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1 • • .-Solinst ' 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 MTBE 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 off-site location of the MTBE plume. (LUST). The site is located in a narrow valley underlain Because the impacted supply well is not directly 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 shut down. 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 transect13 CMT wells midway between the LUST and history,prior land use, operating records of nearby water the impacted ted well. Because , the uncertainty the transect was installed the wells, as well as published information regarding the location of the dissolved plume, geology,hydrogeology,and groundwater extraction in the T arc downgradient the known release area(Figure site vicinity was also reviewed. Thee CMT wells were installed on 25-foot centers, and ' extended to an average depth of 50 feet. The wells were installed in continuously-cored borings drilled with a sonic drill rig. 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 MTBE was identified, flowing within the shallow alluvium(Figure 4). x w"u 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. ' �• e T 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 arc to ensure that no portion of the suspected impact of the dissolved contaminant plume on the ' plume escaped detection. downgradient industrial well. A ML]N MLl.3 AI CPTH ' N410 NN MLB (CPTH MW MLi NLl NL-0 ML-11 H LI] ML-13 ICPT101 ICPTd) (CPT-91 MI) ICIA (CPT 3) (CPT-q �I O Mir ' YILF:OO 3L YYT2r rvamx.woxmnxlW 1! INI ' Figure 4. Isoconcentration contours of MTBE measured in groundwater samples collectedfmm the CMT wells. A thin,narrow plume of dissolved MTBE was identified within the shallow younger alluvium. In this figure,the MTBEplume is flowing out of the page toward the reader. ' N. m o.�W. For further information, please contact:Solinst Canada Ltd } �g 09/00 35 Todd Road Georgetown,ON,Canada L7G 4R8 E-mail:instruments@so1m4com TEL:(905)873-2255 or(800)661-2023 FAX.'+1(905)873-1992 or(800)516-9081 Web:www.solinst.com rJ+