Tracer test feasibility assessment : Frongoch Mine tailings lagoon

The feasibility of tracer testing at the Frongoch Mine site was investigated during a two day site visit. A walkover survey identified a potential tracer injection point where a surface stream sinks into tailings deposits. Water discharges through a culvert approximately 50 m from the sinking stream. The origin of the water in the culvert is unknown but it is thought to drain the tailings area. Tracer testing could be used to determine whether this is the case. A successful tracer test would demonstrate the travel times from the stream sink to the culvert, demonstrating whether there is a rapid preferential flowpath or whether groundwater flow occurs more slowly. Quantitative results (tracer mass balance and breakthrough curves) could be obtained from the Frongoch Stream gauging site. Tracer testing using Sodium Fluorescein dye is not recommended at the Frongoch mine site due to loss of ~ 75% of the fluorescence, which is likely to be due to the presence of high concentrations of metals within the groundwaters. Bacteriophages are likely to be the best tracer at this location, and any tracer should be tested in groundwater samples from the site prior to a tracer test. Single borehole dilution testing was carried out to assess the suitability of two boreholes as tracer injection points. Results indicated slow dilution times suggesting that tracer injected into these boreholes is unlikely to be detected at the culvert. Groundwater velocities were faster in the glacial deposits (~ 2–20 m/d) than in the tailings (~0.4 m/d), although there are no porosity data available for the glacial deposits therefore there is more uncertainty in the calculated groundwater velocity.