The scientists published their findings, which summarize two years of wastewater and groundwater monitoring data, in the journal ACS ES&T Water. The data was made public as it was collected.

Many treatment facilities in the Florida Keys perform initial biological and chemical treatment of wastewater and then inject it into shallow wells, less than 100 feet underground. In theory, remaining nutrients, such as inorganic phosphate, would adsorb or adhere to the surface of the porous bedrock as the sewage plume moves underground before reaching coastal waters, the scientists said .

But Penn State researchers and other groups of scientists have detected potential wastewater contamination in groundwater and coastal waters, suggesting that current wastewater treatment and disposal techniques may be insufficient. Citing previous studies by other researchers and preliminary data from this study by Penn State researchers, an environmental group sued the city of Marathon, Florida, in 2022 over alleged pollution from shallow wells. The city agreed to settle the lawsuit by abandoning the use of such wells.

In 2021, Penn State scientists installed monitoring wells around the injection site of a wastewater treatment facility in the city of Marathon and gathered two years of data on nutrients, dissolved ions and man-made compounds, such as the artificial sweetener sucralose and pharmaceuticals, in groundwater. and coastal waters.

They found that the surface injection process removed more than 90 percent of soluble reactive phosphorus (SRP), a type of inorganic phosphate. But both SRP and sucralose were detected in coastal waters, indicating incomplete wastewater disposal, the researchers said.

“Our results suggest that the use of shallow injection as a mechanism for disposing of treated wastewater should be re-evaluated in facilities with large discharge capacities,” said Miquela Ingalls, assistant professor of geosciences at Penn State and corresponding author of the study. “Other analytical and quantitative approaches like those we used here could help determine whether wastewater injection can be considered the direct equivalent of a point source contaminant discharge.”

The Clean Water Act prohibits the direct discharge of contaminants into fresh water – such as sewage dumped from a pipe into a river. But the question of whether something is considered the equivalent of a direct release is complicated and involves factors such as how far the water has to travel and the path it takes, the researchers said.

In the Florida Keys, water passes through bedrock made of porous carbonate material made from ancient coral reefs that can bind phosphate to its surface through a process called adsorption.

“The idea is that any remaining phosphate that was not removed in the initial processing steps, once pumped into the ground, will be adsorbed onto the bedrock surface and removed from solution,” Ingalls said. “We investigated the effectiveness of this remediation mechanism by studying the efficiency and permanence of phosphate adsorption.”

The scientists said about 75% of the SRP was removed from the plume during the first 10 days of transit through adsorption. A slower elimination mechanism in which SRP is incorporated into calcium phosphate minerals, like those that make up our bones and teeth, has brought the total phosphate removal efficiency above 90%.

The researchers also injected a fluorescent green dye to trace the movement of wastewater from the injection well to all of the sampling well sites.

Groundwater in the Florida Keys has high salinity due to its proximity to the ocean and is therefore very dense, scientists said. When less dense wastewater is injected underground, it quickly rises to the surface.

This is a problem because contaminants or nutrients that were not removed during initial treatment or adsorbed onto bedrock can move directly to coastal waters along the coastline, the scientists said.

“It’s kind of a confluence of issues because of the geography of the Keys,” Ingalls said. “You have this salty groundwater that brings the less dense sewage to the surface. And the Keys themselves are such narrow stretches of land that once they come back to the surface, there’s very little transport distance before returning to the ocean.”

Ingalls said the team continues to analyze data collected from the shallow injection wells and is currently focusing on levels of nitrogen, another wastewater pollutant.

“With phosphate, it’s about the chemical bond to the carbonate bedrock,” she said. “With nitrogen, it’s just the microbial communities that live underground and consume nitrate and other nitrogen species. The reason to study both is that both can have similar negative impacts on Clean water Both can cause eutrophication, which increases algae growth and low-oxygen conditions that harm fragile, shallow marine ecosystems.

Lee Kump, John Leone dean of the College of Earth and Mineral Sciences and professor of geosciences, as well as Kate Meyers and Megan Martin, who received their master’s degrees from Penn State in 2023 and 2022, respectively, also contributed to this work .

The U.S. Environmental Protection Agency funded this work.