Notifying residents of unsafe nitrate levels in tap water lets them decide if they want to purchase bottled water or install filters that remove nitrate and other contaminants. Credit: Iakov Filimonov|Dreamstime.com.
A recommended switch from tap to bottled water in the local newspaper is a powerful reminder that we can’t take safe drinking water for granted. Such recommendations are published about 600 times a year in the United States. Among the most common reasons is a violation of the federal Safe Drinking Water Act when public water testing detects elevated nitrate levels.
This happened in the rural Kansas hometown of assistant professor Jeff Hadachek while he was a graduate student in California. It planted the seed for two questions that became part of his dissertation research: Do public water quality reports cause behavioral changes with potential health benefits? And if so, are these benefits evenly distributed within affected communities?
“I decided to analyze if nitrate violations caused avoidance behavior in the form of increased bottled water sales,” says Hadachek. “Quantifying the health benefits of this behavioral change helps us assess the economic value of safe drinking water.”
The vast majority of nitrate violations occur in rural America because nitrogen is mostly used as fertilizer for agricultural crops. Since plants need nitrogen to grow, these fertilizers increase yields and lower consumer prices of staple foods. The downside: Excess nitrogen not absorbed by plants leaches into the soil and dissolves into groundwater as nitrate, where it can persist at unsafe levels for up to 60 years. Public water systems drawing from underground aquifers rather than surface water account for 95% of nitrate violations.
Nitrate is one of about 90 federally regulated water contaminants. Its maximum allowed level of 10 milligrams per liter stems from a 1951 report linking it to blue baby syndrome. This condition restricts blood oxygen flow and can be fatal in formula-fed infants. Although blue baby syndrome is rare today, nitrate’s acute health threat makes it distinct from many other contaminants whose standards were derived from studies of chronic exposure. It also explains why public water utilities have to notify their customers within 24 hours of detecting violations.
The recommended response is a switch to bottled water or installation of a household water filter. The average duration of nitrate violations—135 days, or 4.5 months—is much longer than for bacterial contamination, which usually resolves within a few days.
To quantify avoidance behavior, Hadachek used scanner data from 48,000 retailers selling bottled water. He merged 10 years of data on sales and nitrate violations by zip code, enabling an analysis of 3,244 U.S. counties from 2010 to 2019 at high spatial resolution. The retailers ranged from low-cost and convenience stores to regional, national and mass merchandise grocery stores, with the exception of family-owned small businesses. Hadachek added census-tract average income data to test if changes in bottled water sales varied by socioeconomic status. (Census tracts of 2,500 to 8,000 residents are defined by the U.S. Census Bureau.)
To estimate the short-term health impacts of avoiding unsafe water, he used monthly per-county infant mortality rates from the Centers for Disease Control and Prevention. These rates describe the probability of newborns dying before the age of one and are considered regional indicators of community health. Causes of death were unavailable for this study, but drinking nitrate-contaminated water during pregnancy increases the risk of preterm birth and low birth weight, two of the leading causes of infant deaths.
The analysis focused on bottled water sales during post-violation periods at all stores located in the same city as the public water utility. It compared these “treatment” sales with “control” sales during pre-violation periods, estimated as average sales at the same stores during the week that started 21 days before the notification.
Bottled water sales increased by an average of 32% during post-violation periods, with a maximum increase of 67% after three weeks, and returned to baseline after nine weeks. Consumers spent an estimated $4.5 million annually on bottled water, equivalent to $7 per affected resident.
This behavioral change had public health benefits: Infant mortality rates in counties with at least one violation declined by an average of 11% during the first post-violation month and returned to baseline one to two months later. The decline, and the increase in bottled water sales, was greatest in counties with an average income in the top 25%. Those in the bottom 25% had smaller increases in bottled water sales and no significant change in infant mortality.
“This means residents with the strongest avoidance behavior saw the greatest improvement in health measures and vice versa,” says Hadachek, noting that more work is needed to determine if income accounts for most of the differences in county health outcomes.
The health outcomes represent both residents on public water and those using private wells, which are common in rural areas. These wells don’t have federal testing requirements but often draw from more shallow parts of the same aquifer. Thus, public water nitrate violations may extend to privately-sourced water, and notifications likely reach both sets of residents. Hadachek estimated the benefits of these notifications as $160 million per year, exceeding the annual spending on bottled water by a factor of 35.
To alleviate the nitrate problem, researchers have studied socially optimal levels of nitrogen fertilizer and the use of cover crops to reduce leaching. In high-intensity agricultural areas, installing household filters that remove nitrate and other contaminants—ideally with financial assistance for low-income residents—is a more sustainable solution than long-term bottled water purchases. Public water utilities with persistent violations may opt for a third solution: consolidation with a nearby system or technology upgrades.
“My Kansas hometown ultimately installed a $4 million groundwater treatment system,” says Hadachek. “When such solutions aren’t immediately feasible, easily accessible information about drinking water quality provides large social benefits for relatively low costs.”