Citation
Sinkholes, hydrogeology, and ground-water quality in northeast Iowa

Material Information

Title:
Sinkholes, hydrogeology, and ground-water quality in northeast Iowa
Creator:
Hoyer, Bernard E.
Hallberg, George R.
Publisher:
Iowa Geological Survey
Publication Date:
Language:
English

Subjects

Subjects / Keywords:
Regional Speleology ( local )
Environment ( local )
Genre:
Monograph
serial ( sobekcm )
Location:
United States

Notes

General Note:
Illustrations. The physical setting in both the Karst and Shallow-Bedrock regions present potential hazards for ground-water contamination. Any management strategies developed for protection of these water resources must consider both of these settings, which in total constitute about 6,800 square miles of land overlying important bedrock aquifers."
Restriction:
Open Access
Original Location:
Northup Database Collection
General Note:
See Extended description for more information.

Record Information

Source Institution:
University of South Florida Library
Holding Location:
University of South Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
K26-03717 ( USFLDC DOI )
k26.3717 ( USFLDC Handle )
8912 ( karstportal - original NodeID )
OCLC16521142

USFLDC Membership

Aggregations:
Karst Information Portal

Postcard Information

Format:
Serial

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Full Text
Description
Illustrations. The physical setting in both the Karst
and Shallow-Bedrock regions present potential hazards for
ground-water contamination. Any management strategies
developed for protection of these water resources must
consider both of these settings, which in total constitute
about 6,800 square miles of land overlying important bedrock
aquifers."



PAGE 1

SINKHOLES, HYDROGEOLOGY, AND GR OUNDWATER QUALITY IN NORTHEAST IOWA G.R. Hallberg and B.E. Hoyer Iowa Department of Natural Resour ces, Geological Survey Bureau, Open File Report 82-3, 1982, 120 p. ABSTRACT N ortheastern Iowa is considered to have Iowa's most abundant supplies of good quality groundwater. The most widely accessible aqui fers (water-producing geologic uni ts) are comprised of carbonate (limestone or dolomite) rocks. Un fortunately, these carbonate aquifers are much more susceptible to contamination from sources at th e land surface than other types of aquifers. This is because the groundwater flows through openings in the rock, en larged by chemical solution, which range in size from microscopic fractures to large caves. When these larger ope nings extend to the land surface, they form depressions, called sinkholes. The open sinkholes provide a direct conduit for surface waters, and contaminants which they may carry, to run directly into the underground cavities in the carbonate rocks, and join the groundwater system. Where sinkholes are abundant they form distinct landforms, collectively called karst topography Documented, local occurren ces of serious groundwater contamination in the karst area have raised the concern of whether or not regional groundwater contam ination is occurring. if so, are regional or local control measures necessary to a lleviate the problem? These are vita l questions because water-quality p roblems may impact public health as well as the region's economic well-being. To address these issues a systematic analysis wa s undertaken for he karst regions and the carbonate aquifers in 22 counties in northeast Iowa. Pertinen t geologic, hydrologic and water quality data were compiled, and analyzed, including over 14,000 water analysis records provided by the University Hygienic Laboratory (UHL). The distribution of over 12,700 sinkholes was mapped. "Soil-materials" cover the bedrock to depths varyi ng from 0 to 500 feet, but the sinkhol es are only found in certain areas where the "soil-materials" are less than 30 feet thick. There are three main areas of sinkhole concentrations: one in the area of exposure of the Ga lena aquifer, in southwestern Allamakee County, and adjacent areas; and two areas in the Silurian-D evonian aquifer, in southern Clayton County and adjacent areas, and adjacent to the Cedar Ri ver, mainly in Floyd and Mitchell Counties. Results of the geological studies were used to subdi vide the area into three geologic regions: Karst-areas with significant concentrati ons of sinkholes; Shallow Bedrock--a reas with less than 50 feet of "soil" covering the bedrock, but with few sinkholes; and Deep Bedrock--areas with more than 50 feet of "soil" covering the bedrock. Groundwater in the Karst and Shallo w Bedrock areas exhibits signifi cantly higher concentrations of nitrate than in the Deep Bedrock areas, particularly to depths of 150 feet. The greatest differences occur in the 50-99 foot depth range, where the medium nitrat e concentration in the Karst regions (34 mg/L) is 1.8 times greater than in the Shallow Bedrock regions (19 mg/l) and nearly 6 times greater than in the Deep Bedrock regions (6 mg/l). Below 100 feet the Karst and Shallow Bedrock areas show similar levels of nitrate. This is attributed to the direct fl ow of nitrate in surface wa ters into sinkholes in Karst Page 1of 3 GSB OFR 82-3 Abstract 3/14/2011 file://\\dnriacfs01\webdata\ p ubs\abstracts\of r -1982-3.htm

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regions, combined with significant diffuse recharge of nitrate to the aquifer in both the Karst and Shallow Bedrock regions. For persp ective, the median nitrate values from all areas are below the 45 mg/l drinking water standard. Fo r the study area, 18% of all samples exceed 45 mg/l. Within the different geologic settings, 25% of analyses from the Karst areas, 19% in the Shallow Bedrock, and 15% in the Deep Bedrock areas exceeded 45 mg/l. Much of the excessive nitrate contam ination is localized to individual wells, but nitr ate levels are clearly elev ated regionally as well. The source of the nitrat es is clearly man's activity; natural background levels of nitrate are generally less than detectable. Little data is av ailable regarding other widely used chemicals. What data there is indicate that pesticides, albeit in low concentrations, are entering th e groundwater system is unclear, as are the possible health effects of these low concentrations. The physical setting in both the Karst and Shallow Bedrock regions present potential hazard s for groundwater contam ination. Any management strategies developed for protection of these water resources must consid er both of these settings, which in total constitute about 6,800 square miles of land overlying important bedrock aquifers. The relationship between th e sinkholes and the groundwater flow system in the carbonate aquifers suggests that the bulk of these surface contaminants in the karst regions should be contain within the shallow portion of the flow system. This may, in part explain why significant nitrate contamination is confined to relatively shallow depth (less than 150 f eet). However, because of the lack of detailed data about the aquifers, an alternative wh ich must be considered is that the deeper portions of the aquifer show less contamination because th ere has not been enough time for the nitrates to diffuse this deep. Further research is needed on the nature of bacterial contamination of groundwater in the Karst areas. Analysis of bacterial data indicates that bacter ial contamination of rural water supplies requires attention. Thirty-five percent of al l analyses from UHL for the study ar ea do not meet health standards. This contamination appears to be primarily relate d to problems in individua l rural domestic water systems, but in Karst regions may be increased by influx of surface water. The magnitude of chemical and bacterial contamination of an individual well is also related to problems of poor well construction maintenance and/or we ll placement. Contamination of a well from surface sources may also introduce contaminants into the aquifer. Shallow wells, less than 50 feet deep, statistically show high nitrate values regardless of their geologic setting. Sh allow wells throughout Iowa, regardless of the aquifer involved, are susceptible to contamination by nitrates, and indeed are exhibiting significantly high levels of nitrate contamination. Groundwater in the Karst areas is r eadily susceptible to c ontamination from hazardous substances which locally may be discharged at the su rface. On the regional level, nitrates bacteria and pesticides are the three general contaminants of concern for public health. Both point and non -point sources can be identified. Land use patterns and other studies suggest that non-point sources, pr imarily infiltration, tile drainage, and water and sediment runoff from agricult ural lands are the most significant. Point sources, however, should be eliminated where possible. There are existing rules and regulations to control these p oint sources, but many of these rules are difficult or impossible to enforce. Non-point source problems are particularly difficult to resolve, and given the complex interaction of climate and farming practices some delivery of these contaminants into the gr oundwater in the Karst areas is unavoidable. Possible control measures or best manageme nt practices (BMPs) must take in to account these complex variables, as well as the needs of particular farm operations, a nd the nature and extent of existing tile drainage. Before any effective management sc heme can be developed, further rese arch must address the details of the delivery and fate of these contaminants in the groundwater system, locally and regionally. Also, there is a pressing need for a wate r -quality monitoring network to pr ovide a base of information on Page 2of 3 GSB OFR 82-3 Abstract 3/14/2011 file://\\dnriacfs01\webdata\ p ubs\abstracts\of r -1982-3.htm

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Iowa's water resources. This should include impr ovements in present wate r-quality data collection schemes. The development of a management plan and BMP's to protect groundwat er quality in these carbonate aquifers will require the integrat ed cooperation of many agencies a nd people. Implementation of any effective measures will require an e ffective program of public education. Iowa Geological Survey 109 Trowbridge Hall Iowa Ci ty, IA 52242-1319 Phone: 1-319-335-1575 Fax: 1-319335-2754 Page 3of 3 GSB OFR 82-3 Abstract 3/14/2011 file://\\dnriacfs01\webdata\ p ubs\abstracts\of r -1982-3.htm