When farmers put nitrogen fertilizer on their fields it soaks down into the soil and turns into nitrates that feed crops. But when there are too many nitrates, water from rain or irrigation carries those extra nutrients past the point where roots can reach and eventually to the aquifer below.
For the cities and towns that depend on the underground aquifer or surface water for their drinking water, that can be a big problem.
On the High Plains Aquifer in Nebraska, for instance, there are pockets of nitrates pushing far, past the legal limit of 10 parts per million. In central Nebraska, a pocket of high nitrate water is flowing right toward the wells where the city of Hastings draws its drinking water.
“They get up to the 50 part per million range,” says Marty Stange, the environmental supervisor for Hasting’s utilities system. “They’re five times the drinking water standard. Those are really areas of high concern for us.”
Too many nitrates are a health hazard, particularly for infants whose blood can lose its ability to absorb oxygen. So nitrates must be reduced or removed, but cleaning nitrates from the city’s water is a huge expense. When nitrate levels rise above the safe drinking water limit, Des Moines, Iowa, fires up a filtering system that costs thousands of dollars to operate each day. That’s rarely possible for small cities and towns dotting farm country.
Stange and his team have designed a unique line of defense. They’re using wells strategically placed on the outskirts of town to act as a filter for the aquifer, skimming off nitrates as they move toward the city.
Nitrates tend to collect on the top of the aquifer. So unlike most wells, this one pumps from the top, middle, and bottom all at once in order to dilute high-nitrate water.
Clean water will be pumped back down into the aquifer so the high nitrates never reach the wells that supply Hastings residents and high-nitrate water leftover from the cleanup will be used to irrigate and fertilize an alfalfa field.
“As the plants are growing, they will consume up those nitrates,” Stange said. “We’re going to take lemons and make lemonade basically.”
The city of Hastings will spend $46 million to build the system, Stange says, which is a big hit to a city of just 25,000 people.