By Cheryl A. Naus, Daniel G. Driscoll, and Janet M. Carter
Water-Resources Investigations report 01-4129
Prepared in cooperation with the
South Dakota Department of Environment and Natural Resources and the West Dakota
Water Development District
The full report is available in PDF format.
The Madison and Minnelusa aquifers are two of the most important aquifers in the Black Hills area because of utilization for water supplies and important influences on surface-water resources resulting from large springs and streamflow- loss zones. Examination of geochemical information provides a better understanding of the complex flow systems within these aquifers and interactions between the aquifers.
Major-ion chemistry in both aquifers is dominated by calcium and bicarbonate near outcrop areas, with basinward evolution towards various other water types. The most notable differences in major-ion chemistry between the Madison and Minnelusa aquifers are in concentrations of sulfate within the Minnelusa aquifer. Sulfate concentrations increase dramatically near a transition zone where dissolution of anhydrite is actively occurring.
Water chemistry for the Madison and Minnelusa aquifers is controlled by reactions among calcite, dolomite, and anhydrite. Saturation indices for gypsum, calcite, and dolomite for most samples in both the Madison and Minnelusa aquifers are indicative of the occurrence of dedolomitization. Because water in the Madison aquifer remains undersaturated with respect to gypsum, even at the highest sulfate concentrations, upward leakage into the overlying Minnelusa aquifer has potential to drive increased dissolution of anhydrite in the Minnelusa Formation.
Isotopic information is used to evaluate ground-water flowpaths, ages, and
mixing conditions for the Madison and Minnelusa aquifers. Distinctive patterns
exist in the distribution of stable isotopes of oxygen and hydrogen in precipitation
for the Black Hills area, with isotopically lighter precipitation generally
occurring at higher elevations and latitudes. Distributions of 
18O
in ground water are consistent with spatial patterns in recharge areas, with
isotopically lighter 18O
values in the Madison aquifer resulting from generally higher elevation recharge
sources, relative to the Minnelusa aquifer.
Three conceptual models, which are simplifications of lumped-parameter models, are considered for evaluation of mixing conditions and general ground-water ages. For a simple slug-flow model, which assumes no mixing, measured tritium concentrations in ground water can be related through a first-order decay equation to estimated concentrations at the time of recharge. Two simplified mixing models that assume equal proportions of annual recharge over a range of years also are considered. An “immediate-arrival” model is used to conceptually represent conditions in outcrop areas and a “time-delay” model is used for locations removed from outcrops, where delay times for earliest arrival of ground water generally would be expected. Because of limitations associated with estimating tritium input and gross simplifying assumptions of equal annual recharge and thorough mixing conditions, the conceptual models are used only for general evaluation of mixing conditions and approximation of age ranges.
Headwater springs, which are located in or near outcrop areas, have the highest tritium concentrations, which is consistent with the immediate-arrival mixing model. Tritium concentrations for many wells are very low, or nondetectable, indicating general applicability of the timedelay conceptual model for locations beyond outcrop areas, where artesian conditions generally occur. Concentrations for artesian springs generally are higher than for wells, which indicates generally shorter delay times resulting from preferential flowpaths that typically are associated with artesian springs.
In the Rapid City area, a distinct division of isotopic values for the Madison
aquifer corresponds with distinguishing 18O signatures for nearby streams,
where large streamflow recharge occurs. Previous dye testing in this area documented
rapid ground-water flow (timeframe of weeks) from a streamflow loss zone to
sites located several miles away. These results are used to illustrate potential
errors that may result from the simplified conceptualization of this complex
ground-water setting with dual-porosity hydraulic characteristics. For Rapid
City sites with timeseries data, minimal variability in 18O
values corresponded with tritium data indicative of dominant proportions of
older water. Other sites showed response to temporal 18O
trends in streamflow recharge, with tritium data indicating larger proportions
of modern recharge. Several large production wells located near the isotopic
transition zone had changes in 18O
values indicative of changes in capture zones associated with recent production.
Evaluation of major-ion and isotope data indicates that regional flowpaths
for the Madison aquifer are essentially deflected around the study area, with
the possible exception of the southwestern and northwestern corners. Two wells
just north of the study area clearly show influence of regional flow, and a
well just within the study area shows possible influence. Large artesian springs
near the northern axis of the uplift show no regional influence and are concluded
to be recharged within the uplift area. Ion concentrations for wells west of
the study area in Wyoming indicate deflection of regional flowpaths, with minor
influence possible for several wells. The 18O
values for large springs along the southern axis of the uplift essentially preclude
regional influence, which also is supported by ion chemistry, and indicate potential
recharge areas extending along the entire southwestern flank of the uplift.
Low, but detectable, tritium concentrations in these springs along the southern
axis confirm the influence of recharge from within the study area, but indicate
relatively long traveltimes.
Hydrographs for 9 of 13 well pairs are fairly well separated and do not indicate direct hydraulic connection between the Madison and Minnelusa aquifers. Comparison of geochemical information provides no evidence of extensive mixing resulting from general, areal leakage between the aquifers.
Aquifer interactions can occur at artesian springs, which discharge about one-half of average recharge to the Madison and Minnelusa aquifers in the Black Hills area. Various investigators have hypothesized that the Madison aquifer is the primary source for many artesian springs, based on geochemical modeling. The Madison aquifer is inferred as the primary source for several springs where artesian conditions in the Minnelusa aquifer are precluded by nearby outcrop sections. For many springs, quantifying relative contributions from each aquifer is hampered by geochemical similarities between the Madison and Minnelusa aquifers, especially near recharge areas. For some springs, high sulfate concentrations indicate Minnelusa influence, but may result from dissolution of Minnelusa minerals by water from the Madison aquifer.
Generally higher hydraulic head in the Madison aquifer, in combination with gypsum undersaturation, is concluded to be a primary mechanism driving interactions with the Minnelusa aquifer, in areas where artesian conditions exist in the Madison aquifer. Upward leakage from the Madison aquifer probably contributes to general dissolution of anhydrite deposits in the Minnelusa aquifer and development of breccia pipes, which enhances vertical hydraulic conductivity. Breccia pipes are a likely mechanism for upward movement of large quantities of water through the Minnelusa aquifer at artesian spring locations and many exposed breccia pipes of the upper Minnelusa Formation probably are the throats of abandoned artesian springs. Dissolution processes are an important factor in a selfperpetuating process associated with development of artesian springs and preferential flowpaths, which initially develop in locations with large secondary porosity and associated hydraulic conductivity, with ongoing enhancement resulting from dissolution activity.
Outward (downgradient) migration of the artesian springs probably occurs as upgradient spring-discharge points are abandoned and new ones are occupied, keeping pace with regional erosion over geologic time. In response, hydraulic heads in the Madison and Minnelusa aquifers also have declined over geologic time. Artesian springflow and general leakage are concluded to be important factors in governing water levels in the Madison and Minnelusa aquifers. Artesian springs are especially important in acting as a relief mechanism that provides an upper limit for hydraulic head, with springflow increasing in response to increasing water levels.
Section available in pdf:
Abstract
Introduction
Purpose and Scope
Previous Investigations
Acknowledgments
Description of Study Area
Geologic Setting
Hydrologic Setting
Regional Setting
Local Setting
Methods Used and Data Sets Considered
Geochemistry of Madison and Minnelusa Aquifers
Major-Ion Chemistry
Distribution of Major Ions
Saturation State
Evolutionary Processes
Isotope Chemistry
Background Information and Isotopic Composition of Recharge Water
Stable Isotopes of Oxygen and Hydrogen
Tritiurn
Conceptual Mixing Models
Description of Models
Limitations of Models
Areal Flowpaths, Ages, and Mixing Conditions
Isotope Distributions and General Considerations
Stable Isotopes
Tritium
Site-Specific Considerations
Rapid City Area
Headwater Springs
Northern Black Hills Area
Southern Black Hills Area
Regional Flowpaths
Interactions Between Madison and Minnelusa Aquifers
Interactions at Well Pairs
Hydraulic Considerations
Geochemical Considerations
Synopsis of Interaction Processes
Summary and Conclusions
References
Supplemental Information
For additional information write to:
District Chief
U.S. Geological survey
1608 Mt. View Road
Rapid City, SD 57702
Copies of this report can be purchased from:
U.S. Geological Survey
Branch of Information Services
Box 25286
Denver, CO 80225-0286
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