A Farmer Drilled a Well into an Open Aquifer Composed

A farmer drilled a well into an open aquifer composed of sand and gravel. He installed a septic system downslope from the drinking well (see diagram) . A few years later the septic system started to leak. Water tests showed that the well water was clean and uncontaminated by bacteria present in the septic system. Why did the septic system not contaminate the drinking water supply?

A) The bacteria were drowned in the groundwater.
B) The groundwater flow carried water away from the well.
C) Gravel has a low permeability that makes it difficult for bacteria to travel from the septic system to the well.
D) The septic system is not located in the aquifer's recharge zone.