A cave formation discovered in South Williamson County during a road construction project was filled and sealed off in late October after county and state officials determined leaving it intact could present an environmental hazard.

On June 13 construction crews unearthed the entrance to the cave while excavating earth for a storm sewer drain adjacent to the ongoing RM 620 road widening project. Per state law, construction near the discovery immediately halted so that officials from Williamson County, the Texas Department of Transportation and the Texas Commission on Environmental Quality could assess the cave and devise a plan of what to do next.

Following more than two months of geological and hydrological review, TxDOT submitted a recommendation to the TCEQ on Aug. 23 that the cave be filled in order to prevent future contamination of the Edwards Aquifer—the vast underground natural well system that provides an estimated 1.7 million Texans with water.

"Based on a review of the options presented by TxDOT, TCEQ agreed that filling the [cave] with concrete but leaving the bottom porous and permeable with gravel/rocks was the safest and most protective option," the TCEQ noted in an official statement. "Written approval was issued on Sept. 13, 2013, to TXDOT."

Williamson County retains the services of Kemble White, senior project manager for SWCA Environmental Consultants, for analyzing the environmental impacts of its development projects. White said in the case of the cave found underneath RM 620, determining a plan of action came down to finding the "least bad solution."

"There was no good option; there was not going to be a good outcome for that cave," White said. "In our last meeting with the TCEQ, everybody was just kind of sick about it. It is a wonderful little thing that we found there."

According to surveys conducted by White and a team of geologists and engineers representing the county, state and road project contractor, the cave's dimensions measured 96 feet long by 64 feet wide with ceilings of at least 10 feet in height. The cave's interior was divided into multiple chambers and featured an abundance of stalagmite and stalactite mineral formations formed over millions of years from groundwater draining through the cave's ceiling, White said.

Like most of western Williamson County, the cave resides in the Edwards Aquifer recharge zone, so any water—or contaminants—that run through it spill directly into the underground water supply. Because of the need to protect the aquifer, the question of how to manage the cave took on larger implications than simply maintaining the geological formations it contained, White said. According to state rules, once a construction project disturbs an aquifer recharge source, authorization from the TCEQ is required for "methods proposed to protect the sensitive feature and the Edwards Aquifer from potentially adverse impacts to the water quality."

Over the course of examining the cave's features and after several consultations with project engineers, White's team devised a series of options. The list included leaving the cave alone and rerouting the storm sewers, installing an at- or above-grade bridge system over the cave, rerouting the road around the cave, or filling and capping the cave.

After examining the options, the county and TxDOT officials felt trying to preserve or work around the cave presented a greater environmental threat than simply filling it with stone and sealing it with concrete at its entrance.

"When we opened up the entrance, it destroyed a bunch of [the structural] elements, and [construction] would continue to do so," Williamson County Precinct 1 Commissioner Lisa Birkman said. "If we tried to go around it they would probably just hit more caves."

Ultimately, White said, the county and state could find no way for the road and cave to coexist next to one another.

"The overall goal of the regulatory program is to protect the entire aquifer," he said. "[By preserving the cave] you are talking about leaving a sensitive geologic feature that is basically a contaminant pathway down to the aquifer. Basically it would be sitting there waiting for an accident to happen—a tanker truck to flip over—it is basically a liability."

The likelihood of encountering caves connected to the Edwards Aquifer and discovering areas that may include federally protected endangered species has been a concern for developers and Williamson County officials for more than a decade. In 2008, after several years of working with the U.S. Fish & Wildlife Service, the county was granted approval for a federally mandated Habitat Conservation Plan. The HCP essentially allows the county to establish a series of preserves that include habitats for all known endangered species in the area. In exchange, the county is given the right to build over new habitats that are discovered during road, utility, school or private development projects.

The HCP is managed by the Williamson County Conservation Foundation and includes seven preserves encompassing more than 500 acres with dozens of caves and hundreds of aquifer recharge features.

"We in effect have a preserve of caves that we will maintain forever," said Gary Boyd, director of environmental programs for Williamson County. "We've got enough habitat protected that no catastrophic event is going to deter the long-term preservation of the endangered species."

The HCP is funded through development fees. In the case of RM 620, the county paid $540,000 into the plan for the right to avoid the federal review process—which can often take months and result in millions of dollars in construction delays—when it discovered the cave.

"The HCP provides for a 'no surprises' provision, meaning that if there is a cave that happens to be there it can be assessed and closed per water regulations," Boyd said. "All they had to do was contact TCEQ, determine what the most effective way to deal with the void so that the aquifer was not harmed and then proceed without having to take any more steps for endangered species."