I have 5 sentences I want to combine.
I want to know if there are any problems with the way that I combine them.
1. Another hazard is groundwater.
2. Water might first leak into a burial site.
3. It might cause canisters to leak.
4. It might then carry radiation into aquifers.
5. The aquifers are underground.
My way to combine them is
Another hazard is groundwater which might first leak into a burial site, cause canisters to leak, and then carry radiation into underground aquifers.
In reviewing your posting, I would agree that your sentence is "correct." However, remember also that when you are writing, you want not only to include information, but you also want your work to be of interest if it is to be read by others. This can be done without altering the content of the information you are sharing or the tone that your writing requires to be considered formal writing.
You have achieved the sound of formal writing: there are no cliches, colloquialisms or slang used. However, to make it more interesting, you can easily alter the word order in your sentence(s). While simple writing uses a "subject-verb" word order, it can be switched around, where a dependent clause or a prepositional phrase can be added to the beginning of the sentence, followed by a more traditional word order for the remainder of the writing. There are almost endless possibilities. When you really want to grab your audience, use a variety of sentence structures.
Repetitious sentence patterns [are] monotonous. ...most common patterns [are]...simple subject+verb sentences.
I would also make sure that the context of your information is very clear.
As an example, you might write:
Though it is often desirable, groundwater can be a hazard if it finds its way into a burial site, thereby causing canisters to leak and radiation to be carried into aquifers that hold fresh untainted water.
Phrases like, "Though it is often desirable" and "thereby" are not required, but they make the sentence more interesting and more sophisticated, even though you have not altered the content of the sentence as much as its "tone." ("Tone" is the author's feeling toward his subject: here it is serious.) I also added a reference to what aquifers are, which simply provides a context for the sentence and points to the danger of the things you mention, such as "hazard," "leak" and "radiation."
It is not necessary to write this way: your sentence covers the basics, but with few additions and changing the sentence's "syntax" (word order), or structure, your writing can be more interesting and engaging.
Your answer is fine, but a little better might be
"Another hazard is groundwater, which might first leak into a burial site, causing canisters to leak, and then carry radiation into underground aquifers."
Another hazard is groundwater which might first leak into a burial site and then it might causes canisters to leak and carry radiation into aquifers which are underground.