‘Hydrologists should be happy.’ Big Supreme Court ruling bolsters groundwater science

A new U.S. Supreme Court ruling puts groundwater science at the center of decisions about how to regulate water pollution.

Today, in a closely watched case with extensive implications, the court ruled six to three that the federal Clean Water Act applies to pollution of underground water that flows into nearby lakes, streams, and bays, as long as it is similar to pouring pollutants directly into these water bodies. The decision came after a sewage treatment plant in Hawaii claimed that the landmark environmental law covered only “point sources” of pollution, such as an effluent pipe that dumps polluted water in a stream, lake, or bay, and not polluted groundwater that seeps into water bodies.

The decision is a win for environmentalists, who feared the court might side with the Trump administration’s argument that the law didn’t apply to groundwater. In the decision, Justice Stephen Breyer wrote that groundwater pollution was subject to federal water-quality regulations as long as the connection to surface waters was the “functional equivalent” of a direct discharge such as a pipe.

The new standard will hinge on scientific studies of how water and chemicals move underground, says Pat Parenteau, an environmental law expert at Vermont Law School. “It puts the science front and center,” he says. “The hydrologists should be happy.”

That’s partly because questions of when the law applies will come down to the scientific details of individual cases. The court’s decision spells out criteria that might influence whether groundwater pollution meets the new standard and is covered by the law. Those criteria could include how quickly and how far the pollution moves from its source before reaching surface water, the underlying geology, and how much the chemicals are diluted or broken down. “The difficulty with this approach, we recognize, is that it does not, on its own, clearly explain how to deal with middle instances,” Breyer writes.

Tracing the movement of pollutants through groundwater is tricky work, says Thomas Harter, a hydrologist at the University of California, Davis, who helped draft an amicus brief to the court from several scientific associations. Much of what’s happening is underground, obscured except for data gathered from test wells and geologic samples drilled from boreholes. Impacts can take decades to materialize as water and contaminants slowly seep through the earth.

Computer models coupled with direct observations of water flow, pollution levels, and geology can predict how chemicals are likely to flow and interact with the chemistry of surrounding rocks. But, “There are huge uncertainties to this,” says Harter, who studies groundwater pollution from California farms. “It’s not unlike trying to predict the weather.” (During oral argument of the case in November 2019, however, Breyer remarked that briefs laying out the science of tracking groundwater pollution had impressed him. “The scientists really convinced me they’re geniuses and they can trace all kinds of things,” he said.)

The new legal yardstick won’t be that different from what state and federal regulators have done for decades, predicts David Henkin, a Honolulu-based attorney for the environmental law firm EarthJustice, who argued the case before the Supreme Court. Previously, regulators held that the federal law applied whenever they found a “direct hydrologic connection” between groundwater and surface water. “Science always has and will continue to play an important role in understanding the bounds of the Clean Water Act,” Henkin says.

Parenteau, however, suspects the new ruling will place even more emphasis on the science, because it spells out a longer list of potential factors that can shape decisions about which activities are regulated.

A technical test case

The case before the court illustrates just how science-intensive these questions are. The dispute revolved around a municipal sewage treatment plant on Maui that each day pumps roughly 15 million liters of treated wastewater into underground wells 1 kilometer from hotel-lined beaches. Scientists and residents had suspected since at least the mid-2000s that wastewater was flowing through groundwater and into the ocean, injecting nitrogen that sparked algae growth and damaged coral reefs.

But it took an ambitious research push by scientists at the University of Hawaii, Manoa, to show decisively that it was happening. “We threw everything at it that we have,” says geochemist Craig Glenn, who led the research.

They found seaweed growing near the shore there with high concentrations of a nitrogen isotope associated with human activity such as wastewater and fertilizers. Airplanes and drones equipped with infrared cameras showed a plume of unusually warm water emerging in the ocean just southwest of the treatment plant. Researchers dumped a dye, fluorescein, into the wells and monitored the marine waters along the beaches. The dye first turned up in the ocean nearly 3 months later, with concentrations peaking at between 9 and 10 months after it was inserted. In a 500-page report, the scientists concluded that more than 60% of the wastewater dumped into wells was reaching the ocean. The dye study was “the thing that really put the nail in the coffin for this case,” Glenn says.

Environmental groups sued Maui county, arguing it needed to get a federal water quality permit for the plant, which would have essentially required it to clean up the pollution leaks. But the county resisted, resulting in a federal lawsuit in which it argued that the Clean Water Act didn’t apply because the law only covers point sources.

Today, the court rejected that argument and sent the case back to a federal district court for reconsideration. There, a judge had previously ruled in favor of the environmental groups. Now, the question is expected to be exactly how the Maui county plant will bring the discharges into compliance with federal law.

In a statement, Maui County Mayor Michael Victorino noted the court did not overtly rule the county needed a water quality permit. “We look forward to further clarity from our local regulators and working collaboratively to protect our waters.”

The U.S. Environmental Protection Agency said in a statement that it was reviewing the court’s call for further clarity from the agency. But it said the ruling “unfortunately leaves some uncertainty for the public, including private property owners.”