Surface Coal Mining Effects on Ground Water Recharge (1990) / Chapter Skim
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6. Quantifying Ground Water Recharge
6. Quantifying Ground Water Recharge
Pages 81-94
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From page 81... ...
TECHNIQUES FOR ESTIMATING GROUND WATER RECHARGE Ground water recharge at a mine site can be estimated, in principle, by a variety of techniques. Tracing water that enters the surface soils as it percolates through the soils and underlying material (vadose zone)
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From page 82... ...
-82TABLE 6.1 Experimental Measurements Required to Determine Ground Water Recharge Measurement Instrument or Technique Onsite data -- atmospheric Precipitation Atmospheric pressure Air temperature Relative humidity Wind speed Net radiation Onsite data -- subsurface Hydraulic head Water table Water content of soil Bulk density Soil temperature Water characteristic Hydraulic conductivity Laboratory data Soil texture Particle density Tipping-bucket rain gauge Barometric-pressure transducer Thermistor probe Relative-humidity probe Anemometer Fritschen-type net radiometer Piezometer and duplicate tensiometer nests Water-table well Neutron probe, gravimetric method, and gypsum blocks Gamma probe and core method Thermocouples Tensiometers and neutron probe Determined from changes in water content and tension of a bounded soil volume during drainage and/or evaporation. Hydrometer method Pycnometer method
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From page 83... ...
Direct measurement and observation of the migration of water from the land surface to the water table require installation of instruments to detect variations in the water content of soil with depth, from the land surface to the water table, over an extended period of time. Generally, these methods
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From page 84... ...
An equation written in terms of ground water recharge for the saturated zone is where GWin GWR GWout AGW Storage GWin + GWR = GWOut + ~GW Storage ground water inflow rate; ground water recharge; ground water outflow rate; and change in ground water storage. (The units of all above terms are length/time.)
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From page 85... ...
Another method for estimating ground water recharge involves the measurement of water flow through the vadose zone. The flux of water through the vadose zone is a function of the ability of the medium to transmit water, its unsaturated hydraulic conductivity, and the driving force, the hydraulic gradient.
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From page 86... ...
Methods for Southeastern Massachusetts Study Method Water Water Location Tritimn Table Budget This study [Knot and Site 1 66.3 -- - Ol~mpio, 1986] Site 2 >42.4 -- - Site 3 -- 52.1 - Guswa and LeBlanc (1985)
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From page 87... ...
To compute recharge from such plots, the estimated total change in the water table, Ah, is multiplied by the specific yield and the surface area over which the change is estimated to occur. Analysis of stream baseflow recession curves has also been utilized to estimate basin ground water recharge (Fetter, 1988; Figure 6.2~.
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From page 88... ...
-88TABLE 6.3 S,~mmary of Mechanisms That Lead to Fluctuations in Ground Water Levels Uncon- Hunan- Short- Long- Climatic fined Confined Natural induced lived Diurnal Seasonal term influence Ground water recharge X X X X (infiltration to the water table ) Air entrapment during X X X X ground water rechargo Evapotranspiration and X X X X phreatophytic con~umption Bank-storage effects X X X X near streams Tidal effects near X X X X oc eans Atmospheric pressure X X X X X effects External loading of X X X confined aquifers Earthquakes X X X Ground water pumpage X X X X Deep-well injection X X X Artificial recharge; X X X leakage from ponds, lagoons, and land ft lls Agricultural irriBati on X X X X and drainaSe Geotechnical drainaBe X X X of open pit mines, slopes, tunnels SOURCE: Freeze and Cherry, 1979.
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From page 89... ...
1979 1 1980 1981 J O 1 982 L J A J O 1 983 FIGURE 6.1 Hydrograph of monthly ground water levels and bar graph of monthly precipitation. SOURCE: Knot and Olimpio, 1986 .
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From page 90... ...
· I . I I .u I I ~ S O i~ D I F `\ ~ `, I I ~ S O ~ ~ I f ~ ~ ~ I I ~ S O ~ D I f Runoff Year 1 Runoff Year 2 Runoff Year 3 FIGURE 6.2 Semilogarithmic stream hydrographs showing base flow recessions.
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From page 91... ...
Water level data spanning the time period of interest are required for calibration. APPLICATION OF TECHNIQUES TO SURFACE MINING SITES Attempting to quantify ground water recharge at mine sites requires an appreciation for the strengths and limits of monitoring techniques, the impact of the selected mining technique on the hydrologic system, and the constraints of the climatic and geologic setting.
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From page 92... ...
Such sites do not lend themselves to detailed vadose zone monitoring, principally because unsaturated consolidated formations and secondary fracture porosity and permeability make standard instrumentation unuseable or unreliable. While there may be special situations where the technique would be valuable (e.g., monitoring the effectiveness of spoil-segregation techniques)
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From page 93... ...
Post-mining analysis is further h~mn~red he the time ~ ~ delay in the ground water system recovery period, which may mask recharge events. Implementation requires measurement of the level of the water table, which may be quite difficult to obtain in zones of secondary fractures.
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From page 94... ...
Uncertainty in the measurement of relevant parameters results in an uncertainty in recharge estimates that likely exceeds changes in long-term recharge due to mining if proper reclamation practices are followed. Also, it must be recognized that at some mine sites that are small or are located in dry climates, reliable recharge quantification may be outside the realm of current technology.
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