This isn’t ideal, because we want to protect drinking water from contamination. The drinking water would typically be above areas in which waste is injected, so we want an impermeable layer above the area in which waste is injected. It doesn’t matter whether there aren’t any permeable layers below the impermeable rock. We want impermeable rock between the drinking water and the area in which waste is injected.

We’re told that “under the best conditions,” wastes are injected into areas with salt water and “separated by impermeable rock strata from aquifers containing drinkable water.” We’re told earlier that the area with salt water is permeable. We’re also told that aquifers containing drinkable water are typically above the areas in which waste would be injected. So having an impermeable layer above the permeable rock in which waste is injected would be ideal.

We want to protect drinking water from waste, so we want an impermeable layer above the waste and below the drinking water. (C) doesn’t describe a layer of impermeable rock below drinking water and above the area in which waste is injected. In addition, deep-well injection does require at least some permeable rock that has salt water. So it’s not clear whether deep-well injection is possible if impermeable rock extends from the surface down to 300 meters.

A depth of 300 meters or less isn’t ideal, because this is the range in which drinking water aquifers might exist.

A depth of 1,800 meters or more isn’t ideal, because this is the depth at which other methods are more cost-effective.