#8 - Fluoride enters a region’s groundwater when rain

[Administrator]

Complete Question Explanation

Must Be True—CE. The correct answer choice is (D)

This stimulus contains a fact set discussing the presence of fluoride concentrations in
groundwater. First, we learn that fluoride enters the groundwater when rain dissolves minerals in
the soil containing fluoride. Next, we are told about a recent study, which showed that fluoride
concentrations in groundwater are higher “where the groundwater also contains a high concentration
of sodium.” This finding was true when other factors, including but not limited to rainfall and
concentrations of fluoride-bearing minerals, were held constant.

The question stem identifies this as a Must Be True question, but the language of the question stem
is sufficiently qualified to affect our view of the answer choice. Specifically, we are told to select the
answer choice containing what can “most reasonably be concluded” from the stimulus. This is a less
stringent requirement than is normally the case in Must Be True questions. So, we need to open up
our prephrase to let in more possibilities.

The stimulus indicates that there is a correlation between high concentrations of sodium in the
groundwater and significantly higher concentrations of fluoride. We know the mechanism for
the appearance of fluoride in the groundwater: rain dissolves the fluoride-bearing minerals in the
soil. So, it appears that there is some connection between the higher concentration of sodium and
the dissolution of the fluoride-bearing minerals. However, as we know from our experience with
causal reasoning on the LSAT, we need to be very cautious about inferring the presence of a causal
relationship from a correlation.

In this case, however, the stimulus is clear that even when the other possible causal factors, the
“other relevant variables,” are held constant, the result is the same. This information eliminates
potential alternate causes and leaves us with the reasonable, if not justified, conclusion that there is a
causal connection between the sodium and the dissolution of fluoride-bearing minerals that results in
the increased concentration of fluoride in the groundwater.

Answer choice (A): The stimulus did not provide any other potential source of fluoride in the
groundwater, so we cannot infer that the fluoride-bearing minerals are not its primary source.

Answer choice (B): Rainfall was one of the factors held constant by the study. So while we can say
that rainfall is not the source of the observed increase in fluoride concentrations in groundwater, we
cannot say that rainfall has no effect on those concentrations.

Answer choice (C): Here, we have no basis to infer this comparison between the dissolution rates
of sodium-bearing minerals and fluoride-bearing minerals. Sodium-bearing minerals were not
mentioned in the stimulus at all. Rather, the stimulus mentioned groundwater containing sodium.

Answer choice (D): This is the correct answer choice because it indicates a causal connection
between sodium and an increased level of fluoride in the groundwater, in keeping with our prephrase.

Answer choice (E): Here, the answer choice refers to levels of sodium in the soil. However, the
stimulus only talked about the level of sodium in groundwater.

[BostonLawGuy]

I have read and understood the very thorough reasoning behind this question. Can someone help me out with what I am missing?

As was pointed out, Sodium is not found in the soil, but only in the groundwater, emphasizing the fact that these are two different locations.

Therefore, it seems to me that Sodium cannot help Fluoride-minerals dissolve as Fluoride had already dissolved when it was in the soil, BEFORE it entered the groundwater where the Sodium is located.

In short I don't see how the minerals that release Fluoride come into contact with Sodium as they are located in two different areas.

[Brook Miscoski]

Boston,

The stimulus does not make any distinction between the soil and where groundwater is found. In fact, the gist of the stimulus is that rainfall enters the soil and is thus groundwater. The sodium, fluoride-bearing minerals, and water are all in the same place.That is also the correct definition of groundwater, but all you need to do is let the stimulus speak for itself.

The OP is pointing out that the observation in the stimulus was an observation of the water, not that they were in different places. There are other reasons you can eliminate E, which I take to be the focal point of your question.

One reason that E is wrong is that the stimulus doesn't mention sodium-bearing minerals. Another reason that E is that it reverses the relationship between sodium and fluoride. We know to expect to find sodium around high fluoride levels; we don't know to expect to find fluoride around high sodium levels.

[Leela]

I understand why D is correct and A, B, and C are wrong. My instinct in eliminating E was also that it discussed the soil, rather than the groundwater, which doesn't help the researchers' findings. However, reviewing this I can't help but think E could be okay, considering that groundwater is in soil. Could someone please explain why we can't make this connection? Additionally, hypothetically, if we could make this connection between soil and groundwater, would E become a better answer choice than D?

[Dave Killoran]

Hi Leela,

In this case, you know LSAC doesn't see them as the same, so in studying a problem like this, the best approach is to try to determine why they don't see it as identical. Analyzing their past decisions is the best way to understand what they will do in the future!

In this case, they make it clear fomr the start that they see groundwater and soil as separate entities: "Fluoride enters a region’s groundwater when rain dissolves fluoride-bearing minerals in the soil.." (italics added). Since they've separated the entities, that's why we can't make this connection. Think of it like worms in the dirt: they are commingled, but are clearly separate entities

If they were the same, I could probably allow for (E) if it said "higher" vs "high," but it's not a hypothetical I love either way and wouldn't have stated it that way if I was trying to create a correct answer choice. Better to focus on why they are different ion this case.

Thanks!

[Agent00729]

I was deciding between D and E, but ruled out D because it only says that it increases the rate, not the actual total amount. How does the increased rate increase the total amount? it seems like the total can still be the same in all soil, just the sodium soil would get there faster

[KelseyWoods]

Hi Agent00729!

So yes, eventually, maybe all of the fluoride-bearing minerals in the soil will completely dissolve and there will be no fluoride left to enter the groundwater. But the stimulus is comparing these areas at a singular moment in time. We know that at that moment, the amount of rainfall and the concentrations of the fluoride-bearing minerals in the soil are the same between these areas. The differences between these areas are the concentration of fluoride in the groundwater and the concentration of sodium in the groundwater. If the amount of rainfall and the concentration of the fluoride-bearing minerals are the same, then the only thing that explains why at this moment in time we have a higher concentration of fluoride in the groundwater in some areas is that the higher concentration of sodium speeds up that process of the fluoride-bearing minerals dissolving in the rainwater.

Hope this helps!

Best,
Kelsey

[Agent00729]

Thank you, that clears it up!

[mab9178]

Hi,

Brook Miscoski provides another reason for eliminating answer-choice E:
"Another reason that E is that it reverses the relationship between sodium and fluoride. We know to expect to find sodium around high fluoride levels; we don't know to expect to find fluoride around high sodium levels."

Respectfully and humbly, when I abstract the logic from answer-choice E and the last sentence of the argument, I do not see an illegal reversal in E.

The word "where" introduces a sufficient condition!

The second/last sentence of the argument states:
"In a recent study, researchers found that when rainfall, concentrations of fluoride-bearing minerals, and other relevant variables are held constant, fluoride concentrations in groundwater are significantly higher in areas (HCF) WHERE the groundwater also contains a high concentration of sodium (HCNa)."

Diagramming the logical structure of the last sentence: HCNa----->HCF

E states:
"Soil that contains high concentrations of sodium-bearing minerals (HCNaBM) also contains high concentrations of fluoride-bearing minerals (HCFBM)."

Diagramming E : HCNaBM----->HCFBM

The issue with E, in my humble opinion, is that fluoride-bearing minerals are distinct from fluoride, and sodium-bearing minerals are also not the same as sodium.

In other words, the argument's last sentence draws a conditional rule between fluoride concentrations and sodium, whereas E draws a logically matching structure BUT between the "sodium-bearing minerals" and the fluoride-bearing minerals."

In retrospect, based on the argument, we can infer that a groundwater that contains a high concentration of sodium guarantees (or is sufficient to guarantee) significantly higher concentrations of fluoride in that very groundwater (the higher concentrations of fluoride in the same groundwater is the necessary part).

The flaw in E --"Soil that contains high concentrations of sodium-bearing minerals also contains high concentrations of fluoride-bearing minerals." -- is not that it commits an illegal reversal between the fluoride and the sodium.

The flaw is that E's sufficient part is "Soil that contains high concentrations of sodium-bearing minerals," and its necessary condition is "soil that contains high concentrations of fluoride-bearing minerals." And fluoride-bearing minerals and fluoride are not the same, and sodium-bearing minerals are also not the same as sodium.

Please let me know if I am wrong?

Thank you

[Adam Tyson]

You're not wrong, mab9178! It's that switch from the groundwater (in the stimulus) to the minerals (in answer E) that ruins that answer. The ratio on the minerals does nothing to explain the ration in the water!