Thoughts From Engineers: On Shaky Ground: The Effect of Human Activity on Drinking Water
The results of a study examining the relationship between land use and perfluoroalkyl and polyfluoralkyl substances (PFAS) contamination were published in the Journal of Environmental Science and Technology in February 2022 (bit.ly/PFAS2022). The expansive study, based mostly in the Eastern United States, showed a strong relationship among certain hydrologic metrics, specific types of human activity such as landfills and fire-fighting test sites, and the presence of PFAS compounds. PFAS compounds serve as the current poster child for chemicals not easily eradicated by advanced water treatment, but there are countless others lurking in our water supplies—many regulated, many not.
This research and others like it show the importance of protecting lands that provide source water for public drinking supplies. It’s not enough just to regulate, monitor and treat contaminants—our country’s drinking water, point-source and nonpoint-source control laws are a principal framework for this—but to minimize the point of entry altogether. New York City managers, for example, know that millions of minor—and not so minor—human activities in the watershed can mean “death by a thousand cuts” to the city’s exceptional drinking water quality, which is why this city goes to extraordinary lengths to protect it. How our largest U.S cities evolved to be the population centers they are today is a function of historic settlement patterns, geography, early industries and many other factors. Not surprisingly, the word “variable” is key here, which describes not only the cities themselves, but the source of their water supplies.
Analysis of Potential Source-Water Contamination
In a landmark analysis published in December 2021 in Nature Communications (go.nature.com/3KjUXp5), the authors used multiple data sources to develop source-water analyses for drinking water supplies for 116 of the largest cities (i.e., more than 150,000 people) in the United States. Geospatial data layers detailing the existence of human activity—such as agricultural, industrial and human residential areas—were combined with the city’s spatially referenced water supply catchments to understand the relative contribution of each, including groundwater, to the city’s total water supply. The study then quantified the relative percentage of point and nonpoint contributions of each catchment relative to the city’s total public water supply. The study used U.S. polluting facilities data, wastewater treatment plant discharge estimates, and land-cover data to generate point-source and runoff estimates for different land activities.
According to the authors, the decision to focus on facilities and operations associated with specific sectors such as livestock, mining, and the oil and gas industries provided a more-accurate understanding of the potential for contamination than by looking narrowly at specific contaminants currently regulated by the Environmental Protection Agency.
A Comparison of Watersheds
According to this research, the average large city in the United States uses surface waters for nearly 81 percent of its total supply. Roughly 75 percent of the cities in this study use more surface water than groundwater. Surface waters also make up 100 percent of the supply for more than half of the largest cities in the United States.
Among the 116 largest U.S. cities, large variability exists in the quality of their source watershed(s). New York City and Boise, Idaho, receive water from pristine watersheds with minimal human activity. This also is true of many cities in the Pacific Northwest and California (e.g., Seattle, Portland and San Francisco) that draw water from mountain ranges or remote watersheds poorly adapted to human development. Interestingly, research findings show that more than 50 percent of the cities in this study acquire water from watersheds that are more than 80 percent pristine with nonpoint contributions at less than 5 percent and treated wastewater effluent inputs at less than 2 percent.
The cities on the other end of the spectrum that receive higher contributions of potentially contaminated point or nonpoint sources tend to be located in the Midwest and the South. Indianapolis, Atlanta and Houston, for example, are highly ranked in terms of receiving high proportions of both point and nonpoint contamination sources. Large cities in the Midwest dominate in terms of water supplies with greater than 60 percent of the total supply originating from nonpoint sources. Examples include Des Moines, Iowa; Fort Wayne, Ind.; and Columbus, Ohio—all of which draw most of their water supply from watersheds heavily used for agricultural purposes. None of the cities studied was found to have water supply catchment areas dominated by more than 50 percent non-agricultural land use, but five cities located in the South and Mid-Atlantic regions—Atlanta and Tampa, Fla., for example—had more than 20 percent of their watersheds dominated by urban land uses.
The Value of Data, Planning and Preemptive Action
This study examined the potential for water contamination in source watersheds, but it’s obvious several factors can and do act to limit and eliminate water contamination—from best-management practices to enforcement of environmental laws to advanced drinking water treatment technologies to other measures. This research, however, serves an important purpose in creating a baseline of information about the source water of some of our largest cities. Character of existing and proposed land activities, hydrogeology and other geophysical characteristics should be at the forefront in guiding future development in these critical watersheds.
As sophisticated as our drinking water treatment technologies are, there are limits to what can be done, and the term—“forever chemical”—is grimly final. Relying on a purely reactive institutional approach will, in time, lead to an irreversibly damaged public resource. Above all, now and in the years to come, it pays for cities to meticulously track and analyze data as well as take a proactive role in source watershed and water-supply protection.
About Chris Maeder
Chris Maeder, P.E., M.S., CFM, is engineering director at CivilGEO Inc.; email: [email protected]