Deep Well Injection

Answer choices

1. A

   Below is Not a Concern19% picked this

   At 300 meters or more below the surface, the area contains a layer of impermeable rock below which

   We inject the waste at a minimum depth of 300m, because we're trying to make sure it's beneath any of our precious water supplies, which are found above that. So it doesn't really matter to the author if the rock below the injection site is impermeable, because the author isn't really worried about waste leaking down into the Earth's mantle. She's worried about it leaking up into our precious aquifers.

   19%
2. B

   Correct58% picked this

   At 300 meters or more below the surface, the area contains a layer of permeable rock above which there is

   Why this is right

   We inject the waste at a minimum depth of 300m, because we're trying to make sure it's beneath any of our precious water supplies, which are found above that. So if the rock above the injection site is impermeable, then there's no way for the waste to leak up and get into the aquifers above. This answer is similar to what's being described in the first sentence of the 2nd paragraph. The other fact we need to understand it is in the middle of the 1st paragraph: wells are drilled to a minimum depth of 300 meters -- the minimum depth that generally puts the injected waste as a safe distance below any aquifer.

   Skill tested: Application · how this choice captures the passage's function is the move to repeat next time.

   58%
3. C

   Everything is Impermeable15% picked this

   The area contains one or more layers of impermeable rock extending from near the surface to a depth

   This one is tricky. Whether something is permeable or impermeable refers to whether or not it's completely solid, or whether liquid can pass through it (and essentially be stored in it). The first sentence of the 2nd paragraph is saying that we inject the waste into rock strata that is saturated with salt water. If we're injecting into rock strata that is saturated with salt water, then we're injecting into permeable rock. This makes some common sense. If we drilled into solid rock, then we would only be able to dump our liquid waste into that column we've drilled. But if we drill down into a permeable layer of rock strata, we can dump a lot more waste in there, because it will seep into all the nooks and crannies of that rock strata (mixing with the salt water). So this answer choice is describing an area with lots and lots of impermeable rock, and the problem is we need a permeable strata in order for it to hold a bunch of our liquid waste, but we need that permeable strata walled off from other things so that it doesn't leak and ooze into other strata where we don't want waste.

   15%
4. D

   Opposite, if anything4% picked this

   At a depth of 300 meters or less, the area contains an aquifer into

   We're normally injecting waste at a minimum depth of 300m because generally water is always found above that. But this answer is talking about water that is below 300m, putting it at greater risk of getting tainted by leakage from the waste well.

   4%
5. E

   Too Deep4% picked this

   The area contains a layer of impermeable rock overlain by a layer of permeable rock at a depth

   We're told near the end of the 1st paragraph that "wells are rarely deeper than 1800 meters, because it's too expensive". So this answer seems to be talking about what's going on below 1800 meters, which is too deep for affordable drilling, so not an ideal area.

   4%