A sanitary landfill is located above a deposit of clay that is 3 m thick andover

Question

A sanitary landfill is located above a deposit of clay that is 3 m thick andoverlies an aquifer that provides drinking water to a small town. A zone of leachate-contaminated groundwater has accumulated at the base of the landfill. Observations in theaquifer beneath the clay indicate a steady piezometric level of 2,400 m above mean sea level.The surface of the water table in the landfill is 2,411.1 m. The hydraulic conductivity of theclay is approximately 2 x 10-10 m/s, and the effective porosity is 13 percent (connected pores).Estimate how long it will take for nonreactive contaminants to move through the clay into theaquifer. Express you answer as a range of values that you consider to be reasonable in light ofthe available data. The maximum time for leachate to move through the clay layer would be tosimply assume no dispersion and compute the travel time of water to move through the claylayer.

Explanation / Answer

specific discharge q = kj
where q= specific discharge
k= hydralic conductivity = 2*10-10m/s
j = hydralic gradient = (2411.1-2400)/3=11.1/3
j= 3.7
q =2*10-10(3.7) = 7.4*10-10m/s
velocity V = q/n
where n = porocity =13% = 0.13
V = 7.4*10-10/0.13 = 56.923 *10-10 m/s
time taken for contamination t = L/v
where L is thickness of landfill = 3m
t = 3/ 56.923*10-10 = 0.0527*1010sec

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