At some point in the Middle Jurassic period, about 169 million years ago, magma crept through the Earth and crystallized over what researchers believe was about one million years — a blink in geologic time — to form an intrusion known as a batholith. Several shallow copper deposits developed within the area, the foundation of what would become the Anaconda Copper Mine.

These geologic processes might seem abstract, but they linger over the site today. They factor into an ongoing debate over who is responsible for cleaning the mine, where waste containing uranium and arsenic migrated through antiquated unlined ponds to a local water supply. The complicated geology and mineralization of the area — including the presence of uranium — makes it challenging to trace exactly where mine contamination begins and where it ends.

Nearly two decades since the mine’s last operator declared bankruptcy, workers are starting to cut off contamination pathways under an EPA deal deferring oversight of the vast cleanup to state regulators. In the coming months, the Nevada Division of Environmental Protection (NDEP) will start deciding how Atlantic Richfield Company, better known as ARCO, should address pollution caused by the Anaconda Copper Company, which merged with a subsidiary of the oil giant in 1977. ARCO is now a subsidiary of BP, previously known as British Petroleum.

The trickiest and potentially most long-lasting decision will come in selecting how to address the contaminated groundwater, an invisible plume — with areas of elevated uranium concentrations — that extends north of the mine and travels below residences and irrigated agricultural fields.

"A big outstanding issue is what's going to happen to groundwater plume," said John Hadder, director of the Great Basin Resource Watch, a watchdog group that questioned the deferral, the deal in 2018 that transferred oversight of the mine cleanup from the EPA to the state.

Today there are more than 300 wells testing levels of uranium and arsenic in groundwater on and off the site. Testing released earlier this year showed that, in several cases, many off-site wells display levels of uranium above the drinking water standard of 30 micrograms per liter.

Uranium is a radioactive element, and its primary exposure pathways, according to the EPA, are through ingestion or inhalation of dust. That is why residents have raised concerns about the quality of their well water (many of them drink bottled water) and why former mine workers could be concerned about historic working conditions. A health consultation was published in 2006 but its conclusions were limited, pending more data. Some areas of waste rock were cleaned so dust is likely less of an issue than it was historically. State regulators said health assessments are required during the cleanup process and are “critical elements” of guiding the remediation.

The health risk is not the only unanswered question.

First regulators must resolve a hydrological question: How did the uranium get there?

The question is important because defining the extent of mine-impacted groundwater will inform to what extent Atlantic Richfield is responsible for addressing it. Already, the company is arguing the scope of pollution from the mine is about half of what was previously modeled in 2017.

In mid-July, a contractor for the company filed a report suggesting the mine is responsible for about three miles of the roughly seven-mile plume. The research argued that naturally occurring minerals and certain agricultural practices from fields adjacent to the mine have contributed to the presence of contaminants. It also suggested agricultural use can cause abrupt changes in contaminant levels, citing a recent uptick in a well near the Yerington Paiute Tribe’s land.

State regulators conditionally approved the report, but attached questions asking the company to clarify and justify the findings in a draft investigative report, due in October. That study will be critical in setting the stage for deciding how to remediate the groundwater. The state has not yet reached a conclusion on how far mine contaminants have traveled through the groundwater or whether agriculture is liable for their spread, according to emailed responses to questions.

In a letter, an NDEP official challenged the company, saying the October report “must present a more conservative interpretation” of where the mine-impacted groundwater begins and ends.

Residents and activists are following the issue closely.

Atlantic Richfield pays to deliver bottled water to about 120 residents, part of an interim measure that first began in the early 2000s. NDEP, building on an administrative process that started with the EPA, is considering whether to wind down the program. With more knowledge about the scope of the plume, the state is unsure it can justify requiring ARCO to provide bottled water.

The state recently proposed criteria under which residents would be eligible to get bottled water going forward. Under the plan, deliveries would stop for nearly all domestic well-owners.

Atlantic Richfield paid to expand Yerington’s municipal system to connect domestic well users who had their water supplies contaminated. It also reimbursed some residents for property losses as part of the class-action settlement in 2013. A company spokesperson said it did not request ending bottled water service until “connections to municipal water” were made.

Jeff Collins, who leads the division's corrective actions bureau, said in an August letter that ongoing research “shows the mine-impacted groundwater within the plume boundary does not influence the majority of domestic well owners currently receiving bottled water.”

Most residents within the plume are connected to the municipal water system. For the remaining three to four residents within the plume — who have access to the municipal system but do not want to connect to it — the state is recommending Atlantic Richfield pay to connect them to a water system. If they decide not to connect, then the company can stop bottled water service.

The calculus can be complicated though. Switching to city water usually means giving up a statutory water right, often considered part of one’s property and central to everyday living.

“I can understand the desire to keep your own well,” said Senate Minority Leader James Settelmeyer, a Republican who has considered legislation to deal with domestic wells in tainted water.

For those outside the plume, there is now a local debate over how to pay for clean water. A recent city of Yerington agenda item looked at providing water to about 20 residents, but the issue was tabled for more discussion about costs, according to the Mason Valley News.

Hadder said that regardless of whether residential wells are within the plume, the state should be mindful of moving too quickly away from a program that has existed for more than a decade.

“This is kind of a tricky subject,” Hadder said. “This bottled water program would never have occurred had it not been for the mine itself. If the mine hadn’t been there, that doesn’t mean there wouldn’t be some arsenic and uranium in the groundwater. But it [has set] up a certain dynamic, where the state needs to work with the community and [the company] to resolve that.”

Whatever option the state decides to select would have to be legally enforceable. Based on the most current testing of the plume and even a conservative interpretation of its boundary, NDEP administrator Greg Lovato said he was not sure “EPA could issue the same order now and enforce it.” This is especially true for residents outside of the plume.

The proposed criteria are not set in stone. In a statement, the division said the “decision for discontinuing bottled water service may change pending [the] outcome of studies and analyses being completed.”

The Anaconda Copper Mine sits within the Yerington Paiute Tribe’s historic land — as does the contaminated plume. Most reports suggest the plume stops short of the tribe’s current land, but it sits within the same aquifer used by the tribe. Its members are closely connected with the Walker River Paiute Tribe. They share hunting, natural resources, and familial connections. For years, they have called on state regulators to take seriously the impact of contamination.

Amber Torres, chairman of the Walker River Paiute Tribe, said the deferral to the state “blatantly slapped [the tribe] in our face” and remains concerned that the cleanup process will not fully protect her tribe’s health. She said she hoped that the federal government would remain the lead agency, as the federal government — not the state — has a trust responsibility to sovereign tribes.

But she doesn’t see the deferral going away any time soon.

“I see them charging full steam ahead, whether we agree or not,” Torres said. “It is what it is. I'm not a big gaming tribe. I don't have supplemental income to fight the federal government and state. So now we have to figure out how to be at the table with our wants and needs.”

The state is still negotiating memoranda of understanding with the Walker River Paiute Tribe and Yerington Paiute Tribe. Those agreements will outline the tribes’ role in the clean up and how they will receive funding to provide comments on technical reports and studies. Under the deferral agreement, the tribe’s reimbursement could be reduced significantly from what the tribe had received under the EPA. Some see this as part of a longer attempt to sideline the tribe.

Two months before the deferral deal was signed, a BP lawyer sent an email offering feedback on a draft version. The email, reported by Audubon Magazine, said among its disagreements was “providing for ‘substantial participation’ by the tribes in off-reservation response actions.”

More than a year-and-a-half since deferral, the state memos have yet to be signed. There is “an ongoing dialogue” over the agreements, according to Attorney General Aaron Ford’s office.

The Yerington Paiute Tribe, with land adjacent to the groundwater plume and an unlined drain where mine spills have occurred, entered into a confidential settlement with Atlantic Richfield earlier this summer, according to federal court records. Since then, the Yerington Paiute Tribe has not commented extensively on deferral. At a tour of the Anaconda mine hosted by the Progressive Leadership Alliance of Nevada last month, Yerington Paiute Chairman Laurie Thom said the tribe had indicated to Gov. Steve Sisolak that it was in support of deferral at this time.

She said the tribe has received assurances that it will have a seat at the table.

“That, to me, is progress,” Thom said. “We didn’t have that before.”

Recent court decisions could further bolster the tribe’s standing. In 2017, the Ninth Circuit Court of Appeals ruled in favor of expanding tribal groundwater rights, though a federal District Court judge recently put limitations on the ruling. The scope of that decision is still being litigated.

In a statement to The Nevada Independent, Sisolak said that he had “directed NDEP to continue regular communication with both the Yerington and Walker River Paiute Tribes to ensure their concerns are adequately addressed.”

As complicated as it is to define the mine’s contamination, the state will soon face an even more difficult decision — choosing what to do about it. State regulators are not scheduled to make a decision until 2023, but it is central to why the deferral agreement received the criticism it did.

In deciding the proper remediation, the state will conduct a feasibility study on what is practical and a legal analysis to account for other water users that have pumping rights. Lovato said the state’s goal is for the aquifer to be “protected and restored to the maximum extent practicable.”

State regulators have a variety of options. They could actively treat — and clean — the water through targeted pumping. They could also leave the plume, allowing it to naturally attenuate, a potential way to reduce the severity of groundwater plumes over a significant period of time.

“The jury is out as to what is going to happen,” Hadder said.

Regardless of what remedy is selected, the aquifer will likely require monitoring for a long period of time. Hadder remains concerned that the deferral agreement did not set more concrete goals for aquifer restoration and that Atlantic Richfield could walk away if the cleanup takes decades.

Lovato, who came to the state agency after 14 years with the EPA’s San Francisco office, said that NDEP’s goal is to complete construction on the cleanup by 2029, one decade after deferral. But people have misinterpreted this end date, he said, as when all cleanup activity would stop.

“It’s not like people walk away in 2029,” he said.

“We're not on the hook for anything related to groundwater,” he added. “I can tell you that.”

Funding for the cleanup is coming from hazardous waste fees paid by a facility near Beatty. Atlantic Richfield must reimburse the agency for most of its costs overseeing the cleanup. But critics say this gives Atlantic Richfield leverage over the state. The EPA often has other funding sources to turn to if a company is stalling. The state’s resources are more limited, they note.

For some lawmakers, the debate over groundwater strikes at a larger question about the state’s responsibility, especially when tainted waters shares an aquifer with a federally-recognized tribe.

In an arid state, water sustains life for people and habitat, says Sarah Peters, a Democratic assemblywoman from Reno and an environmental consultant for the tribes. Even if everyone has access to safe drinking water, she said, it feels wrong to leave contamination for the future.

“It shouldn’t be the next generation’s responsibility,” Peters said.