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CASE STUDY After site assessment with cone penetrometer testing(CP-T 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 'rom a known leaking underground fuel storage tank xas known about the off-site location of the MTBE plume. (LAST). The site is located in a narrow valley underlain Because the impacted supply well is not directly W young alluvial sediments.Groundwater occurs at a depth downgradient of the LUST site, the hydrogeoicgist Di 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 :nap depicting the LUST and vicinity is shown in Figure 1. gradient), in the two months since the impacted well was prior to beginning ;.he field investigation, a conceatuai shut down. model of the site was developed that incorporated all The site owner authorized the consultant to install a ransect of 13 CMT wells midway between the LUST and relevant informatioor• about the tank construction, release the impacted well. Because of the uncertainty in the history, prior land use, operating records of nearby water location of the.dissoived plume, the transect was instaiiea wells, as well as published information regarding the in an arc downgradient of the known r eiease area'Figure 1?. geology,hydrogeoiogy, and groundwater extraction in the The CMT wells were installed on 25-toot centers, and site vicinity was also reviewed. e. ended to an average depth of 50 feet. The wells were installed in continuously-cored borings drilled with a sonic . 14 I al ll 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 \��A measured in the CMT wells, confirming the downward jhydraulic gradient. A thin, narrow plume of MI BE was identified, flowing within the shallow alluvium(Figure 4). Non-detect(ND)values for MTBE were recorded in sampiing C ocints on all sides of the plume, thus ensuring that nc portion of the plume escaped the monitoring :network. .'sing the MTBE concentration data and the hydraulic Figure i.Site layout. A transect 3 CWT wells was installed properties of the shallow alluvium, the flux of IX73E between:he LUST and the impacted water well. The transect was emanating from the source area alas estimates. 'anis oriented peroendicuiar ro the groundwater-low direction, but calculation preyed to be a powerful way of assessing the .vas extended in an arc to ensure that no portion of the suspected _mpact of the dissolved contaminant plume or. the plume escaped detection. downgradient industrial wet. MLf7®-. M47A A (cFr n I MPT-'IA M41Y Mli MLA MLa MLi ML-2 .NLS NL-11 M412 ML-i3 ML-1a T.PT.1G1 (cp-s) ;CP'-il IcPT-ll IcPT..n ;CPT-21 MPT-_) J so N ED IF- W ^ 1 _— I ----__.__.__._..__.-_ 1B0 3GJ111r=9 YO f� I ]0 N NQl'1C?t,w0 NOW20MTAL Figure 4. 'soconcentration contour,-of.VTBE measured in groundwater samples collected f>om the C:TT wells. A thin, narrow plume of dissoived MTBE was identified within the shallow younger alluvium. in thus ligere, the_LITRE plume is flowing out of the page toward the reader. Prinm,n Canam For frerther information, please contact:SoHnst Canada LtdII •V 09/00 35 Todd Road Georgetown, ON° Canada L7G 4R8 E-mail:instrurnentsLsolinsz com ,72;-L:(905)873-22-5.5 or(800)661-2023 F.,LY +1(90)873-1992 or(800)516-9081 Lev:wwwso&mcom 4 w