5. When developing a coal mine, the local aquifer needs to be completely dewater
ID: 1712621 • Letter: 5
Question
5. When developing a coal mine, the local aquifer needs to be completely dewatered and water needs to be pumped out of the mine for the duration of the operation. For an aquifer with an area of 100 acres, a thickness of 100 ft and a porosity of 0.2, how much water is released during initial dewatering? Given the catchment area is 10,000 acres, we can assume all recharge into the catchment now releases into the mine Given 1200 min yr of rainfall, 50% of the rainfall becomes ET and 25% of the rainfall becomes surface runoff, how much water needs to be continuously pumped out during operation, at steady state? (5 points) eoaer needs to be coatinuously pumped out dutinge Discuss where the released water can be directed to (optional 2 points) where the released water can be directed to. (optional 2 points)Explanation / Answer
Solution:
Area of Aquifer = 100 Acres = 100 x 43560 ft2 = 43.56 x 105 ft2
Porosity = 0.2
Amount (Volume) of water Release = Areaof Aquifer x Thickness x Porosity
= 43.56 x 105 ft2 x 100 ft x 0.2
= 87.12 x106 ft3 Ans
Total rainfall = 1200 mm/year
Rainfall percolates in Aquifer = 1200 x (1 - 0.50 - 0.25) = 300 mm/year
Volume of Rainfall Percolates in Aquifer Per Second = 300/(25.2x12) ft x (10000 x 43560 ft2)/(365 x 86400)
= 13.595 ft3/sec
Amount of water needs to be continuously pumped out = 13.595 ft3/sec + Rate of discharge of Aquifer water Answer
Rate of discharge of water from Aquifer = Volume of water / duration in which Aquifer water is to be dewatered
Here, duration in which Aquifer water is to be dewatered is Not Given in Problem
The Released water can be directed to Streams/rivers nearby.
or If the Reservoir are present then it can be stored there localy.
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