Q: What is the correct verb to use in my research paper?

The first sentence is slightly tricky. Typically, a mass noun, like "soil" or "water" would take a singular verb. However, in this case, although both the nouns in the subject are mass nouns, they are still two different nouns. So technically, the verb should be plural. Take for instance a sentence like "Water and alcohol are common solvents used in cosmetics." The subject has two mass nouns that are treated as distinct. However, sometimes such subjects are considered as a single unit rather than as having distinct entities. In such cases, it may be acceptable to use a singular verb, for example, "Bread and butter is her favorite breakfast." If you think of "fuel-stained soil and street waste" as one whole entity, then the use of the singular verb is fine.

According to most widely used scientific style guides, units of measurement should be treated as collective singular nouns. Thus, in the second sentence, the verb should be singular even if the noun "kg" (kilograms) is plural. So whether you say "15 mL" or "fifteen milliliters," the verb should always be "was."

To understand how verb tenses should be used correctly in the "Methods" section of your paper, read this short helpful post.