Nitrate concentrations in the Hastings area increased during a five-year period, but the change from furrow to pivot irrigation led to reduced nitrate in the vadose zone.
Daniel Snow, a professor with the University of Nebraska Water Sciences Laboratory, discussed at the Hastings City Council work session on April 19 findings of a recently completed study that compared nitrogen concentration profiles between 2011 and 2016.
The vadose zone is the depth that separates the unsaturated zone from the crop root zone down to the water table.
“We collect cores, and then we analyze those cores to give us an idea of the profile of nitrate in the unsaturated zone,” he said.
Nitrates flow northwest to southeast.
The 2011 study, conducted by Roy Spalding and Jeff Toavs of the university, took cores reaching depths of 45 to 60 feet. It evaluated the relationship between land use and nitrate leaching.
The highest accumulations occurred beneath gravity-irrigated corn.
Spalding and Toavs believed changing water management likely had led to lower rates of leaching beneath pivot-irrigated fields.
A subsequent study in 2016 revisited 32 locations and estimated changes in the quantity and distribution of nitrate and other contaminants. The 2016 study looked at differences in residential versus agriculture and sites that converted from gravity to pivot irrigation.
There is a a lot of sand and gravel, salt and clay in the vadose zone.
“It’s not a uniform soil profile like you might expect,” Snow said. “There’s actually a lot of complexity in the vadose zone.”
Overall, there was an increase in vadose zone nitrates.
“The bottom line, when you look at all the sites, we saw an increase in vadose zone nitrate by about 50% at these 32 locations,” he said.
Five urban sites saw a decrease by about 44%, from 480 to 270 pounds of nitrate per acre.
Under cropland, which was 23 of the 32 sites, nitrate concentrations increased by about 30%, from 400 pounds per acre to 520 pounds per acre.
The largest increase was under a gravity-irrigated site, from 420 to 1,400 pounds per acre.
One of the sites converted from gravity to pivot irrigation saw a 170-pound decrease in the top 60 feet.
“We were curious if it would reduce the amount of nitrate in the vadose zone, and it appears to,” Snow said.
The study found among cumulative nitrate in the top 65 feet is 320 pounds of nitrate per acre at residential sites, 540 pounds per acre at pivot-irrigated sites, and 700 pounds per acre at furrow-irrigated sites.
Furrow irrigation uses twice as much water as pivot irrigation.
“We take that to mean nitrate mass is directly related to water use,” Snow said.
The study tried to do some simulations.
The model was calibrated with moisture and nitrate at three locations.
According to the study, the fastest movement occurred under furrow irrigation at 2.6 feet per year and the slowest is under unirrigated land at 1.2 feet per year.
The model assumes homogenous uniform flow.
“It’s unlikely that happens, so these are conservative estimates,” Snow said.
Groundwater uranium levels in the wellhead protection area exceeded the maximum contaminant level of 30 parts per billion.
The study found a correlation between vadose zone uranium and arsenic and change in stored nitrate under pivot, furrow and residential land use.
The authors believe nitrate application is mobilizing uranium in the unsaturated zone and that is getting to the water table.
Snow estimated residential areas in general show lower amounts of nitrate leaching, because there is less fertilizer used and turf is a very good scavenger for nitrate with a more dense root.
Marty Stange, environmental director for the city of Hastings, also spoke about selenium contamination.
New regulations are being proposed by the Nebraska Department of Environment and Energy to control selenium in wastewater discharge.
With the work the city has done with the nitrate and uranium issues in Hastings, it is understood that selenium also may be mobilized due to nitrates and biological activity.
The city knew there was some varying selenium results in the municipal wells and thus did a historical data search.
Data was gathered from municipal wells, Superfund studies and work on the former Naval Ammunition Depot.
Hastings currently is assessing cooling tower operations at Whelan Energy Center as required by the discharge permit.
Regulatory discharge limits are 20 micrograms per liter.
The discharge limit is set at 17 micrograms per liter for winter operations.
The cooling tower discharge over the past four years is 10 to 50 micrograms per liter.
Cooling tower operations concentrate selenium in the wastewater.
It is expected the next permit will require treatment. The application for the permit will be submitted this summer.