Wastewater has nutrients that can lead to excessive algae growth, resulting in harmful algal blooms and pollution in oceans and other bodies of water. A recent study by researchers from Penn State investigated the movement of these nutrients from disposal sites in the Florida Keys, and the findings are already influencing wastewater management in the area.
Wastewater has nutrients that can lead to excessive algae growth, resulting in harmful algal blooms and pollution in oceans and other bodies of water. A recent study by researchers from Penn State investigated the movement of these nutrients from disposal sites in the Florida Keys, and the findings are already influencing wastewater management in the area.
The researchers published their findings, which compile two years of monitoring data on wastewater and groundwater, in the journal ACS ES&T Water. The data was released to the public as it was collected.
Many treatment plants in the Florida Keys conduct initial biological and chemical treatments of wastewater before injecting it into shallow wells located less than 100 feet underground. The scientists explained that, ideally, remaining nutrients like inorganic phosphate would cling to the porous limestone bedrock as the wastewater moves underground before reaching coastal areas.
However, researchers from Penn State and other institutions have found signs of potential wastewater contamination in groundwater and nearshore waters, indicating that current treatment and disposal methods might not be adequate. In 2022, an environmental group filed a lawsuit against the city of Marathon, Florida, over suspected pollution from shallow wells, citing earlier studies and preliminary findings from this research. The city agreed to settle by phasing out the use of these wells.
In 2021, scientists from Penn State set up monitoring wells around a wastewater treatment facility in Marathon and collected two years’ worth of data on nutrients, dissolved ions, and man-made compounds, including the artificial sweetener sucralose and pharmaceuticals in groundwater and nearshore waters.
The study revealed that while the shallow injection method removed over 90% of soluble reactive phosphorus (SRP), a type of inorganic phosphate, both SRP and sucralose were found in nearby waters, indicating that wastewater removal was not fully effective, according to the researchers.
“Our results suggest that the use of shallow injection for disposing treated wastewater should be reexamined at facilities with large discharge volumes,” said Miquela Ingalls, an assistant professor of geosciences at Penn State and the study’s corresponding author. “Further analytical and quantitative methods like those used in this study may help assess whether wastewater injection effectively serves as a direct discharge of contaminants.”
The Clean Water Act prohibits the direct discharge of pollutants into freshwater, such as sewage spilling into a river. However, deciding if a discharge is considered direct involves complex factors, such as the distance the water must travel and the route it takes, according to the researchers.
In the Florida Keys, the water passes through porous carbonate bedrock, which is composed of ancient coral reefs that can bind phosphate through a process known as adsorption.
“The idea is that any phosphate that isn’t removed during initial treatment should stick to the bedrock’s surface after being pumped into the ground,” Ingalls explained. “We investigated how effective this process was in terms of phosphate adsorption efficiency and permanence.”
The scientists reported that around 75% of the SRP was removed from the wastewater plume within the first 10 days through adsorption. A slower process, whereby SRP is integrated into calcium phosphate minerals similar to those found in bones and teeth, increased the total phosphorus removal rate to over 90%.
The researchers also injected fluorescent green dye to track wastewater movement from the injection well to various sampling sites.
Due to its closeness to the ocean, groundwater in the Florida Keys has high salinity and is very dense. Consequently, when the less dense wastewater is injected underground, it quickly rises back to the surface.
This poses a problem because any contaminants or nutrients not removed during initial treatment or that didn’t bind to the bedrock can move directly to nearshore waters, the scientists indicated.
“This situation is compounded by the Keys’ unique geography,” Ingalls noted. “The salty groundwater causes the lighter wastewater to rise to the surface, and given the narrow land nature of the Keys, there isn’t much distance for it to travel before it returns to the ocean.”
Ingalls mentioned that the team continues to analyze the data collected from shallow injection wells, focusing on nitrogen levels, another pollutant found in wastewater.
“With phosphate, it’s about the chemical binding to the carbonate bedrock,” she stated. “For nitrogen, it’s mainly about the microbial communities in the subsurface that process nitrate and other nitrogen forms. Studying both is essential as they both can negatively impact clean water by causing eutrophication, which leads to increased algae growth and low-oxygen conditions that are detrimental to delicate marine ecosystems.”
Lee Kump, the John Leone Dean in the College of Earth and Mineral Sciences and a professor of geosciences, along with Kate Meyers and Megan Martin, who earned their master’s degrees from Penn State in 2023 and 2022, respectively, also contributed to this research.
This research was supported by funding from the U.S. Environmental Protection Agency.
