A sophisticated study blending 35 years of well contamination tests shows high levels of toxic nitrate is turning up a little less frequently in Dane County-area drinking water.

But because the substance is so widespread and hazardous to human health, scientists from five state and local agencies who conducted the study are urging broader efforts to limit farm fertilizer use that is the primary source of contamination.

More than one in five tests of wells providing water to homes, churches, schools, bars and restaurants exceeded the safe limit from 2010 through late 2014.

That’s down from 30 years ago when more than one-third showed unsafe levels, but still more than twice the statewide rate.

A 43-page report summarizing the landmark study also recommends more testing of water from more than 20,000 residential wells in the county, especially in highest risk areas, which the scientists mapped with greater precision than ever before.

“It’s still a problem, but the fact that it has leveled off shows that some of these efforts have worked,” said Dick Lathrop, a UW-Madison expert on fresh water systems who served as one of the study’s principal investigators. “It should give Dane County a chance to have more conversations with farmers.”

But another expert suggested that government efforts to entice farmers into statewide programs designed to control fertilizer and manure spreading aren’t likely to substantially offset powerful market forces that drive up nitrate pollution.

When prices rise for corn, which requires heavy applications of nitrogen-based fertilizer, farmers quickly convert acreage and boost spreading, said Kevin Masarik, a UW-Extension outreach specialist focused on helping rural well water users.

Unless consumers are willing to pay more for corn and the things it goes into — such as ethanol, dairy products and goods sweetened by high-fructose corn syrup — farmers will continue to increase fertilizer use, he said.

“When someone gets their water quality report back and it’s above the standard they think the farm next to them is doing something illegal, but it’s probably just more the reality of the systems we have in place,” Masarik said. “That’s the reason why it’s been 30 years and there hasn’t been much progress made.”

Acute risks to infants

Drinking water contaminated with more than 10 milligrams per liter of nitrate poses acute risks to infants and women who are pregnant, a possible risk to fetuses in early stages of pregnancy, and a longer-term risk of serious disease in adults.

Nitrate is the state’s most widespread groundwater contaminant, and its extent and severity have been increasing, top state agency officials appointed to the Wisconsin Groundwater Coordinating Council said in their 2015 report to the Legislature.

It’s not just residential wells that are at risk. In Dane County alone there are 232 facilities such as churches, bars, day cares and restaurants with wells, most of which draw water from the shallow aquifer, typically 100 to 300 feet down, that is most vulnerable to nitrate pollution.

Statewide, the number of public water systems with unsafe nitrate levels increased slightly to 57 in 2014, requiring officials to provide bottled water, post notices, replace wells, install treatment, or take other corrective actions, the groundwater council report said.

Previous studies have estimated that 90 percent of nitrate in groundwater comes from spreading of synthetic fertilizers and dairy manure on farm fields, with most of the remainder from septic systems.

About 31 percent of the state’s 9 million crop acres are covered by nutrient management plans, according to a November report of the Department of Agriculture. The plans require soil testing and other practices to control nutrients that can end up in lakes, streams and groundwater.

But the plans require adherence to UW-Extension nutrient recommendations aimed primarily at maximizing farm profits by avoiding spending too much on fertilizer or manure spreading, said Masarik, who worked last year on a nutrient planning rules update as a member of the Wisconsin Standards Oversight Council.

Most farmers try to limit fertilizer costs even if they aren’t enrolled in nutrient planning programs, which are mandatory only for farmers enrolled in certain subsidy or preservation programs, and those who have been caught polluting or have 1,000 or more animals.

Dane County has 130,000 acres enrolled in plans, more than all but four other counties. The county has subsidized construction of two manure digesters designed to generate electricity while reducing the nutrients in manure before it is applied to fields, said Kevin Connors, director of the Dane County land and water resources department.

“We’re not out of the woods, but we’ve made progress,” Connors said. “The ag community should be proud.”

Millions from the state

The state has allocated millions of dollars to farm “cost-sharing” that includes nutrient planning. The groundwater council, in its 2015 report to the Legislature, said more plan acreage would significantly reduce nitrate pollution.

Farmers applied more than 200 million pounds of nitrogen in excess of UW-Extension recommendations in 2007 alone, according to a state Department of Agriculture estimate. Still, research is mixed on whether the standards can keep wells below the 10 milligram per liter nitrate limit, the groundwater council report said.

Increased attention to the affect of farm practices on water quality led to an increased 2009 state budget for Department of Agriculture conservation efforts, including nearly $3 million for nutrient management. But subsequent budget lapses and shortfalls cut funds, and requests at one point outstripped available money by $3.5 million, the report said.

More recently, higher corn prices led to acreage being taken out of conservation programs that encouraged crops like legumes and alfalfa, which take up more nitrogen before it can reach groundwater, the report said. Wisconsin Agricultural Statistics Service reported that corn acreage rose to 4.1 million in 2011, up from 3.6 million in 2006.

Nitrates move quickly to groundwater

Another major component in fertilizer and manure, phosphorus, contributes to abnormal algae growth in lakes and streams when it runs off the land with rain and snowmelt. Nitrate behaves differently. Relatively little lingers near roots where it can be absorbed. Water washes it down into shallow groundwater that is the source of drinking water for one-quarter of Wisconsin residents.

An estimated 9 to 10 percent of Wisconsin wells have tested over safe limits for nitrate, Masarik said.

For the recent study covering Dane County and small areas of surrounding counties, scientists crunched more than 61,000 well test records dating back to the 1970s.

“The idea was to pull together a lot of data we knew existed, but hadn’t been pulled together before and to look for hot areas,” the UW’s Lathrop said.

About 15 percent of the study area had groundwater with nitrates over 10 milligrams per liter from 2010 through late 2014, down from 18 percent on average from 1985 to 1990.

At the same time, the well tests showing less than 2 milligrams per liter increased, Lathrop said. The study also calculated that 75 percent of tests found concentrations of 8 milligrams per liter or less since 2010, down slightly from 30 years ago.

The study’s lead author was Cory McDonald, a state Department of Natural Resources research scientist. The DNR communications office didn’t make McDonald available to discuss the report. Other agencies involved were the UW-Madison Center for Limnology, Public Health Madison and Dane County, Wisconsin Geological and Natural History Survey and Capital Area Regional Planning Commission.

The study mapped nitrate hot spots, but contamination can vary over very short distances, said co-author Kirsti Sorsa, the health department’s environmental health labs supervisor.

“Because of the variability, it’s still important that people know they should test their wells,” Sorsa said.” Very few people do test their water. We certainly advocate more testing.”

Residential well owners are responsible for testing their own drinking water, said DNR spokesman George Althoff. The DNR encourages annual testing for nitrate, total coliform and E. coli bacteria, he said.

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Steven Verburg is a reporter for the Wisconsin State Journal covering state politics with a focus on science and the environment as well as military and veterans issues.