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This report synthesizes information on the regional ground-water flow and geochemistry in the Midwestern Basins and Arches aquifer system in parts of Indiana, Ohio, Michigan, and Illinois. Aquifers that compose this water-table aquifer system include glacial aquifers and an underlying, areally extensive carbonate-rock aquifer. Water within the aquifers is most commonly a Ca-Mg-HCO3 type or a Ca-Mg-SO4 type. In general, the distribution of hydrochemical facies within the aquifer system is controlled by the mineralogy of the aquifer material, rather than by a chemical evolution of water along general directions of regional ground-water flow.Some ground-water flow systems within the aquifer system provide base flow to streams in response to ground-water recharge events. Other (often deeper) ground-water flow systems respond minimally to variations in ground-water recharge from precipitation and provide a fairly constant supply of water to streams. Streamflow hydrographs and base-flow duration curves were used to estimate such components of base flow in selected streams for long-term steady-state conditions in the aquifer system. Mean sustained ground-water discharge (discharge from fairly stable ground-water flow systems) ranges from 3 to 50 percent of mean ground-water discharge (discharge from all ground-water flow systems) to the selected stream reaches. These percentages indicate that 50 to 97 percent of base flow in the streams within the study area can be attributed to transient ground-water flow systems, which typically have a major component of local-scale flow. Because ground-water flow across the external boundaries of the aquifer system is minimal, such percentages indicate that most ground-water flow in the aquifer system is associated with seasonally transient local flow systems.Results of a ground-water flow model that was calibrated by use of regression methods and that simulates regional flow systems within the aquifer system (approximately 10 percent of total ground-water flow in the aquifer system) indicate that most water (99 percent) in simulated regional flow systems is from recharge at the water table. Most water (78 percent) discharges from simulated regional flow systems to the principal streams. Less than 3 percent of water in simulated regional flow systems discharges to the Ohio River, Lake Erie, or downdip areas in the Illinois (structural) Basin. Simulations also indicate that most of the Midwestern Basins and Arches aquifer system is characterized by alternating regional recharge and discharge areas at intervals of less than every 10 miles along the dominant regional trends of the potentiometric surfaces in the aquifers. Such alternating regional recharge and discharge areas result in the absence of long flow paths from the very highest regional potentiometric levels to the very lowest regional potentiometric levels. The presence of tritiated ground water (less than 50 years old) across most of the aquifer system also indicates that the aquifer system receives recharge across most of the study area.The northeastern part of the aquifer system near Lake Erie differs from the rest of the system with respect to regional ground-water flow and chemistry. Specifically, part of the northeastern part of the aquifer system can be characterized as a broad area (tens of miles) of weak regional discharge (less than 0.5 inch per year). Results of the regional ground-water flow model indicate that regional flow systems have a limited ability to carry ground water away from this area; thus precipitation is prevented from recharging the regional flow systems in this part of the aquifer system. Some ground water recharged during Pleistocene glaciation was found in this area. Sulfide concentrations and sulfur isotope data, which indicate that extensive sulfate reduction has occurred in the aquifer system within this area, confirm that only minimal recharge of this part of the aquifer system has taken place over a long period of time.The longest simulated ground-water flow paths within the aquifer system (nearly 50 miles) were also identified in the northeastern part of the aquifer system. These flow paths terminate at Lake Erie. On the basis of carbon-14 data, some of the oldest waters (approximately 13,000 years) were found at the downgradient end of these particularly long flow paths. The area near these flow paths is the only area within the aquifer system in which a systematic increase in ground-water ages was observed in the general direction of regional ground-water flow. Not all of the regional flow paths in the northeastern part of the aquifer system are particularly long. The very oldest waters that were found in the aquifer system (approximately 38,000-45,000 years) are associated with comparably short ground-water flow paths (approximately 10 miles). These waters are present beneath the Maumee River Basin and indicate that parts of the aquifer system beneath the Maumee River Basin are fairly stagnant.
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Forward
Abstract
Introduction
Purpose and scope
Approach
Acknowledgments
Geohydrology
Geologic setting
Hydrologic setting
Ground-water use
Aquifers and confining units
Ground water
Hydraulic characteristics
Levels
Recharge
Discharge
Regional ground-water flow
Conceptual model
Numerical model
Model design
Discrete geohydrologic framework
Boundaries, sources, and sinks
No-flow boundaries
Specified-head boundaries
Specified-flux boundary
Head-dependent flux boundaries
Approach to mapping regional recharge and discharge
Parameterization
Model calibration
Procedure
Estimates of parameter values
Simulated hydraulic heads
Simulated flows
Model discrimination
Reliability of parameter estimates
Simulated regional ground-water flow
Geochemistry
Spatial distribution of dissolved-solids concentrations and hydrochemical facies
Dissolved-solids concentrations
Hydrochemical facies
Relation between dissolved-solids concentrations and hydrochemical facies
Geochemical and hydrologic processes, by water type
Calcium-magnesium-bicarbonate water
Calcium-magnesium-sulfate water
Sodium-enriched water
Sodium-chloride and sodium-calcium-chloride water
Insights into regional flow provided by geochemical data
Major solutes
Isotopes
Sulfur isotopes
Oxygen and hydrogen isotopes
Tritium and carbon isotopes
Variation in ground-water chemistry with depth
Summary and conclusions
References cited
The text and illustrations for this report are presented here in pdf format
Cover, title page, Foreword, and table of contents--pdf--260KB
Abstract and Introduction--pdf--64KB
Regional ground-water flow--pdf--1,946KB
Summary and conclusions--pdf--64KB
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