I. GROUNDWATER QUALITY AND HYDROGEOLOGY OF DEVONIAN-CARBONATE AQUIFERS IN FLOYD AND MITCHELL COUNTIES, IOWA
R.D. Libra, G.R. Hallberg, G.G. Ressmeyer, B.E. Hoyer
Iowa Department of Natural Resources, Geological Survey Bureau
Open File Report 84-2, 1984 149 p.
The Floyd-Mitchell Study is the third phase of an assessment of the hydrogeology and groundwater quality of karst-carbonate aquifers in northeast Iowa. The project was jointly funded and conducted by the Iowa Geological Survey and the U.S. Environmental Protection Agency, with cooperation from the Iowa Department of Water, Air and Waste Management.
Devonian rocks form the carbonate aquifer in the study area. A revised analysis of Devonian stratigraphy indicates the aquifer may best be described as a three-part system. The components of this system are informally termed the lower, middle, and upper aquifers, and are separated by relatively low permeability shale units. The regional effectiveness of these shales as confining beds is unclear. However, limited extant data suggest that a partially confined, deep flow system is present in the lower aquifer, generating strong upward flow components locally.
Four geologic regions are delineated in the Floyd-Mitchell area--Deep Bedrock, Shallow Bedrock, Karst, and Incipient Karst. These areas present unique interrelationships between surficial geology, hydrology, and the resultant groundwater quality. The distribution of karst topography is controlled by depth to bedrock and the distribution of Devonian strata. Prominent karst development occurs where "Upper Cedar Valley" limestones are close to the land surface. In other areas where bedrock lies at shallow depth, the Devonian units are dolomite, and karst features are less common or absent.
Fifty wells, distributed over the four geologic regions, were selected for an initial site and water-quality inventory. Surficial well construction and placement were examined, and water samples were collected for nitrate and bacteria analyses. These wells were chosen from the Iowa Geological Survey well log file; therefore, the location of completed intervals with respect to stratigraphy were known. Based on the results of this inventory, twenty wells were selected for a monthly monitoring network. Three surface-water sites and two drainage tile outlets were also included in this network. Monthly sampling included nitrate and bacteria analyses. Samples for pesticide analysis were also collected, on a less frequent basis.
Monthly monitoring of groundwater quality from the various geologic regions in the Floyd-Mitchell area reinforces the conclusions of prior studies in northeast Iowa, and shows that the use of these regions provides a valid means of defining the relationships between surficial geology, hydrology, and resultant groundwater quality. Data from the "protected" Deep Bedrock Region provide a background against which the water quality from other "less protected" regions may be evaluated. Properly constructed wells in this region produce water with <5 mg/L nitrate. The highest and relatively constant levels of nitrate in groundwater occur in the Incipient Karst Region (68 mg/L, annual mean). This region is characterized by high rates of infiltrating recharge which leaches nitrates from shallow soil horizons. Karst-area groundwater shows the next highest levels of nitrate (41 mg/L), and the greatest variability of concentrations. This variability is caused by different recharge mechanisms which operate in open-karst areas. In Shallow Bedrock regions, groundwater contains intermediate concentrations of nitrate (22 mg/L), less than the Karst/Incipient Karst regions, but significantly greater than the Deep Bedrock areas.
Pesticides were not detected in wells from the protected Deep Bedrock region. Pesticides were detected at least once in over two/thirds of the wells in the other geologic regions--Shallow Bedrock, Karst, and Incipient Karst. In almost all instances, pesticides were delivered to groundwater through infiltration processes. Total pesticide concentrations generally ranged up to 2.0 µg/L. Locally, however, groundwater containing 4.5 to 21 µg/L total pesticides was detected, evidently caused by infiltration leaching from fields which received excessive chemical applications. Pesticides detected in well water samples include Atrazine, Sencor, Lasso, Bladex, and Dual. All of these pesticides were present in samples collected prior to the 1983 application of pesticides, indicating the potential for year-round pesticide persistence in groundwater and shallow soil horizons.
Both nitrate and pesticide concentrations show seasonal variations with higher concentrations occurring during wet (recharge) periods of the year. Analysis of nitrate-pesticide concentrations, from all parts of the hydrologic system in the area, indicates a relationship between these contaminants. Local variables and the environmental characteristics of the various chemicals complicate this relationship. Thus, nitrate concentrations in groundwater are not an accurate predictor of pesticide presence in groundwater.
Hydrologic responses and trends in contaminant levels are very similar, both in groundwater and surfacewater, in the Floyd-Mitchell and Big Springs area, demonstrating the regional applicability of the findings of this report and prior studies. These findings indicate that groundwater in the combined Shallow Bedrock-karst areas are being degraded by the infiltration of surface-applied chemicals. These regions constitute over 6800 square miles (17,600 sq. km) of the land surface in northeastern Iowa; strategies developed to protect groundwater resources must consider this entire area, not just the "sinkhole" or karst areas.
II. DEVONIAN STRATIGRAPHY OF NORTH-CENTRAL IOWA: A GEOLOGIC FRAMEWORK FOR THE DEVONIAN AQUIFER AND KARST SYSTEMS IN FLOYD-MITCHELL COUNTIES
B.J. Witzke and B.J. Bunker
Iowa Department of Natural Resources, Geological Survey Bureau
New lithostratigraphic interpretations of Middle-Upper Devonian carbonate rocks across north-central Iowa suggest that a three-part aquifer system may exist in this region. Limited observations of potentiometric heads in the Charles City area of Floyd County support this contention. Cored intervals of Devonian strata at Mason City (Cerro Gordo County), as well as numerous exposures across northern Iowa, have been of critical importance for recognizing and defining lithostratigraphic units across a large portion of the area. Devonian rocks in this region are presently divided into five formations, which in ascending order include: the Spillville, Wapsipinicon, Cedar Valley, Shell Rock, and Lime Creek formations.
Fracture permeability of porous, vuggy dolomites of the Spillville Formation results in this interval being utilized as an aquifer, informally termed the "lower aquifer," across much of northern Iowa. The abundance of shaley and argillaceous strata in the overlying Wapsipinicon Formation suggests that it serves as an upper confining unit for the "lower aquifer." The apparent high potentiometric surface of the "lower aquifer" in Floyd County, and the westward and southward pinchout beneath the Wapsipinicon Formation, largely restricts vertical groundwater flow in this aquifer.
The Cedar Valley Formation can be informally subdivided into five members, designated in ascending order, Units A through E. Units A, B, and C exhibit a prominent cyclic character, and the stratigraphic contacts between them are drawn at the top of each depositional cycle. Each cycle records a major transgressive-regressive sequence. Fossiliferous dolomite intervals record deposition in open-marine carbonate shelf environments during successive transgressive phases, whereas laminated, interclastic, and brecciated carbonates record deposition in shallow, restricted subtidal and tidal flat settings during each regressive phase. Unit A may represent a separate hydrologic subsystem ("middle aquifer") across most of northern Iowa. It is confined below by shales and shaley carbonates of the Wapsipinicon Formation, and above by the "Rapid Shale," which is stratigraphically placed in the upper part of Unit A. Intervening carbonate strata (Units D and E) between the top of Unit C and the base of the Shell Rock or Lime Creek formations are informally included in the "Upper Cedar Valley Formation." Strata (Unit E) included in the "Upper Cedar Valley" may be correlative, in part, with the Shell Rock Formation. "Upper Cedar Valley" and Shell Rock strata in areas of shallow bedrock display prominent to incipient karst development in portions of Floyd and Mitchell counties. These karstified areas are developed in regions where Units D and E are predominantly limestone. The relative vertical continuity of carbonate strata in Units B and C, and "Upper Cedar Valley" (Units D and E) suggest that these strata may form part of a single aquifer system ("upper aquifer") which is confined below by the "Rapid Shale."
The Lime Creek Formation is subdivided into three members, which in ascending order include: Juniper Hill, Cerro Gordo, and Owen. The Juniper Hill Shale and the Cerro Gordo Member, where it is predominantly a shale, form a prominent confining unit for the underlying "upper aquifer" (Unit B through Shell Rock) in western Floyd and Cerro Gordo counties. Shales of the Cerro Gordo member are laterally replaced to the west by carbonates and together with the overlying Owen carbonates may form part of a local aquifer system informally termed the "Lime Creek aquifer." This aquifer is confined beneath Devonian shales of the Yellow Spring Group to the southwest in north-central Iowa.
Cretaceous rocks locally and unconformably overlie Middle and Upper Devonian strata across northern Iowa. These rocks are assigned to the Windrow Formation and include three general lithologies: 1) claystone or mudstone; 2) sandstone; and 3) conglomerates. Where Windrow claystones/mudstones are well developed, they may serve locally to confine the underlying "upper aquifer" or the "Lime Creek aquifer." Local occurrences of friable Windrow sandstones could also conceivably yield water to shallow wells in portions of north-central Iowa.