I. HYDROGEOLOGIC OBSERVATIONS FROM MULTIPLE CORE HOLES AND PIEZOMETERS IN THE DEVONIAN-CARBONATE AQUIFERS IN FLOYD AND MITCHELL COUNTIES, IOWA

R.D. Libra and G.R. Hallberg


Iowa Department of Natural Resources, Geological Survey Bureau
Open File Report 85-2, 1985, 32 p.

ABSTRACT


Previous investigations in northeast and north-central Iowa have identified areas where surficially derived contaminants, particularly nitrates and commonly-used pesticides, have caused degradation of groundwater quality (Hallberg and Hoyer, 1982; Hallberg et al., 1983, 1984; Libra et al., 1984). These studies showed that the depth to bedrock aquifers, and the presence or absence of karst features, have a major influence on the potential for surficial contaminants to reach groundwater. Geologic regions based on these criteria were delineated. In parts of north-central Iowa, where Devonian carbonate strata form a widely used multi-aquifer system, stratigraphic relationships also influence the extent and degree of groundwater contamination (Witzke and Bunker, 1984; Libra et al., 1984). In addition, areas with significant numbers of agricultural drainage wells (ADWs) are present in parts of north-central Iowa. The ADWs also must be considered as potential sources of groundwater contamination. ADWs have been shown to deliver tile-drainage water and surfacewaters containing nitrate, pesticides, and microbial contaminants to groundwater (Musterman et al., 1981; Baker and Austin, 1984).

To further the evaluation of ADWs as a groundwater contaminant source in the Floyd-Mitchell County area, a refined understanding of the affect of the Devonian stratigraphy on groundwater flow and quality was needed. Therefore, four core-holes, completely penetrating the Devonian sequence were drilled during the summer of 1984. The coring and subsequent geophysical logging of the coreholes allowed for detailed stratigraphic analysis and correlation of the Devonian strata. Packer tests were run in three of the coreholes to obtain in situ information on aquifer hydraulics, the degree of interconnection of individual aquifers, and to allow for water-quality sampling of individual aquifers. These three holes were then completed as piezometer nests, to allow for continued monitoring of water levels and quality within individual aquifers at each site. In addition, at two of these sites, piezometers were installed in the surficial, unconsolidated glacial materials overlying the Devonian units. This report will summarize the information obtained during the drilling, core analysis, packer testing, and sampling of the test holes, and will also present preliminary monitoring results from the piezometers nests.

 

 

II. STRATIGRAPHIC FRAMEWORK FOR THE DEVONIAN AQUIFERS IN FLOYD-MITCHELL COUNTIES, IOWA

B.J. Witzke and B.J. Bunker


Iowa Department of Natural Resources, Geological Survey Bureau

ABSTRACT


Prior to the research drilling program in Floyd and Mitchell counties in northern Iowa, reported in this paper, a general stratigraphic framework for the Devonian aquifers was proposed by Witzke and Bunker (1984). This framework served as a basis for subsequent geohydrologic studies. In particular, the Devonian carbonate rock sequence was subdivided into a three-part aquifer system based on limited, extant data. Continuous rock cores of Devonian strata were drilled at four localities in the Floyd-Mitchell county area during the summer of 1984 to provide more detailed stratigraphic control. Nested piezometers were installed and packer tests were completed in three of these holes to provide critical information on the head relationships and water quality within the Devonian aquifers.

In general, the Devonian stratigraphic framework proposed by Witzke and Bunker (1984) was substantiated by these studies during the past year. Stratigraphic units first recognized in a rock core from Mason City, Cerro Gordo County, and at surface exposures in the Floyd-Mitchell County area were clearly recognizable in the four new core-holes. Because of the similarities, most of the details of the stratigraphy need not be reiterated here, and the reader is referred to our earlier report (Witzke and Bunker, 1984) for more information.


References:

Baker, J.L., and Austin, T.A., 1984, Impact of agricultural drainage wells on groundwater quality: Project 2450, I.S.U.-E.R.I. - Ames-85183, 239 p.

Hallberg, G.R., and Hoyer, B.E., 1982, Sinkholes, hydrogeology, and groundwater quality in northeast Iowa: Iowa Geological Survey, Open File Report 82-3, 120 p.

Hallberg, G.R., Hoyer, B.E., Libra, R.D., Bettis, E.A., III, and Ressmeyer, G.G., 1983, Additional regional groundwater quality data from the karst-carbonate aquifers of northeast Iowa: Iowa Geological Survey, Open File Report 83-1, 16 p.

Hallberg, G.R., Libra, R.D., Ressmeyer, G.G., Bettis, E.A., III, and Hoyer, B.E., 1984, Temporal changes in nitrates in groundwater in northeastern Iowa: Iowa Geological Survey, Open File Report 84-1, 10 p.

Libra, R.D., Hallberg, G.R., Ressmeyer, G.R., and Hoyer, B.E., 1984, Groundwater quality and hydrogeology of Devonian-Carbonate aquifers in Floyd and Mitchell counties, Iowa: Iowa Geological Survey, Open File Report 84-2, 106 p.

Musterman, J.L., Fisher, R.A., and Drake, L.D., 1981, Underground injection control in Iowa: project termination report: U.S. EPA Grant No. G007165-01, 237 p.

Witzke, B.J., and Bunker, B.J., 1984, Devonian stratigraphy of north-central Iowa: Iowa Geological Survey, Open File Report 84-2, p. 107-149.