Geologic Characterization

Geologic Characterization

Of the Silver Maple Claims

Park City, Utah

Introduction

A geologic characterization of the Silver Maple claim was necessary for the determination of tailing thickness and volumes. This effort was accomplished in two phases. In July 2002, the Bureau of Land Management (BLM) initiated a non-intrusive surface geophysics survey which was followed by confirmation drilling in December 2002. The National Science and Technology Center (NSTC) of the BLM conducted the geophysical survey and contracted the drilling to Dynamac Corporation, a Maryland-based engineering firm.

The geophysical survey utilized two-dimensional electrical resistivity technology to image spatial variations and thickness of the tailing in the subsurface. This geophysical approach has several significant advantages over drilling, such as speed of data acquisition, its non-intrusive nature, but also its mobility in areas of where access is difficult for a drill rig, such a wetland. Though drilling typically provides very accurate data, e.g., drill core or tangible hand samples, a drill location and its information are limited in a lateral sense. Two-dimensional resistivity data, however, offers relatively laterally continuous data which often provides a more comprehensive understanding of subsurface conditions. Taking advantage of all these positive attributes, this characterization effort combined the two approaches to obtain the best results possible.

The geophysical survey consisted of five electrical resistivity survey lines which transected the site, north to south, at various locations. Because the wetland environment severely limited access of a drill rig, only six boreholes were drilled to confirm the presence of tailings and its thickness, the degree of heavy metal contamination via X-ray Fluorescence (XRF) analysis, and depth to bedrock. Information from the borings was also used to correlate with resistivity results and provide thickness estimates in areas inaccessible by the drill rig. The six borings were placed near two resistivity lines (SM-1a and SM-7).

Methodology

Direct Current Electrical Resistivity Methodology

Electrical resistivity surveys measure the electrical resistive property of earthen materials. In other words, the ability of the material to inhibit an electrical current which is the reciprocal of electrical conductivity. Subsurface environments are analogous to a series of electrical resistors, but of unknown arrangement and dimensions. The specific resistivity method utilized collected two-dimensional data in order to delineate the spatial distribution and thickness/depths of these resistors. Though the magnitude of resistivity is important, often it is the information obtained from the lateral and/or vertical changes, i.e., spatial distribution, of the data that is more diagnostic. Inferences from spatial variations of resistivity are made of the distribution of the subsurface stratigraphy and sediment facies. It is up to the user, however, to ascertain the cause of the changes in the data. Therefore, it is important to develop conceptual models of the geologic setting and the effects it may have on the geophysical signature. Such a model is important at this site, because possible interference caused by the electrically conductive surface water, the various types of tailing wastes, and the natural underlying topography could potentially produce unique results.

Electrical resistivity surveys can be employed in various electrode arrangements (arrays), though the basic resistivity principals are the same for all the possible arrays. For this survey, a schlumberger array was used. In simple terms, this linear array involves the application of an electrical current to the ground via two source electrodes. The potential difference created at the surface is measured between two receiving electrodes, located at a known distance. The potential difference (voltage drop) produced by the current, as it encounters earthen materials (resistors), is measured and used to determine the resistivity of the material encountered. The effective depth of a sounding is sequentially increased by increasing the distance between electrode pairs. The further this distance, the greater the vertical interval (depth) in which the bulk of the current flows. As the electrode separation is incrementally changed, differences or contrasts of resistivity are notable when changes in the geo-electrical properties are encountered. These changes are inferred as geologic boundaries caused by changes in lithology, sediment grain-size, moisture and/or chemical boundaries due to changes in water chemistry. For this site, these changes could be caused by the presence of the organic muds, tailings, waste rock, water chemistry and underlying stratigraphy. Because the electrical current will prefer to flow in areas of low resistivity, such as clay or salts, the resistivity value will be a relatively lower magnitude as compared to sand or gravel. The apparent resistivity value is recorded by the instrument and is measured in ohm-m.

The sequential changes in electrode arrangement are performed automatically by specialized equipment. For this study, an automated multielectrode system was employed which consisted of a Sting R1 resistivity meter and the Swift automatic switching box and multielectrode cable; all are manufactured by Advanced Geosciences Inc. in Austin, Texas (Attachment 4 Cover sheet).

Data Acquisition

The resistivity data were collected in a two-dimension fashion using a 1-meter electrode spacing. For quality assurance, the resistivity instrumentation was programmed to collect a single data point twice. If the two values were within 2% of each other, the data point was recorded, otherwise, the data point was skipped. Data gaps (skipped data) did occur on several lines during this survey due to poor electrode coupling to the ground. However, in most cases a particular target or boundary could be viewed across the skipped portion of the data profile. Portions of all transects were submerged into the organic muds of the wetland, and in some cases were submerged by as much as 4ft of water.

Each resistivity line was mapped using global positioning system (GPS) instrumentation (Trimble Geoexplorer) with the accuracy less than1 meter. The perimeter of the wetland was also mapped with GPS.

Data Analysis

Interpretations made from unprocessed resistivity field data (pseudosections) can be tricky, but more accurate interpretation methods rely upon computer processing generally known as data inversion. The inversion process basically accounts for the geometry of the electrodes, voltage etc. and suggests the “true resistivity” rather than the apparent resistivity presented in the pseudosection. Inversion can be further controlled by being correlated to known information such as drill log information. If no control is available, the inversion process can still be used, and in either case, a “best fit model” is generated. Again, in all cases, it is up to the user to determine if the processed data profile is possible given the existing knowledge of site conditions (site model). All survey lines were inverted and plotted in the field so real-time modifications to survey strategies, if necessary, would be possible. Though drill log information was obtained, their results weren’t utilized to control the inversion processing because acceptable correlations were possible without their addition. As presented in the results, the amount of error between the drilling the resistivity was approximately 1ft to the top of bedrock.

The resistivity data obtained during this survey were inverted using a smoothing algorithm provided in processing software licenced to the BLM. This software is Interpex Software (IP2DI version 4.12) of Golden, Colorado.

The unique challenge was the wetland environment and the abundance of standing water. Submerging electrodes in water and mud of the wetland was required in many locations. The specific conductance of the surface water (1,800 to 2,200 umho/cm) was sufficient of being detected by the survey and also suggested groundwater would also have an effect. However, because a resistivity result is a bulk measurement, the addition of sediments would provide a sufficient difference from that of water alone, enabling the water to be discerned from those of saturated sediments or tailings. What was found to be of more benefit for the separation of signatures was the interbedding of the tailings.

The geophysical survey consisted of five electrical resistivity survey lines which transected the site, north to south, at various locations (Plate 1).

Drilling

Six borings were drilled using a Geoprobe continuous drive spoon drill rig (Fgure 1). Conetec drilling company of Salt Lake City, Utah performed the drilling. Dynamac Corporation was the engineering contractor that logged the borehole using the United Soils Classification System. The drilling effort focused on two locations of the site, where access was readily available and a significant amount of tailing material is visible on the ground surface. The first site is located near a small pond immediate downstream from the local playground and is the location for BH-1, -2 and -3. The second drill site is located at the beaver pond approximately 300m downstream. This was the location of BH-4, -5 and -6. An additional borehole (BH-7) was planned for the north end of transecting SM-10, but the drill rig encountered access difficulties and this boring was never drilled. Each flight of the drilling was photographed using a digital camera and each drill location was recorded with a Global Positioning System (GPS), specifically, a Trimble Pro XR.

Representative samples of the drilling core were shipped back to NSTC for additional testing using X-ray Fluorescence (XRF) and pH testing; and additional resistivity values were obtained from hand samples via a resistivity test box.

Results of these tests are found in the drill logs. Specific resistivity testing was performed on three of these samples to aid in the interpretation of the resistivity profiles.

Figure 1. Geoprobe drill rig drilling BH-1.

Site Conceptual Model

A site conceptual model is important for designing the geophysical survey’s acquisition parameters and understanding the data obtained in the field. Moreover, it’s instrumental in the interpretation of the data. The objective is to determine the spatial variations and thickness of the tailings within the study area. Only site characteristics which may potentially influence the resistivity signature, data quality and interpretation are discussed further.

Tailings latent with heavy metals are known to exist across the site area. While previous studies focused within the Prospector Square (upstream) very little site specific information is available for this study area. In a previous study (Mason, 1989) a well was drilled adjacent to the Silver Creek and located somewhere in the vicinity of transect SM-7 which is the present-day beaver pond. The drill log for this well suggests an interbedding of sand and poorly mixed gravel. Tailing are not noted in this log or in any of the additional drill logs which are known to have been drilled through the tailings of Prospector Square.

The tailings would naturally be deposited atop the native alluvium, but the dynamics of the fluvial wetland environment mixing of the alluvial sediments with the tailings is possible and thereby may influence the resistivity. The underlying the unconsolidated stream sediment and tailings is bedrock, the Woodside Shale (Crittenden, 1971).

Some form of earth moving activities had previously occurred in the drainage as observed in a 1962 aerial photograph (see figure 4 of this report). In this photograph a series of circular features are obvious and appear to be the result of the unloading of dump trucks or dredged piles of sediment. Nonetheless, remnants of these piles could appear as lenticular shapes in a resistivity profile, and depending on the type of material these lenticular shapes could be either resistive or conductive. Lastly, surface water at the time of the survey had electrical conductance ranging from 1,800umho/cm to 2,100 umho/cm (4 ohm-m) which is capable of affecting the resistivity of the saturated alluvial sediments. Regardless of the sediment type, the coarse-grained sediments such as sand and gravel would have a greater resistivity value than the finer-grained sediments and tailings.

The resistivity profiles are capable of presenting information regarding the local stratigraphy; the relative thickness of the layers; and their spatial distribution.

As with any geophysical survey, the ability to discern layers or objects depends upon a contrast of geophysical properties, otherwise with similar resistivity properties, different objects would appear as a single homogeneous body or layer. To assist in identifying the resistivity of these tailings, several discrete samples were collected from the drill core and tested in a portable resistivity test-box. Because the test box samples can’t exactly replicate the compaction, moisture, and temperature of in-situ tailings, the test results aren’t identical, but are within the same magnitude of the layers identified in the profiles as tailings. These test results varied between 1,200 and 3,800 ohm-cm (12 to 38 ohm-m) which generally agrees with the resistivity results of the tailings discussed in the profile results (6 to 28 ohm-m).

Results and Discussions

The results of each survey line are discussed individually and are accompanied with the relevant borehole information. The resistivity profiles and borehole logs are presented in Plate 1. The color scheme used in the profiles is color-balanced to aid visual, qualitative comparisons amongst the various survey lines. The color scheme assigns low resistivity values blue hues, and ramps upward in resistivity value to red hues. It is important to note that a sharp boundary between low (blue) resistivity to high resistivity (red) will contain a color gradation containing green and yellow caused by the steep or sudden color ramping. This gradation is very similar to the compressed grouping of contour lines along a cliff or steep hill of a topographic map. Contour lines are also present on the profiles and help highlight these steep resistivity gradients. Transects 2, 3 and 9 were not evaluated via geophysics and were not mapped via GPS. Locations of the survey lines and boreholes are presented in Plate 1.

Three borings, BH-1, -2 and -3a, were placed in the area of the southern end of this resistivity transect; two east of the line and one to the west. The boreholes were distributed across a prominent tailing area, and though not directly on-line with SM1a, the sediment sequence presented by the drilling information is very similar to each other as well as to the vertical sequence of resistivity data. Based upon visual inspection of drill core and XRF results, the tailings extend from the ground surface to the top of bedrock.

Tailings found in the three boreholes were stratified and varied in color and texture. BH-1 is located approximately 25ft west of electrode #1. Though discerning between native sediments and tailings was difficult at times, particularly in the top soil, XRF results identified this shallow interval to be tailings. Tailings identified on the drill log exist from 2.5ft (0.8m) to 10ft (3.2m), with bedrock being found at 10 ft. BH-2 was drilled approximately 45 ft east and perpendicular to electrode #9. Tailings in this boring are present from 1.5ft (0.4m) to 11ft (3.4m), which was the top of bedrock. BH-3 encountered similar sediments/tailings and bedrock at 9.5ft. Because this borehole is located 100ft from the resistivity line, it was not used to correlate with the resistivity results. In summary, based upon drilling and XRF information, tailings in this area exist from 0ft to bedrock, which varied from 9.5ft to 11ft.

Surface material varied from moderate resistivity (40 to 59 ohm-m) to low (5 to 20 ohm-m). At the beginning of the transect, between electrode # 1 and #11, and at the end, between electrodes #34 and #40, the moderate resistivity likely correlates to sparsely vegetated organic sandy clay (CL) noted in both drill logs. Beyond electrode #11, low resistivity (13 to 18 ohm-m) seems to correlate with an increase in vegetation and saturated surface conditions. Standing water was encountered at electrode #20, and had a resistivity of 25 to 30 ohm-m. Though the surface water was conductive enough (2,200 umho-cm which is equivalent to 4 ohm-m) to be detected by the resistivity, no apparent readings of the water were observed, probably because the water was of insufficient thickness (2ft or less). Correlations between the drilling the resistivity profiles are shown on Plate 1.

Of particular importance is the spatial distribution of the low resistivity interval (13 to 18 ohm-m) which correlates to the tailings. Found in the middle of this transect is the densely vegetative saturated surface of the drainage. This interval is represented as the low resistivity interval that extends from the ground surface to the top of bedrock which ranges from 30 to 40 ohm-m. This low resistivity interval extends laterally to both ends of the resistivity profile and correlates to the tailings noted in the drill logs for BH-1, -2 and -3a. At both ends of the profile, however, a thin veneer of greater resistivity covers the low resistivity interval. This occurrence is likely caused by the influence of sandy sediment along the margins of the drainage. This sandy sediment increases the range of resistivity (29 to 51 ohm-m) and is found on top of this low resistivity interval noted in the center of the profile.

Approximately 4ft below the ground surface and beneath electrodes #22 and #31 -#35, a thin horizon of greater resistivity occurs. Subtle indications of this horizon are noted on SM-4 and SM-6. This horizon suggests some interbedding of the tailings is possible and may include some gravel.

Because 2D resistivity surveys provide more lateral information than drill holes, the geometry, e.g., geomorphology, of subsurface features can be observed. Two lenticular bodies of greater resistivity (138 to 986 ohm-m) are found beneath electrodes #16 and #27. The base of both bodies occurs at 11ft which correlates to the depth to bedrock found in BH-2. Drill logs from both BH-1 and -2 indicate gravelly horizons are present which would account for the coarse-grained marker horizon and these bodies. A third lenticular body (275 ohm-m) occurs beneath electrode #5. Though the resistivity profile doesn’t completely cover this anomaly, its depth appears slightly greater than the other two. It is possible this last anomaly is the sewer line which may have been keyed slightly into the bedrock. Because of their greater resistivity, these bodies are likely consisting of coarse-grained sediments such as sand and gravel. Historical photographs of the site indicate dump-truck-like piles of sediments exist in the wetland.

One of the assumptions of electrical resistivity is that the layers or boundaries of the subsurface are fairly horizontal. When violations of this assumption are encountered, anomalous features are produced in the inverted profile. Such features occur in this profile as the diagonal extensions of elevated resistivity emanating downward from the lenticular bodies.

Transect SM-4 (schlumberger array, 1-meter electrode spacing)

This transect is located approximately 80m downstream of transect SM1a. This transect begins outside the perimeter wire fence, with electrode #2 being placed just inside the fence. The transect ends approximately 6ft from the wire fence on the north side of the drainage. Due to the saturated conditions, boreholes were not possible along this transect. Dense vegetation and saturated conditions occurred along the entire transect and only a few shallow (less than 2ft) water tracks existed. No visible signs of the sewer line were present, but according to construction records should be present on the southern end of the transect.

Many of the same features found in SM-1a are found on this profile. The thin interbed of greater resistivity (sand and gravel) found in SM-1a occurs at the same depth (4ft) in this profile and is found at the beginning and end of this transect. Three gravel lenses are present in this transect and reside on top of bedrock, which is 11 ft deep. Processing artifacts are again created by these gravel lenses. An anomaly of greater resistivity is found below electrode #6. This anomaly appears similar to the lenticular gravel lenses, but at shallower depths 1.5m (5ft). The sewer line may be the cause of this resistivity anomaly or it’s a gravel bar associated to the storm event and its marker bed.

Due to the dense vegetative cover on this transect there were no visible signs of the tailings on the ground’s surface. However, the low resistivity interval (6 to 28 ohm-m) of this line appears to be the equivalent to the low resistivity interval found to be tailings by the boreholes near Transect SM-1a. Another similarity of the low resistivity intervals of the two transects is the geomorphology of this interval; it extends laterally across both transects with approximately the same depths and thickness (consistently 11ft).

SM-6 (schlumberger array, 1-meter electrode spacing)

This transect occurs in the mid-portion of the site and just east of the circular piles noted in Figure 4 (the 1962 historical photograph). This transect was densely vegetated with willows, cattails and grasses. Standing water, though abundant, never exceeded 1ft in depth. A sewer line manhole is located west of electrode #6, approximately 25ft. Soils near this electrode were rocky which would increase the resistivity. This transect crossed through the southern wire perimeter fence between electrodes #3 and #4. No boreholes were drilled along this transect.

A relatively large body of elevated resistivity (>100 ohm-m) is found between electrode #11 and #22. Based upon its resistivity, this body is more coarse-grained and likely corresponds to one of the piles noted in the 1962 historical photograph. Tailings along this transect are consistently 10 ft thick. The resistivity signature of the sewer line, or that of another pile is present between electrode #5 and #7, and at a depth of approximately 7ft to 10ft. A small segment of the thin interbed is found at a depth of 4.4, crosses the southern top of the predominant gravel pile.

SM-7 (schlumberger array, 1-meter electrode spacing)

This transect was placed to run into the large beaver pond. Electrodes #1 to #7 were on grassy tailings, #7 to #9 were at the water margin. Electrodes #9 through #40 were under water. Shallow water (2ft or less) was encountered from electrodes #9 to 20. The depth gradient than increased to as much as 5ft at electrode #30 and a boat was used to place the remaining electrodes.

Three boreholes were drilled in the vicinity of this transect (BH-4, -5 and -6). BH-5 was placed at the start of the transect, 10 ft east of electrode #2. Though the borehole log indicates tailings from 2.3ft to 3.8ft, XRF results show elevated Zn in the upper 1ft to 2ft interval. Total tailing thickness found in this boring is approximately 5ft. However, elevated Zn (664 mg/kg) is found in the bedrock interval 5.5ft to 8ft. XRF and drilling results for BH-6 show tailings exist only in the upper 2ft, while results for BH-4 show much greater concentrations and tailing thickness extending from the ground surface to bedrock (5.2ft). Slightly elevated concentrations of Pb (1,070 mg/kg) and Zn (607 mg/kg) are also found in bedrock. Drilling results show variability in the tailing thickness ranging from 2ft to as much as 5.2ft, and possibly more if considering bedrock XRF results.

Resistivity results generally agree with stratigraphic sequence noted in the boreholes logs. A range of relatively low resistivity (14 to 28 ohm-m) correlates to the tailings interval, and with depths ranging from 2.5ft to as much as 8ft under the pond. Standing water of the beaver pond starts at electrode #9 and extend to the end of the transect. Some the resistivity signature may be caused by the more expansive water body, but this would be restricted to the upper 1m and the lowest resistivity values do occur at the surface of the pond. The end of the transect had many “skipped” resistivity data points which may have been caused by the thick soft bottom mud which caused difficulty ensuring the electrodes had a good electric coupling to the ground.

SM-10 (schlumberger array, 1-meter electrode spacing)

This transect was the easternmost of the resistivity transects. A manhole cover was noted approximately 20ft west of electrode #34. A 1-meter deep water channel exists between electrodes #6, and #8. The substrata of this channel was very rocky. A berm of tailings is located between electrode #16 and #19, and immediately adjacent is an irrigation channel (between electrodes #19 and #22). Very hard dry ground is present between electrodes #29 through #50. The soils in this area are crushed Woodside Shale. Borehole BH-7 was proposed in this area by was cancelled due to access difficulties. The transect end 25ft short of the wire perimeter fence.

Depth to the top of bedrock along this transect is consistently found at 3m (9.6ft). Two bodies of high resistivity are located on top of the bedrock. These are found below electrodes #5 and #6 and #14 and #17. Both appear to be gravel piles, but have more moderate resistivity (64 to 86 ohm-m) which may suggest these are natural fluvial deposits.

Data found at depths greater than 6 ft, and between electrodes #30 and #50 are of relatively poor quality due to data skips. The actual cause of the skips is unknown, but could have been related to the drying of mud on these electrodes. The sewer line crossed at electrode #34 and appears to have been backfilled with sediments of greater resistivity. Crushed Woodside Shale was exposed on the ground surface at this location and would be of greater resistivity than the tailings. Unfortunately, the drilling effort was not able to confirm the presence of the tailings that is suggested to exist in the 1.8m(6ft) to 3.8m (12ft) interval.

Tailing thickness along this transect appear to increase from 5 ft at the southern side of the wetland to possibly 12 ft on the northern side, though data on this northern end is lacking.

Conclusions

Because of the excellent correlations between the drill logs and resistivity data (1ft error), accurate thickness estimates of tailings can be made where drilling was not possible. The majority of the resistivity survey lines crossed saturated soils and was typically being submerged 1ft or less, but to as much as 5ft in the beaver pond (SM-7). Though the surface water had a specific conductance of 1,800 to 2,200 umho/cm, and capable of being detected, its thickness was typically insufficient to cause interpretation problems. However, in areas where the depth of water was greater (3ft or more) the resistivity reading was very similar to that of the tailings, but due to the interbedding of the tailings, the two could be differentiated.

Both drill logs and resistivity profiles indicate some interbedding of sediments, and tailings typically extending from ground surface to the top of bedrock. A gravel deposit was commonly found to reside on top of bedrock, but because of the more continuous lateral coverage provided in a resistivity profile, the shape of these deposits is observed as lenses or piles of gravel. This observation is consistent with historical aerial photographs which show dump-truck piles in the wetland area of the site. These gravel piles have subsequently been buried by finer grained sediments, primarily tailings, as confirmed by drilling logs and XRF sample results. XRF results of the borehole samples indicate elevated heavy metal concentrations in nearly all of the fine-grained sediments and most of the gravel deposits.

Across the site, bedrock is chocolate, reddish brown shale known as the Woodside Formation. XRF results of the bedrock material didn’t show significant of heavy metal concentrations.

Throughout the site the thickness of tailings extends from the ground surface to the top of bedrock. There is a slight thinning of the tailings in the downstream direction, starting at 11ft in the west end and ending with 4.8 to 7 ft at the east end of the site. Though some native sediment interbedding may occur, the natural mixing of these native uncontaminated sediments with tailings has rendered all of the fine-grained sediments contaminated. The source of the gravel piles which exist on top of the bedrock is unknown, and based upon the XRF results contain elevated concentrations of heavy metals. These deposits could be assumed to be waste rock and are included in the total volume calculations of the tailings.

Several volume estimates of the tailings were generated in order to provide a range of possible scenarios. This is necessary to address the changing tailing thickness and some uncertainty of the lateral extent of the tailings in the northeast side (Area-C)of the drainage where neither drilling nor resistivity was implemented. To accommodate these thickness changes into the volume estimate, the study area was divided into three Areas (A, B, C). The division between Area-A and -B occurs at SM-6, with an average thickness of 11ft of tailings being applied toward the western segment (Area-A) and 7ft to the east. Based upon the above possibilities the total volume of tailings ranges from 141,651yd³ to 177,541yd³(Plate 1).