GROUNDWATER AND SURFACE WATER MONITORING IN THE BIG SPRING BASIN 1996-1999: A SUMMARY REVIEW
Huaibao Liu, R.D. Rowden, R.D. Libra
Iowa Department of Natural Resources, Geological Survey Bureau
Technical Information Series 44, 2000, 121 p.
Big Spring basin is a 103 mi2 (267 km2) groundwater basin
located in Clayton County, northeast Iowa.
Precipitation, groundwater and surface-water discharges, and the
concentrations and loads of various agricultural contaminants have been
monitored within and around the basin since 1981.
This report summarizes the monitoring results at Big Spring, Turkey
River, Silver Creek, and tile line outlet L22T during WYs 1996 through 1999, and
Roberts Creek during WYs 1993 through 1999.
monitoring results have shown a time lag between annual precipitation and annual
groundwater discharge from Big Spring. This
was confirmed by the discharge and precipitation data obtained during WYs 1996
through 1999. After receiving below
normal annual precipitation during WYs 1994 and 1995, the Big Spring basin had
30.59 inches of rainfall in WY 1996, which was 1.31 inches greater than WY 1995
and 2.38 inches below the long-term average.
The groundwater discharge at Big Spring decreased from 30,013 ac-ft in WY
1995 to 28,143 ac-ft in WY 1996. In
WY 1997, precipitation within the Big Spring basin totaled 38.29 inches, which
was 5.32 inches above the long-term average.
However, annual groundwater discharge at Big Spring continually decreased
to 22,943 ac-ft, the smallest annual discharge at Big Spring since WY 1991.
The relatively wet conditions continued in WYs 1998 and 1999.
Annual precipitation totaled 41.21 inches in WY 1998, which was 8.24
inches greater than the long-term precipitation average.
Correspondingly, groundwater discharge increased to 35,713 ac-ft for this
water year. During WY 1999, annual
precipitation within the Big Spring basin was 39.99 inches.
Although this precipitation was 1.22 inches lower than that in WY 1998,
annual groundwater discharge at Big Spring increased to 37,133 ac-ft.
The non-linear relationship between annual precipitation and groundwater
discharge from Big Spring results from variations in discharge-controlling
factors, including precipitation intensity, timing and distribution patterns,
antecedent soil moisture, groundwater storage, and evapotranspiration rates.
quality, especially the annual fw mean concentrations and loads of nitrate-N,
can be affected by variations in discharge.
Annual fw mean nitrate-N concentrations and loads showed trends similar
to groundwater discharge during most of the monitoring period.
However, the annual fw mean nitrate concentration for Big Spring
increased from 45.3 mg/L (10.1 mg/L as NO3-N) in WY 1995 to 46.4 mg/L
(10.3 mg/L as NO3-N) in WY 1996, while annual groundwater discharge
decreased from 30,013 ac-ft to 28,143 ac-ft.
The nitrate-N contamination and load increased significantly along with
annual groundwater discharge in WY 1998. In
WY 1999, the nitrate-N concentration and load decreased slightly as annual
groundwater discharge increased. These
divergences may be related to regional antecedent conditions, such as the amount
of groundwater flux during the previous water year.
nitrate, atrazine trends did not coincide with changes of annual groundwater
discharge. Annual atrazine
concentrations and loads from Big Spring varied significantly from WY 1995
through WY 1999, but remained relatively low compared with WYs 1990 and 1991.
In WY 1997, the annual atrazine load decreased to 10.5 pounds, which was
the third-smallest annual load since WY 1982.
In WY 1998, the annual fw mean atrazine concentration at Big Spring
decreased to 0.12 mg/L, which was the lowest level recorded during the
monitoring period. In addition, increasing numbers of non-detections of atrazine
occurred at Big Spring and other monitoring sites during recent years.
These may be the result of reductions in atrazine application, landuse
improvements, and the replacement of atrazine by new pesticides, such as
acetochlor, within the Big Spring basin. Reduced
sampling frequency may also be affecting the accuracy of atrazine fw mean
concentration and load calculations.