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Municipal Groundwater Drinking Supply Protection using Cone Penetrometer Exploration Methods

Andrew I. Strutynsky
STRATIGRAPHICS, Glen Ellyn, Illinois

Kay Gilmer, Chris Osborne, and Jason Romp
Ohio Environmental Protection Agency, South Eastern District Office, Logan, Ohio

Rick Darr
Lawhon and Associates, Columbus, Ohio

ABSTRACT: The Ohio Environmental Protection Agency (OEPA) has the responsibility to oversee public water systems in Ohio. When chlorinated solvents were detected in the municipal wellfield of Bridgeport, OEPA decided to try cone penetrometer exploration methods to study the problem. Penetrometer stratigraphic profiling, with groundwater, soil and soil gas sampling, and field GC/MS chemical analysis, were used to map the plume impinging on the wellfield, and locate the source of the plume. This method was then applied to studies of contaminant plumes affecting the wellfields of Coal Grove and the Belmont County Sanitary Sewer District #3, among others. This paper will present penetrometer equipment descriptions, exploration program philosophy, and short case histories of these investigations.

INTRODUCTION

OEPA investigations were undertaken at the Bridgeport, Coal Grove and Belmont County Wellfields, among others, to locate the sources of contaminant plumes affecting the wellfields. Another investigation objective was to gather data to evaluate potential response actions to ensure safer water supplies for the future. The choice of exploration method would be primarily controlled by four factors: 1) cost effective plume delineation - it was important that stratigraphy and groundwater chemistry be rapidly evaluated, to allow subsequent exploration points to be located based on actual site conditions, rather than on an arbitrary grid pattern. This approach would avoid expensive, multi-phased exploration typical of many groundwater studies; 2) very low contaminant concentrations - this would require use of relatively sophisticated field analytical equipment; 3) exploration in urbanized areas - it was important that methods be as unobtrusive as possible; and 4) geological conditions - wellfield aquifers were 15-30 m (50-100 ft) deep, and both gravelly and heaving sands might be encountered.

The consultant (Lawhon and Associates) recommended, and OEPA approved, use of a cone penetrometer exploration system (Strutynsky and Sainey, 1991) along with a field analytical laboratory to achieve the goals of the program. The consultant and OEPA provided senior professionals to immediately evaluate all exploration results and plan subsequent exploration locations, while the penetrometer company (STRATIGRAPHICS) provided a geotechnical engineer or hydrogeologist to evaluate stratigraphy and recommend sampling procedures. OEPA also provided oversight of the entire program.

Initial groundwater samples would be collected around the most contaminated well or lateral, in the case of the Ranney Well, to determine the direction of plume transport. A series of upgradient exploration lines would then be run transverse to the expected axis of the plume to delineate the plume and identify its source. A number of penetrometer soundings, depending on encountered geological conditions, would be performed to develop stratigraphic cross-sections. Groundwater samples would be collected at multiple depths at each location to determine the vertical distribution of contamination within the aquifer. Field analytical testing would allow a 3-dimensional profile of the plume to be rapidly developed, allowing subsequent exploration locations to be chosen based on the complete analytical database. Penetrometer soil and soil gas sampling could be performed, in addition to groundwater sampling, to confirm potential source areas.

CONE PENETROMETER SYSTEM

The STRATIGRAPHICS cone penetrometer system was designed in 1986 for use during both geo-environmental and geotechnical exploration programs in difficult soil conditions. The heavy (240 kN and 300 kN or 24 and 30 ton) truck mounted rigs are fully self-contained, including data acquisition systems, dry and wet work areas, water tanks, steam cleaners, decontamination and grouting systems, separate rodstrings for sounding and sampling, optional dynamic rod driving, and heavy duty downhole equipment for use in glaciated terrains (Fig 1). Cone penetrometer (direct push) systems require no borehole to advance probes and samplers, and result in little exploration derived waste. Downhole equipment is decontaminated during retrieval using an automatic rodwasher, and open hole is pressure grouted. Most exploration activities are performed inside an enclosed portion of the rig, providing all-weather capability and a low visual presence. Truck mounted penetrometer systems can be very productive, with as much as 400 m (1300 ft) of stratigraphic logging per day, with depth capacity exceeding 60 m (200 ft). As many as 18 groundwater, or up to 30 soil or soil gas samples can be acquired in a day.

High resolution, continuous soil profiling (sounding) for geo-environmental exploration is most often performed by STRATIGRAPHICS using the indirect Piezometric Cone Penetration Test with soil Electrical Conductivity (CPTU-EC, Fig 2). The cone tip and friction sleeve resistance measurements (CPTU-EC) are evaluated for soil types and geotechnical parameters (Douglas and Olsen, 1981). The piezometric measurement (CPTU-EC) allows evaluation of soil saturation, hydraulic conductivity, potentiometric surfaces, and soil types (Saines et al, 1989). The soil Electrical Conductivity measurement (CPTU-EC) provides information on soil moisture in vadose zone soils and indications of groundwater quality in saturated soils. The EC measurement has proved very useful in exploration for inorganic (metal, brines and landfill leachate) contamination (Strutynsky et al, 1998) and somewhat useful in LNAPL or DNAPL exploration (Strutynsky et al, 1991).

The STRATIGRAPHICS Penetrometer Sampler can be deployed in groundwater, soil or soil gas sampling modes by using interchangeable components. The groundwater sampler is a heavy wall, shielded sampler. The shield prevents cross-contamination of the sample and screen clogging. The shield is retracted to allow groundwater to flow through a 0.5 m (20 inch) long screen into the barrel. Sample can be decanted from the barrel or can be pumped to the surface using an inertial pump. The sampler is typically tripped out after each sample to allow thorough decontamination. The rate of groundwater inflow and equilibrium levels can be recorded using a small diameter pressure transducer. Inflow test results can be analyzed using rising head slug test solutions.

The soil sampler consists of a barrel, sealed with a locking piston. The piston is unlocked with a wireline tool to obtain the sample. The sampler is tripped out of the hole for sample extrusion and decontamination. The soil gas sampler is a smaller version of the groundwater sampler. Samples are contained within Tedlar bags, or are routed through portable vapor analyzers.

ANALYTICAL TESTING

A Hewlett-Packard Model 5890 Series II Gas Chromatograph (GC) and a Hewlett-Packard Model 5971 Series Mass Spectrometer (MS) were operated by the analytical company (Aqua Tech Environmental Laboratories- ATEL) in a trailer mounted, on-site field laboratory. A Tekmar 2000 purge and trap device, and a Tekmar 2016 auto sampler for automated sample handling, were also used. The samples were analyzed using EPA method 524.2 with a detection limit of 0.5 ug/l. The contaminants of concern (COC’s) varied for each site, but were primarily chlorinated solvents, and their breakdown products.

BRIDGEPORT WELLFIELD INVESTIGATION

The Bridgeport wellfield is located in southeastern Ohio, along a channel of the Ohio River, and serves 3600 residents. The wellfield is within the unglaciated Appalachian Plateau Ohio River Aquifer. Beginning in 1989, very low levels of TCE and cis-1,2-DCE were detected during routine wellfield monitoring; soon PCE was also detected. After multiple well sampling events confirmed the continual presence of the COC’s, OEPA decided to perform an investigation during the summer of 1994.

Ten CPTU-EC soundings for stratigraphy, 26 dissipation tests for hydraulic conductivity and potentiometric surfaces, 52 groundwater (from 57 attempts), and 9 soil gas samples were acquired during the course of a 13 day field program. Soil gas samples were obtained in vadose zone soils around the suspected source after the groundwater sampling program defined the limits of the plume.

Evaluation of the CPTU-EC soundings revealed variability (sand and cinder fills, buried refuse, or clay) at shallow depths. Shallow soils were typically moist to wet. Deeper stratigraphy was more uniform, and typically consisted of sands and silty sands, with some local gravelly layers. Saturated conditions were typically found below depths of 10-13 m (35-45 ft). The aquifer was characterized as a continuous water table aquifer, with few, apparently discontinuous, aquiclude interlayers. Bedrock was typically encountered within about 21-26 m (70-85 ft) of the surface.

CPTU-EC soundings in the identified source area indicated somewhat different stratigraphy, with finer grained soils predominating, and significant thicknesses of interlayered sands, silts and clays. Bedrock was also found at a shallower depth (18 m or 59 ft), consistent with a steep slope of a buried bedrock valley, as indicated by nearby rock outcrops.

Plume concentration maps were developed at three different depth intervals. In the upper interval, concentrations ranged from less than 0.5 ug/l, to as high as 9,700 ug/l total COC’s at the source area. The upper portion of the plume angles away from the wellfield following regional ground water flow. In the middle interval, concentrations ranged from less than 0.5 ug/l, to as high as 1,900 ug/l total COC's near the center of the plume (Fig 3). In the lower interval, concentrations ranged from less than 0.5 ug/l to 105 ug/l total COC's, again with the highest concentrations near the center of the plume. The results indicated that while most of the COC’s are drawn towards the wellfield through the middle depth interval, contamination of the wellfield is actually occuring in the lower depth interval .

The investigation identified the source of the contamination as a dry cleaner located about 365 m (1200 ft) northwest of the wellfield. OEPA positioned 6 permanent monitoring wells along the plume. These are used as an early warning system, allowing wellfield operators to modify wellfield production to lessen capture of the plume.

COAL GROVE INVESTIGATION

The Coal Grove wellfield is located on the banks of the Ohio River in southern Ohio. The four production wells are configured within about 1/2 hectare (1.5 acres), and serve 4700 residents. The production wells are within alluvial deposits associated with the Ohio River and a tributary, Ice Creek. TCE and DCE have been detected in some of the production wells since 1988. One production well (CG-2) was taken out of service in 1989 due to high contaminant levels, and has been intermittently pumped to waste to control contamination of the other production wells. Five monitoring wells had been installed upgradient of the wellfield. TCE and DCE detections increase with distance upgradient.

Upgradient (southeast) of the wellfield is a coal dock. Further upgradient is a closed facility which had operated for numerous years as a truck terminal and as a tanker truck repair and cleaning operation. In 1993, USEPA conducted an emergency removal action at this facility . During the removal action, on-site wastes were found to contain various volatile organic compounds (VOC’s), including TCE. OEPA determined that additional groundwater data were required to protect the wellfield from further damage. The penetrometer method successfully used at Bridgeport was chosen for use at Coal Grove. The investigation had to proceed in two parts (1994 and 1995) due to lack of permission to enter the tanker truck cleaning facility.

Fourteen CPTU-EC soundings for stratigraphy, 38 dissipation tests for hydraulic conductivity and potentiometric surfaces, and 61 groundwater samples (from 64 attempts) were acquired during the course of the first 12 day field program. CPTU-EC soundings revealed 5.5-14 m (18-46 ft) of silty clay, underlain by granular soils. The gravelly sand to silty sand aquifer saturated thickness varied from about 1.2 m (4 ft) downgradient to 13 m (43 ft) upgradient of the wellfield. The aquifer ranged from water table to confined, depending on surficial clay thickness. Bedrock was encountered at the base of the aquifer, at depths between 17-26 m (55-87 ft).

The results from field GC/MS analytical testing on obtained samples showed that the VOC plume followed a linear flow path, apparently originating to the southeast of the coal dock facility, on the northern boundary of the closed tanker truck cleaning facility (Fig 4). The plume narrowed with distance upgradient of the wellfield. The ability to rapidly obtain data from the field analytical lab, as well as stratigraphic information from the cone penetrometer, was instrumental in locating this narrow plume. As the investigation proceeded, it became apparent that if sampling had been strictly conducted on the sample grid which was originally surveyed at the site, the narrow, most contaminated portion of the plume would have been entirely missed, with significantly different conclusions as to the source of groundwater contamination.

To complete this investigation and confirm the source of the VOC’s, the first program was used to support an administrative search warrant for exploration at the closed tanker truck cleaning facility. With the unexpected accompaniment of local TV news reporters, the warrant was served, the property was entered, and additional exploration was conducted. Four CPTU-EC soundings for stratigraphy, 6 dissipation tests for hydraulic conductivity and potentiometric surfaces, 37 groundwater samples (from 37 attempts), and 30 soil samples were acquired during the second 10 day field program.

The second program showed that the high concentrations of VOC’s continued in a very narrow plume less than 15 m or 50 ft wide to a corner of the closed tanker truck cleaning facility's wastewater treatment plant. This confirmed that the source of the VOC contamination was the tanker truck cleaning facility. Following this sampling effort, OEPA conducted a pump test at the wellfield. The data obtained from the pump test, along with the groundwater analytical data, were used to evaluate future impacts to the wellfield. OEPA concluded that contaminant levels at the wellfield could be expected to remain constant for some time into the future. However, it was also concluded that, without the continued pumping to waste of production well CG2, higher contaminant levels would be expected in other production wells. OEPA is currently evaluating both technical and legal means to address the threat to the Coal Grove wellfield.

BELMONT COUNTY SANITARY SEWER DISTRICT 3 RANNEY WELL INVESTIGATION

The Belmont County Sanitary Sewer District 3 serves about 25,000 people in southeastern Ohio with water from a single Ranney Collector Well (BC#3). This well is located near the Ohio River, and consists of six laterals, which are screened near bedrock. The well can supply up to 25 million liters (6.5 million gallons) of water per day. Contaminants cis-1,2-DCE, 1,1,1-TCA, PCE and 1,1-DCA have been detected in the well.

The Ohio Department of Transportation (ODOT) discovered VOC contamination in soil and groundwater during an environmental investigation of a nearby property (Site) during a highway relocation project. The Site is about 365 m (1,200 ft) southwest of BC#3. Various chemicals, including chlorinated solvents and petroleum products, were distributed from a bulk plant located at the Site. ODOT had detected the following VOC’s in Site soils and groundwater: VC; PCE; TCE; cis-1,2-DCE; trans-1,2-DCE; 1,1,1-TCA; and BTEX compounds. Initial results indicated that VOC’s were migrating in ground water north-northeast from the Site toward BC#3, and towards the southeast with regional groundwater flow.

ODOT notified OEPA of its findings in early 1996. During the fall of 1996, OEPA performed a field investigation to achieve the following goals: 1) identify all sources of BC#3's contamination; 2) determine the vertical and horizontal extent of contaminant plumes, 3) determine rates of migration; and 4) determine whether the suspect Site was impacting BC#3. The investigation was successful in achieving all these goals.

A major portion of the investigation was conducted using penetrometer exploration methods developed during the Bridgeport and Coal Grove investigations. Six CPTU-EC soundings for stratigraphy, 8 dissipation tests for hydraulic conductivity and potentiometric surfaces, and 55 groundwater samples (from 58 attempts) were acquired during the course of the 10 day field program. The CPTU-EC soundings revealed fine grained soils to depths of about 11-14m (38-47 ft), followed by a confined, gravelly sand aquifer. Groundwater sampling was conducted to determine the lateral and vertical extent of contamination within the aquifer.

The most common groundwater contaminant was found to be cis-1,2-DCE, with concentrations as high as 4,200 ug/l. The following cis-1,2-DCE detections were observed between the Site and BC#3: 300 ug/l about 135 m (440 ft) south of BC#3; 360 ug/l about 40 m (140 ft) south of BC#3; and 90 ug/l about 15 m (50 ft) south of BC#3. Data showed that the plume was being drawn northward from the Site, directly against the regional ground water flow direction, and into the end of the western lateral of BC#3 (Fig 5). This unusual, reversed flow path reflects the very large capture zone of the high capacity Ranney Well. The investigation showed that VOC’s are migrating from the Site to BC#3 via ground water flow paths primarily in the upper portion of the aquifer. It also showed that the Site is apparently solely responsible for the BC#3's cis-1,2-DCE contamination.

ODOT and OEPA agreed that excavation and off-site disposal of contaminated soils and debris prior to highway construction would best remediate the Site. Source removal, as opposed to treatment, was selected both to complete the highway project on time and to eliminate further contamination of the aquifer. The source removal took place during the summer of 1997. About 20,000 tons of contaminated, non- hazardous, solid waste and about 1,550 tons of hazardous waste were excavated and removed from the Site.

CONCLUSIONS

Thirty four CPTU-EC soundings, totaling 685 m (2250 ft) of data; 205 groundwater, 9 soil gas, and 30 soil samples (3755 m or 12210 ft of sampler deployment) were obtained during the 45 days of field exploration for these 3 projects. Penetrometer costs (less mobilizations) totaled about $133,000. A total of 244 samples, plus numerous QA/QC samples, were analyzed by the field laboratory using GC/MS. The cost for the field analytical laboratory totaled about $45,000 (less mobilizations). While this cost is comparable to costs for off-site analyses with a 24 hr turnaround, having analytical results within 15-30 minutes of sample acquisition allowed optimal placement of subsequent exploration locations. Accurate targeting of exploration locations was the key factor in significantly decreasing overall cost and duration of these investigation programs.

OEPA rapidly and cost-effectively investigated contaminated municipal wellfield groundwater supplies by the use of high capacity penetrometer stratigraphic profiling, rapid penetrometer groundwater, soil and soil gas sampling, a sophisticated field analytical laboratory, and immediate evaluation of acquired data by senior professionals. Complex contaminant plumes, which often followed unusual groundwater flow paths, were characterized within days rather than months or years. Plumes were delineated, sources identified, and realistic models developed for planning long term monitoring and site remediation activities. OEPA has used these projects during in-house training sessions as examples of innovative and effective groundwater exploration.

 

REFERENCES

Douglas, B.J., R.S. Olsen, 1981. Soil Classification using Electric Cone Penetrometer. Cone Penetrometer Testing and Experience, ASCE.

Saines, M., A.I. Strutynsky, and G. Lytwynyshyn, 1989. Use of Piezometric Cone Penetration Testing in Hydrogeologic Investigations. Presented at the First USA/USSR Hydrogeology Conference, Moscow, USSR

Strutynsky, A.I., T.J. Sainey, 1991. Use of Piezometric Cone Penetration Testing and Penetrometer Groundwater Sampling for Volatile Organic Contaminant Plume Detection. Petroleum Hydrocarbons and Organic Chemicals in Groundwater: Prevention, Detection and Restoration. API/NWWA.

Strutynsky, A.I., R. Sandiford, D. Cavaliere, 1991. Use of Piezopmetric Cone Penetration Testing with Electrical Conductivity Measurements (CPTU-EC) for Detection of Hydrocarbon Contamination in Saturated Granular Soils. Current Practices in Ground Water and Vadose Zone Investigations, ASTM.

Strutynsky, A.I. R. Glaccum, L. Conklin, and B. Baker, 1998. Chloride Mapping using Geophysical and Cone Penetrometer Methods. Proceedings of the First International Conference on Site Characterization, Atlanta, GA