Among the VOCs found in groundwater, which are encountered most frequently and in the highest concentrations?

• A. THMs and haloacetic acids
• B. 1,1,1-Trichloroethane and trichloroethylene
• C. 1,1-Dichloroethylene
• D. Benzene

Answer: (B)

In groundwater, chlorinated solvents are the VOCs you’re most likely to see at detectable levels and at higher concentrations. 1,1,1-Trichloroethane and trichloroethylene are classic examples of this group. They were widely used in industry and can persist and migrate through soil into aquifers, so many contaminated sites show both compounds together and at relatively high levels.

Disinfection byproducts like trihalomethanes and haloacetic acids come from treating water with chlorine, not from the groundwater itself, so they aren’t typically the highest-concentration VOCs in raw groundwater. Benzene and 1,1-dichloroethylene do appear in groundwater, but they’re not as consistently prevalent or as elevated across a broad range of sites as TCE and TCA.

So the most frequent and higher-concentration VOCs in groundwater are the chlorinated solvents, namely 1,1,1-trichloroethane and trichloroethylene.