Agriculture
and Groundwater:
The View from Big Spring
by Robert D. Libra
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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.
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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.
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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.
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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.
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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.