The Free Press, Mankato

In a state rich in water resources, there haven’t been a lot of worries about water supplies.

But in recent years the Department of Natural Resources, cities and others have raised alarms about the overuse of deep aquifers and about pollution dangers to surface water.

Hundreds of feet below southern Minnesota, the Mount Simon aquifer provides drinking water for more than 1 million area residents, including Mankato. As a recent MPR story noted, that aquifer has dropped by as much as 200 feet in some areas since pioneer settlement times.

While there’s no danger of the aquifer drying up, officials worry about the growing use of the aquifer to provide drinking water and water for watering lawns and crops.

Mankato has long aimed to extract a limited supply of water from Mount Simon, using about 25 percent of its total and relying on shallow wells near the Blue Earth and Minnesota rivers for the majority of its drinking water.

Those wells draw water from just a few dozen feet below the rivers.

But in recent years nitrate levels in the rivers means the water drawn from those wells contains higher amounts of nitrates. Mixing that water with the pure water from the deep aquifer has been enough to keep overall nitrate levels in drinking water at a safe level. But with those rising nitrate levels in rivers, cities such as Mankato may either have to take more aquifer water or build an expensive nitrate-filtering plant.

Fortunately the state has been paying more attention to the long-term health of deep aquifers. A 1989 law restricts the number of new Mount Simon wells that can be dug. In north-central and northern Minnesota, where some aquifers are more fragile, the DNR has been limiting new wells used to irrigate cropland. That’s a necessary move. While crops are a valuable part of the state’s economy, it is irresponsible to grow them in places and soil types that require heavy irrigation for them to survive.

But more attention needs to be paid to rising nitrate levels in rivers and lakes. That problem not only puts greater demand on water from deep aquifers but also causes algae growth in rivers and lakes that can destroy aquatic life.

Nitrates come from fertilizer that runs into the rivers and lakes, as well as from outdated individual septic systems.

City residents have a role in reducing the amount of grass clippings and fertilizer runoff from their lawns that go down storm sewers and end up in the rivers or lakes. And cities should continue their efforts to build more holding ponds that slow water runoff and allow pollutants to settle out of runoff water before it goes into rivers and lakes.

But scientific research shows a large amount of the nitrogen is caused by the increase in more efficient farmland drainage systems. The network of underground field tile quickly send water from millions of acres of farmland into ditches that flow into area rivers. That drainage water contains fertilizer from fields. But the increase in drainage also makes rivers rise very high, very fast. That causes dirt from river banks to erode into the river, carrying with it a lot of naturally occurring nitrogen that used to be held in the stream bank.

While increased rainfall amounts in recent years contributes to the erratic rise in rivers, scientists have found the increased precipitation has a relatively small impact on the rising rivers compared to the increased flow from farm drainage.

For the safety of drinking water, the future of aquifers and the health of rivers and lakes, the state and federal governments, city residents and farm organizations need to do more to address the nitrate problems.

Communities such as Lake Crystal have done a good job at educating residents to keep grass clippings and leaves from getting into storm drains. Other cities should do likewise. And there are farm drainage designs that do more to slow the release of water to rivers. Those systems can require more land to be used to store water on the surface and can be more expensive. Programs to provide financial assistance to farmers to improve their drainage systems as well as tighter requirements on drainage systems are in order.

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