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.