Groundwater Remediation Achieved through a 25-Year CERCLA-to-Drinking Water Program
Of the many responsibilities with which a water utility is regularly tasked, ensuring public health is always at the forefront. In fact, public health is tied to just about everything a water utility does. But sometimes, fulfilling that responsibility doesn’t come in the form of common capital projects like pipeline rehabilitation, fixing leaks, installing new meters, etc.
Often, the task can be much more immediate, especially when groundwater contamination is a concern.
Such has been the case for Tucson Water in Arizona, which has responded to isolated groundwater contamination issues for decades. Tucson Water is the municipal water supplier for the City of Tucson and much of the surrounding area. The nearly 30 billion gallons of drinking water delivered by the utility to its 722,000 customers each year is pumped from area aquifers, although about 90 percent of that current volume is native groundwater blended with recharged Colorado River water. In its roughly 390-sq-mile service area, Tucson Water operates:
- 206 active production or standby groundwater wells;
- 57 potable storage facilities capable of storing nearly 305 million gallons;
- 4,600 miles of pipelines; and
- Hundreds of booster stations and pressure control valves that give the utility maximum flexibility in water delivery.
In recent decades, Tucson Water has responded to the presence of various groundwater contaminants in an isolated portion of its service area by constructing multiple treatment initiatives.
TCE Discovery
In 1981, the utility and the U.S. Environmental Protection Agency (EPA) confirmed the presence of Trichloroethylene, an industrial solvent commonly known as TCE, in wells in the vicinity of Tucson’s airport. The chemical, used in aircraft maintenance activities in the area from the 1940s to 1970s, entered the aquifer after being improperly disposed of in open, unlined industrial pits or directly on the ground. Drinking water contaminated with TCE has been linked to liver and kidney damage, cancers such as leukemia, and birth defects.
As a result of TCE contamination, 11 city wells and several private wells were shut down. A year later, in 1982, the EPA added the airport site to its Superfund National Priorities List under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). The site and the subsequent treatment facility built as the result of a consent decree are known as the Tucson Airport Remediation Project (TARP).
Discovery of the contaminant set the stage for further investigations and feasibility studies that took place throughout the late 1980s, leading to implementation of a TCE remedy. By 1990, backed by funding from all parties included in the consent decree, Tucson Water designed and built wellfields, pipelines and a central treatment plant to address the TCE contamination. Construction took place in the early 1990s and, by 1994, nine remediation wells and a packed column aeration facility became operational.
According to Jeff Biggs, strategic initiatives division administrator for Tucson Water, funding for the project came largely from the U.S. Department of Defense, which was found to be the primary party responsible for the contamination. At the time, roughly $35 million was placed in a trust fund to be used to finance aspects of the cleanup, as well as ongoing annual operation and maintenance of the facility.
1,4-dioxane Discovery
In 2002, another contaminant, 1,4-dioxane, was also detected in the TARP area. 1,4-dioxane is a stabilizing agent used in industrial solvents common in aircraft maintenance activities. The EPA does not regulate 1,4-dioxane but it does issue advisories as guidelines for water utilities. In an effort to mitigate this new potential public health risk, the utility began blending contaminated groundwater with potable water from uncontaminated city wells, reducing 1,4-dioxane levels in the resulting product below the EPA’s then-established drinking water health advisory of 3.0 parts per billion.
The EPA issued a revised drinking water health advisory for 1,4-dioxane of 0.35 parts per billion in 2011. Utility leaders quickly determined that more aggressive steps were needed to manage 1,4-dioxane in the TARP wells, and Tucson’s mayor and city council approved a utility proposal to build an Advanced Oxidation Process (AOP) water treatment facility.
Although the 1,4-dioxane contamination occurred within the same aquifer treated by the TARP facility, city leaders learned that the existing CERCLA funds were intended only for construction and operation of a facility to treat TCE. Design and construction of the AOP facility would cost approximately $19 million. Recognizing that a court-ordered cleanup plan could take years, further endangering public health, the mayor and city council decided to fund the project through capital improvement program funds. City officials would negotiate later for reimbursement from those responsible for the contamination.
“First and foremost, this was a water quality issue and the City of Tucson took it upon itself to clean it up and treat it,” says Biggs, who led the project. “We were delivering that water to the public. As a drinking water utility, we were responsible for cleaning it up.”
The AOP facility was built adjacent to the existing TARP plant. The 4,500-sq-ft facility was completed on schedule and within budget in 2014. Together, the two facilities are capable of purifying over 8 million gallons of water a day, which is subsequently delivered to approximately 60,000 customers within Tucson Water’s service area. AOP technology combines ultraviolet light with hydrogen peroxide, creating a strong oxidant that removes not only 1,4-dioxane from the underground plume at the TARP wellfield, but also removes TCE.
“[The] new AOP Water Treatment Facility is an important investment in our community’s water quality and long-term water reliability,” said Tucson Mayor Jonathan Rothschild at the time of the facility’s completion. “Tucson Water’s commitment to ensuring that customers have enough safe, high-quality water today and in the future is a strategic approach that we fully support.”
As of 2017, the city had recouped 90 percent of the funds used to build the AOP facility from the Department of Defense, Biggs says.
Perfluorinated Compounds
As if that wasn’t enough, another contaminant was discovered in the TARP area. This time, the groundwater was found to have perfluorinated compounds – in this case perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) – and the city soon needed to take action to make sure it was meeting EPA guidelines.
“We thought, here we go again,” says Biggs. “We had to do something about this.”
The utility discovered that the granular activated carbon (GAC) component of the AOP facility, which was intended to quench hydrogen peroxide in the AOP process, also had the serendipitous effect of removing perfluorinated compounds from groundwater. Despite this working for a few years, however, perfluorinated compounds eventually saturated the GAC and the amount being removed decreased. At the same time, EPA published new drinking water health advisories for PFOA and PFOS, making the levels detected in water produced by the TARP and AOP plants too close for comfort.
After investing in upgrades to its GAC, the city today is removing PFOA and PFOS to levels below the EPA’s health advisory. Now working with Carollo Engineers, Tucson Water is looking to design a new permanent facility that would add more efficient GAC capabilities.
Fulfilling Its Mission
For more than 25 years, the City of Tucson and other stakeholders, such as its Unified Community Advisory Board (UCAB) for community engagement, have been working closely to resolve issues related to the cleanup process at the TARP site.
Biggs says that while the operation is ongoing, it demonstrates a long-term project undertaken by a water utility to do whatever it takes to ensure safe drinking water requirements are met for the public, despite financing uncertainties.
“I think this shows that – as a drinking water utility – we knew what we had to do, and we went out and did it. And then afterwards, you reach for the money from the entity responsible for the contamination,” he says, adding that this is the approach the city is again taking to address PFOA and PFOS contamination.
Tucson Water reports that the water being produced by the TARP and AOP treatment facilities continues to meet all federal drinking water standards and is safe for drinking, cooking and bathing. Tucson Water performs thousands of tests each year on water samples from across its service area and reports the results to customers, elected officials and the Arizona Department of Environmental Quality.
Since 1994, the city has completed the remediation of 51.3 billion gallons of groundwater and the removal of 5,625 lbs. of TCE. Since 2014, it has removed 115 lbs. of 1,4-dioxane and significantly decreased the overall TCE and 1,4-dioxane contamination at the TARP site. In 2015, Tucson Water’s AOP Water Treatment Facility was awarded the Grand Prize in Design from the American Academy of Environmental Engineers and Scientists, among several other accolades.
Andrew Farr is the managing editor of Water Finance & Management, published by Benjamin Media in Cleveland, Ohio. He has covered the water sector in North America for eight years and also covers the North American trenchless construction industry for sister publications Trenchless Technology and NASTT’s Trenchless Today.
