Geophysical response of filled sinkholes, soil pipes and associated bedrock fractures in thinly mantled karst, east-central Illinois

Transcription

1 Geophysical response of filled sinkholes, soil pipes and associated bedrock fractures in thinly mantled karst, east-central Illinois Shawkat Ahmed Æ Philip J. Carpenter Abstract Karst aquifers are often protected by a thin mantle of unconsolidated sediment. Soil pipes and sinkholes may breach this natural protective barrier and open pathways for contaminants to quickly reach bedrock aquifers. Geophysical surveys offer a quick and noninvasive way to identify these features; such surveys may also be sequenced to reveal increasing detail in critical areas. At a study site in east-central Illinois, electromagnetic (EM) surveys mapped high conductivity anomalies over filled sinkholes and soil pipes that penetrated the unconsolidated cover. Two-dimensional inverted resistivity sections, made over these anomalies, depict filled sinkholes and soil pipes as conductive zones above deeply weathered bedrock fractures. Borings verified the geophysical models and suggest high conductivities associated with the filled sinkholes are the result of enhanced moisture near active soil pipes. EM surveys also identified conductive zones in the overburden above a probable bedrock fracture linking sinkhole areas 0.5 km apart. Resistivity and EM methods, used in a phased and sequential manner, thus proved useful in mapping filled sinkholes and in delineating the vertical and lateral connections between soil pipes and hydraulically active bedrock fractures. Keywords Sinkhole Æ Geophysics Æ Soil piping Æ Macropores Æ Karst Æ Illinois, USA Received: 15 July 2002 / Accepted: 10 March 2003 Published online: 22 May 2003 ª Springer-Verlag 2003 S. Ahmed Æ P. J. Carpenter (&) Department of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115, USA phil@geol.niu.edu Tel.: Fax: Introduction Recharge to covered karst aquifers largely occurs through openings such as macropores and soil pipes that connect bedrock fractures to the earth s surface. Such recharge often is rapid and downward percolating water may contain human and animal wastes, pesticides, and urban runoff that may contaminate the aquifers. Landfills, unregulated dump sites, tailing piles, waste lagoons and septic systems have often been placed within or above sinkholes containing soil pipes linked to bedrock fractures (Aley 1972). Major karst aquifer contamination in the US, southern France, southern Poland, western Turkey and eastern China have resulted from these practices (Vaute and others 1997; Kacaroglu and Gunay 1997; Zhu and others 1997; Rozkowski 1998; Drew and Hotzl 1999; Zhou and others 2000; Peterson and others 2002). A particularly good example of contaminant migration along soil pipes is described by Krothe and others (1999). In this case, polychlorinated biphenyl compounds (PCBs) from a Bloomington, Indiana landfill migrated downward along soil pipes from the base of a sinkhole to a bedrock fracture. Hydraulic activity at the bedrock-soil interface may also initiate subsidence and collapse of soils. Therefore, it is also important from an engineering standpoint to locate and characterize soil pipes, filled sinkholes, and buried bedrock fractures, as they may reactivate in the course of time. This paper explores the usefulness and accuracy of resistivity and shallow electromagnetic (EM) surveys for characterizing karstic recharge features at a field site in east-central Illinois. The specific objective of this investigation is to test the ability of EM and resistivity methods to map filled sinkholes associated with soil pipes connecting hydraulically active bedrock fractures to the surface. These methods are used in a phased and sequential approach to reveal increasing detail about the subsurface in critical areas. The study site is typical of many areas around the world where active karst systems are mantled by thin soil cover. In these settings, noninvasive geophysical techniques have the potential to detect karst aquifer recharge features such as soil pipes, fractures or filled sinkholes without disturbing the land or contaminating the karst system. Surface geophysical surveys do not eliminate the need for boreholes, but such DOI /s Environmental Geology (2003) 44:

2 Fig. 1 Map of the Perry Farm Park showing Sinkhole Areas 1 and 2 (SA1 and SA2) and the location of geophysical surveys. Sinkhole symbols indicate the approximate location of filled or partially filled sinkholes and not their size. Inset shows locations of geophysical surveys, borings and soil samples for SA1 surveys may be used to place borings and wells in optimal locations and to extend information from existing boreholes to more fully characterize the three-dimensional nature of these systems. The resolution of these geophysical methods have improved in recent years to the point where contaminated zones a few meters wide can be identified along conduits at depths of up to 30 m (e.g., Ogilvy and others 2002). Relatively few geophysical studies have specifically examined the response of features such as soil pipes or filled sinkholes. Seismic refraction surveys, which can detect both sharp and gradational changes in velocity within soil, or between rock and soil, have mainly focused on mapping the relief of the bedrock surface, or on identifying bedrock collapse structures (e.g., Steeples and others 1986; Odum and others 1998; Thompson and others 2001). Electrical methods have also been extensively used in shallow karst investigations. Cook and Van Nostrand (1954) investigated apparent resistivity profiles over filled sinks with various types of resistivity arrays, primarily for mineral exploration. Benson and La Fountain (1984) related diffractions in the upper 4 m of a ground-penetrating radar (GPR) record to piping that channeled infiltration to a major cavity at a depth of m. Stewart and Parker (1992) found GPR surveys useful for locating potential karst drains and subsidence features in a covered karst aquifer system in southern Florida. Yuhr and others (1993) employed EM surveys over a paleokarst collapse feature in west Texas to define the extent of the doline, as well as the presence of clay-filled joints within the depression. Panno and others (1994) used resistivity profiling to identify sinkhole-prone areas in southern Illinois through correlation of high resistivity zones with air-filled cavities in the overburden that had formed over a bedrock conduit. Roark (1997) used GPR to delineate the internal structure of sinkholes in Missouri. Carpenter and others (1998) used resistivity, GPR and EM surveys to identify filled sinkholes that were potential recharge features near Oak Ridge, Tennessee. Buried sinkholes were also mapped using ground-penetrating radar in Egypt and Jordan respectively by El-Behiry and Hanafy (2000) and Batayneh and others (2002). Recently a number of papers have used two-dimensional (2D) resistivity imaging to examine sinkholes and the underlying weathered, or epikarstal, bedrock. For example, tomographic imaging of fractures at the bedrock surface below sinkholes is critically examined in Zhou and others (2000). Labuda and Baxter (2001) discuss the pros and cons of using 2D and 3D resistivity techniques to delineate a variety of karst features, including buried or filled sinkholes. Geological setting The site for this study is the Perry Farm Park in Bourbonnais, east-central Illinois. This site was chosen on the basis of easy access, bedrock outcrops (rare in northern and eastern Illinois), thin soil cover and relative absence of cultural interference. Karstic Silurian Racine Formation dolomite subcrops within a few meters of the surface beneath the park. Karst features in the dolomite include caves and solutionally enlarged fractures. The Perry Farm Park, shown in Fig. 1, covers approximately 69 hectares. Dolomite bedrock is exposed along Bourbonnais Creek in the northwest part of the park where it forms 4 8 m high cliffs in a narrow gorge. Fracture trends measured in the gorge show dominant orientations of approximately N60 W (North 60 West), N80E and N30E (Ahmed and others 1999). 706 Environmental Geology (2003) 44:

3 Approximately 1 5 m of glacial till, outwash deposits and wind-blown loess cover bedrock across the park (Paschke 1979; Byers 1995; Frankie 1997). Soil pipes form within these unconsolidated sediments above hydraulically active bedrock fractures. The pipes may extend to the surface with apertures of 3 10 cm, or they may enlarge underground until they collapse, forming cover-collapse sinkholes. In the eastern part of the park, which is the focus of this paper, about 4.5 m of unconsolidated sediment covers bedrock, and sinkholes have formed over the soil pipes. Over the years, these sinkholes have been filled with sediment and other debris and presently exhibit little or no topographic expression. However, new soil pipes are forming in the debris and sediment filling these sinkholes. Sediment is only 1 2 m thick in the western part of the park and soil pipes generally extend to the surface without sinkhole development. Geophysical surveys were concentrated over filled and partially filled sinkholes in the eastern portion of the Perry Farm Park, designated Sinkhole Areas 1 and 2 (SA1 and SA2) in Fig. 1. At least three sinkholes previously existed in SA1 but were filled by sediment from former farming operations and disposal of compost. These filled sinkholes were located by interviewing persons who previously worked at the Perry Farm before it was converted into a park. Anecdotal evidence of subsidence, subterranean sounds and emerging cool air suggests reactivated soil piping in the sinkhole fill with pipes connecting the surface with openings in the bedrock, even though these pipes were not visible at the time of the geophysical surveys. Sinkholes in SA2 are only partly filled and, in some cases, soil pipes connecting them to bedrock fractures are visible. Photos of SA1, SA2 and a soil pipe are shown in Fig. 2. Hydrogeological setting Silurian dolomite bedrock is the primary aquifer in Kankakee County (Stuart and others 1990). Most groundwater moves along irregularly distributed joints, fractures, solution cavities, and bedding plane openings within the upper 30 m of the bedrock. Surface waters percolate to the bedrock primarily through soil pipes, fractures, and other macropores. Figure 1 shows drainage divides in the Perry Farm Park. Drainage in the northern part of Perry Farm Park is dominated by Bourbonnais Creek, which is structurally controlled. Most of the rest of the park is drained by sheet flow or subsurface flow to the west and southwest (Byers 1995). Several intermittent drainages discharge water into the Kankakee River along the west and southwest margins of the park. The elevation of the water table beneath the Perry Farm Park is poorly known. Seismic refraction surveys in SA1 by Ahmed (2002) suggest the unconsolidated overburden is generally unsaturated, although small areas of perched water may exist within the unconsolidated overburden. Piezometers installed in SA1 and SA2 Fig. 2 a SA1, looking to the east. Sinkholes are filled in this area. b SA2 showing partially-collapsed pits and sinkholes. c Soil pipe near SA2 during this study (discussed in more detail below) suggest the static water table lies within the bedrock, approximately at the level of Bourbonnais Creek, a few meters above the Kankakee River, which is the base level for this karst system. Environmental Geology (2003) 44:

4 Geophysical data collection, processing and interpretation Electrical resistivity soundings Several vertical electrical soundings (VES) were used to estimate the thickness of the unconsolidated overburden and its electrical properties in SA1 and SA2. Four Schlumberger VES and three Wenner VES were made across SA1 and one Schlumberger VES was made in SA2. Most of the soundings in SA1 were inverted for 3-layer models with a thin high-resistivity topsoil layer ( ohm-m and about 0.3 m thick), overlying a lower resistivity combined loess and till layer (29 40 ohm-m and about m thick), overlying weathered bedrock (420 1,110 ohm-m). The sounding made in SA2 was also modeled with three geoelectrical layers: in this case a relatively conductive topsoil layer (18 ohm-m) about 1.4 m thick is underlain by a 9.5 m thick silty and clayey till unit (resistivity about 34 ohm-m), which in turn, is underlain by bedrock with a resistivity of about 2,000 ohm-m. Borings made in these areas after the soundings found excellent agreement between the predicted and actual thickness of these geoelectrical layers. The depth of the upper interface (topsoil-loess/till) and the lower interface (loess/till-bedrock) predicted from the resistivity soundings had errors of less than 15% when compared to the boring logs, in most cases. Electromagnetic conductivity surveys EM and GPR surveys are fast, flexible and generally accurate in karst investigations. In most areas, these methods would probably be the first choice for investigators seeking to map filled sinkholes. GPR surveys with 50 and 100 MHz antennas in area SA1, however, were disappointing. Clayey sediments filling the sinkholes strongly attenuated the radar signals and trees in SA1 produced a myriad of air-wave reflections that interfered with subsurface signals. However, EM surveys with a Geonics EM31 conductivity unit (coil spacing 3.77 m) worked well at the site. EM measurements were made with the EM31 at waist level (about 1 m above the ground) and computations for response depth over the relatively conductive soils in the eastern part of the park suggest that about 70% of the vertical dipole (VD) response is from the upper 5 m and 75% of the horizontal dipole (HD) response is from the upper 2 m. The VD conductivity thus reflects the unconsolidated cover and uppermost weathered bedrock, whereas the HD conductivity only represents the upper unconsolidated sediment. In- phase secondary magnetic fields were negligible, suggesting buried metals are probably not present beneath the surveyed areas. The VD and HD conductivities represent a lateral area roughly equivalent to the intercoil spacing (McNeill 1980), which in this case is about 4 m. In SA1 most EM conductivity measurements were collected on a 4m x 4m grid. Essentially all the EM data in SA1 were collected on June 17 and June 22, This was a dry period with no measurable rainfall. An EM calibration line in the eastern part of SA1 was repeated at the beginning of each field day, and through most of the summer of 1999, to insure the EM31 was calibrated properly and to monitor changes in ground conductivity over time due to climatic variations. Stations were spaced 2 and 4 m along this calibration line. Two reconnaissance EM conductivity profiles (ECP1 and ECP2) were also made between SA1 and SA2, as shown in Fig. 1. These were collected at the end of July Rainfall totaling 5.7 cm occurred between the collection of the EM data in SA1 and collection of EM data along ECP1 and ECP2, so anomalous zones along these lines may, in part, be due to rainfall or shallow moisture. Although actual conductivity values for ECP1 and ECP2 are not plotted in this paper, anomalous zones along these lines will be discussed in a later section. Contoured EM conductivity maps were plotted for both vertical dipole (VD) and horizontal dipole (HD) coil orientations to differentiate near-surface from deeper response, as shown in Figs. 3 and 4. Probably the most prominent anomalies on the VD map in Fig. 3 are the linear low conductivity zones trending north-south and east-west, possibly due to roots and air-filled macropores associated with vegetation along former fence lines, or due to soil compacted from vehicle travel along the fence lines. Another low conductivity anomaly running EW between y=0 10 m and x=0 to )84 m is a dirt road containing sandy, compacted soil. High conductivity anomalies occur over and near the filled sinkholes (x=40 80 m, y=0 20 m). Apparent conductivities over the sinkholes are generally above 20 ms/m in both the HD and VD; conductivity values away from the filled sinkholes drop by 2 4 ms/m. The filled sinkholes in SA1 also appear to lie along an approximate N80E trend, possibly coincident with a deeper bedrock fracture. Another linear zone of high conductivity trends approximately N15W and may represent a north-northwest trending fracture below the soil cover. White arrows indicate these trends in Figs. 3 and 4. Soil borings Borings and soil samples were collected over the high conductivity anomalies defined by the EM surveys, and adjacent non-anomalous areas, to determine the cause of the anomalies. Two-dimensional (2D) resistivity sections from dipole-dipole surveys were also used to locate the borings (the dipole-dipole lines were collected before the borings but are discussed in the next section so that data from the boring logs may be plotted on the resistivity sections). The first set of nine soil borings were made to a depth of 2 m using a hand auger within SA1 (designated A-I in the inset in Fig. 1). Samples were collected at five depths (0, 0.5, 1.0, 1.5 and 2.0 m) at six locations (A-E) along the EM calibration line for water content determinations and at two depths (1.0 and 1.5 m) for grain size determinations. These depths correspond to the main response depth for the HD orientation, which showed somewhat better defined conductivity anomalies over the filled sinkholes than the VD orientation. Three other borings (G, H, and I) were also made within SA1 along a north-south profile 16 m to the west. boring G was made 12 m north of the reported location of a filled sinkhole, H 708 Environmental Geology (2003) 44:

5 Fig. 3 Vertical dipole apparent ground conductivity in SA1 measured with the EM31. Locations of a road, tree-lines, dipole dipole resistivity profiles and the EM calibration line are also shown, along with the approximate location of filled sinkholes (cross-hatched white circles). Arrows indicate possible alignment of high conductivity anomalies Fig. 4 Horizontal dipole apparent ground conductivity in SA1 measured with the EM31. Arrows indicate possible alignment of high conductivity anomalies. Approximate locations of filled sinkholes are shown as cross-hatched white circles was made 4 m to the south of it and boring I was made directly over the filled sinkhole. Results of the water content and grain size analysis for all samples are shown in Tables 1 and 2. Deeper soil cores, extending from the surface to the top of fresh bedrock, were obtained at 8 locations in the park, designated borings 0 through 7 (boring 00 was substituted for boring 6). These were continuous direct push core samples obtained using an AMS Powerprobe Model 9600 with a Geoprobe Macro-Core sampler. Most of the eight cores were obtained over EM anomalies and along the dipole dipole resistivity lines in SA1. One core was obtained in SA2. Grain size and water content analyses were not performed on these cores as they were compacted by vibration during penetration and subsequent storage, and also exhibited minor soil mixing. Three soil layers (topsoil, loess [mainly silt] and till) were clearly evident in the cores and these overlie weathered dolomite bedrock across the sampled portions of SA1. In SA2 (boring 7) approximately 3 m of gravelly sediments were encountered below the till layer, just above the weathered dolomite bedrock. These sediments may represent glacial outwash, or simply bedrock regolith. The boundary between weathered and fresh bedrock is quite distinct in most places. Weathered bedrock is highly altered, contains many fractures and voids, and tends to be soft, friable and even powdery. Fresh bedrock is hard crystalline dolomite without iron oxide stains, with no or tight fractures, and no voids. It broke into hard flat chips under the Power Probe. The cores show most subhorizontal fractures and possible soil pipes in the m depth range. Soil pipes in the cores were identified from iron stains along fractures, fractures or voids containing topsoil or cobbles, or occasionally from lost core. Vertical soil pipes were not identified in the cores. Soil pipes appear to be concentrated at the boundary between the silty layer and the underlying till, perhaps due to the lower permeability of the till. The depth of the top of weathered bedrock averaged 4.4 m in SA1 and was remarkably constant across this part of the park. However, bedrock below the filled sinkholes was weathered much more intensely than bedrock away from the sinkholes. A clay-filled cave probably occurs within the weathered bedrock between borings 00 and 1, as these Environmental Geology (2003) 44:

6 Table 1 Gravimetric water contents (%) along the EM calibration line for various depths Samples 0.0 m 0.5 m 1.0 m 1.5 m 2.0 m A B C D E F Table 2 Grain-size distribution for samples collected in SA1 at various depths Samples Sand (%) Silt (%) Clay (%) A (1.00 m) B (1.00 m) C (1.00 m) D (1.00 m) E (1.00 m) F (1.00 m) A (1.50 m) B (1.50 m) C (1.50 m) D (1.50 m) E (1.50 m) F (1.50 m) G (0.65 m) G (1.00 m) G (1.50 m) H (1.00 m) H (1.75 m) H (2.00 m) I (1.00 m) I (1.40 m) I (2.00 m) borings intersected the clayey cave-fill as well as weathered dolomite. This cave extends from a depth of m and possibly extends at least 20 m to the southwest, where it thins to about a 0.3-m radius filled with clayey sediment (as encountered by borings 0 and 2). Other bedrock fracture zones below the filled sinkholes were identified by iron-oxide stains (indicating water movement) or fillings of brownish-gray or greenish-gray clay. In some cases bedrock fractures also contained topsoil. Piezometers were installed in borings 1, 2, 3, 4, 5, and 7 to measure water levels near the bedrock surface. A 0.3 m long screen with a sand pack was placed just above the bottom of the holes and water levels were periodically measured. Water was only encountered in borings 2, 4, 5 and 7. The shallowest water levels were 3 4 m below the surface, measured in borings 4 and 5. These piezometers were dry during dry weather, however, suggesting this shallow water table may be perched. More constant water levels were measured throughout the year about 10 m below the surface at borings 2 and 7, well within the dolomite bedrock. This bedrock water level is similar to water levels in Bourbonnais Creek, a few meters above the Kankakee River level, and may represent the static water table below the park. Dipole dipole (2D) resistivity profiles The positions of two dipole-dipole resistivity profiles, DD1 and DD2, are shown in Figs. 1 and 3. These were collected within SA1 to obtain detailed resistivity crosssections through the line of EM conductivity anomalies and to test two-dimensional (2D) imaging against the ground truth provided by borings. The surveys were performed with an Advanced Geoscience, Inc. Sting R1 / Swift earth resistivity meter and automated multielectrode switching system. The dipole-dipole array was chosen based on previous work that showed good resolution of epikarstal fractures and caves with this configuration (Roth and others 1999; Zhou and others 2000; Labuda and Baxter 2001). A total of 28 electrode positions with a dipole spacing of 3 m were used. The raw apparent resistivity dipole-dipole data were inverted and interpreted using the rapid two-dimensional (2D) resistivity inversion least squares method (program RES2DINV, ver. 3.3, Loke 1998) to acquire a 2D earth resistivity inversion solution. The inversion program produces a 2D cross-section model with a grid of resistivity values. The program automatically determines the distribution of the resistivity cells and their sizes so that the number of cells does not exceed the number of data points. Cell positions are related to data point distributions. The output of the inversion is in the form of a resistivity-depth model consisting of rectangular cells with estimated true resistivity values. Resistivity values are then commonly plotted at the center of each cell and contoured. The end product is a contour map of inverted earth resistivity along a 2D vertical slice, i.e., a 2D tomographic image. These 2D images may still contain distortions and artifacts of the modeling process. Values near the base of the section, for instance, often incorporate 3D effects or offline information projected onto the section due to lateral current spreading at wide electrode spacings. Anomalies near the edge of models are also suspect since there are usually few data points available for the inversion in these areas. Different numbers of iterations with different inversion parameters are necessary to insure an anomaly near the edge is not a processing artifact. Some of the inversion parameters that may be changed to test a solution s robustness include damping factor, flatness filters and the initial model for the inversion (Loke 1998). Mesh sizes may also be changed, within certain constraints. DD1 (Fig. 5) was made in a northwest-southeast orientation, nearly perpendicular to the strike of the anomalous EM conductivity zone in SA1. Historical information (Vic Johnson, personal communication,1999) suggests the center of a sinkhole, or sinkhole complex (now filled), at about x = 42 m along this line (42 m from the northwest end). In DD1 the buried sinkhole appears as a conductive zone (darkest shades) above a conductive trough in the bedrock, which may indicate an enlarged fracture and its alteration halo. Borings 0 and 3 encountered relatively fresh bedrock at depths of 6.6 and 4.7 m, respectively. This corresponds approximately to the 400 ohm-m resistivity level on the 2D images. Boring 0 also encountered soil 710 Environmental Geology (2003) 44:

7 Fig. 5 Inverted dipole dipole resistivity profile DD1 across a filled sinkhole in SA1. The approximate extent of the sinkhole, the interpreted bedrock surface, a possible clay-filled fracture and soil pipes encountered by borings are also indicated. Boring numbers (BH #) are shown above each log Fig. 6 Inverted dipole dipole resistivity profile DD2. The approximate extent of the sinkhole, the interpreted bedrock surface, fractures encountered by the borings and soil pipes are also indicated. Boring numbers are shown below each log and lithologic symbols are shown in Fig. 5 pipes or fractures at depths of 1.6 and 3.8 m, as well as a small clay-filled void at about 6.3 m depth. This void could be part of the wider fracture encountered by borings 00 and 1 approximately 15 m to the east that was discussed in the previous section. The projection of this clay-filled fracture along strike is plotted on Fig. 5. Resistivity profile DD2, shown in Fig. 6, was made eastwest, subparallel to the trend of the filled sinkholes in SA1. This profile crosses the filled sinkhole at about x=27 31 m (i.e., from the west end DD2). Borings 0 and 2 are along this line as well. These borings encountered soil pipes at depths of m in the unconsolidated overburden. The soil piping and filled sinkhole zone is highly conductive (very dark shades) on the 2D section. A somewhat thicker zone of weathered bedrock, encountered by boring 2, appears on the 2D section as a slight thickening of lowresistivity material between x=21 36 m. A high resistivity zone indicated by the bulls eye between x=36 and 48 m may indicate more solid or fresher dolomite bedrock in that area. Borings suggest a bedrock fracture zone is present between x=24 and 30 m at 5 7 m depth, in addition to the fractures encountered by boring 0 discussed above. This fracture zone is shown as a dashed line on Fig. 6. The thickening of shallow conductive material on the western end of this profile could be caused by another filled sinkhole or soil pipe near the west end of DD2, at about x=5 m. It is interesting to compare the 2D resistivity sections at their intersection point. Resistivity values are between 30 and 80 ohm-m for the unconsolidated cover representing sinkhole filling materials for both sections. Below the top of bedrock at about 4.5 m, however, resistivity values are about 200 ohm-m lower along line DD1 than they are along DD2. This could be the result of anisotropy. The fracture zone and weathered bedrock below the sinkhole appears much more prominently on DD1. It could be easily missed in DD2, which appears to just show a gently undulating bedrock surface. The high resistivity bulls eye on DD2, centered at about x=42 m at a depth of 10 m, probably represents the same high resistivity zone centered at about 10 m depth at x=66 m along DD1. It is not known whether this high resistivity zone is directly below the lines or represents the projection of an offline high resistivity zone representing relatively unweathered bedrock. Resistivity values in the 2D resistivity profiles are consistent with conductivity values measured with the EM31 across the same areas. For example, an average VD conductivity of 24 ms/m recorded over the filled sinkholes in SA1 corresponds to a resistivity of 42 ohm-m. A typical VD conductivity of 21 ms/m over non-sinkhole areas near SA1 corresponds to a resistivity of 48 ohm-m. These resistivity values appear in the upper 2 3 m of the 2D inverted resistivity profiles DD1 and DD2. Environmental Geology (2003) 44:

8 Discussion Source of conductivity anomalies over filled sinkholes and soil pipes Enhanced soil moisture in the vicinity of active soil pipes within the filled sinkholes may be the primary cause of the high conductivity values recorded over these features. Figure 7a shows EM conductivity on July 6, 2000 for both the HD and VD orientations along the EM calibration line in SA1; Fig. 7b shows the average moisture content in the upper 2 m along the EM calibration line on the same date. The highest moisture contents occur over the filled sinkholes, which also exhibit the highest conductivity values. When moisture content is plotted as a function of apparent conductivity the regression coefficient r 2 for the fit with the HD data is 0.60 and for the VD data is 0.46, suggesting that water content correlates reasonably well with apparent ground conductivity measured in the HD orientation. The VD regression coefficient is somewhat lower probably due to the influence of bedrock on the VD conductivity values. Similar relationships have been established between moisture content and EM conductivity for similar types of sediments in other settings (Kachanoski and others 1988). Rhoades and others (1976) showed that changes in volumetric water content of 2 5% may result in conductivity changes on the order of 1 2 ms/m, depending on the conductivity of the soil water. Other observations support the hypothesis that higher electrical conductivity over the sinkholes is due primarily to elevated soil moisture. On June 22, 1999, the effect of rainfall on EM measurements was observed by comparing conductivity values along the EM calibration line before and after a thunderstorm that produced only about 0.5 mm of precipitation (this occurred after data for the EM grid in SA1 was collected). Both HD and VD conductivity values showed increases of about 2 ms/m after the storm. Also, conductivities periodically measured along the EM calibration line in SA1 generally show a decrease in apparent conductivity away from the sinkholes and a conductivity increase over the filled sinkholes as the summer of 1999 progresses. This is especially evident in the VD conductivity data, as shown in Fig. 8. Conductivity in the non-sinkhole area drops from about ms/m between June 17, 1999 and August 6, 1999, whereas over the sinkholes apparent conductivity increases from about 23 ms/m to over 27 ms/m during this same time period. Data collected on June 29 does not follow this pattern for some reason. The uniformly lower conductivity recorded on June 29 may reflect a 2 mm rainfall immediately preceding the survey or a calibration problem with the EM31 on that date. Conductivity changes of 2 4 ms/m are very small, and in many surveys would be regarded as fluctuations due to noise. However, in this study the conductivity increases observed after the June 22 thunderstorm, the consistent trend of anomalies in the contoured conductivity data in Figs. 3 and 4, the comparison of conductivity and water content in Fig. 7 and the repeatability of the conductivity pattern in Fig. 8 all support the significance of these small apparent conductivity variations. Thus moisture appears to be influencing conductivity values, especially over the filled sinkholes. Moisture in the unconsolidated cover may be flowing toward the sinkholes in SA1 during the summer months, increasing electrical conductivity of sediments within and around the sinkholes. Another possibility is that moisture is condensing around soil pipes within the sediment-filled sinkholes, as these pipes convey cooler air to the surface. Alternative hypotheses were considered as well. The summer of 1999 was very hot and dry. In some climates drying of the soil under these conditions precipitates conductive salts. There was no visual evidence of precipitation of salts within the sinkhole soils, however. It is also unlikely such salts would be precipitated from evaporating soil water since Paschke (1979) reports extremely low salinities for soil waters extracted from the Whalan loam, the soil series mapped at the surface in SA1. An increase in clay or silt content could also cause increased conductivity over the sinkholes. Grain sizes from the upper 2 m in SA1 are shown in Table 2. When these data are analyzed, however, no consistent pattern emerges as to differences in grain size between sinkhole and nonsinkhole areas for the upper 2 m. For example, at a depth of 1 m, samples taken over the easternmost filled sinkhole (E and F) had the following average grain size distribution: 18.4±0.6% sand, 49.0±0.5% silt and 32.6±0.1% clay. Uncertainties represent 1 standard deviation from the mean. Samples collected away from the sinkhole (A D) at 1 m depth exhibited an average grain size distribution of 13.9±1.3% sand, 51.3±2.3% silt and 34.8±1.1% clay. Therefore, at 1-m depth the sinkhole sediments contain more sand and less clay. However, samples collected at 1.5-m depth show a somewhat different distribution. Sinkhole samples (E and F) have 13.8±0.4% sand, 49.6±1.1% silt and 36.7±1.3% clay whereas nonsinkhole samples contain 12.4±1.2% sand, 54.7±1.6% silt and 32.9±1.1% clay. Therefore, the deeper sinkhole sediment samples have less sand and more clay. Samples collected along G-I are similarly complex. Overall, though, the sinkhole sample I appears to have more sand and less silt than the nonsinkhole samples. Another way to look at these grain size distributions is to examine the variation in grain sizes in nonsinkhole samples and compare this to sinkhole samples. For example sand varies from % in nonsinkhole samples A-D at a depth of 1.0 m, silt varies from % and clay varies %. Average values for sinkholes E and F easily fall within these ranges. Grain size values for sinkhole sample I are more like nonsinkhole samples A-D, than the sinkhole samples E and F. The only sample with an unusually high sand content is sample F, taken at a depth of 1.0 m over a filled sinkhole. Samples collected at location F at 1.5 m, however, are not distinctive. In conclusion, it appears that any grain size differences between sinkhole and nonsinkhole areas are slight and are masked by the overall lateral heterogeneity in grain sizes. 712 Environmental Geology (2003) 44:

9 Fig. 7 a Apparent conductivities measured on July 6, 2000, along the EM calibration line in SA1. b Average water contents in the upper 2 m from soil samples collected along the EM calibration line on July 6, 2000 Connection of filled sinkholes to bedrock fractures Two-dimensional resistivity surveys show low resistivity zones associated with weathered bedrock fractures directly below the filled sinkholes. Borings 0 and 1 penetrated about 4.5 m of sinkhole fill and underlying sediment Environmental Geology (2003) 44:

10 Fig. 8 Vertical dipole EM conductivity profiles along the EM calibration line for various dates during the summer of 1999 Fig. 9 Map of the Perry Farm Park showing a possible hydraulic connection or zone of bedrock fracturing between SA1 and SA2, based on high conductivity anomalies (light gray segments) along reconnaissance EM profiles, the distribution of soil pipes and sinkhole locations containing subhorizontal fractures and possible soil pipes. Below the bedrock surface the borings encountered several more meters of weathered bedrock containing cavities, solutionally enlarged subhorizontal fractures containing reworked topsoil and loess, and alteration stains from percolating water. Vertical soil pipes were not encountered. However, the presence of topsoil and loess in bedrock fractures suggests the subhorizontal soil pipes and fractures must be linked by vertical conduits below the sinkholes. Thus, both moisture and soil material may be continually piped down into the bedrock from the sinkholes. Connections between sinkhole areas Two EM-31 conductivity reconnaissance profiles, ECP1 and ECP2, were made in generally north-south directions between SA1 and SA2 to identify possible high-conductivity anomalies similar to those observed over filled sinkholes in SA1. Measurements were made every 4 m along these profiles. Anomalously high VD and HD conductivity values were recorded along the following segments, measured from the south end of ECP1: 20 28, 64 76, and m. A conductivity high, possibly representing a filled sinkhole or soil pipe, occurs m from the south end of ECP2. Anomalous high conductivity segments of these lines are shown in light gray in Fig. 9, along with the distribution of soil pipes observed at the surface and filled sinkholes. The conductivity profile shapes for the VD and HD are very similar, suggesting the high conductivity materials are fairly close to the surface in the unconsolidated sediment cover. The anomalies could represent filled sinkholes, as in SA1, or soil pipes above a deeper bedrock fracture that would probably be within the dashed area shown in Fig. 9. Alternatively, these high conductivity areas could represent perched water within the soil cover since profiles ECP1 and ECP2 were collected only a few days after a 5.7 cm rainfall. 714 Environmental Geology (2003) 44:

11 Conclusions Geophysical surveys were employed in a phased and sequential approach at the Perry Farm Park to reveal increasing detail about the subsurface. In the first stage resistivity soundings were used to characterize the vertical resistivity structure and EM surveys were used to quickly identify areas of anomalously high conductivity over the presumed location of filled sinkholes. The second stage of the work included using high-resolution dipole-dipole resistivity surveys to obtain 2D resistivity images of the subsurface over these EM anomalies. Finally, borings were made over the EM anomalies along the 2D resistivity lines to verify the geophysical interpretations and to obtain even more detailed subsurface information below the filled sinkholes and adjacent areas. The EM and resistivity surveys revealed high ground conductivity over filled sinkholes containing active soil pipes. These high conductivity anomalies are roughly aligned, and may be the shallow manifestations of deeper hydraulic activity along enlarged bedrock fractures. Enhanced soil moisture is probably the main reason for high apparent conductivity recorded over these features. Closer examination of these anomalous areas using 2D inverted resistivity sections and borings show that soil pipes exist in the sinkhole fill and below the filled sinkholes and that enlarged, altered bedrock fractures lie directly below the sinkholes. A clay-filled fracture or cave was also encountered by one boring below a filled sinkhole. Some fractures in the weathered bedrock contained topsoil and other shallow sediments, indicating active conduits extending to the surface. In conclusion, EM and resistivity surveys were successful in noninvasively identifying filled sinkholes, soil pipes and associated deeper bedrock fractures. The geophysical methods also helped to define possible vertical and lateral connections between the soil pipes and hydraulically-active bedrock fractures. Further work at this site should include imaging the extent of bedrock fractures with a deeper-penetrating EM unit, such as the Geonics EM34 ground conductivity meter. This instrument will more easily differentiate anomalies caused by the upper shallow soil cover from those generated in the upper bedrock. Additional 2D resistivity surveys with automated dipole-dipole arrays would also be very useful in further delineating the extent of bedrock fractures and caves beneath the filled sinkholes and their connections with soil pipes at the surface. Soil moisture concentrations could also be monitored in and around the filled sinkholes and the percolation of water through shallow conduits to deeper bedrock fractures monitored. Collection of soil moisture contents with an automated tensiometer, along with periodic EM or resistivity transects, will also help to establish hydraulic connections between the cover materials and underlying bedrock. Acknowledgments The authors would like to thank the Bourbonnais Township Park District, the Northern Illinois University Graduate School and the Department of Geology and Environmental Geosciences for funding this project. James Cooper of Argonne National Laboratory, Chris Augustine, Liang Chen and Rebecca Vershaw of Northern Illinois University provided valuable assistance in collection of field data for this project. Mike Konen of the NIU Geography Department provided valuable assistance in the analysis of soil samples. We would also like to thank an anonymous reviewer for suggestions that greatly improved this manuscript. 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