Question

The compressibility of water  is 4.4 x 10-10 Pa-1. The compressibility  for clay, gravel and granite are approximately 10-6, 10-8, and 10-10 Pa-1, respectively. What is the percentage of water released from storage due to decompression of the aquifer skeleton and due to decompression of water for a clay, gravel and granite confined aquifers? The porosities are 35%, 25%, and 1% for clay, gravel and granite aquifers, respectively. The aquifers have a similar thickness of 20 m and cover a land area that has a length of 80 km and width of 125 km. How much water will be released from each aquifer, in m3 and in km3, if the head in the aquifers dropped by 5 ft?

Answer 1

Given

Thickness of aquifer (B) = 20 m

length = 80 km

width = 125 km

Area of aquifer = length * width = 80 * 125 = 10000 km² ; head in aquifer (h) = 5 ft

in the above picture we already derived the final expression for

volume of water released for clay aquifer as 3.434 * 10^-3 * A* B* h

= 3.434 * 10^-3 * 20* 10000*0.001524 ( 5 ft = 0.001524 km )

(Thickness will be in m because unit of specific storage is m^-1 and remaining are in km )

= 1.046 km³  = 1.046 * 10⁹ m³ ( 1 km = 1000 m that implies 1 km³ =10⁹ m³ )

volume of water released for gravel is 2.556 * 10^-5 *A*B*h

= 2.556 * 10^-5 * 10000*20*0.001524

= 7.79 * 10^-3 km³ = 7.79*10⁶ m³

volume of water released for granite is 5.29 * 10^-8 * A*B*h

= 5.29 * 10^-8 * 20*10000*0.001524

= 1.612 * 10^-5 km³ = 1.621 * 10⁴ m³