1. A gas station has a leaking underground storage tank which has leaked gasolin
ID: 293506 • Letter: 1
Question
1. A gas station has a leaking underground storage tank which has leaked gasoline into ground water. The approximate depth to water is 20 feet below ground during average conditions and the seasonal fluctuation in ground water levels is + 5 feet from the average, thus levels fluctuate by about 10 feet over the course of a year. There is a confining layer at a depth of 50 feet. Describe what would be a reasonable well construction including screened interval depth and total well depth for site monitoring wells to monitor ground water quality in association with the release of the gasoline. Where would you put wells? How deep will they be? This questions could be answered in different ways.
Gas Station Drinking Well Volatilization Microbial Gas Tank Gas Leak Degradation Mobile Phase Sorption On to Soil Dissolved Phase Groundwater Flow (Q)Explanation / Answer
The depth of water table is 20 feet with a fluctuation of +/- 5 feet. It means in post-monsoon season when water level will come up, the depth of water table will be 15feet and in pre-monsoon summer season it will go down to 25feet. The confining layer is at 50feet depth, so this will be the limit of this unconfined aquifer. So to monitor ground water quality in association with the release of gasoline, we should construct at least two monitoring wells. The first well should penetrate only this unconfined aquifer upto 50feet depth with screen between 15-50feet depth. And this monitoring well should be in the direction of the ground water flow i.e, along the gradient of the ground water flow. A second monitoring well should be constructed in the aquifer below this aquifer; the confined aquifer. In this well there should be casing to check the gasoline mixed water to enter the well and the screen should be below the confining layer of 50feet. The sole purpose of this well is to monitor ground water in the lower aquifer and to check whether the gasoline mixed water from the unconfined aquifer has leaked into this aquifer.
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