Tajadas wrote:I understand - I think - why B is correct, but why is C incorrect? I thought as long as there is at least one layer of impermeable rock above the injection site that should be good.
Andy's basic reasoning above is correct. The passage tells us that "Under the best conditions, wastes are injected into rock strata saturated with salt water and separated by impermeable rock strata from aquifers containing drinkable water" and "Most of these wells are drilled to a depth of at least 300 meters—the minimum depth that generally puts the injected waste at a safe distance below any aquifer." So wells need to be injected into the permeable strata that is already saturated with salt water at a depth of at least 300 meters and with a layer of impermeable rock separating it from the aquifers.
The problem with answer choice (C) is that it states that the area contains "impermeable
rock extending from near the surface to a depth of at least 300 meters." We need permeable rock to drill the well into, not 300 meters of impermeable rock. We just need that layer of impermeable rock separating the well from the aquifers.
Hope this helps!