The stimulus does tell us the amount of leachate that escapes is usually unpredictable, but it lacks direct information about how predictability would help solve the problem. (A) would be much more promising if the stimulus featured a claim like "if the quantities of leachate could be predicted, sewage plants' capacity to handle the substance would increase."

This mashes up two unrelated claims. The stimulus tells us:

1. Leachate develops when water permeates a landfill site
2. If the landfill's capacity to hold liquids is exceeded, leachate will escape into the environment

So both of (B)'s clauses reflect text in the stimulus – it's just that those pieces aren't connected by an "if, then" relationship.

This is a much too strong restatement of the stimulus' claim that not all sewage plants are capable of handling the highly contaminated water. Properly translated, that claim reads:

Some sewage treatment plants are not capable of handling leachate.

Negating an "all" claim yields a "some" claim.

The plants incapable of handling leachate aren't necessarily receiving any leachate.

The stimulus tells us most leachate is sent to sewage plants of some description. The stimulus also tells us there are some sewage plants that can't handle leachate. None of the stimulus' claims prevent us from sticking to the sewage plants that can handle leachate.

The support for (E) comes from the stimulus' second sentence. If and only if marks a biconditional claim. Broken down, it reads:

1. If the landfill overfills, leachate escapes
2. If leachate escapes, the landfill has overfilled

Both these claims act as normal conditionals, meaning (among other things) that they can each be expressed in the contrapositive:

1. If leachate doesn't escape, the landfill hasn't overfilled
2. If the landfill doesn't overfill, the leachate won't escape

That bolded piece matches (E).