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.

ABSTRACT

The 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. 

Previous 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.

Water 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.

 Unlike 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.