When fracking liquid waste is left in pools on the surface, _ can evaporate into the air and contribute to pollution

The answer is "toxic chemicals".

Explanation:

Hydraulic fracture uses compounds such as sand to prevent fractures from closing, chemical additives, and several hundred chemical products, some very toxic and carcinogenic, that perform the function of preventing gas and oil from becoming contaminated. They are also used to prevent corrosion. By depositing fracking liquid waste in the pools, the most volatile products tend to evaporate contributing to air pollution. Some of the elements contained in this wastewater are selenium, radio, and lead.

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what is the question here. i think you messed up.