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USGS dye‑tracer study proposed for Fraser River to map groundwater inputs
Summary
USGS researchers (presented by county water quality manager) proposed an October dye‑tracer study in the Fraser River reach to identify groundwater inputs; presenters said the fluorescent dye is used at low, non-toxic concentrations and results would guide targeted groundwater sampling and management.
Catherine Morris, Grand County water quality manager, presented a proposed USGS (NGWOS) dye‑tracer study for the Fraser River designed to identify locations where groundwater enters the channel and quantify contributions to baseflow. She explained the approach: inject a fluorescent dye at a controlled concentration, measure the downstream concentration with fluorometers, and interpret dilution patterns to locate groundwater inputs. "They ramp it up until they reach a static or stable concentration…and as that concentration travels downstream if there's an input of groundwater it's going to dilute," Morris said.
The USGS team would pair the dye run with age‑dating techniques (tritium/hydronium and carbon‑14) at targeted groundwater sites to learn whether groundwater is recharging at historic rates or showing signs of depletion. Morris said the dye concentrations planned (tens of parts-per-billion) are within commonly used, non-toxic ranges and noted Colorado Parks and Wildlife and other agencies have used fluorescent tracers without observed ecological harm. Towns downstream (Fraser, Winter Park) were consulted and notification to private landowners and stakeholders would be part of the plan before field work.
