The migration of leachate from mining operations through the ground is an issue of
concern to the mining industry, regulators, and public, particularly where leachate
constituents may be moving into surface waters. In many geologic settings, seepage
enters surface water invisibly through submerged fractures and bottom sediments.
Until now there were no practical methods for identifying these subsurface flows.
Knowing the location and contaminant flux of offsite seepage can be important in
estimating the degree of contamination in an area, and in designing programs for
useful monitoring, remediation and reclamation. By identifying and quantifying
subaqueous seeps, it should be possible to reduce costs of hydrogeological
investigation and monitoring.
A new reconnaissance method for detection of acid mine drainge (AMD) has been
evaluated near mine operations near Sudbury and Timmins, Ontario. An
electrical-conductance, bottom-contacting probe (known as the sediment probe) was
towed behind a slowly moving boat over more than 21 line-kilometres of lake and river
bottom.
The evaluation has been successful, both as a test of the method and as a preliminary
identification of groundwater discharge areas at the two study sites. The method
effectively solves the problem of identifying discharge of AMD in surface waters and,
by quantifying the groundwater and solute-transport, it has provided estimates of
impact at points of discharge.
The sediment-probe method depends on two conditions: 1) groundwater-contaminant
plumes and surface waters differ in electrical conductivity, and 2) upward advection
moves the groundwater signatures within centimetres of receiving surface waters.
The method was used to locate eight areas of leachate discharge. These were studied
quantitatively, to evaluate the utility of the probe and provide site-specific information.
Some targets were confirmed by measuring the porewater electrical conductivity 20 to
120 cm below the sediment/water interface. Other targets were confirmed using direct
measurements of flux, using seepage meters. Still others were confirmed by measuring
upward gradient, moderately high hydraulic conductivity and solute chemistry.
The discharge conductivities ranged from l2 820 to 43 mS/cm and from 6.9 to 4.8 pH.
Some discharges contributed nickel in concentration ranging as high as 9.5 ppm to the
surface waters.
In order to attribute many of the discharges to leachate from mine tailings, waste rock,
septic tanks or road salt, it will be necessary to do additional chemical and isotopic
work using the existing piezometers. The authors and industrial partners hope to
conduct major-ion, metal and isotopic analyses, to distinguish sources and provide
contaminant concentrations for better flux estimates.
Specific findings:
1. Sediment-probe results, supported by quantitative measurements, show that
groundwater of elevated electrical conductivity is entering Lake Kamiskotia along
two-thirds, or 1.5 km, of the northeastern shoreline. This shoreline discharge could
contain AMD, road salt, septic-tank effluent or waters that are naturally high in
dissolved solids.
2. Bottom-water samples below the outlet of Lake Kamiskotia in the Little Kamiskotia
River indicate that AMD may be entering the river 300 m upstream of any obvious
damage to the terrestrial environment.
3. Several sources of nickel input to the Onaping River were identified on the river bed.
At one location, a crude but illustrative calculation showed that 12 kilograms of nickel
enter the river each year over a 50 m2 bottom area.
4. Recommendations for further work include: (a) analysis of existing samples to
determine sources of high dissolved solids water entering the studied surface waters,
and (b) collection of additional samples for chemical and isotopic analyses.
Helium-3/tritium analysis using mass spectrometry should be used to determine the
groundwater residence time for the discharging waters. Some of the suspected AMD
may, in fact, be natural discharge.
