When it rains, and even when it pours and floods like it did in late 2018 and through much of 2019 in Nebraska, the precipitation that hits the surface doesn't always contribute to recharging the state's groundwater supply.

In fact, the majority of precipitation doesn't reach those depths. So when does it most often happen? A recent study led by Mikaela Cherry, a doctoral candidate in the University of Nebraska-Lincoln's School of Natural Resources, found that winter precipitation reloads most of the state's groundwater supply.

And learning when groundwater is recharging could reinforce efforts to limit how much nitrate reaches the supply, Cherry said. Cherry said that exploring the water quality wasn't part of the initial plan, but it turned out to be a key unforeseen benefit.

"When we approached the project, it was more about finding the 'when,'" she said.

Cherry started her doctoral program two years ago, and her adviser, groundwater hydrologist Troy Gilmore, told her about a trove of data collected in 2011 by former UNL faculty member John Gates, who coordinated with Nebraska's natural resources districts to gather nearly 800 groundwater samples from across the state that year.

"[Gilmore] said, 'Here you go,' and that's how I started working on it," Cherry said.

The samples allowed for an examination of the isotopic composition of groundwater found throughout Nebraska, and further allowed Cherry and her research team to play a match game of sorts.

They compared about 600 of the groundwater samples with corresponding precipitation data. By analyzing the groundwater and precipitation samples' respective oxygen-18 and hydrogen-2 — or deuterium — isotopes, the researchers were able to determine when groundwater samples showed a similar isotopic makeup to winter or summer precipitation.

Although there were some areas where groundwater supplies recharged from summer precipitation, the majority of the state saw more recharge during winter.

Why winter?

Nebraska's summers typically see more precipitation than the winter months, so why does the recharge happen more often during a drier season? There are several reasons, Cherry said. One is that potential evapotranspiration from soil and plants exceeds rainfall during summer months, meaning that there isn't excess water available that can become groundwater recharge.

Human impact is another reason. Nebraska's groundwater supply is frequently used for irrigation-powered agricultural operations. In areas of intensive canal irrigation where the Platte River is the source, Cherry found what appeared to be a nongrowing (winter) season recharge ratio that actually is mostly Platte River water recharged through canals. However, in some intensively pivot-irrigated areas, Cherry said, the water cycle has shifted the recharge signature toward the growing season.

"We're not really sure why," Cherry said. "It might be that people are overirrigating, and you have precipitation plus irrigation that leads to recharge in the summer. Or it might be that something to do with the process of irrigation is altering the isotopic composition of groundwater, so it just appears to be more summer-based. Different types of irrigation can impact it differently, but we're not quite sure why."

Understanding nitrate contamination

Cherry said the research team was surprised that the data showed when the groundwater recharged in areas with different types of irrigation on such a fine scale. That information could help reinforce water management practices that strive to limit and reduce nitrate levels in Nebraska's groundwater supply, she said.

Formed when nitrogen and oxygen combine, nitrate is prevalent in manure and inorganic fertilizers. Nitrate is water-soluble and passes through soil to the groundwater table, where it can settle for decades. If nitrate levels are high in drinking water supplies, consuming it can have adverse health effects, especially for infants. Treating nitrate-rich drinking water is possible but costly.

"By understanding what times aquifers are recharging and vulnerable to contamination, we can improve water quality best management practices," Cherry said. "An example of this might be fall fertilizer application. If we know that recharge is occurring in the nongrowing season, then applying fertilizer during this time might mean more contamination than if the fertilizer was applied in the spring.

"Another issue is groundwater quantity. Climate change might shift precipitation patterns and timing as well as lengthen the growing season, which would impact the overall amount of precipitation that becomes groundwater. Since the majority of the state relies on groundwater for both drinking water and irrigation, these are areas where more information would help inform management decisions."

The team's paper was published in the March issue of Hydrological Processes. It was written by Cherry and features contributions from Gilmore; Aaron Mittelstet, assistant professor in the Department of Biological Systems Engineering; Didier Gastmans and Vinicius Santos of Sao Paulo State University; and John Gates of CropMetrics.

Funding for the research was provided by USDA's National Institute of Food and Agriculture and by the Nebraska Environmental Trust.

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