The Hydrology of Pit Lakes Lee C. Atkinson - Southwest Hydrology

The Hydrology of Pit LakesLee C. Atkinson, Ph.D. – Hydrologic Consultants, Inc. of ColoradoWhen mining and dewatering activitiescease in any surface mine below the localwater table, the excavation will begin tofill with groundwater and form a pit lake.The hydrologic system, both the groundwaterand surface-water components, willthen begin to establish a new equilibriumthat might or might not be similar to preminingconditions.The rate at whichthis occurs and theextent to which premininghydrologicconditions arerestored depends onthe local hydrogeology, the size of the pit,the magnitude and duration of dewateringduring mining, and the climaticconditions. In any case, pit lakes act asartificial “windows” in the water tableand, because of evaporation, becomeessentially permanent points of groundwaterwithdrawal.Hydrologic Budget for a Pit LakeThe hydrologic budget (or water balance)for a pit lake can be described in terms ofvolumes (i.e., fluxes x time) by theequation:V in =V out + V e + V ∆swhereV in = volume of ground-water inflowV out = volume of ground-wateroutflowV e = volume of evaporation from pitlake surfaceV ∆s = change in volume of storage(change in water level x area)This budget is shown in Figure 1. It isassumed that engineered drainage ditchesand diversions will keep surface water outof a pit both during and post-mining. Ofcourse, in some mines there may beoccasional and presumably short-livedinflows of surface water, but they are notincluded in the generic water balance for atypical pit lake.14 • September/October 2002 • Southwest HydrologyThe rate of ground-water inflow isproportional to two primary factors: 1) thehydraulic conductivity (or permeability) ofthe rocks comprising the pit and 2) thehydraulic gradient between the water levelin the pit at any point in time and somedistant point where the water level hasbeen essentially unaffected by mining andThe potential worst case may occur where both inflow andevaporation are moderate and there is outflow of evapoconcentratedwater with high concentrations of certain constituents.dewatering. The latter is a function of thehydraulic conductivity and storageproperties of the intervening materials andthe time since dewatering began. The nearpithydraulic gradient, induced bydewatering and pit infilling, changes overtime, with the steepest gradient occurringat the end of dewatering, when pit lakerecovery begins. This is shownschematically in Figure 2. Because thehydraulic gradients are steepest duringearly pit lake recovery, the rate of infillingis greatest during early recovery anddecreases with time. The rate of infilling isalso a function of the volume per unitdepth of the pit, which increasessignificantly as the pit lake fills, a functionof the conical shape of the pit. If a) thehydraulic conductivity is relatively large,b) dewatering has occurred for a relativelylong time (say 10+ years), and c) thestorage of the rocks is low (which is thecase for most bedrock), then the time ofinfilling can be quite long.Evaporation can be a major variable in thepit lake infilling process. As used here, theterm includes the net loss of water fromthe pit lake surface (evaporation less directprecipitation) and any evapotranspirationfrom either aquatic or phreatophyticvegetation associated with the pit lake. Insome cases, the evaporation can beestimated from pan rates or publishedvalues for shallow lakes. In other cases,the process is much more complicated andfactors such as water temperature, airtemperature, wind speed, and humidity,and even lake factors including shape,fetch, shading, and seasonal turnover, mustbe considered. In areas where either thewater temperature is geothermallyelevated or pan rates do notrepresent the winter months, asimple pan rate might not beapplicable.EvapoconcentrationEvaporation from the pit lakesurface causes concentrationsof dissolved constituents within the pitlake water to increase over time. This isdescribed by the approximate, transientrelationship:C t = M t M t - 1 + ∑(V in C in -V out C t - 1)=V t V t - 1 + V ∆swhereC = concentrationM = mass in solutionV = volume of water in pit laket = timeThe evapoconcentration factor typicallyranges from 10 to 40 times theconcentration in the ground-water inflow.In the case of constituents such as arsenic,selenium, and many trace metals,evapoconcentration is a majorconsideration.Terminal or Flowthrough?An issue closely related toevapoconcentration is whether a pit lakewill ultimately have outflow. Pit lakes arereferred to as terminal when evaporation issuch that the pit lake will always be aground-water sink. A flowthrough lake hasdowngradient outflow. It should be notedthat, depending on the gradient, outflowcould begin well before the pit lakereaches equilibrium. Again, this depends

on the size of the lake, hydraulic conductivityof the adjacent rock, and the climaticconditions.Whether or not a pit lake is terminal orflowthrough can have significantly differentenvironmental consequences. If inflow issmall and evaporation is high, it is likely thatevapoconcentration will be relatively high, butunlikely that outflow will occur. This resultsin a standing body of potentially undesirablewater quality. Conversely, if inflow is highand evaporation is low, significant outflowwill occur that most likely will haveexperienced little evapoconcentration of itsdissolved constituents. The potential worstcase may occur where both inflow andevaporation are moderate and there is outflowof evapoconcentrated water with highconcentrations of certain constituents. In anycase, evaporation from the pit lake will resultin some loss of water resource relative to preminingconditions and, regardless of whethera pit lake is terminal or flowthrough, someevapoconcentration will most likely occur.Hydrologic Impacts of Pit LakesA possibly non-intuitive effect of pit-lakeinfilling is that the dewatering-induceddrawdown actually continues to increase inlateral extent during the early stages of pitinfilling. The ultimate extent of drawdown cancontinue for several years after the dewateringceases. This is illustrated in Figure 3 using the10-foot drawdown isopleth of the water table,a criterion for quantifiable impact used in theEIS process in some Western states.Hydrologic Budget of a Pit LakeFigure 1Pre- and Post-Groundwater LevelsFigure 2Drawdown Associatedwith Dewatering and PitLake InfillingFigure 3If drawdown induced by dewatering and pitlake infilling impacts surface-water bodies,the maximum extent of the impact alsocontinues past the end of dewatering. This isshown in Figure 4.Although the general concepts of thehydrology of pit lakes and their impacts onthe quantity and quality of water resources arequite simple, no general conclusions should bedrawn. Each pit lake is unique depending onthe local hydrogeology, the size of the pitlake, and the climatic conditions.Potential Impacts onNearby Stream FlowContact Lee Atkinson at 303-969-8033 orlatkinson@hcico.comFigure 4September/October 2002 • Southwest Hydrology • 15