#7 - Concept Reference, Must Be True

[Administrator]

Complete Question Explanation

The correct answer choice is (B).

Answer choice (A):

Answer choice (B):

Answer choice (C):

Answer choice (D):

Answer choice (E):

This explanation is still in progress. Please post any questions below!

[Tajadas]

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.

[andyykijun]

The question is asking about the ideal characteristics. "Ideal" can be interpreted in many ways, and possibly the option the author would like the most would be to shove contaminants into the earth's core forever. But since this is not in any of the options nor plausible, you should focus on a central concept all the answer choices are dealing with: permeability.
Permeability appears in two sentences: "Wells are drilled into porous and permeable rock strata..." in the first paragraph and "Under the best conditions, wastes are...separated by impermeable rock strata from aquifers." in the second paragraph. Here we know that: First, wastes can be injected only through permeable rock strata, and second, impermeable rock strata should separate it from above.
Answer choice A and C leaves no room for injection. This can be deduced from the first mention of permeability.
D is just nonsense. Aquifers should be separated.
E notes 1800 meters. Shoving toxins below 1800 meters would be nice. But since we know that plausibility should be a consideration, it should be eliminated. If plausibility were not a consideration, the ideal option would be to shove it as deep as possible.
B is plausible. Also, permeable strata for injection and impermeable strata for separation exists. The ideal answer choice.

[KelseyWoods]

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.

Hi Tajadas!

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!

Best,
Kelsey

[wdrosenfeld@yahoo.com]

wouldn't it be that the layer of impermeable rock above the permeable rock layer would make it so the there is no permeable layer of rock to drill into? is it that the drill cannot drill through impermeable rock, isn't that why C is wrong?

[Jeff Wren]

Hi wdrosenfeld,

Answer C is wrong for a couple reasons. First, it's important to understand that when the answer states "the area," it is referring to the exact area where the hazardous waste will be injected.

What we know from the passage is that ideally the drilling is generally done at least 300 meters below the surface, rarely deeper than 1,800 meters deep, require a porous and permeable rock strata already saturated with salt water, and having an impermeable rock strata above it to separate it from any aquifers.

Answer B best captures these details.

With Answer C, the area being described is actually the area from the surface to the 300 meters depth (meaning this answer is describing the wrong area in terms of the depth). It is also describing the area as having on or more impermeable rock strata, which would prevent the hazardous waste from being injected as you point out.

[TootyFrooty]

This was a very difficult question since it's pulling from multiple paragraphs and its hard to read the entire passage again and many times during the questions due to time constraints. What's the best strategy here because I missed the whole permeable impermeable stuff.

[Chandler H]

Hi TootyFrooty!

You're right that this question is difficult. This question could certainly take you a bit longer than average, but I'll walk you through a strategy that would make answering this question as time-efficient as possible.

So, we know that this is a reference question, because it's asking us to choose a phrase that "most accurately describes" a concept from the passage. Therefore, the first thing we want to do is find where that concept is referenced. In this case, the concept is "the ideal characteristics of an underground area suitable for the deep-well injection of hazardous wastes."

Now that we know that, we can go back and start skimming the passage. Just a few sentences into the very first paragraph, you'll see this section:

In this method, wells are drilled into porous and permeable rock strata that are already saturated with salt water. Liquid wastes are then injected into the rock strata. 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, in this case a rock stratum containing drinkable water. Such wells are rarely deeper than 1,800 meters, because below this depth it is more cost-effective to consider an alternative method of disposal.

From this section, we know that:
1. Wells are drilled into porous, permeable rock strata that are saturated with salt water
2. Most wells are drilled to a depth of at least 300 meters, because the well needs to be at a safe distance below drinkable water
3. Most wells are not drilled deeper than 1,800 meters

Just using the second two criteria (at least 300 meters, no more than 1,800) we can eliminate answer choices (C), (D), and (E). Answer choice (A) has us drilling into a layer of impermeable rock; answer choice (B) has us drilling into a layer of permeable rock. Ignore everything else! We already have our answer—the first paragraph tells us that "wells are drilled into porous, permeable rock." Answer choice (A) has us drilling into impermeable rock; therefore, it can't be right. Answer choice (B) is the only one left, and is the correct answer.

Does that help explain the strategy to attack this question?