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Use of Angled Drilling Techniques to Characterize Fractured Crystalline

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Use of Angled Drilling Techniques to Characterize Fractured Crystalline Use of Angled Drilling Techniques to Characterize Fractured Crystalline Bedrock and Minimize Migration of Suspected Non-Aqueous Phase Liquids (NAPL) David Finney, P.G., Jeffrey Hershberger, P.G., and Peter Nangeroni, P.E., ESS Group, Inc. Abstract Several monitoring wells were installed at varying angles using rotasonic and rotary coring methods, to further characterize groundwater quality in bedrock, and to provide compliance monitoring locations (CMLs) directly beneath former unlined disposal trenches suspected to contain non-aqueous phase liquids (NAPLs). Drilling was initiated using rotasonic drilling techniques adjacent to impacted areas at angles and locations that were calculated such that impacted deposits, residual source materials and NAPLs would not be encountered during advancement of borings to their target locations within the bedrock. A dense non-aqueous phase liquid (DNAPL) contingency plan was developed and implemented in order to identify DNAPL and prevent any further mobilization as a result of continued drilling activities. Packer testing was performed along the length of the boreholes and detailed fracture logs were developed to better understand the variability in hydraulic conductivity of the fractured bedrock in these portions of the Site. Monitoring wells were installed at depth within the angled boreholes, and hydraulic head information as well as groundwater chemistry data collected from these locations has been used to gain further understanding of the groundwater flow paths beneath former disposal areas. These data have also been used to further improve the understanding of the relationships between the unconsolidated glacial deposits and the underlying fractured crystalline bedrock at the Site. Introduction Chemical mobilization in the subsurface as a result of site investigation activities is a legitimate concern, when investigating properties where there has been a chemical release. Specifically, when completing soil borings and installing monitoring wells within a suspected source area it is important to take the necessary precautions to minimize the potential that NAPLs encountered during drilling will be mobilized to greater depths as a result of continued advancement of the boring. The remobilization of the NAPL can adversely impact groundwater quality and affect sampling results from any well installed at that location or hydraulically downgradient of that location. When the presence of NAPL is suspected in the subsurface, NAPL field screening methods can be used during soil boring advancement in an effort to confirm the absence of NAPL at a given interval of soil before continuing with the advancement of the boring. These methods include, but are not limited to, visual observation, hydrophobic dye shake testing and centrifugation, UV fluorescence analysis, total ionizable compound (TIC) screening, and field laboratory analysis (Cohen and Mercer, 1993; Griffin and Watson, 2002). Once NAPL has been detected, boring advancement may be terminated. Alternatively, drilling methods may be modified to incorporate telescoping and sealing techniques that isolate layers where NAPL has been identified, while allowing boring advancement to the desired depth. These methods can be incorporated into a NAPL Contingency Plan that will help reduce the likelihood of remobilization of NAPLs. The goal of the recent boring program, described herein, was to install monitoring wells within the fractured crystalline bedrock beneath two former unlined disposal trenches suspected to contain NAPL. The monitoring wells were designed for use in monitoring hydraulic heads and groundwater quality within the bedrock directly beneath the former trenches. These data are used to gain a better understanding of the bedrock lithology and quality, and to evaluate the extent of interaction between the overlying unconsolidated deposits and shallower more weathered bedrock. The objective of the source area remediation is tied to the comparative quality of groundwater found in the source areas versus the underlying unconsolidated deposits and/or crystalline bedrock. Therefore, the groundwater results from these bedrock monitoring wells are also being evaluated as part of an effort to establish a remedial target for the shallower groundwater within the unconsolidated deposits. Due to the importance of establishing monitoring points that were truly representative of bedrock conditions directly beneath the former disposal trenches, it was determined that, in addition to a NAPL Contingency Plan, additional measures would be taken when designing and implementing the drilling program in order to prevent 825 mobilization of NAPLs during the drilling process. Specifically, drilling was initiated using rotasonic drilling techniques outside of the previously delineated trench areas at angles and locations that were calculated such that impacted deposits, residual source materials and NAPLs would likely not be encountered during advancement of borings to their target depths within the bedrock. By circumventing the source areas all together the likelihood of remobilizing NAPL as a result of the drilling process was greatly reduced. This paper (1) describes the process of selecting the appropriate start locations and drill angles; (2) outlines the drilling and hydraulic testing methods used; (3) presents the methods for characterization of the bedrock lithology, quality and hydrogeology; (4) discusses the ultimate uses for the hydraulic and water quality data collected at these monitoring locations; and (5) presents an evaluation of the effectiveness of the methods and lessons learned. Site Setting The Site is a former farm, which allegedly received up to 10,000 drums, and an undetermined volume of bulk wastes, including chlorinated solvents, which were reportedly disposed in six trenches over an 8-acre area. Soil and groundwater are impacted primarily by a wide variety of volatile and semi-volatile organic compounds (VOCs and SVOCs). Initial cleanup involved the removal of over 10,000 drums and excavation of the six former disposal trenches. Since that time, over 235 soil borings and over 400 wells have been installed at the Site as part of various investigations and remedial actions. Soil impacts extend to bedrock in some portions of the disposal trenches and groundwater impacts extend into bedrock across the Site. The presence of DNAPL has been suspected at the Site, but has not been confirmed. A Source Remedy is currently being implemented to address the impacts associated with the former disposal trenches. The Management of Migration (MOM) remedy includes hydraulic containment of the Concentrated Plume (TVOC greater than 1 mg/L) and monitored natural attenuation (MNA) for the Dilute Plume. The Record of Decision (ROD) for the Site anticipated three years of Source Remedy operation and 20 years to meet the groundwater cleanup levels (MOM and Dilute Plume). Methodology Drill Locations and Drilling Angles Three (3) bedrock angle wells were installed during this drilling program (MW-104 BRI, MW-105 BRI, and MW-106 BRI). The primary objective of installing bedrock wells MW-104BRI (northern portion of Trench A), MW-105BRI (southern portion of Trench A), and MW-106BRI (northern portion of Trench B), was to locate the well screens directly beneath the Source Areas. Each of these wells was drilled at a fixed angle adjacent to and beneath the known limits of impacted soil within the former disposal trenches. The wells were drilled in an approximately North-South orientation perpendicular to the predominantly East-West orientation of previously mapped joint sets. The specific locations, angles, depths and well construction information associated with each boring are described below. MW-104BRI was drilled at a 53 degree angle from horizontal from just north of the edge of the asphalt pavement in the northern end of the Trench A. The boring was terminated approximately 86 feet along the borehole, or approximately 68.7 feet below ground surface (bgs) between two existing 4-inch diameter monitoring wells. A monitoring well was constructed with a screen that spanned 72.0 to 84.0 feet along the borehole, covering an approximate 9.6-foot vertical interval from 445.94 to 436.34 feet mean sea level (MSL). MW-105BRI was drilled at a 42 degree angle from horizontal from just southwest of the edge of the asphalt pavement in the southern end of the Trench A. The boring was terminated approximately 115.5 feet along the borehole, or approximately 77.3 feet bgs in the vicinity of PZ-204. A monitoring well was constructed with a screen that spanned 99.5 to 114.5 feet along the borehole, covering an approximate 10-foot vertical interval from 465.38 to 455.38 feet MSL. 826 MW-106BRI was drilled at a 55 degree angle from horizontal from just north of the edge of the asphalt pavement in the northern end of the Trench B. The boring was terminated approximately 102 feet along the borehole, or approximately 83.6 feet bgs just southwest of dual phase extraction well ED-132D. A monitoring well was constructed with a screen that spanned 89.0 to 101.0 feet along the borehole, covering an approximate 9.8-foot vertical interval from 456.60 to 446.80 feet MSL. The locations of the angle wells at the ground surface and the approximate lateral locations of the center of the completed well screens are shown on Figure 1 and well construction information is summarized below in Table 1. Figure 1. Site Layout Table 1 Summary of Well Construction Information In addition to the previously
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