FRACTURE TRACES AND PRODUCTIVITY OF MUNICIPAL WELLS IN THE MADISON LIMESTONE, RAPID CITY, SOUTH DAKOTA

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1 Proceedings of the South Dakota Academy of Science, Vol. 87 (2008) 261 FRACTURE TRACES AND PRODUCTIVITY OF MUNICIPAL WELLS IN THE MADISON LIMESTONE, RAPID CITY, SOUTH DAKOTA Perry H. Rahn Department of Geology & Geological Engineering South Dakota School of Mines & Technology Rapid City, SD ABSTRACT Between 1991 and 2002 the City of Rapid City drilled eight wells to the Madison Limestone. The yield of these wells ranges from 150 to 2,580 gpm. Aerial photographs were used to test the hypothesis that more productive wells are located on fracture traces. The conclusion of this study is that there is no difference. The overlying Spearfish Formation probably masks any fractures that may be present in the Madison Limestone. Keywords Madison Limestone, fracture traces, water wells INTRODUCTION Lattman and Parizek (1964) found that wells drilled to Cambrian and Ordovician limestone and dolomite in Nittany Valley, Pennsylvania, have greater yield if they were located on fracture traces. Fracture traces are natural linear features that have lengths of less than one mile. Seen on aerial photographs, they typically include topographic features such as linear stream and gully reaches, or linear tonal features in farm or ranch lands (Figure 1). The higher yielding wells are associated with solution-enhanced fracture zones possessing higher permeability. For example, Siddiqui and Parizek (1971) found the mean productivity (gpm per foot drawdown per foot static saturated thickness) was approximately 2 gpm/ft/ft for nonfracture wells and 200 gpm/ft/ft for fracture trace wells. In a study of caves in the Black Hills, South Dakota, Greene and Rahn (1999) found transmissivity anisotropy mimics cave passageway orientation developed along fracture zones. There is reason to believe that these fractures would be visible on aerial photographs. Using rose diagrams, Miller (2005) found some correlation between joints on outcrops and cave passageway orientation in the Madison Limestone, but found only limited correlation of fracture traces with these parameters. Rahn (1992) studied private wells in various geologic settings including Precambrian rocks of the Black Hills; he found that the specific capacities of wells on fracture traces were approximately three times those of wells not

$262 Proceedings of the South Dakota Academy of Science, Vol. 87 (2008) Figure 1. Sketch showing relationship of fracture traces to bedrock fractures (Lattman and Parizek, 1964). on fracture traces.$

2 262 Proceedings of the South Dakota Academy of Science, Vol. 87 (2008) Figure 1. Sketch showing relationship of fracture traces to bedrock fractures (Lattman and Parizek, 1964). on fracture traces. Aerial photographs were used to test the hypothesis that more productive wells are located on fracture traces. Rapid City Wells The City of Rapid City has two wells (RC-3 and RC-4) drilled in the 1930s to the Minnelusa Formation. Beginning in 1991 the city embarked on a program to supplement its existing water supply by drilling eight wells (RC-5 to RC-12) to the Madison Limestone (Figure 2). The drilling depths to the Madison Limestone range from 1,292 ft to 3,280 ft (Table 1). During 2000 to 2005 these wells supplied approximately 10 cfs (nearly half of the water used) for municipal supply (Rahn 2006). The wells show increasing nitrate concentration, presumably from nearby onsite-waste water systems (Rahn 2006). Test pumping for the eight Madison wells (Table 1) shows a wide range of yield for these wells. Well RC-9 was pumped at 2,580 gpm. Well RC-7 yielded only 150 gpm, and ultimately was not utilized as a production well. Well RC-12 is presently not hooked up to the municipal water system. For this paper, the specific capacity was determined in order to compare well yields for the eight wells (Table 1). The specific capacity is simply defined as the yield (gpm) divided by the drawdown (ft). Figure 3 is a geologic cross section showing the geology of this area. The Madison Limestone is approximately 420 ft thick in this area (Rahn 1987). Formerly called the Pahasapa Limestone, this Mississippian carbonate is primarily a dolomite (Rahn 2005). The upper part of this stratum is karstic and is very permeable (Greene and Rahn 1999).

3 Proceedings of the South Dakota Academy of Science, Vol. 87 (2008) 263 Figure 2. Map of the western part of Rapid City showing location of municipal wells (from Rahn, 2007; modified from Greene, 1999, and Anderson et al., 1999). The Madison Limestone is highlighted; its potentiometric surface (feet above sea level) and general ground water flow direction are shown.mb = Meadowbrook Gallery, GS = Girl Scout Gallery, and JS = Jackson Spring.

4 264 Proceedings of the South Dakota Academy of Science, Vol. 87 (2008) Figure 3. Geologic cross section (from Rahn, 1992). The arrow shows the general location of municipal wells drilled to the Madison Limestone (Mp). Table 1. Rapid City Madison well data. (1) Yield Static Pumping Drawdown Specific Capacity Well No. Depth (ft) (gpm) Level (ft) Level(ft) (ft) (gpm/ft) (2) RC-5 1,292 1, RC-6 1, RC-7 3, RC-8 2, RC-9 (3) 1,050 2, RC-10 1,790 1, RC-11 1, RC-12 (4) 1,323 1, (1) From Anderson et al. (1999) unless otherwise noted. (2 ) Specific Capacity = Pumping rate/drawdown. (3) RC-9 is a flowing artesian well with a static level of 85 ft above the ground (Ron Barber, pers. comm., 5/14/07). The pumping level was not measured due to the line shaft leading to the pump. A crude estimate of the pumping level is 200 ft, based on Burns and McDonnell (2007) who noted that the submersible pump is at a depth of 380 feet below the well house floor. (4) RC-12 data from Miller (2005) and Stacey Titus (pers. comm., 3/29/07). Fracture Traces Vertical black and white aerial photographs are available for the western part of Rapi d City at an approximate scale of 1:24,000. Early photos, taken in 1938, were utilized for the most part, supplemented with more recent photos. Presently much of the western part of Rapid City is urbanized, and hence older photographs are much more useful in this endeavor. Figure 4 shows a typical fracture trace from a 1952 air photo. Figure 5 is a map showing the location of fracture traces as interpreted from the air photos. The locations of the eight Madison wells are also shown.

5 Proceedings of the South Dakota Academy of Science, Vol. 87 (2008) 265 Figure 4. June 2, 1952, aerial photograph of area along Sheridan Lake Road in Rapid City. The arrow shows the location of a prominent fracture trace.

$Map of western Rapid City area showing 29 fracture traces (solid lines) and location of 8 municipal Madison wells.$

6 266 Proceedings of the South Dakota Academy of Science, Vol. 87 (2008) Figure 5. Map of western Rapid City area showing 29 fracture traces (solid lines) and location of 8 municipal Madison wells. The dashed line is a fault mapped by Cattermole (1969).

7 Proceedings of the South Dakota Academy of Science, Vol. 87 (2008) 267 RESULTS Table 2 summarizes the results of this study. The specific capacity data for the nine municipal wells are divided into three groups. Four wells were not located on a fracture trace (Group #1); their specific capacity averaged 7.03 gpm/ft. Three wells were located on a fracture trace (Group #2); their specific capacity averaged 4.32 gpm/ft. [Note: the three wells in Group #2 were not all precisely on a fracture trace, but were very close or aligned along the strike of a fracture trace.] One well (RC-6) was located on the intersection of two fracture traces (Group #3); its specific capacity was 1.84 gpm/ft. A cursory examination of Table 2 indicates there is not much difference between the specific capacity of the three groups. In fact, the two best wells, RC-9 and RC-12, are not located on a fracture trace. A t-test shows no difference (at the 95% level of significance) between Group # 1 and #2. Thus the wells on fracture traces in this area are no more productive than wells not on a fracture traces. Table 2. Specific capacity (gpm/ft) for three groups of wells. Group #1 (Wells not on fracture trace) Group #2 (Wells on fracture trace) Group #3 (Well on intersection of two fracture traces) 0.29 (RC-7) 5.45 (RC-5) 1.84 (RC-6) 1.73 (RC-8) 5.11 (RC-10) 17.1 (RC-12) 2.64 (RC-11) 9.0 (RC-9) x 7.03 gpm/ft 4.32 gpm/ft 1.84 gpm/ft DISCUSSION This study of the western Rapid City area seems to indicate that fracture trace analysis is not useful for locating a productive Madison well. However, the limestone aquifer in this area is quite deep. In general it is overlain by about 500 ft of the Minnelusa Formation, 100 ft of Opeche shale, 40 ft of Minnekhata Limestone, and hundreds of feet of Spearfish Formation, a red shale. The wells farthest to the east (RC-7 and RC-8) have approximately 2,000 ft of sedimentary rocks on top of the Madison Limestone. The overlying sedimentary rocks probably mask any fractures that may exist in the Madison Limestone. I recommended that future studies of Madison Limestone well yields and fracture traces be conducted on the actual outcrops of the Madison Limestone.

8 268 Proceedings of the South Dakota Academy of Science, Vol. 87 (2008) LITERATURE CITED Anderson, M.T., D.G. Driscoll, and J.E. Williamson Ground water and surface-water interactions along Rapid Creek near Rapid City, South Dakota. U.S. Geological Survey, Water-Resources Investigations Report Burns and McDonnell Water facilities planning and Jackson Springs water treatment facility reconstruction. City of Rapid City, Project No. WO4-1425/CIP 50570, Phase 1B- Facility Plan, Burns and McDonnell Engineering Company. Cattermole, J.M Geologic map of the Rapid City West quadrangle, Pennington County, South Dakota. U.S. Geological Survey, Map GQ-828. Greene, E.A Characterizing recharge to wells in carbonate aquifers using environmentally and artificially recharged tracers. U.S. Geological Survey, Water-Resources Investigations Report C, p Greene, E.A., and P.H. Rahn Localized anisotropic transmissivity in a karst aquifer. Ground Water 33: Lattman, L.H., and R.R. Parizek Relationship between fracture traces and the occurrence of ground water in carbonate rocks. Journal of Hydrology 2: Miller, S.L Influence of geologic structures and stratigraphy on groundwater flow paths in the karstic Madison aquifer in the Rapid City area of South Dakota. PhD Thesis, South Dakota School of Mines and Technology, 191 p. Rahn, P.H Geologic map of the Rockerville Quadrangle, Pennington County, South Dakota. Geological Society of America, Map and Chart Series, MCH062, 16 p. Rahn, P.H Permeability of the Madison aquifer in the Black Hills area. Final Report to the Groundwater Research and Public Education Program, South Dakota Department of Environment and Natural Resources, Vermillion, South Dakota, 131 p. Rahn, P.H Chemical weathering and land denudation of Paleozoic carbonate rocks in the Black Hills, South Dakota and Wyoming: Proceedings, South Dakota Academy of Science 84: Rahn, P.H. 2006, Nitrate in Rapid City s water supply. Proceedings, South Dakota Academy of Science 85: Rahn, P.H Future water supplies for Rapid City. Proceedings South Dakota Academy of Science 86: Siddiqui, S.H., and R.R. Parizek Hydrogeologic factors influencing well yields in folded and faulted carbonate rocks in central Pennsylvania. Water Resources Research 7:

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