Ecological studies have shown that the current landfill that serves

This is an 'explain the discrepancy' problem - the indicators are the word "surprisingly" in the argument, the word "justification" in the question, and the fact that the argument doesn't include any support for the environmentalists' position. To solve this, you'll need to do two things: understand the environmentalists' position, and then find an answer choice that gives a good reason for them to take that position.

The environmentalists prefer the old landfill to the new landfill. So, the right answer will have to explain why, from an environmentalist's perspective, keeping the old landfill is good or creating a new landfill is bad.

(E) shows that the new landfill is bad, but not from the perspective of the environmentalists. There isn't any logical reason for the environmentalists to care about the cost of the land.

(D) tells us why the county might want the support of environmentalists. That's interesting, but the right answer has to tell you why the environmentalists actually did choose to give their support. It's not enough to just show that their support would be good, you have to explain why they give their support.

(A) looks like it says that the old landfill is good. However, (A) doesn't actually give you good information about the rate of pollution. It's lower than initially reported, but since we don't know what the initially reported rate was, that doesn't mean anything to us. It could be quite low, or it could still be very high (but just a bit less high than we thought.)

(C) is similar to (A). Since we don't know anything about the level of pollution from coal plants, we can't get any good info by comparing the groundwater pollution to it. The level of groundwater pollution could still be quite high, even if it's not as high as the coal pollution. Plus, the argument only gives us certain options: old landfill, or new landfill? This answer choice seems to suggest that we shouldn't be worrying about the landfills at all, which doesn't help us decide whether the old one or the new one is better.

That leaves (B). (B) is correct because it suggests that the environmentalists voted for the old landfill in order to ensure that it keeps being maintained, which will prevent pollution.