Anthony C. Runkel and Robert G. Tipping,
Minnesota Geological Survey Minnesota Geological Survey (MGS) Scientists
Robert Tipping and Anthony Runkel recently completed a two-year hydrogeologic study of the Prairie du Chien Group and underlying Jordan Sandstone. The Legislative Commission on Minnesota Resources funded this project, the goal of which was to characterize groundwater flow in the Prairie du Chien Group, and underlying Jordan Sandstone of southeastern Minnesota. The text below serves as a summary of the project results that will be described in greater detail in a Minnesota Geological Survey Report of Investigations that is being prepared..
SIGNIFICANCE OF PROJECT
The Prairie du Chien Aquifer is a widely used groundwater source that is under a well-documented threat of degradation. Nitrate levels exceed the safe drinking water standards in several rural areas as a result of infiltration of fertilizers. The aquifer is also highly susceptible to contamination in urban areas, including much of the Rochester and Twin Cities metropolitan areas. Specific sites where the Prairie du Chien Aquifer is known to be contaminated include a landfill in northern Olmsted County, at the former Twin Cities Army Ammunition Plant in Ramsey County, and beneath sewage treatment ponds in Wabasha and Winona Counties. Sites under active investigation by the Minnesota Pollution Control Agency (MPCA) include the City of Faribault (Rice County) where municipal wells draw contaminated water from the aquifer, and at Koch Refinery in Dakota County where petroleum products have traveled through the Prairie du Chien Aquifer toward the Mississippi River. A common problem to investigators at these sites is the difficulty encountered in predicting the paths of groundwater flow through the Prairie du Chien.
Despite the well-documented susceptibility of the Prairie du Chien to contamination and the widespread use of this unit as an aquifer, prior to our investigation there had been no comprehensive characterization of groundwater flow. The occurrence and distribution of fracture flow paths, nature of recharge, and internal hydraulic variability were virtually unknown. As a result, groundwater managers did not have the fundamental information necessary to develop aquifer protection strategies.
The results of this project have these practical applications: 1) For delineation of hydrogeologic setting and wellhead vulnerability of municipal water supplies as mandated by State Wellhead Protection Rules written by the Minnesota Department of Health (MDH). 2) To improve on-going bedrock hydrogeologic studies by the Department of Natural Resources (DNR), United States Geological Survey (USGS), and the MGS that currently suffer from inadequate understanding of fracture flow. Such studies include pollution sensitivity maps and recharge delineation investigations of the most widely used aquifer system in southeastern Minnesota. 3) To provide accurate hydraulic parameters of fractured aquifers for use in computerized groundwater models such as those currently being produced for the City of Rochester (by the USGS), and for the metro area (by MPCA). These range from city-scale (e.g. Rochester) to regional-scale (e.g. Twin Cities Metro) models. 4) To provide information necessary for effective regulatory guidelines for the construction and abandonment of water wells. 5) To provide a framework for site-specific investigations including sewage treatment lagoons, feedlot sites, leaking tanks, spills, and hazardous waste disposal sites.
OUTLINE OF SPECIFIC PROJECT RESULTS:
Result 1: Field investigations of outcrops and existing boreholes in southeastern Minnesota
Our field investigation of the Prairie du Chien Aquifer provided important new information on the distribution of cavities and how groundwater moves within them. Examination of outcrops and cores in 11 southeastern Minnesota counties allowed us to document marked variability in the abundance, size and connectivity of cavities that provide pathways for groundwater in saturated conditions. Cavities are preferentially developed in the middle to upper part of the Prairie du Chien, whereas its lower part contains relatively few cavities (e.g. Figure 1).
The differential distribution of cavities within the Prairie du Chien is reflected by karstic attributes at the land surface, and by groundwater hydraulics and chemistry. A collaborative project partially sponsored by Wabasha County (County Geologic Atlas Program) demonstrated that sinkholes, which are potential pathways for the rapid introduction of contaminants, are preferentially developed where the middle to upper Prairie du Chien lies directly beneath a thin cover of glacial drift. Borehole geophysical tests of nine water wells in five counties showed that ambient groundwater flow is relatively strong in the cavity-riddled middle to upper Prairie du Chien, and markedly more subdued in the lower part that contains relatively few cavities. Groundwater chemistry data compiled from previous work, and collected from 28 wells in 10 counties as part of this project, demonstrated that the lower Prairie du Chien commonly separates distinct hydrogeochemical facies. For example, groundwater water above the lower Prairie du Chien is often strongly impacted by anthropogenic constituents such as nitrate, chloride, and tritium whereas water below it is not. Exceptions typically occur where the lower part of the Prairie du Chien lies near the bedrock surface and water travels relatively quickly downward through vertical fractures into the underlying Jordan aquifer.
Result 2: Drilling of boreholes for rigorous scientific testing.
Three scientific boreholes drilled in Rice (Figure 2), Washington, and Olmsted counties allowed us to collect detailed hydraulic measurements of the Prairie du Chien Aquifer. Natural gamma, electric, caliper, EM flowmeter, and video logs were collected at all three boreholes. Temperature and conductivity logs were also collected at two of the boreholes. Discrete interval packer tests (10 ft intervals) indicated that hydraulic conductivity ranges over at least four orders of magnitude: intervals with few secondary pores have a conductivity of 10-1 ft/day or less, whereas intervals with well developed secondary pores can have a conductivity of 103 ft/day or greater. The low conductivity intervals behave as aquitards that separate discrete aquifers with potentiometric levels that differ by as much as 9 ft.
The results of our tests at all three sites indicate that the middle to upper part of the Prairie du Chien is characterized by a relatively dynamic flow system in which groundwater travels chiefly along a few thin intervals of very high hydraulic conductivity, separated by aquitards of low conductivity. The middle of the Prairie du Chien contains a zone of particularly high conductivity, greater than the measuring limits of standard hydraulic tests. At one borehole, near Northfield Minnesota (Figure 2), an individual conduit system less than five feet thick in the middle of the Prairie du Chien accommodated water travelling under ambient conditions at rates greater than 12 gallons/minute. The lower Prairie du Chien at all sites tested served as a low-conductivity confining unit.
Result 3: Interpretation and synthesis of data, and compilation of maps, cross-sections, and reports
Synthesis and interpretation of the data collected, together with a re-evaluation of existing data (geologic maps, dye traces, stream gauging, pump tests, chemical studies, and the distribution of springs and sinkholes), demonstrate that the Prairie du Chien is hydrogeologically more complex than commonly believed. At the same time it has attributes that are generally predictable across much of southeastern Minnesota. The upper part of the Prairie du Chien is best considered a karstic aquifer, which we call the Shakopee Aquifer. It is characterized by abundant cavities that accommodate a relatively dynamic groundwater system in which flow occurs predominantly along a few discrete intervals of high conductivity that are separated from one another by aquitards. The lower part of the Shakopee Aquifer has a particularly well-developed system of interconnected, high-conductivity features and therefore is a regionally important pathway for groundwater. In contrast the lower part of Prairie du Chien is best considered a low conductivity unit, the Oneota confining unit, that has the ability to hydraulically separate the underlying Jordan aquifer from the Shakopee aquifer.
Our results have significant practical applications for citizens across much of southeastern Minnesota, where over 15,000 domestic, municipal, and commercial wells draw water from the Prairie du Chien Group and Jordan Sandstone, making them the most widely used sources of groundwater in that part of the state. Over 100 communities rely entirely, or in part, on the Prairie du Chien or Jordan aquifers, and the information provided by this study can be used for the development of wellhead protection plans required by the Minnesota Department of Health (MDH). Our characterization of the distribution and magnitude of conductivity can also be used by groundwater investigators at the United States Geological Survey (USGS), Minnesota Pollution Control Agency (MPCA), and Minnesota Department of Natural Resources (MNDNR)-Division of Waters. These agencies can use the results to more accurately model travel times, assess vulnerability to contaminants and regulate the construction of water wells for the purpose of aquifer protection. Our characterization can also be used as a general framework from which site-specific investigations of contamination can be conducted. In addition, the techniques we used to study the aquifer are currently scheduled to be adapted by the MDNR as part of a groundwater monitoring project, and as part of an in-progress 2001 LCMR project (W-22) in which the effects of high volume pumping at quarries is investigated.
DISSEMINATION OF PROJECT RESULTS
Our results have been or will be disseminated in a variety of fashions. Some results were presented to the Southeastern Minnesota Water Resources Board on Sept 5, 2001 and to the Board of Commissioners of the same group on Nov 19, 2001. A presentation was made at the Midwest Groundwater Conference in Madison, Wisconsin on Oct 22, 2001 and an abstract summarizing our results was published as part of the proceedings from that meeting. A general article written for a broad audience was published in the summer 2001 issue of Inventing Tomorrow, a magazine of the Institute of Technology at the University of Minnesota. An on-site video of our borehole testing and a summary of the project were shown on the television program Environmental Journal.
A scientific report is being prepared that will include a comprehensive description of our data and interpretations, along with discussions that outline the practical value of the data to the citizens of Minnesota. The final report will be a Minnesota Geological Survey Report of Investigations (RI series). These reports are distributed widely to state and local units of government, state agencies, and libraries. They are available to the general public at the MGS, and are covered by major scientific abstracting services. An electronic version of the report will also be posted on the MGS web site in PDF format.
Captions to two figures
Figure 1. Large interconnected dissolution cavities along the Shakopee-Oneota contact at a quarry near Plainview, Minnesota. Our investigation demonstrates that cavities such as these are preferentially located at the Shakopee-Oneota contact. In saturated subsurface conditions they accommodate large volumes of water that can travel at rapid rates.
Figure 2. Logs of scientific test hole at Northfield, Minnesota. Open borehole exposes the Prairie du Chien Group and Jordan Sandstone. Flowmeter data was collected under ambient groundwater conditions with the tool moving up hole at a rate of 10ft/min (trolling log), as well as with the tool at stationary positions. Note that water enters the borehole chiefly through the Jordan Sandstone near the bottom of the hole, and travels up the hole, past the entire Oneota Dolomite, with negligible loss, at rates greater than 20 gallons per minute. This upflow exits the borehole through large dissolution cavities at a discrete horizon that approximates the Oneota-Shakopee contact. Such a borehole flow pattern, in conjunction with information from the caliper log, static water levels, temperature logs, and chloride concentrations all strongly indicate that the lower part of the Oneota Dolomite confines the Jordan Sandstone at this site. The Prairie du Chien Group and Jordan Sandstone therefore contain at least three distinct hydrogeologic units: A lower, Jordan Aquifer, and middle Oneota confining unit, and an upper Shakopee aquifer. Similar borehole tests in other parts of southeastern Minnesota also demonstrate the presence of three or more distinct hydrogeologic units within the Prairie du Chien and Jordan.