LULING, Tex. — Amid the dry weeds on a 470-acre ranch here, a rusted head of steel pokes up, a vestige of an oil well abandoned decades ago. Across the field stand two huge, old wooden oil tanks, one of them tilting like a smokestack on the Titanic.

“Basically I get 61 acres here I can’t do anything with,” said Stuart Carter, the landowner, who is in a legal dispute with the oil producer operating on part of his ranch over who should clean up the site. Mr. Carter fears that the oil well, probably dating to the 1930s, could create a pathway for saltwater or oil to contaminate the groundwater.

Abandoned oil field equipment is a common problem in Texas, which is home to vast numbers of old wells that were never properly sealed. Some remain from the heady decades of the early- to mid-20th century, before current standards kicked in. In recent decades, regulators have worked to plug the old wells so they do not act as a conduit for liquid pollutants to enter groundwater. But some fear that the recent surge in oil drilling, brought about by the modern practice of hydraulic fracturing, will set off worrisome encounters with the old wells.

“Not every unplugged well leads to pollution, but a high percentage of wells that are left unplugged do present pollution hazards,” said Scott Anderson, an oil and gas expert based in Austin with the Environmental Defense Fund.