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N I SOIDIst CASE STUDY After site assessment with cone penetrometer testing (CPT) Expedited Assessment of an equipment, the investigators identified two ages of alluvium beneath the site. Because of the contrast in permeability b1TBE Release between the two ages of alluvium, the site hydrogeologist In November 1999 , low concentrations of MTBE were suspected that the MTBE plume may have been migrating detected in a private industrial well located downgradient preferentially within the upper more permeable unit. Little from a known leaking underground fuel storage tank was known about the offsite location of the MTBE plume. (LUST). The site is located in a narrow valley underlain Because the impacted supply well is not directly l by young alluvial sediments. Groundwater occurs at a depth downgradient of the LUST site , the hydrogeologist of approximately 8 feet, and flows toward the southeast suspected that the dissolved plume may have drifted at a velocity estimated to be about half a foot per day. A southeast, away from the well (following the regional flow map depicting the LUST and vicinity is shown in Figure 1 . gradient), in the two months since the impacted well was Prior to beginning the field investigation, a conceptual shutdown. model of the site was developed that incorporated all The site owner authorized the consultant to install a relevant information about the tank construction, release transect of CMT wells midway between the LUST and history, prior land use, operating records of nearby water the impacteedd well. Because of the uncertainty in the wells, as well as published information regarding the location of the dissolved plume, the transect was installed geology, h dro eolo in an arc downgradient of the known release area (Figure 1). site vicinity y g gy, and groundwater extraction in the The CMT wells were installed on 25-foot centers, and snity was also reviewed. extended to an average depth of 50 feet. The wells were installed in continuously-cored borings drilled with a sonic I drill dg. A 4-inch-diameter temporary steel casing was advanced, which was ideal for installing CMT wells. Thirteen CMT wells were installed in just four days at the locations shown in Figure 1 . Groundwater elevations were measured in the CMT wells, confirming the downward hydraulic gradient. A thin, narrow plume of N(TBE was - identified, flowing within the shallow alluvium (Figure 4) . . I Non-detect (ND)values for MTBE were recorded in sampling points on all sides of the plume, thus ensuring that no portion of the plume escaped the monitoring network. Using the MTBE concentration data and the hydraulic - Figure 1. Site layout. A transect of 13 CMT wells was installed properties of the shallow alluvium, the flux of MTBE between the LUST and the impacted water well. The transect was emanating from the source area was estimated. This .oriented perpendicular to the groundwaterflow direction, but calculation proved to be a powerful way of assessing the was extended in an are to ensure that no portion of the suspected plume escaped detection, impact of the dissolved contaminant plume on the idowngradient industrial well. ML-10 .IAI wP* » .vii, �,3 ML ,. - , [ m rim «Ai. , i i (Cs,� � � a .11 f ;r� ------_ --�-- o l Figure 4. Isoconcentration contours of iTlBE measured in groundwater samples collected from the CMT wells. A thin, narrow plume of dissolved MTBE was identified within the shallow younger alluvium. In this figure, the MTBE plume is flowing out of the page toward the reader. l Pantoa in �niuia For further information, please contact: Solinst Canada Ltd r 09/00 35 Todd Road, Geo etovvn, ON, Canada L7G 4R8 E-mail: insii-ttnren 0 rg linst.cam TEL: (905) 873-2255 or (800) 661-2023 FAX- +1 (905) 873-1992 or (800) 516-9081 Web: www.saln vtcom