The source of carcingens here is the military. The cancer tends to be confined to those exposed.

[LeftInTX]

The Defense Department has publicly acknowledged that during the Vietnam War era it stored Agent Orange at the Naval Construction Battalion Center in Gulfport, Mississippi, and the former Kelly Air Force Base in Texas, and tested it at Florida’s Eglin Air Force Base.

https://www.afcec.af.mil/Home/BRAC/Kelly/Cleanup/

After more than 30 years of investigation and cleanup, the environmental cleanup program is in the final stage of completion at the former Kelly Air Force Base in San Antonio, Texas. All site investigations are complete, all decision documents have been signed, and cleanup is complete at more than 90 percent of all sites.

For the remaining sites, remedies are in place to remove contaminants from the soil and groundwater, ensuring protection of human health and the environment. More than 600 monitoring locations provide data to be sure the public is protected and the groundwater treatment systems are working properly. The Air Force is committed to this effort and environmental regulators provide oversight and certification.

During its 83 years as a military aviation, training, supply, and maintenance complex, several areas of Kelly were environmentally impacted. The contaminants found at Kelly are typical of manufacturing and maintenance facilities--primarily perchloroethylene (PCE) and trichloroethylene (TCE), used as degreasers.

Contaminants entered the soil and groundwater at the facility through leaks, spills, and approved operating and disposal practices of the time. Over time, the contaminants migrated into the groundwater resulting in contaminant plumes in the shallow zone, about 30 feet deep.

Some areas where oil extraction and refining are prohibited and wells are far away have fairly high rates, like Real and Edwards County. In some counties where there is a lot of pumping, like Webb County, it is quite low.

Surprisingly Houston has lower cancer rates than San Antonio and Austin.
https://www.cancer-rates.com/tx/

There are numerous causes of cancer and it isn't just oil pumping and refining.

https://www.asbestos.com/cancer/state-cancer-rates/