remedial Investigation. The objectives were to quantify and locate major zones of upwelling groundwater and to assess acute exposures to aquatic organisms from the upwelling groundwater. A segment of the river upstream of the mine was also part of the investigations to evaluate natural effects on the river from drainages containing hydrothermal scars.
Groundwater upwelling was evaluated at various levels of sophistication.  Differential stream flow gauging was used to identify gaining and losing reaches.  Natural Radon-222 activity in groundwater and surface water combined with stream flow gauging measurements were used in a mass balance mixing approach to further refine zones of upwelling.  Lastly, results from in-stream tracer-dilution studies were used where changes in tracer concentration approximate the change in stream flow and groundwater upwelling.
An in situ toxicity study of upwelling groundwater was conducted in the river at major upwelling areas.  Mini-piezometers were used to measure upwelling and to sample groundwater within the streambed.  Aquatic organisms (mayflies) were placed into flow-through chambers and secured to the streambed.  The in situ exposures lasted for four days and acute toxicity was determined from observed mortality rates. Results indicate that acute toxicity could not be attributed to the discharging groundwater in the vicinity of the mine.  The only significant reduction in survival was from an upwelling zone upstream of the mine.  Primary metals concentrations and pH in upwelling groundwater, however, suggest that chronic toxicity remains a concern along the mine.