Effects of Geologic Location on
Groundwater Quality Within the Big Spring Basin, Northeast Iowa
by H. Liu, R.D.
Rowden, and R.D. Libra
Abstract
Five private wells (VD24, F08, T17, GL01, and JSW) were selected to illustrate the variation of water quality in the Big Spring area, which is a 267 km2 (103 mi2) groundwater basin located in a karst region in Clayton County, northeast Iowa. All of these wells penetrate the Ordovician Galena limestone aquifer, but have different geologic settings within the basin. Sampling for nitrate and common pesticides at these wells started in WY 1981, generally on a monthly basis. Sampling at GL01 and JSW ended in WY 1994, but continues at the three other wells.
The water quality varies significantly due to well locality. The frequencies of atrazine detection (>0.1 mg/L) range from 3% at VD24 to 97% at T17. The average nitrate concentration varies from 12 mg/L at GL01 to 63 mg/L at T17. Data from T17 are also compared with data from a tile line site (L22T) and a surface water site (F47) within the basin. The results show that nitrate concentrations at T17 are significantly greater than the concentrations from surface water, and very close to the concentrations at the tile line site. The atrazine concentrations at T17 are usually more stable, but greater than concentrations from the tile line water and close to concentrations of the surface water.
Results from this study indicate that geologic location is one of the most important factors affecting groundwater quality within the Big Spring basin. Well T17 is located in a sinkhole area with extremely shallow bedrock, and where the Maquoketa shale unit above the Galena aquifer is missing. In this area, following extreme rains or snowmelt, infiltration along with runoff captured by sinkholes is diverted into the Galena aquifer. This allows relatively high concentrations of agricultural contaminants to move more directly from the land surface and shallow soil zone into the aquifer. Other wells have better water quality because they are located in an area either without nearby sinkholes or where the Galena aquifer is confined by the overlying Maquoketa shale unit.
I. Introduction
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| Figure 1. The Big Spring basin is a 267 km2 (103 mi2) groundwater basin located in Clayton County, northeast Iowa. A network of sites, including groundwater, surface water, tile lines, and springs, was established by the Geological Survey Bureau of the Iowa Department of Natural Resources (IDNR) and cooperating agencies in 1981 to monitor water quality. Five private wells were selected to illustrate the water quality variation within the basin. |
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| Figure 2. Landuse within the
Big Spring basin is essentially agricultural. Forty to
fifty percent of the basin is annually cropped to corn,
typically in rotation with alfalfa. Small- to
moderate-sized hog and dairy operations are common.
Agriculture is the main factor impacting the groundwater
quality within the basin. Photo by Aimee Donnelly. |
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| Figure 3. Groundwater movement is from the north to the south within the basin. Over 85% of the groundwater exiting the basin into the Turkey River is discharged through Big Spring, the largest spring in Iowa. |
II. Geology of the Big Spring Basin
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| Figure 4. The Big Spring basin is located in the Paleozoic Plateau landform region. The bedrock in this area is Ordovician and Silurian carbonate strata (limestone and dolomite). The main aquifer used by residents is the Ordovician Galena Group, with an average thickness of 67 m (220 ft) in the study area. |
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| Figure 5. In the west portion of the basin, the Galena aquifer is overlain by the Ordovician Brainard Shale Member of the Maquoketa Formation and/or Silurian dolomites. In the east portion, the Galena is overlain by lower limestone/dolomite members of the Maquoketa Formation, or covered by Quaternary loess or glacial till only. The Brainard Shale retards the downward movement of groundwater towards the Galena aquifer, while the shaly carbonates of the lower Maquoketa are generally connected with the aquifer. |
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| Figure 6. Fractured limestone
of the Galena aquifer in the Big Spring Basin. Photo by Art Bettis. |
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| Figure 7. A sinkhole that
formed in 1981 in the Big Spring area. Photo by Greg Ludvigson. |
Figures 6 & 7: Fractures and sinkholes are common in the Big Spring basin. Sinkholes occupy about 11% of the basin area, and are especially concentrated near the outcrop area of the contact between the Galena Group and the lower Maquoketa Formation in the eastern and central portions of the basin.
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| Figure 8. The Galena aquifer is recharged by both infiltration and runoff which can enter sinkholes in some areas. The groundwater quality is affected by location within the basin. Groundwater is of lower quality in sinkhole areas than the groundwater in the rest of the basin. |
III. Results
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| Figure 9. Results from five private wells within the Big Spring basin. |
Sampling for nitrate and pesticides at five private wells within the basin started in WY 1981. Sampling at GL01 and JSW ended in WY 1994, but continues at F08, T17 and VD24. Results show significant variation among the wells. The average nitrate concentration ranges from 12 mg/L at GL01 to 63 mg/L at T17. The frequencies of atrazine detection vary from 3% at VD24 to 97% at T17.
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| Figure 10. Nitrate concentrations from wells T17, F08, and VD24. |
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| Figure 11. Atrazine concentrations from wells T17, F08, and VD24. |
Figures 10 & 11: Comparison of water quality for F08, T17 and VD24 shows that the well T17 has the greatest concentrations for both nitrate and atrazine, followed by well F08. Well VD24 has the lowest nitrate and atrazine concentrations. The data also indicate that atrazine concentrations in this area have gradually declined since WY 1991. Nitrate concentrations have been relatively stable at F08 and VD24 following WY 1991, and slowly declined at T17 since WY 1993.
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| Figure 12. Comparison of nitrate concentrations between well T17 and tile line site L22T. |
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| Figure 13. Comparison of atrazine concentrations between well T17 and tile line site L22T. |
Figures 12 & 13: Comparison of well T17 and tile line site L22T shows that the two sites have very similar nitrate concentrations except for some periods during WYs 1989-1991. Well T17 has generally greater atrazine concentrations and significantly fewer non-detections than L22T.
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| Figure 14. Comparison of nitrate concentrations between well T17 and surface water site F47. |
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| Figure 15. Comparison of atrazine concentrations between well T17 and surface water site F47. |
Figures 14 & 15: Comparison of well T17 and surface water site F47 shows that the two sites have similar atrazine concentrations with concentrations from T17 being more stable. Well T17 has significantly greater nitrate concentrations except during some periods in WYs 1990-1991.
IV. Conclusions
The Big Spring basin is located in a karst area. The eastern portion and part of the central portion of the basin have very shallow carbonate bedrock with sinkholes. Infiltration through a thin soil zone along with runoff may enter the Galena aquifer by fractures and sinkholes, and significantly affect water quality in these areas (Fig. 16).
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| Figure 16. Groundwater
recharge and discharge in karst regions. Illustration by Pat Lohmann. |
Water quality varied among the five studied wells because of their geologic settings. Well GL01, located in an area where the Galena aquifer is confined by the Maquoketa shale unit, contained the smallest contaminant concentrations. Well VD24, located above a sinkhole area, also had good water quality. Well F08, located in a shallow bedrock area but without nearby sinkholes, had moderate water quality. Water quality of well T17, located in a sinkhole area, was significantly poorer than other wells due to the more direct connection between the land surface and the aquifer.
Compared to tile line site L22T and surface water site F47, well T17 had generally greater atrazine concentrations than tile line water, and greater nitrate concentrations than surface water. The concentrations of nitrate and atrazine from T17 were more stable, suggesting that the aquifer is buffered, or somewhat less responsive to precipitation than the tile line and surface water sites.
The concentration trends at the private wells indicate that atrazine concentrations have gradually declined since WY 1991, while nitrate concentrations have remained relatively stable in the Big Spring basin during recent years.
Presented as a poster at the Sixth National Nonpoint-Source Monitoring Workshop, September 21-24, 1998, Sheraton Four Points Hotel, Cedar Rapids, Iowa.