The Complicated Challenge of
MTBE Cleanups
The way in which an MTBE plume responds to altered pumping rates at municipal wells can cause further contamination problems, some unexpected. LISA S. DERNBACH
5 1 6 A • DECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
s an increasing number of gasoline releases are identified, so are occurrences of methyl tert-butyl ether (MTBE) plumes in groundwater. In California, MTBE plumes reaching drinking water wells have been a significant environmental problem since at least 1995. In the past five years, nearly 20 California community water systems have been affected by MTBE releases. Communities on the south shore of Lake Tahoe have, unfortunately, had more than their share of such problems: By the summer of 1999, 13 municipal wells were shut down as a result of actual or threatened MTBE contamination. Understanding how MTBE plumes behave is proving crucial to the design and implementation of effective remediation strategies and for protecting freshwater supplies.
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tensive investigations are conducted (1-6). Many of these studies have been prompted by the detection of MTBE in drinking water wells. Although researchers have slowly come to understand the manner in which MTBE plumes reach drinking water wells, the behavior of MTBE plumes after municipal well pumping rates are altered has been less documented. A better understanding of the latter issue could aid decision makers in their deliberations about what actions should be taken to remediate contamination. A common reaction by well owners is to shut down a drinking water well in which MTBE is present. This may be done with or without the foresight that such an action can lead to altered MTBE plumes affecting other receptors. The behavior of two MTBE plumes in groundwater at South Lake Tahoe after pumping rates had been modified in municipal wells and these plumes' effect on other receptors show why it is necessary to be able to predict the behavior of MTBE plumes when evaluating the risks of modifying pumping rates. Lessons learned in South Lake Tahoe have changed how California regulators now oversee gasoline releases.
Conceptual cross section of South Lake Tahoe showing a spreading MTBE plume (red).
Yet despite this necessity, practitioners in the environmental field have been slow to learn that MTBE plumes behave differently than aromatic hydrocarbon plumes and to change investigation methods. For many years, researchers studying MTBE plumes emulated investigations of aromatic hydrocarbons, such as benzene. Water table monitoring wells were used to evaluate the lateral extent of MTBE plumes, which were thought to migrate farther than aromatic hydrocarbon plumes, but not many times the distance. MTBE plume thickness was rarely investigated because the thickness of aromatic plumes was often considered negligible compared to their lateral extent. Fortunately, this situation is changing because the behavior of MTBE plumes is becoming better understood as more and more ex-
An area susceptible to contamination Lake Tahoe (at right) is located at an elevation of 6229 ft on the eastern crest of the Sierra Nevada. The lake sits in a granitic graben bounded by dormant faults, on the western boundary of the Basin and Range geologic province. Annual precipitation averages 30 in. Two-thirds of Lake Tahoe lie in California and one-third lies in Nevada. The south shore of the lake contains the largest population with about 40,000 residents living near the flat lakeshore, in valleys, and on mountain slopes. Residents on the Nevada side receive municipal drinking water from intakes extending 1000 ft or more into the lake. Most residents on the California side, however, receive drinking water from municipal wells provided by three water purveyors. The largest water purveyor, the South Tahoe Public Utility District (STPUD), operated 34 municipal wells in 1997, scattered in developed areas. Some of these municipal wells dated back to the 1950s. DECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 5 1 7 A
FIGURE 1
Pumping and MTBE plume migration Pumping pulled the MTBE plume (dark green; as seen in 1997) to the Arrowhead Wells; after the Arrowhead Wells had ceased pumping, the MTBE plume reacted by changing direction (light green; as seen in 2000).
Interestingly, gasoline constituents from releases before the addition of MTBE were never detected in drinking water wells in the basin. Even sites with several feet of gasoline-free product floating on the water table did not affect wells. A likely explanation is that aromatic hydrocarbon characteristics do not lend themselves to extensive migration, as is seen with MTBE. Benzene, toluene, ethylbenzene, and xylenes are less soluble than MTBE. Furthermore, these aromatics are more susceptible to attenuation mechanisms such as dilution, dispersion, and degradation. In the Tahoe basin, the largest plumes extended 300-500 ft from the source area, or only one-third to one-sixth the distance of the MTBE plumes.
Arrowhead municipal wells
Source: Adapted from data in References 15) and [12).
The Beacon Station in Meyers.
Municipal wells in South Lake Tahoe are susceptible to MTBE for many reasons. • Most municipal wells are located close to commercial areas with gas stations. This is because there is limited land area for development, as Lake Tahoe occupies about 80% of the basin. • The concept of wellhead protection did not gain widespread acceptance until the 1980s after most municipal wells were already in place. • All municipal wells are installed in geologically young valleys (less than 10,000 years old) that have only 100-260 ft of glacial and alluvial sediments and that lack extensive aquitards. • Municipal wells installed before 1970 were shallow (about 120 ft deep), lacked thick sanitary seals, and were sometimes screened across thin aquitards. • Shallow groundwater, within 4 ft of ground surface in some areas, serves as an excellent medium for dissolving and transporting petroleum hydrocarbons released from underground storage tank systems. The municipal wells in South Lake Tahoe affected or threatened by MTBE plumes included one or more of the above factors. • The chemical properties of MTBE, such as high solubility and poor degradation and attenuation, enabled it to reach the municipal wells. 5 1 8 A • DECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
In September 1997, MTBE was detected at 1.4 ug/L in two municipal wells in the community of Meyers. The municipal wells, called Arrowhead Wells 1 and 2, pumped at a combined rate of 800 gallons per minute (gpm). Arrowhead Well 1 extended to a depth of 130 ft with a 63-ft screen and was set in an unconfined aquifer composed of permeable glacial outwash and alluvial fill, containing mostly interbedded sand. Arrowhead Well 2 extended to 210 ft below ground, with a 60-ft screen. The well screen was set across a 20-ft-thick, silty-clay aquitard at 155 ft. The source of MTBE was eventually traced to a piping release at an operating gas station. An unknown amount of gasoline leaked into the soil for about two weeks before the release was repaired. Gasoline reached groundwater at the 16-ft depth, which was within the capture zone of the municipal wells and affected by drawdown. MTBE in groundwater traveled 1300 ft in 30-50 days to reach the municipal wells (see Figure 1). The estimated capture rate was 26-43 ft/day. The direction of migration to the municipal wells was cross-gradient to the regional groundwater flow direction, which is toward the north. After verifying MTBE concentrations, the water purveyor shut down the municipal wells. A groundwater sample collected at the gas station five months after the release showed 28,000 ug/L of MTBE, along with other gasoline constituents (7). The responsible party for the gas station conducted an extensive groundwater investigation eight months after the release occurred. Groundwater samples were collected from temporary borings on a horizontal grid spacing of 100 ft using a Geoprobe rig. Sampling occurred at 20, 40, 60, and 80 ft below ground surface. Investigation results showed trace levels of MTBE, from 0.6 to 1.7 ug/L, in groundwater from the gas station toward the municipal wells, in the northeast direction (8). The results also showed MTBE concentrations up to 3500 ug/L from the gas station toward the Upper Truckee River, to the northwest, in the expected natural groundwater flow direction. This information indicated that a "shadow" MTBE plume was still detected in groundwater eight months after the municipal wells ceased pumping. When no longer controlled by the Arrowhead Wells, the plume swung about 100° from the east to the expected regional groundwater flow direction in the north (see
Figure 1). The natural groundwater flow velocity is 1-3 ft/day for the Meyers area, depending on the season. Interestingly the BTEX plume extended horizontally only 200 ft from the gas station only onesixth the extent of the MTBE plume. The investigation also found MTBE concentrations at more than 100 ft below the surface near the municipal wells, but less than 70 ft in the direction of the Upper Truckee River. These data imply that the plume dove in the water column from the gas station when acted on by pumping forces from the municipal wells. Yet, when pumping ceased and only the natural groundwater flow was active, the plume acquired a 40-ft thickness from the top of the water table. The plume thickness was likely caused by a combination of surface infiltration of precipitation and a minor vertical gradient component. Because the Upper Truckee River flows to Lake Tahoe, a drinking water source, the first remedial action taken at the gas station included plume containment. Groundwater is pumped by three extraction wells at a combined rate of 60 gpm. Extracted groundwater is treated by air stripping and carbon polishing before being discharged to a constructed subsurface infiltration gallery. Remedial actions at the gas station, however, only contain the on-site MTBE plume; the offsite MTBE plume continued migrating to Lake Baron, 1500 ft away (9). Plans are under way to install an offsite remediation system to expand plume containment. Further actions, such as excavation and soil vapor extraction, are also being considered to remediate the source area beneath the gas station.
TABLE 2
MTBE impacts at Tata Lane well No. 4 MTBE concentrations steadily increased over two years in Tata Lane Well No. 4 until finally reaching the state action level in 1998.
Source: Adapted from data from the South Lake Tahoe Public Utility District.
FIGURE 3
The effect of remedial extraction pumping In 1997, the MTBE plume (dark blue) was being pulled cross-gradient to groundwater flow direction to the Tata Lane Well No. 4; the extent of the MTBE plume (light blue; as seen in 2000) is considerable after Tata Well No. 4 ceased pumping for several months and was restarted at a lower pumping rate. The plume has since commingled with an MTBE plume (purple) from another gas station acted on by natural groundwater flow.
Tata Lane Well No. 4 In June 1996, MTBE was detected at 1.3 ug/L in a municipal well in the city of South Lake Tahoe, 5 miles from the Arrowhead wells. The source of MTBE was unknown. Over the course of the next year, MTBE levels steadily increased to 26 ug/L as the municipal well pumped at 70 gpm (see Figure 2). The municipal well, called the Tata Lane Well No. 4, remained in operation because its water was already undergoing well head treatment by air stripping to remove chlorinated hydrocarbons. In September 1997, the STPUD conducted a groundwater investigation by collecting water samples from temporary borings between the municipal well and potential gasoline sources. The source of the MTBE was traced to an operating gas station that was located 1500 ft away and cross-gradient to the regional groundwater flow direction (see Figure 3). During the previous year, the responsible party had detected MTBE at 30,000 ug/L in groundwater beneath the gas station (10). The sources of gasoline releases were eventually identified in August 1998, as leaky seals in turbine pumps, overfilling of an underground storage tank, and poor piping connections beneath dispenser islands. Two years after the first MTBE detection, the Tata Lane Well No. 4 was shut down when concentrations exceeded 35 pg/L, California's drinking water action level (see Figure 2). The length of time that it took for the MTBE plume to first reach the municipal well cannot be calculated because releases at the gas station had been ongoing and it is unknown when
Source: Adapted from data in References (3) and 161.
The USA Station (now Unilocal) in South Lake Tahoe.
DECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 5 1 9 A
MTBE first entered the groundwater. Depth to the groundwater at the gas station ranges from 6 to 18 ft, and the hydraulic gradient is 0.046. The natural groundwater velocity is 0.5-1.5 ft/day in this area of the city, depending on the season. Before the municipal well ceased pumping, the responsible party for the gas station was directed by regulators to define the plume in three dimensions. Multilevel HydroPunch groundwater samples were collected from temporary borings using a hollowstem auger rig. Groundwater was sampled every 20 ft from the water table to the bottom of the municipal well at 127 ft and along the expected length and width of the plume. The investigation determined that the aquifer consists of three water-bearing zones separated by thin clayey-silt aquitards {11). Each zone is about 30-40 ft thick and composed of interbedded sand to silty-sand layers, deposited as alluvial fill. FIGURE 4
Subsurface view of MTBE plume A profile view of the MTBE plume, not to scale, diving beneath the water table toward the screen section of Tata Well No. 4 reveals why the plume was initially not detected by off-site shallow monitoring wells.
Source: Adapted from Reference 16).
The groundwater investigation found that the plume started to dive beneath the water table at 150 ft from the release areas at the gas station (see Figure 4). This explained why the plume was initially not detected by off-site shallow monitoring wells. The diving plume stayed above a laterally extensive aquitard that sloped toward the Tata Lane Well No. 4. Closer to the municipal well, the plume was detected down to the depth of the screen from 87 to 127 ft. The plume's average vertical thickness was 30 ft, and the maximum width was 600 ft. Because of these findings, multidepth monitoring wells were installed at every monitoring location. The Tata Lane Well No. 4 was shut down for threeand-a-half months. During this time, the water purveyor and regulators realized that without plume containment from the pumping well, unchecked plume migration would threaten six other nearby municipal wells within 1000 ft of the plume boundary. All municipal wells were then shut down. With the STPUD's permission, the responsible party pro5 2 0 A • DECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
posed to use the Tata Lane Well No. 4 as a remedial extraction well for containing plume movement. Twelve smaller-sized extraction wells were also installed in each water-bearing zone between the gas station and the municipal well. The combined pumping rate of these intermediate extraction wells was 120 gpm. Extracted groundwater was sent to three remediation systems and treated with granular activated carbon before being disposed to the sewer system and a constructed leach field. The corrective action plan approved by regulators stated that the former municipal well would extract groundwater at 70 gpm. Instead, the well was actually pumped by the responsible party at 15 gpm. This resulted in a smaller capture zone at the former municipal well, affecting only about 400 ft of the plume. The 1100-ft plume length from the gas station that was not affected by pumping was acted on by natural groundwater and had three effects: the plume renewed migration, the plume path shifted about 40 degrees from the northwest to the north direction, and the former diving plume migrated at the water column depth that it occupied before pumping ceased. Subsequent investigations revealed that the plume had migrated more than 2000 ft from the gas station, and the plume width had expanded to 1500 ft (Figure 3) (12). The northernmost 1000 ft of the MTBE plume was consistently detected between 40 and 75 ft below ground surface. MTBE was also found above action levels in one of the other municipal wells not in operation. With the former municipal well pumping at 15 gpm, the responsible party had expected the offsite extraction wells to contain plume migration. However, the plume migrated more than 2000 ft from the gas station, indicating that the off-site extraction wells were not effective. Indeed, aquifer tests later confirmed that the capture zones for some of the extraction wells had decreased with time as screens slowly clogged with sediments and iron fouling, and pumps failed. Part of the plume had, in essence, slipped between the capture zones of the off-site extraction wells. When the responsible party began pumping the former municipal well at 70 gpm to comply with orders by regulators, eight months had passed since Tata Lane Well No. 4 was shut down by the water purveyor. Unfortunately, pumping at 70 gpm was no longer fully effective as a remedial action because the plume had expanded beyond die capture zone of Tata Lane Well No. 4. Although most of the western half of the plume was contained by the former municipal well, the eastern half was still acted on by natural groundwater flow. The responsible party has since installed six additional off-site extraction wells to expand plume capture. Further actions are also necessary to remediate the source area beneath the gas station. The air spargesoil vapor extraction system in operation during 1998 and 1999 was not as effective as hoped. Unexpectedly wet winters with above-average precipitation raised the water table above remaining soil contamination, resulting in very little soil vapor extraction. Because groundwater temperatures are typically in the upper
40 °F (-4.4 °C) range for most of the year, air sparging has been ineffective at removing MTBE. In spring 2000, the responsible party installed a dewatering system, consisting of nine dual-vacuum extraction wells, to lower the water table to the depth of soil contamination at 33 ft. The soil vapor extraction system has been expanded to remove residual petroleum contamination in the exposed soil. Regulators hope the combined remedial approach will achieve site cleanup within 5-8 years.
The experiences in South Lake Tahoe have made regulators, w a t e r purveyors, and others a w a r e of h o w MTBE plumes behave w h e n captured by drinking w a t e r wells.
decrease with time as screens clog with sediments and biofouling, enabling MTBE to migrate in groundwater past the extraction wells. And before modifying pumping rates at drinking water wells, it is essential to predict the MTBE plume's behavior and evaluate potential risks. Altering pumping rates may actually threaten additional receptors. These risks must be weighed against the risks of allowing pumping to continue, which may in return require well head treatment or use of drinking water wells as remedial wells.
Lessons learned
References
The experiences in South Lake Tahoe have made regulators, water purveyors, and others aware of how MTBE plumes behave when captured by drinking water wells. Under the right hydrogeologic conditions, plumes can dive beneath the water table and avoid detection by water table monitoring wells. It is just as important to delineate a plume's vertical thickness as it is to delineate the horizontal extent. And, the rate of plume capture to drinking water wells can be quick. In the case of the Arrowhead wells, the MTBE plume migrated at an estimated rate of 26-43 ft/day. MTBE plumes can behave unexpectedly when pumping rates are altered in affected drinking water wells. The MTBE plume in Meyers changed direction by 100° when pumping ceased at the Arrowhead wells. Although the altered MTBE plume did slow to about the rate of regional groundwater flow for the area, it also threatened additional receptors. Even when not under the influence of well pumping forces, altered MTBE plumes can exhibit a significant vertical thickness caused by infiltrating precipitation and a vertical gradient. This is essential information because a plume's vertical thickness must be known to properly design an effective remediation strategy. Lessons learned in South Lake Tahoe have changed how regulators now oversee gasoline releases. Obviously, any ongoing release must be stopped immediately, and suspected releases must be investigated. All potential receptors, such as drinking water wells and surface waters used for drinking purposes, within a one-half mile radius from the gasoline source need to be identified and sampled for MTBE contamination to a method detection limit of at least the most stringent drinking water standard. MTBE plumes must be delineated vertically as well as laterally by collecting multidepth water samples to the depth of drinking water well screens. Even nondiving MTBE plumes will likely require multidepth monitoring wells to at least evaluate cleanup effectiveness. Capture zones of extraction wells may
(1) Dernbach, L. S. Fate and Transport of MTBE in South Lake Tahoe. Pacific Focus Ground Water Conference Abstract Volume, National Ground Water Association: Columbus, OH, 2000, p. 34. (2) Hurt, K. L. et al. Anaerobic Biodegradation of MTBE in a Contaminated Aquifer. In Proceedings of the Battelle International Conference on In Situ and On-Site Bioremediation Symposium, Batelle Press: Columbus, OH, 1999; pp. 103-108. (3) Newman, B. Kirkwood Meadows: MTBE Contamination at a Diesel Release Site. Abstract of presentation at the 220th National Meeting, San Francisco, CA, 2000; American Chemical Society: Washington, DC, 2000; ENVR 94. (4) Reid, I. B. et al. A Comparative Assessment of the LongTerm Behavior of MTBE and Benzene Plumes in Florida, USA. In Proceedings of the Battelle International Conference on In Situ and On-Site Bioremediation Symposium, Battelle Press: Columbus, OH, 1999; pp. 97-102. (5) Trudell, M. R., et al. Modeling MTBE Transport for Evaluation of Migration Pathways in Groundwater. Abstract of presentation at the 220th National Meeting, San Francisco, CA, 2000; American Chemical Society: Washington, DC, 2000; ENVR 82. (6) Tyner, L. et al. Natural Attenuation of BTEX and MTBE Under Complex Hydrogeological Conditions. In Proceedings of the Battelle First International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Battelle Press: Columbus, OH, 1998; pp. 333-339. (7) Vector Engineering. Submittal of Site Characterization Reportfor Beacon Gasoline Station; Vector Engineering: Carson City, NV, Feb. 1998. (8) Vector Engineering. Report of Findings, Soil and Groundwater Investigation, Meyers Beacon Station; Vector Engineering: Carson City, NV, lune 1998. (9) Secor International. Off-Site Groundwater Characterization Report, Meyers Beacon Station; Secor International: Carson City, NV, April 2000. (10) Park Environmental. Fourth Quarter 1996, Groundwater Monitoring Report, USA Station No. 7; Park Corp.: Citrus Heights, CA, 1997. (11) Pinnacle Environmental Solutions. Groundwater Assessment Report, USA Gasoline Service Station No. 7; Pinnacle Environmental Solutions: Cameron Park, CA, June 1998. (12) IT International. February 2000 Monthly Groundwater Monitoring and Operation and Maintenance of Remediation System Report; IT International: Sacramento, CA, 2000.
Lisa S. Dernbach is an associate engineering geologist at the California Regional Water Quality Control Board, Lahontan Region, in South Lake Tahoe, CA. DECEMBER 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 5 2 1 A
