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1 Flow-System Analysis of the Madison and Minnelusa Aquifers in the Rapid City Area, South Dakota—Conceptual Model By Andrew J. Long and Larry D. Putnam ABSTRACT The conceptual model of the Madison and Minnelusa aquifers in the Rapid City area synthesizes the physical geography, hydraulic properties, and ground-water flow components of these important aquifers. The Madison hydrogeologic unit includes the karstic Madison aquifer, which is defined as the upper, more permeable 100 to 200 ft of the Madison Limestone, and the Madison confining unit, which consists of the lower, less permeable part of the Madison Limestone and the Englewood Formation. Overlying the Madison hydrogeologic unit is the Minnelusa hydrogeologic unit, which includes the Minnelusa aquifer in the upper, more permeable 200 to 300 ft and the Minnelusa confining unit in the lower, less permeable part. The Madison and Minnelusa hydrogeologic units outcrop in the study area on the eastern flank of the Black Hills where recharge occurs from streamflow losses and areal recharge. The conceptual model describes streamflow recharge, areal recharge, ground-water flow, storage in aquifers and confining units, unsaturated areas, leakage between aquifers, discharge from artesian springs, and regional outflow. Effective transmissivities estimated for the Madison aquifer range from 500 to 20,000 ft2/d and for the Minnelusa aquifer from 500 to 10,000 ft2/d. Localized anisotropic transmissivity in the Madison aquifer has tensor ratios as high as 45:1. Vertical hydraulic conductivities for the Minnelusa confining unit determined from aquifer tests range from 1.3x10-3 to 3.0x10-1 ft/d. The confined storage coefficient of the Madison and Minnelusa hydrogeologic units was estimated as 3x10-4 ft/d. Specific yield was estimated as 0.09 for the Madison and Minnelusa aquifers and 0.03 for the Madison and Minnelusa confining units. Potentiometric surfaces for the Madison and Minnelusa aquifers have a general easterly gradient of about 70 ft/mi with local variations. Temporal change in hydraulic head in the Madison and Minnelusa aquifers ranged from about 5 to 95 ft in water years 1988-97. The unconfined areas were estimated at about 53 and 36 mi2 for the Madison and Minnelusa hydrogeologic units, respectively, in contrast to an aquifer analysis area of 629 mi2. Dye-tracer tests, stable isotopes, and hydrogeologic features were analyzed conjunctively to estimate generalized ground-water flowpaths in the Madison aquifer and their influences on the Minnelusa aquifer. The western Rapid City area between Boxelder Creek and Spring Creek was characterized as having undergone extensive tectonic activity, greater brecciation in the Minnelusa Formation, large transmissivities, generally upward hydraulic gradients from the Madison aquifer to the Minnelusa aquifer, many karst springs, and converging flowpaths. Water-budget analysis included: (1) a dry-period budget for declining water levels; October 1, 1987, to March 31, 1993; (2) a wetperiod budget for rising water levels, April 1,The conceptual model of the Madison and Minnelusa aquifers in the Rapid City area synthesizes the physical geography, hydraulic properties, and ground-water flow components of these important aquifers. The Madison hydrogeologic unit includes the karstic Madison aquifer, which is defined as the upper, more permeable 100 to 200 ft of the Madison Limestone, and the Madison confining unit, which consists of the lower, less permeable part of the Madison Limestone and the Englewood Formation. Overlying the Madison hydrogeologic unit is the Minnelusa hydrogeologic unit, which includes the Minnelusa aquifer in the upper, more permeable 200 to 300 ft and the Minnelusa confining unit in the lower, less permeable part. The Madison and Minnelusa hydrogeologic units outcrop in the study area on the eastern flank of the Black Hills where recharge occurs from streamflow losses and areal recharge. The conceptual model describes streamflow recharge, areal recharge, ground-water flow, storage in aquifers and confining units, unsaturated areas, leakage between aquifers, discharge from artesian springs, and regional outflow. Effective transmissivities estimated for the Madison aquifer range from 500 to 20,000 ft2/d and for the Minnelusa aquifer from 500 to 10,000 ft2/d. Localized anisotropic transmissivity in the Madison aquifer has tensor ratios as high as 45:1. Vertical hydraulic conductivities for the Minnelusa confining unit determined from aquifer tests range from 1.3x10-3 to 3.0x10-1 ft/d. The confined storage coefficient of the Madison and Minnelusa hydrogeologic units was estimated as 3x10-4 ft/d. Specific yield was estimated as 0.09 for the Madison and Minnelusa aquifers and 0.03 for the Madison and Minnelusa confining units. Potentiometric surfaces for the Madison and Minnelusa aquifers have a general easterly gradient of about 70 ft/mi with local variations. Temporal change in hydraulic head in the Madison and Minnelusa aquifers ranged from about 5 to 95 ft in water years 1988-97. The unconfined areas were estimated at about 53 and 36 mi2 for the Madison and Minnelusa hydrogeologic units, respectively, in contrast to an aquifer analysis area of 629 mi2. Dye-tracer tests, stable isotopes, and hydrogeologic features were analyzed conjunctively to estimate generalized ground-water flowpaths in the Madison aquifer and their influences on the Minnelusa aquifer. The western Rapid City area between Boxelder Creek and Spring Creek was characterized as having undergone extensive tectonic activity, greater brecciation in the Minnelusa Formation, large transmissivities, generally upward hydraulic gradients from the Madison aquifer to the Minnelusa aquifer, many karst springs, and converging flowpaths. Water-budget analysis included: (1) a dry-period budget for declining water levels; October 1, 1987, to March 31, 1993; (2) a wetperiod budget for rising water levels, April 1, 2 Flow-System Analysis of Madison and Minnelusa Aquifers in the Rapid City Area, SD—Conceptual Model 1993, to September 30, 1997; and (3) a full 10-year period budget for water years 1988-97. By simultaneously balancing these water budgets, initial estimates of recharge, discharge, change in storage, and hydraulic properties were refined. Inflow rates for the 10-year budget included streamflow recharge of about 45 ft3/s or 61 percent of the total budget and areal recharge of 22 ft3/s or 30 percent. Streamflow recharge to the Madison hydrogeologic unit was about 86 percent of the total streamflow recharge. Outflow for the 10-year budget included springflow of 31 ft3/s or 42 percent of the total budget, water use of about 10 ft3/s or 14 percent, and regional outflow of 22 ft3/s or 30 percent. Ground-water storage increased 9 ft3/s during the 10-year period, and net ground-water movement from the Madison to Minnelusa hydrogeologic unit was about 8 ft3/s. INTRODUCTION The Madison and Minnelusa aquifers are the main source of ground water in the Black Hills area (Driscoll and Carter, 2001). The city of Rapid City obtains more than one-half of its municipal water supply from these two bedrock aquifers via deep wells and springs. Numerous additional users in the Rapid City area obtain water from the Madison or Minnelusa aquifers for domestic, industrial, and irrigation usage. Ground-water flow within the Madison and Minnelusa aquifers is complex. Extensive fracturing, solution enhancement, and brecciation contribute to heterogeneity, anisotropic transmissivity, and spatially variable ground-water seepage between the two aquifers. A long-term cooperative study between the U.S. Geological Survey and the city of Rapid City has provided hydrogeologic data and interpretation for planning and management of the Madison and Minnelusa aquifers. Purpose and Scope The primary purpose of this report is to present a conceptual model of ground-water flow in the Madison and Minnelusa aquifers in the Rapid City area leading to a better understanding of the unique concepts involved. The conceptual model consists of discussions of hydraulic properties, hydraulic head, and groundwater flow. A detailed water-budget analysis encompassing WY88-97 characterizes and quantifies recharge and discharge for the study area and is used to refine estimates of hydraulic properties. An additional purpose of the conceptual model was to compile data for numerical modeling and other research efforts in the future. Description of Study Area The study area, which includes Rapid City and the surrounding area, extends north of Elk Creek, to the south near Battle Creek, east of the city of Boxelder, and west into the central Black Hills (fig. 1). Streams flowing from the central Black Hills contribute to streamflow losses that recharge the Madison and Minnelusa aquifers. The western extent of the study area comprises these stream basins, which include Elk, Boxelder, Rapid, Spring, and Battle Creeks. This western area is included in the study area for analysis of streamflow recharge and evapotranspiration. The eastern part of the study area, where the Madison and Minnelusa hydrogeologic units exist, is referred to as the aquifer analysis area (figs. 1 and 2). These hydrogeologic units are described in subsequent sections of the report. Land-surface altitudes range from more than 7,000 ft in the western highlands of the study area to about 3,000 ft in the eastern lowlands. The western extent of the study area is characterized by high relief with predominantly pine and spruce forests. The eastern lowlands comprise approximately the eastern one-half of the aquifer analysis area and are characterized by rolling prairies with bottom lands along stream channels. The outcrops of the Madison and Minnelusa hydrogeologic units are located in the western part of the aquifer analysis area (fig. 2) along the eastern flank of the Black Hills uplift. These outcrops are characterized by high-relief forested areas

Flow-system analysis of the Madison and Minnelusa aquifers in the Rapid City area, South Dakota — Conceptual model