HYDROGEOLOGY
OF WASTE ROCK DUMPS
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Associate Project PA-1
July 1995
EXECUTIVE SUMMARY
Predictions
of whether and when a waste rock pile may start to generate acidic
water, and how long a pile may release elevated concentrations of
metals to the environment, are related on a fundamental level to
an understanding of fluid flow within a waste rock pile. Our analysis
of the hydrogeological properties of waste rock piles is based on
a synthesis of data from four mine sites; Myra Falls, B.C., Island
Copper, B.C., Elkview Mine, B.C., and Golden Sunlight Mine, Montana.
The emphasis in this study is on pile hydrostratigraphy and the
textural properties of the rock mass, spatial and temporal variations
in water content within a waste rock pile, temperature profiles
within a waste rock pile and their response to the infiltration
of water following a rainfall event, and the large-scale hydrogeologic
characterization of a waste rock pile inferred from outflow hydrographs
recorded in toe drains.
Four hydrostratigraphic
models are proposed to characterize waste rock piles; they differ
depending upon material types and construction methods. This framework
differentiates between porous flow in finer sandy gravel materials
and channelized flow in coarser materials. The models are non-segregated
coarse-grained rock piles that transmit water rapidly to the base
of the pile, non-segregated fine-grained rock piles that are likely
to contain a basal saturated zone, segregated rock piles that contain
a fine-grained crest zone that may not permit the passage of significant
quantities of water; and layered, segregated dumps that contain
a finer-grained crest and sandy gravel layers parallel to the face
of the rock pile.
Volumetric
water content is an important characteristic of a waste rock pile.
It appears to be closely associated with the textural properties
of the pile, and it can be used to scan the pile hydrostratigraphy.
For a given waste rock pile, at each depth, values of the water
content appear stable throughout the year. For the data from Golden
Sunlight Mine, attempts to monitor matric potential using heat dissipation
sensors, and to correlate changes in matric potential with rainfall
events, were generally not successful. Temperature appears to be
one of the more reliable parameters to use for tracing the movement
of water in those regions of the pile that are reactive and generating
heat. The fluctuation of the water table in response to infiltration
is affected by the permeability structure of the pile and location
within the pile. The permeability structure of the pile is the spatial
distribution of permeability values within the different regions
of the pile. The data we examine is suggestive of rapid infiltration
of water through waste rock piles, although sampling frequencies
were not adequate to develop precise estimates of fluid velocities.
A methodology
is presented, based on kinematic wave theory that relates the outflow
hydrograph recorded in toe drains to large-scale parameters characterizing
the hydraulic conductivity structure of the waste rock pile. The
outflow in response to an infiltration event is treated as an integration
of the outflows from different channel groups within the pile. Water
transfer from the channels to the finer-grained matrix is taken
account of in the analysis. Application to the Island Copper data
set suggests that the approach holds promise as a means of characterizing
large-scale flow processes in a waste rock pile. Further work is
warranted to improve the model in its representation of channelized
flow, and to apply the method to rainfall events at a number of
different sites to gain insight to the relationship between hydrostratigraphy,
and flow responses.
The most significant
limitation of the existing database is that no single site provided
a complete data record of the important parameters required to characterize
the hydrologic behavior of a waste rock pile, and the frequency
of sampling was often insufficient for our purposes. In our opinion,
to better understand the hydrology of a waste rock pile, the following
measurements should be given priority: water content and temperature
profiles through the unsaturated zone, water table elevation, volumetric
discharge at toe drains, and rainfall and air temperature. Workplans
are presented for three types of monitoring studies; a pile assembly
study, a pile monitoring study, and a pile disassembly study. It
may be advantageous to link these workplans to operations at a low-grade
stockpile. It is important to coordinate the suite of measurements
made prior to and during the disassembly of a pile.
Français
| Contact Us
| Help | Search
| Canada Site
Home | What's
New | CANMET-MMSL
| MMS Site
| NRCan Site
|