MEND Report 4.8.2
September 1994

EXECUTIVE SUMMARY

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.

Last Modified: 2003-11-26		Important Notices

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