Mine Closure - Mining and the Environment: From Ore to Metal - page 828

21.5 PITLAKES
The chemistry of a pit lake is, in part, a rel ection of the groundwater that existed prior to
mining as well as the reactions of the surrounding rock that host the pit lake. Since most
pit lakes do not have much surface water l owing directly into them, groundwater com-
prises most of the inl uent water. Groundwater surrounding the mine provides the appro-
priate comparison for water quality in the pit lake.
Three main factors inl uence the chemistry of the i nal pit lake (the following discussion
is partly drawn from Miller 2002).
First, an important factor of pit lake water quality is the interaction of groundwa-
ter with the wall rock around the pit. In an open pit, oxidation reactions on the exposed
walls may release sulphate, acid and metals into the lake. Additionally, when a pit is exca-
vated below the regional water table, the aquifer in the host rock is dewatered. If present
sulphide-rich host rock will oxidize when exposed to air that is pulled into the evacuated
pore spaces, generating reaction products on the exposed surfaces. As the aquifer recovers
following mining, those oxidation products will be l ushed into the pit lake by groundwa-
ter l owing into the cone of depression, initially as acid rock drainage.
Second, lake chemistry is inl uenced by the pH value. In fact the pH of the pit lake is
the single greatest determinant of the water quality, or toxicity, of a pit lake. As oxidation
of the pit wall rock releases sulphuric acid, calcium carbonate that may be present in the
wall rock also dissolves and neutralizes the acid. Where sufi cient carbonate is present, the
water that enters the pit lake will be near-neutral; if there is a lack of calcium carbonate,
the pit lake will be acidic. Metal solubility, particularly of divalent problematic metals such
as cadmium, nickel, zinc and copper, is much higher at low, acidic pH and can render
water quality poor.
Third, pit lake stability is important. The stability of the pit lake depends on the compari-
son of the stabilizing mechanisms and the disruptive forces, commonly expressed in terms of
the densimetric Froude Number. Fully mixed lakes have a Froude Number greater than 1.
Forces that may disrupt stratii cation of a pit lake include wind and wave action, l ow
through due to runoff into the lake pit, seasonal temperature l uctuation, or potentially
thermal sources at deep pits that potentially could warm the water. Seasonal atmospheric
temperature l uctuation, the cause of mixing of lake waters at higher latitudes, is not a
signii cant factor for tropical lakes. Mechanisms that enhance stable stratii cation of the pit
lake include density stratii cation, relative constant temperature throughout the year and
a small surface area relative to volume and depth. As a result of this stratii cation, higher
salinity water containing most of the heavy metal ions may occur in an anoxic layer at the
base of the pit where reduction reactions may cause metals to precipitate.
The environmental problems related to pit lakes have been recognized in the United
States and elsewhere for only 20 years. PHREEQC, a model developed by the United
States Geological Survey (USGS) i nds wide application in simulating the lake hydraulics
and chemistry. This model has the ability to simulate the pertinent processes occurring in
a pit lake, such as mixing of multiple solutions, precipitation/dissolution of selected solids,
redox reactions, evaporation, atmospheric interaction and adsorption of metals.
It is now recognized that pit lakes represent a commitment of groundwater resources in
perpetuity. Pit lake management is important, particularly in arid climates where water is
the limiting resource for agriculture, domestic watersupplies and wildlife needs. However,
pit lake management is still very much the topic of research. Lime dosing of the pit water
is one measure to improve short-term water chemistry. Accelerating the i lling of the mine
pit (pumping or directing surface water l ow to the opening) as in the case of the Island
Copper Pit in Canada in 1996 ( Case 21.3 ) is another measure.