LSAT 90, Reading Comprehension, Q7, LSATHacks

RC Question 7 Explanation, by LSATHacks

DISCUSSION: This is by far the trickiest question of the passage. Usually, specific detail questions are answered in one part of the passage. But here, the author explains deep well injection site criteria in two places: the middle of paragraph 1, and the start of paragraph 2. It’s easy to get this question wrong if you only look at paragraph 2.

The criteria for an injection site are as follows:

• Lower than 300m so as to be below aquifers (paragraph 1)
• Above 1800m so as to be cheap enough (paragraph 1)
• In an permeable layer full of salt water (paragraph 1, 2)
• This layer should have an impermeable layer above, so that water can’t flow between the injection site and aquifers above. (paragraph 2)

I’ve summed it up in this diagram below:

___________

1. The first part of this is correct: the injection site must be 300m deep or below. The second part is wrong: the start of paragraph 2 says that injection sites must be “separated by impermeable rock strata from aquifers”. Aquifers are above 300m (middle of paragraph 1), so this means that the impermeable layer must be above the injection site.

   So what’s the problem? The problem is the injection site needs permeable layers (look at the diagram above). These permeable layers are below the impermeable layer which separates the injection site from aquifers.

   But what does this answer say? It says there is an impermeable layer below which there are no permeable layers. So this answer directly contradicts a major requirement. We instead need permeable layers below that impermeable layer.

2. CORRECT. We know waste is injected at minimums of 300m, and has to be injected into a layer of permeable rock. We also know that above 300m is where the drinking water is. Sentence 1 of paragraph 2 says that impermeable rock separates drinking water from the waste. So that means the impermeable layer is above the injection site.
3. This has the depth all wrong! Paragraph 1 says that aquifers are from the surface down to 300m, and so therefore injection sites must be below 300m.

   But this answer says that the injection site it above 300m! This puts the injection site square in the middle of aquifers. That’s an awful idea, it would inject deep well wastes directly into drinking water. This answers directly contradicts the central point of the passage, so if you picked it you need to slow down and read more carefully and actually try to picture or draw what they’re describing to you.

4. The entire point of site selection in the passage was to get the injection sites away from aquifers. Whereas this answer implies that we need an aquifer near the toxic waste injection site. That’s bonkers! Aquifers are indeed found at depths of 300m or less, but we don’t need to go seek them out.
5. The 1,800 number is only relevant to tell us that beyond that depth, deep-well drilling is no longer cost-effective. This answer suggests the sites should be at or below 1800m, which is wrong: that’s way too deep.

Hi, I'm Graeme Blake

Founder of LSAThacks; I scored a 177 on the LSAT.

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