Question

3. (Modified version of Problem 5.39 in the text). An unconfined sand aquifer with a surface area of 10,000 m² experiences a 1 m water level drop. The porosity and specific yield for the sand aquifer are given as 0.34 and 0.25, respectively. a. Calculate the volume of water that would be released from the aquifer. b. How much water would remain in the drained portion of the aquifer? Report your answer as a percentage of the total pore space. Ans: a) 2,500 m' b) 26.5%

Answer 1

Given data

An unconfined aquifer with:

Surface area = 10000 m²

^{Water level drop = 1 m}

^{Porosity = 0.34}

^{Specific yield = 0.25}

^{Unconfined aquifer is the one in which a free water surface exist. Unconfined aquifer is the one in which water table is the upper surface of the zone of saturation. Only the saturated zone of this aquifer is important in ground water studies.}

^{The porosity gives a measure of the water storage capability of a formation, not all the water held in the pores is available for extraction by pumping or draining by gravity. The pores hold back some water by molecular attraction and surface tension.}

The actual volume of water that can be extracted by the force of gravity from a unit volume of aquifer material is known as specific yield, S_{y .}

The fraction of water held back in the aquifer is known as specific retention,S_{r .}   

<sub>Volume of water that would be released from the aquifea = Surface Area x Watu level duobx Specific yield = 10000 m² x 4m x 0.25 = 2500 m3 ® pogositya sapagpiping be » n = syt 3r ton » 0.34 = 0.25 + So ( Given in question) Sy= 0.34-0.25 =0.09 portion Volume of water remain in drained of the aquifer = tooted . Surface Area x way water level x Sg = e 10000* Lex 0.09 900 m²

porosity = Volume of void tolal Volume a ma vie * 0.345 in tovoxima Vu = 3400 m² to of water remained in soil as to percentage of total pore spall - You of water retained Vol of voids x 100 or of voide = 900 x 100 3400 260477). = 26.5%</sub>

I hope my answer was satisfactory.