Lessons from practice in the assessment and ... - Springer Link

Geosciences Journal Vol. 12, No. 2, p. 97 − 105, June 2008 DOI 10.1007/s12303-008-0012-y

Lessons from practice in the assessment and remediation of contaminated ground water

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Franklin W. Schwartz School of Earth Sciences, The Ohio State University, Columbus, Ohio 43210, USA Eung Seok Lee Yongje Kim* Groundwater and Geothermal Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 305-350, Korea

ABSTRACT: The famous American humorist Mark Twain once wrote "don’t let school interfere with your education". This paper builds on this theme by examining important lessons that come from work on practical problems of ground-water contamination and remediation in Canada and the United States. We draw lessons from the interesting features of the studies, and mistakes in execution, which have had important consequences. The first case study from Edmonton, Alberta Canada examines a problem of ground-water contamination due to leakage of water contaminated by 2,4-D and other contaminants from a small storage pond. This study highlights the problems related to an inadequate geologic understanding of a site, misunderstandings concerning the advantages and limitations of key tools for site investigation, and how projects can benefit from early and ongoing peer reviews. The second case study examines a problem of sewage contamination related to a deep tunnel system in Milwaukee Wisconsin. This system is designed to store surface-water overflows from an old, combined sewer system. This case study highlights the difficulties in working on unique problems without an effective conceptual hydrogeologic model, the need to always be concerned about the quality of chemical data, and the necessity of being alert to behaviors beyond typical experience. The lessons coming from these case studies have important implications for remedial work being undertaken in Korea and regulatory agencies with oversight of the projects. Key words: ground water, contamination, remediation, oversight

1. INTRODUCTION Mark Twain was a famous American humorist who wrote a variety of books, short stories and essays. His wry observations of the human condition remain part of the American experience. One of the humorous sayings attributed to Twain is “don’t let school interfere with your education”. We have adopted this theme for this paper, namely that lessons from case studies can provide valuable learning experiences not found in classes. Sometimes, the most valuable lessons come from examining mistakes made in studies and the chain of negative events that they set in motion. Hopefully, by understanding the lessons in this paper, readers in Korea can make better decisions in guiding projects. *Corresponding author: [email protected]

The paper examine two case studies – one concerned with the assessment and remediation of a problem of ground-water contamination, and a second with contamination problems associated with the operation of a large tunnel system for the short-term storage of wastewater from combined sewers. What these studies have in common is the inherent complexity in the geologic and hydrogeologic settings, and the practical need of integrating data of many different kinds and quality. In both cases, the problems created in the studies were only sorted out after detailed, independent review. 2. GROUND-WATER CONTAMINATION FROM POND LEAKAGE: EDMONTON, CANADA The first case study examines lessons from a study in Canada concerned with the assessment and cleanup of ground water contaminated by organic contaminants. Dr. Schwartz was asked to undertake a model assessment of how well a pump-and-treat system was working in removing contaminants from the ground water. A chemical plant in Edmonton, Canada produced various organic chemicals with a wastewater stream that contained dissolved contaminants including Cl-, 2,4-D (2,4-Dichlorophenoxyacetic acid; C8H6Cl2O3), and 2,4,5-T (2,4,5-Trichlorophenoxyacetic acid; C8H5Cl3O3). The contaminated water was stored in waste ponds prior to deep subsurface disposal. The unlined waste ponds were dug into a shallow clay unit, which was thought to provide confinement. A problem developed because the wastewater leaked into a shallow sand aquifer. An intensive hydrogeologic investigation was undertaken by a consulting company to define the extent of contamination and to provide the information necessary to install a contaminant recovery system. Many boreholes were drilled, which provided detailed and high quality lithologic logs. Various holes were completed with piezometers to provide hydraulic head measurements and water samples for laboratory analyses. Surface resistivity surveys also provided information on the geologic setting and the contaminant distribution. The lithologic data were not interpreted well and unfortunately, the site assessment

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Franklin W. Schwartz, Eung Seok Lee, and Yongje Kim

Fig. 1. Idealized stratigraphic section showing the distribution of key units.

did not produce a coherent geologic framework. Considerable reliance was placed on geophysical surveys. Overall, knowledge of the site hydrogeology, and distribution of contaminants was poor when the site assessment was completed.

The following paragraphs describe the actual hydrogeologic setting and patterns of contaminant migration. Figure 1 shows an idealized stratigraphic section displaying five key units: (i) Cretaceous bedrock, (ii) Lower Sand Unit, (iii) Lower Till Unit, (iv) Upper Sand Unit, and (v) Upper Till Unit. The Lower Sand Unit and Upper Sand Unit are aquifers and provide permeable pathways for contaminant migration. The other units are predominately clays, which transmit water much more slowly. Figures 2a,b are isopach maps of the Upper and Lower Sand Units, respectively. Notice that the Upper Sand Unit (Fig. 2a) is widely distributed across the site with a marked variability in thickness. As Figure 2b shows, the Lower Sand Unit is much less extensive, and is essentially a long, narrow, buried-valley aquifer. The original site assessment concluded that there was only one aquifer at the site with a highly variable thickness. As will become clear, this error in interpretation

Fig. 2. Detailed description of the site. Panels (a) and (b) show isopach maps in meters for the Upper and Lower Sand Units, respectively. The arrows indicate the generalized directions of ground-water flow. Panels (c) and (d) show 2,4-D plumes. In Panel (c), the shaded area shows concentrations greater than 100 mg/l.


created many problems. The bold arrows on Figures 2a,b show the patterns of ground-water flow in the Upper and Lower Sand Units. In the case of the Lower Sand Unit, ground water flows in a direction that is somewhat different than the upper sand aquifer and is influenced by the geometry of the aquifer. Figures 2c,d illustrate the pattern of spreading of 2,4-D in the two aquifers. As expected, the pattern of spreading of the plume in the Upper Sand Unit away from the leaking ponds coincides with the direction of flow. Contaminants enter the Lower Sand Unit because of downward leakage across the Lower Till Unit, which separates the two sands. Because of this indirect pathway to the source, contaminant concentrations are much smaller in the Lower Till Unit than in the Upper Sand Unit.

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Figure 3a shows the apparently straightforward sevenwell system developed by consultants to remediate the site. The design assumed a single aquifer and a plume with the extent shown in Figure 3a. Consultants determined that the plume did not extend beyond the property boundary because a downstream piezometer showed a non-detect for all the contaminants. This conclusion was wrong because this key piezometer was actually completed in the deeper aquifer. Figures 3b and 3c show where the pumping wells, part of the pump-and-treat system, actually were screened. Two recovery wells were completed in the Lower Sand Unit, and one well was completed in the Upper Sand Unit in an area with little contamination. Geophysics suggested a broader plume than was actually the case from detailed sampling.

Fig. 3. Comparison of differing views of the cleanup and recovery system. Panel (a) shows the larger plume inferred by the consultants from their investigation and the seven wells thought to be installed to collect the contaminants. Panels (b) and (c) show the actual plume the distribution of wells between the Upper and Lower Sand Units. Panel (d) shows the model calculated plume in the Upper Sand Unit.

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Dr. Schwartz completed a model analysis of this contaminant recovery system, using a flow and transport model with the actual site geology and the recovery system in operation. Figure 3d is an illustrative simulation of contaminant migration in the Upper Sand Unit. This analysis found that about 20% of the contaminants in the ground water left the site prior to the installation of the contaminant recovery system. With the present system, an additional 26% of contaminants already present in the ground water could be lost as well. The contaminant recovery system is not effective, because first the wells are misplaced. Second, those wells located in the Upper Sand Unit have small capture zones because of the low hydraulic conductivity of the unit and the small available drawdowns for these wells.

come a time, as now exists in the petroleum industry, where shallow subsurface imaging will become a primary investigative tool. The third lesson reinforces the importance of expert review for complex problems. In the United States, complex investigations often have independent experts who evaluate study plans and offer advice on study strategies, interpretations, and identify potential problems. This study featured errors in investigative design, data interpretation, and study integration that expert review would have uncovered. Our experience is that experts are most effective when fully engaged throughout the study. Schwartz’s expert review at the end of this study served simply to identify the problems in the study after all of the money had been spent.

2.1. Lessons Learned

3. SUBSURFACE STORAGE OF COMBINED SEWAGE EFFLUENT, MILWAUKEE, WISCONSIN

This case study provides three important lessons. Lesson 1 is that “geology matters”. The original site assessment did not provide a coherent geological framework, although data appropriate to this purpose were available. This framework is essentially a conceptual model that provides the basis for understanding what happens between boreholes. Every site investigation has some economic limitation in the number of holes that can be drilled. Geologists through their experience and local knowledge can provide an understanding of what units are present, their pattern of layering, and their lateral extent. The weak geologic interpretation meant that the pattern of contaminant migration couldn’t be determined with confidence. As a result, the contaminant recovery system was flawed causing a significant proportion of the contaminants to bypass the collection system. At the end of four years of assessment, design, and remediation, the regulatory authorities required major changes to the contaminant collection system. Lesson 2 is that one needs to understand the advantages and limitations of key techniques in site assessment and to rely on the primary measurement technologies in assessment. Test drilling, lithological sampling, and sample collection are key to describing the geologic setting. Piezometric measurements provide information to interpret ground-water flow directions and contaminant concentrations. In this study, there was an over-reliance on surface resistivity measurements, and an inability to take advantage of a set of high quality lithologic logs. The geophysical survey provided a simple but erroneous picture of site conditions. The primary test-drilling information with other relevant data will should have formed the basis for producing a coherent picture of the hydrogeologic setting and contaminant distribution. Geophysics has an important role to play in site assessment. However, these approaches need to be integrated with other investigative techniques. In the future, there will

Combined sewers serve many older cities in the United States. A combined sewer is one that carries sanitary wastes to water treatment plants continuously, and storm water runoff from streets and parking lots during rainstorms. These sewers work well except during high intensity rainstorms when a huge volume of water reaching the treatment plant can overwhelm storage and treatment capabilities. Of necessity, untreated wastewaters must be released into nearby rivers or lakes. Typically, this water contaminated by coliform and other contaminants, impacts these water bodies. Environmental organizations are active in the United States to reduce the frequency of combined overflows. A major legal effort was focused on cities of Chicago and Milwaukee to reduce their sewer overflows to Lake Michigan. This beautiful lake is the water supply for many people and an important recreational resource. Both of these cities chose to reduce the frequency of combined sewer overflows through installation of a tunnel system capable of providing temporary waste storage beneath the cities. Figure 4 shows schematically how tunnel storage works to control combined sewer overflows. Without tunnel storage (Fig. 4a), a thunderstorm triggers combined overflows once treatment capabilities are overwhelmed. With tunnel storage, control structures divert the effluent into a deep tunnel system. Once the storm stops, wastewater is pumped from the tunnel system and treated over a period of days (Fig. 4b). Such systems are capable of reducing the frequency and size of overflows, but not eliminating them. In very large storms, the volume of runoff might even use up the storage in the tunnel system at which point, the storage system is closed and effluent is released. Two storms close together can also be problematical because wastewater stored from an earlier storm reduces the ability for the tunnel system to


Fig. 4. Idealization of the problem of combined sewer runoff leading to pollution of Lake Michigan at Milwaukee, Wisconsin (Panel a). The addition of tunnel storage in most events enables the wastewater to be stored below ground for later treatment.

Fig. 5. Map of Milwaukee, Wisconsin showing the location of the tunnel systems for the storage of combined sewer overflows. Generally, the tunnels are aligned along the course of major rivers (from Report of the Exfiltration Panel, 1995).

handle the second storm. The case study here involves the tunnel system created

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for the city of Milwaukee. This system was completed using tunnel-boring machines beneath the city of Milwaukee in 1994 at a cost of 1.4 billion (US) dollars. About 30 km of tunnels were installed with diameters ranging from about 5 to 10 m. Figure 5 shows the map of the tunnels leading to the Jones Island water-treatment plant. The final license for operating the system, which was required from the State of Wisconsin, was withheld because the system was apparently generating ground-water contamination as wastewater stored in the tunnel exfiltrated into the ground water. The threat of A billion dollar lawsuit against the prime consultant on the project prompted the formation of a four-member expert panel to understand the problem from a technical point of view. Unpublished Reports of the Exfiltration Review Panel (1995, 1997) provided the information for the following description. Let’s examine the physical setting in detail. The tunnels were emplaced about 100 meters down from the ground surface in fractured Silurian dolomites (Fig. 6). Where the rock is competent and modestly fractured, the tunnels were unlined. In some places, concrete liners were installed mainly for structural purposes but also to control locally significant ground-water inflows. The tunnels were set significantly below the water table and for this reason, regulators required the emplacement of monitoring wells closely adjacent to the tunnels. Hydraulic heads and water chemistry were monitored on a regular basis with the help of these wells. Figure 7 shows how water level in the tunnel system fluctuated with time. When the tunnel system is empty, the head is at –290 feet (local datum). During a storm event when wastewater is stored in the tunnel, water levels can increase by more than 300 feet to about +15 feet (local datum) – essentially filling up the tunnels to the ground surface. This was the case with events #2 and #3 (Fig. 7). For reference, the Silurian dolomite has a regional head ranging between –50 to +50 (local datum). During the trial operation of the system, tunnel filling apparently caused concentrations of certain constituents in some of the wells to exceed regulatory limits. Figure 8 shows data for one of the wells, CT-MW-01. Exceedances in this case were evident in total dissolved solids, chloride and alkalinity. The concern was then that exfiltration of wastewater from the tunnel was producing pervasive contamination. Regulators were considering asking the contactor to line the entire tunnel system at a large additional cost. The problem of wastewater outflow in the tunnel was a surprise to the original design team. Their analysis suggested that leakage would only occur when hydraulic head in the tunnel was greater than the regional potentiometric surface of the aquifer. Thus, during most tunnel filling events, wastewater would remain in the tunnel. The Report of the Exfiltration Panel (1995), however, concluded that the original analysis was oversimplified. They wrote: “This analysis fails to account for inward

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Fig. 6. East-west geologic cross-section showing Paleozoic bedrock units dipping eastward toward Lake Michigan. The tunnels are mostly developed in the shallowest Silurian Dolomite Unit (from Report of the Exfiltration Panel, 1995).

Fig. 7. Fluctuation of water levels in the tunnel. At about –300 to –290 feet (local datum) the tunnel is dry. The events represent rainstorms in which wastewater from the combined sewer was routed to the tunnel (from Report of the Exfiltration Panel, 1997).

head gradients and the resulting lowered head values in the close vicinity of the tunnel….The trough of depressurized head values in the wall rock along the tunnel line creates a situation where all pressurized conditions in the tunnel will reverse hydraulic gradients from inward to outward and create the potential for exfiltration, at least locally.” The essence of this argument is shown in Figure 9. As the head increases in the tunnel, conditions develop where water in the tunnel flows outward into the surrounding rock. Based on these observations, it was obvious that wastewater spread 10s to 100s of feet away from the tunnels

each time they contained combined sewer effluent. However, the interpretation was complicated by the fact that the chemical data from monitoring wells had serious quality issues. Indeed, the peaks spikes evident in Figure 8 cannot be explained by the chemistry of the wastewater. Further analyses indicate that these and most other spikes were false positives due to (1) mixing up samples either in sampling or during the laboratory analyses, (2) outright errors in the chemical analyses, and (3) variability due to contamination of wells by grouting. As a consequence, much of the chemical data, taken as evidence for exfiltration, simply indicates problems of quality control and quality


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Fig. 8. Chemical hydrographs for well CT-MW-01 showing the time variation in selected water quality parameters versus time. The spikes are considered to be evidence of ground-water pollution by the tunnel (from Report of the Exfiltration Panel, 1995).

Fig. 9. Conceptual model of the water level response with time in a section of the tunnel. As the tunnel fills to –177 feet (local datum) water flows out of the tunnel to fill the potentiometric lows indicated by τ (from Report of the Exfiltration Panel, 1995). 1,2,3

assurance. This conclusion is further supported by the fact that in most partial tunnel-filling events the hydraulic heads in 75% of the wells were always above those in the tunnel. Thus, there is no hydraulic possibility for contaminants to exfiltrate as far as the wells. The expert panel found that really only coliform analyses provided reliable indications of exfiltration. The few times in which exfiltrations could actually be detected in the monitoring wells were when the tunnel was overfilled – water filled the tunnels and inflow shafts right to the ground surface. When the system is operated normally, and not allowed to overpressure (due to filling of the shafts), contamination is generally not evident in the monitoring wells. Exfiltration then is physically possible, but wastewater remains close to the tunnel and within the capture

zone of the tunnel. The only engineering fix necessary was a relatively minor software upgrade to make sure the surface gates to the inflow shafts and tunnel closed at the correct time to prevent overfilling. After the review panel completed their work, additional studies showed a very interesting hydraulic response in the wells. Following the largest tunnel filling events, certain wells showed a permanent decline in water level. Eight wells had declines of between 1.5 to 6 meters and three had declines greater than 6.5 meters. Figure 10 shows a permanent loss of 6.7 meters in well NS- MW-11 through several events. This behavior was serious because if water levels declined in the vicinity of the tunnel then contaminated water might reach out further into the country rock. The panel found that open and closing of the joints dur-

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Fig. 10. Abrupt decline in hydraulic head in monitoring well NS-MW-11 following major tunnel-filling events (from Report of the Exfiltration Panel, 1997).

ing tunnel-filling events likely caused this behavior. Essentially, joints were unable to close to their original aperture because of slight displacements in the rock or the introduction of solids. This interesting problem highlighted another danger in operating this system above design pressures. Permeability increases may occur, which often is only seen in hydraulic fracturing of oil and gas formations. 3.1. Lessons Learned The first lesson in this study is that “hydrogeology matters”. Early conceptual models of how the tunnel system was likely to respond to fill events were oversimplified and basically wrong. Follow on numerical flow simulations were themselves conditioned by incorrect assumptions of likely behavior and had no possibility of providing realistic answers. The kinds of tunnel-ground water interactions observed here were outside of normal geotechnical practice and required a more careful assessment with simple scooping models. As in the previous case study, a critical error early in the study was manifested throughout. Lesson 2 in this study is to always be concerned with the quality of chemical data. Collecting high-quality chemical data requires experience, state-of-the-art techniques, and careful supervision of the laboratories. In most investigations, it is common that this level of diligence is missing and the chemical data are of dubious quality. There are simply so many ways in which errors can be introduced that great skill and attention is required. This study had so many problems with the chemical data that they were hardly useable. Interestingly, the quality of chemical analyses and database didn’t improve after the expert panel identified the errors. In summary, it is necessary for the data on flow and chemistry to provide a consistent interpretation. The study also pointed out the need for conceptual models of what the contaminants are likely to be and how actual problems of contamination would be manifested in

samples. In this study, we showed that the spikes in concentration that were of such great concern to the regulators couldn’t even be produced by the wastewater in the tunnel. As a starting rule-of-thumb, it is probably appropriate to assume there are serious problems with the chemical data. The final lesson in this study is to be alert for behaviors, which are beyond experience. The problem of permeability enhancement around the tunnel because of tunnel filling events was unexpected and potentially a very serious concern to the operation of the system. Because of the uniqueness of the problem, it only came to light after operational data began to become available. In many projects, monitoring data are collected but never used and carefully evaluated. In complex problems, one needs to carefully confirm that the system is operating as constructed. 4. IMPLICATIONS FOR KOREAN PROJECTS The case studies here are examples of the ground-water problems faced by hydrogeologists in the United States and Canada. These are more noteworthy because of the potential cost implications involved in not meeting the project goals. In textbooks, contamination problems appear simple, laid out in a straightforward manner. However, real problems are a different matter. Simple and apparently trivial mistakes can lead to major problems later in a study. The need for cleanups of contamination in Korea is likely as is the case in other industrialized countries like the United States and Canada. In Korea, National Groundwater Monitoring Wells (NGMWs), which comprises 1,965 sites nationwide, have been installed to monitor level of groundwater pollution. Based on estimated transmissivity data, 32% of groundwater flow for NGMWs occurs through fractured-rock aquifers and 68% through alluvial aquifers (Jeon et al., 2005). Of water samples collected from 781 vulnerable sites (such as industrial complex) in 2002, 6.8% did not meet the Korean groundwater quality standards (Korean Ministry of Environment, 2002).


This paper should be instructive for those investigating comparable problems in Korea. The lessons developed in North America will hold in Korea as well. We expect further that sites in Korea will be complex because of the common occurrence of fractured rocks. Investigation of these kinds of systems will require a wide variety of sophisticated tools to address the complexities posed by fractured rocks. ACKNOWLEDGMENTS: This research was supported by the Basic Research Project (08-3211) of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Science and Technology of Korea.

REFERENCES Report of the Exfiltration Review Panel 1995, Milwaukee water pollution abatement program. Unpublished Report, Five Chapters.

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Report of the Exfiltration Review Panel 1997, Milwaukee water pollution abatement program. Unpublished Report, 37pp. Korean Ministry of Environment, July 2002, An integrated measure for preservation and restroration of ground water, Press Release. Jeon, S., Koo, M., Kim, Y., Kang, I., 2005, Statistical analyses of characteristics of Korean aquifer system using pumping test data of the National Groundwater Monitoring Wells (NGMWs), J. of Korean Groundwater and Soil Society, 10(6), 32−44. Manuscript received March 5, 2007 Manuscript accepted March 10, 2008