Agriculture and Groundwater: The View from Big Spring

By Robert D. Libra

Adapted from Iowa Geology 1995, Iowa Department of Natural Resources

Big Spring landscape
The Big Spring watershed in Clayton County is dominated by agriculture.. Photo by Aimee Donnelly

“My uncle Earl’s dream was to find a spring large enough to rear more trout. … In late fall of 1937, my husband Otto went to look at Big Spring. He was awed by its size and called Earl; he too was amazed. … The spring is located at the base of a big bluff, about 550 feet from the Turkey River. It bubbles up through the ground on top of a shelf of layered rock four to five feet higher than the river level. In 1940 … Big Spring deposited 33,000 tons of silt in the big fishing pond. We thought that by closing sinkholes, sediment to the spring could be controlled. We saw dead animals, trash and old DDT containers in those sinkholes that really scared us.”Mary Bankes, The History of Big Spring (1994).

Big Spring is Iowa’s largest, and it has been used as a water source for trout-rearing ever since Mary and Otto Bankes put it to work in 1940. Iowa’s DNR has owned the spring since 1961. Typically, 15,000 gallons of groundwater flow from the spring each minute, fed by fractured rocks of the Galena aquifer. These rocks have been slowly dissolved by circulating groundwaters, forming features such as caverns and sinkholes. Across most of the 100-square-mile groundwater basin drained by the spring, the Galena aquifer lies near the land surface and readily receives downward-percolating water from large rainstorms or snow melt. Sinkholes are present in about one tenth of the basin, and when intense rains generate surface runoff, they capture and direct it into the aquifer. Shallow aquifers are vulnerable to contamination from activities on the land surface. In the Big Spring basin, and across Iowa, the major surface activity is agriculture, and the major contaminants are nitrate (from nitrogen fertilizer) and herbicides used on corn and soybeans.

Sink hole
Sinkholes funnel surface runoff directly into the underground aquifer. Photo by Bob Rowden.

The Iowa Geological Survey began investigating the relationships between agriculture and groundwater quality in the late 1970s. Anecdotal evidence, often from dairy farmers and water-well drillers, suggested that widespread increases in nitrate concentrations were occurring in the extensive, vulnerable bedrock aquifers of northeast Iowa. The Big Spring basin, which is almost entirely agricultural, allowed for a direct study of agriculture’s environmental effects. Equally important, the presence of Big Spring and its definable groundwater basin provided a unique opportunity to measure the volume of groundwater leaving a known area. When discharge volumes are combined with contaminant concentrations, the total amount of a given contaminant carried by the water can be calculated. Since 1981, water quality and discharge and agricultural practices have been tracked in this natural laboratory.

Fractured dolomite quarry wall
Fractured dolomite of the Galena aquifer underlies most of the Big Spring basin landscape. Photo by Lynette Seigley.

Initial investigations in the basin, along with existing data, yielded valuable information on the agriculture – water quality connection. During the 1960s and 1970s the use of chemical nitrogen fertilizer in the basin increased almost three-fold, and nitrate concentrations at Big Spring had increased by a similar amount. By the early 1980s concentrations commonly approached the limit set by the U.S. EPA for drinking water (45 mg/L). Higher concentrations occurred during wetter recharge periods, a pattern also seen in basin wells, streams, and tile drainage. When the amount of nitrate emerging from the basin in surface and groundwater was totaled for typical years, it was equivalent to one-third of the chemical nitrogen fertilizer which basin farmers had applied. Additional losses of nitrate, such as uptake by aquatic plants, also occur and suggested the actual loss from fields was equivalent to half of the chemical fertilizer applied. Herbicides were also detected in the groundwater, with atrazine present in low but detectable concentrations year-round.

Sink hole
Measurements are made of a newly formed sinkhole, caused by collapse of soil and shallow rock into a subterranean cavern. Photo by Art Bettis.

The initial findings at Big Spring resulted in the creation of the Big Spring Basin Demonstration Project, a cooperative effort involving basin farmers and numerous state, federal, and local agencies. The project, which began in earnest in 1987, increased the scope of water-quality and land-use monitoring, and greatly expanded education and demonstration programs aimed at improving the economic and environmental performance of agricultural practices. Improved nitrogen management was an important part of these efforts, as the magnitude of nitrate losses from fields in the basin indicated that reductions in application rates were possible. Indeed, significant reductions have slowly occurred as farmers became more confident that lower rates of fertilization would work for them. Nitrogen fertilizer input declined by a third from 1981 to 1993, from 174 to 115 pounds/acre, with no affect on yields. This represents a two million pound reduction in nitrogen use, saving basin farmers about $360,000 annually.

While nitrogen inputs have decreased significantly, relating these declines to changes in Big Spring groundwater remains a problem. The effects of nitrogen reductions, occurring gradually over a decade, are at present largely lost in the year-to-year climatic variables — particularly rainfall. On an annual basis, nitrate concentrations rise and fall with the volume of water discharging from Big Spring, which is a reflection of the volume of precipitation recharging the aquifer. Nitrate concentrations showed a general decline from 1982 to 1989, but so did the discharge from Big Spring, which reached its lowest point during 1989, the second year of extreme drought. While some of the decline may reflect the decrease in nitrogen applications, the effect cannot be separated from the decrease in recharge and nitrate delivery to the aquifer. Wetter-than-average conditions occurred after the drought, culminating in the “great Flood of 1993.” Nitrate concentrations increased dramatically during this period. This response resulted from both the increased water volume passing through the soil and groundwater system — four times more in 1993 than in 1989 — as well as from leaching of unused nitrogen left over from the drought. Any improvement in water quality resulting from decreased nitrogen applications was again lost in the effects caused by the extreme climatic variations.

Water samples
Water samples from Big Spring, following a heavy rainfall across the basin, show the increase in suspended sediment carried by the emerging groundwater. Photo by George Hallberg.

The studies at Big Spring have provided the nation’s longest running and most detailed record of the relationships between agriculture and water quality. Perhaps the most important thing we have learned is that water quality responds slowly to changes in agricultural chemical inputs, particularly when those changes occur slowly themselves. We need to take the long view when trying to document water quality improvements, and Iowa’s Big Spring helps provide the focus.
Adapted from Iowa Geology 1995, Iowa Department of Natural Resources

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