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HomeMy WebLinkAboutNC0004774_CSA Report Appx C_Source Characterization_20150823Drilling Procedures Drilling Techniques In general, deep borings were advanced using 4'/4 inch inner diameter (ID) hollow stem augers (HSAs) to create a pilot hole for the larger 7 7/8 inch tri-cone mud rotary bit and collect environmental/geotechnical samples. Once refusal was encountered using the smaller ID HSAs, then a 7 7/8 inch tri-cone was advanced to one to two feet below refusal (when feasible) to install the 6 inch PVC outer casing (details provided in Well Installation Procedures below). Once grout was allowed to cure for approximately 24 hours, HQ coring tools were utilized to advance the borehole at least 15 feet below bottom of outer casing. Borehole was then reamed with a 4 or 5 inch diameter roller or tri-cone bit to allow for 2 inch diameter monitoring well installation. Bedrock borings were installed using same drilling techniques as detailed above except the borehole was advanced 50 ft below the bottom of the outer casing. In select locations where bedrock borings were paired with deep borings, wells were double cased (telescoped) to eliminate potential mixing of zone being monitored in the deep boring and zone being monitored in the bedrock boring. To simplify, a 6 in outer casing was installed as mentioned above to bedrock, then boring was advanced 15 to 16 ft below outer casing using a 5 7/8 roller or tri-cone bit to set a 5 in outer casing. Once grout was allowed to cure for approximately 24 hrs the boring was advanced 50 ft below the bottom of the 5 in outer casing. Boring was then reamed using a 4 in diameter roller or tri-cone bit as described above and monitoring well installed. Shallow borings were advance using 4 % in ID HSAs to a termination depth based on observations made during advancement of the deep boring and monitoring well was installed. Monitoring well installation is described in greater detail in the Well Installation Procedures. In addition to HSAs, mud rotary and coring techniques, the use of sonic drilling was utilized to augment conventional drilling techniques. For deep and bedrock well installation, a 6 inch sampler was advanced in 5 to 10-foot sections to gather subsurface material to refusal. A 10 inch recovery bit was advanced to the same depth as the 6 inch bit and advanced and additional 1 to 2 feet to set an outer 6-inch PVC casing. Once grout was allowed to cure for approximately 24 hours, the boring was completed with HQ core tooling as described in the Deep and Bedrock Monitoring Well Installation Procedures. In some cases, shallow wells were installed using sonic drilling technology. The procedure for shallow well installation is similar to the deep and bedrock well installation technique; however, due to the absence of 6 inch outer casing in these wells, a 4 inch sampler and a 6 inch recovery bit were utilized. Well completion procedures for shallow wells are detailed in the following section. Shallow Monitoring Well Installation Procedures Shallow monitoring wells were installed to monitor the regolith zone or ash and are identified with an "S" (shallow) or "SL" (shallow lower) qualifier in the well nomenclature. If the regolith zone was observed to be relatively thick or more than one water bearing zone was encountered during drilling, a second shallow monitoring well was installed to monitor the lower water bearing zone and was identified as the SL well. Shallow monitoring wells were installed at the Site using either hollow stem auger (HSA) or mud rotary (roller or tri-cone bit) drilling techniques. Shallow monitoring wells were installed to monitor regolith aquifer or if installed in a dam, wells were installed to monitor the phreatic surface within the ash basin(s). Each shallow well was constructed with ten feet (ft) to fifteen ft of two inch diameter schedule 40 PVC 0.012-inch slotted pre -packed screen and schedule 40 PVC riser to grade or above grade if needed. A #2 filter pack sand was placed one to two feet above the pre -packed screen, bentonite was placed one to two feet above the sand pack, and borehole was completed to grade with grout via positive displacement method (tremmie grouting). If the boring was in an access path or roadway the well was completed at the surface with a flush mount well cover and locking well cap. If the boring was located off access paths and roadways the well was completed at the surface with a four inch above grade protective casing with lock and bollards to protect the monitoring well. Deep Monitoring Well Installation Procedures Deep monitoring wells were installed to monitor the transition zone (if present) and are identified with a "D" (deep) qualifier in the well nomenclature. Deep monitoring wells were installed using HAS, mud rotary or sonic drilling techniques to approximately one to two feet below refusal where a six inch diameter schedule 40 PVC outer casing was installed to seal off the overburden. Grout utilized for outer casing installation was allowed to cure for twenty-four hours then boring was advanced approximately fifteen feet below the bottom of the outer casing using HQ coring tools. Rock cores were logged in accordance with the Field Guide for Rock Core Logging and Fracture Analysis by Midwest GeoSciences Group and cores were photographed. Each deep well was constructed with five ft of two inch diameter schedule 40 PVC 0.012-inch slotted pre -packed screen and schedule 40 PVC riser to grade or above grade if needed. A #2 filter pack sand was placed one to two feet above the pre -packed screen, bentonite was placed one to two feet above the sand pack, and borehole was completed to grade with grout via tremmie grouting method. If the boring was in an access path or roadway the well was completed at the surface with a flush mount well cover and locking well cap. If the boring was located off access paths and roadways the well was completed at the surface with a four inch above grade protective casing with lock and bollards to protect the monitoring well. Bedrock Monitoring Well Installation Procedures Bedrock monitoring wells were installed to monitor fractured bedrock and are identified with a "BR" qualifier in the well nomenclature. These wells are screened across water bearing fractures within competent bedrock. Bedrock monitoring wells were installed using either HAS, mud rotary or sonic drilling techniques to approximately one to two feet below refusal where a six inch diameter schedule 40 PVC outer casing was installed to seal off the overburden. Grout utilized for outer casing installation was allowed to cure for twenty-four hours then boring was advanced approximately fifty feet below the bottom of the outer casing using HQ coring tools. Rock cores were logged in accordance with the Field Guide for Rock Core Logging and Fracture Analysis by Midwest GeoSciences Group and cores were photographed. Packer testing was performed on select fractures observed in the rock cores. Screen interval was selected based on observations made during coring activities and packer test results. Each bedrock well was constructed with five ft of two inch diameter schedule 40 PVC 0.012-inch slotted pre -packed screen and schedule 40 PVC riser to grade or above grade if needed. A #2 filter pack sand was placed one to two feet above the pre -packed screen, bentonite was placed one to two feet above the sand pack, and borehole was completed to grade with grout via tremmie grouting method. If the boring was in an access path or roadway the well was completed at the surface with a flush mount well cover and locking well cap. If the boring was located off access paths and roadways the well was completed at the surface with a four inch above grade protective casing with lock and bollards to protect the monitoring well. Soil Sampling Procedures For nested monitoring wells, the deep boring was utilized for characterization of subsurface material and collection of environmental and/or geotechnical samples for lab analyses. Standard Penetration Testing (SPT) was conducted utilizing split -spoon sampling techniques at five foot increments with an 18-inch split -spoon sampler. Any environmental and/or geotechnical samples not able to be collected in the deep well were collected in either the shallow or bedrock well that was paired with the deep well. Geotechnical samples consisted of undisturbed (Shelby tube) samples (natural moisture content, grain size with hydrometer, hydraulic conductivity, and specific gravity), sorption samples (Kd/HFO), mineralogy, and index property samples (natural moisture content and grain size with hydrometer). Environmental sampling for soils consisted of Total Inorganic compounds (TI) and Total Organic Carbon (TOC) and environmental sampling for ash consisted of TI, TOC, and Synthetic Precipitation Leaching Potential (SPLP). In situations where there was a single well to installed, as many samples as possible were collected in that boring, but if it was not feasible to collect the necessary samples a second boring (geotechnical boring) was advanced within five ft of the original boring to obtain the remaining samples. Sampling equipment was decontaminated between each sample interval in the same borehole as well as between each boring. Sampling equipment was first rinsed with tap water, washed with a water-liquinox mix and then rinsed a final time with deionized water. All downhole equipment utilized during boring/well installation (i.e.; augers, drill rods, split spoons, etc.) were decontaminated between each borehole to eliminate potential cross contamination between boreholes. A decontamination pad was constructed for field cleaning of downhole tooling. Downhole tooling was decontaminated using a high temperature, high pressure steam cleaner followed by rinsing with potable water. Hydrogeologic Evaluation Testing To better characterize hydrogeologic conditions, falling and constant head tests, packer tests, and slug tests were performed. Data collected from these tests will be used in the groundwater modeling. At select deep/bedrock well locations, horizontal permeability tests (falling or constant head tests) were performed just below refusal in the upper bedrock (transition zone if present). At select shallow/deep/bedrock locations, vertical and horizontal permeability tests were conducted above refusal in varying hydrostratigraphic units. Packer testing was conducted in bedrock borings at select fractures based on observations made during drilling and the rock cores. In general, a double packer system was utilized with generally five to teen feet of separation between packers and a single packer system was utilized to perform a shut in permeability test at the bottom of the borehole. Slug tests were completed in each monitoring well installed to assess hydraulic conductivity. Slug tests were conducted in accordance with requirements of the NCDENR Memorandum titled, "Performance and Analysis of Aquifer Slug Tests and Pumping Tests Policy", dated May 31, 2007. Drilling and Installation Variances Variances from Groundwater Assessment Work Plan 1 I AB-3SL, AB-8SL 2 AB-2BR, AB4-BR, GWA-3BR, GWA- 6BR, GWA-22D 3 AB-10GTB, AB-7GTB 4 AB-41P, AB-81P, BG-21P, 5 GWA-12D (2 borings), AB-91D (1 boring) BG-1 D (2 borings) 6 7 1 AB-41D, AB-51D, AB-61D, AB-71D, AB- 9D, BG-31D, GWA-21D, GWA-31D, GWA-51D, GWA-61D, GWA-12D Source Characterization Eliminated Adl ded Added Added Added I Added Well IDs changed to AB-41BRU, AB- 5BRU, AB-61BRU, AB-71BRU, AB- 9BRU, BG-3BRU, GWA-2BRU, GWA-31BRU, GWA-51BRU, GWA- 6BRU, GWA-12BRU Thickness of ash not sufficient to allow installation of second well screened in ash. NCDENR requested this additional well to fill a data gap. _ A geotechnical boring was added to collect additional soil samples These borings were added to perform permeability tests at these locations These locations were moved because due to unexpected auger refusal This location had to be moved because well boring was drilled in the transmission line right of way and _moved again due to drill rig access. These wells identified as D were changed to BRU because no transition zone was present at these locations and the well screen was set into bedrock.