AECOM Report B&W - N - City of Guelph

City of Guelph2008 Annual ReportClosed Eastview Road Landfill SiteTo better determine if the localized areas that did not meet, or periodically did not meet, their targetelevations would contribute to RUG exceedance at the bedrock, the POLLUTE modelling was revisited withthe more up-to-date data, at that time. To provide conservatism in the model, a concentration for boron of8 mg/L was used. This concentration was considered due to the fluctuations observed at certain locations asstated above. As well, the groundwater elevations used in the model were based on the higher than normallevels observed during 1996 in the outwash below the waste.The POLLUTE modelling was revisited in the northeast area of the landfill (locations 62 through 67) as wellas at location 55 and at location 51. Each location was modelled using the original concentration of 20 mg/L,the conservative concentration of 8 mg/L and the actual average concentration from 1995. The results of thestudy are outlined in the report “Re-Modelling of Leachate Migration in the Tills - Eastview Road Landfill”(GLL 96-362 dated May 1997). The study concluded that there would be no exceedance of the ReasonableUse Guideline concentrations in the bedrock below locations 51, 55 and the northeast area of the landfill,even though target elevations were not attained. Below is a brief discussion on the monitors that once againdid not meet their target elevations, with reference to the revised modelling results (Gartner Lee, 1997), todetermine whether or not the failure to meet the target elevations could result in a future exceedance of RUGin the bedrock aquifer.Location 51Monitor 51-II did not meet its target elevations during 2008 as water levels were slightly above it. The boronconcentrations in the outwash below the landfill in monitor 51-II were found to be slightly higher, averaging6.38 mg/L in 2008, when compared to 6.27 mg/L (1995) used in the revised modelling assessment in 1997,and the 20 mg/L that was used to set the original target elevation. The average downward gradient in 2008at this monitor was about 0.13 m/m, which translate into a downward velocity of about 2.5 cm/a. Thisgradient and downward velocity is higher than in 2004 but similar those observed since 2001 and in since2005.Given the fact that target elevations are slightly above the target elevation, and the boron concentration is onlyslightly higher than in 1995, there is still little overall concern regarding RUG at this location. (Note that the revisedmodelling in 1997 showed that all scenarios, including boron concentrations as high as 20 mg/L (boron in 2008was 6.38 mg/L), still resulted in RUG compliance.) As this is the case, a further assessment with the POLLUTEmodel is not warranted at this time. However, based on the overall review of leachate quality at 51-II, the slightincrease in boron concentrations may be the result of a compromised monitor as discussed earlier in the report.As this is the case, should an increasing trend continue, the monitor should be inspected.Location 63Since 1996, the average concentration of boron in the outwash at this location was well below the ReasonableUse limit of 1.225 mg/L. Therefore, the B7 limit would also be met in the bedrock below this location regardlessof whether or not the target elevation is met. Between 1998 and 2006, samples could not be obtained from thismonitor, as it had been essentially dry. This shows that the leachate has been substantially drawn down in thisarea, which shows the effectiveness of the PLCCS. As well, boron concentration would be expected to besimilar to historical concentrations, which were very low and decreasing since the monitor was installed in 1995.(111414_2ra_apr29-09_city_of_guelph.doc) - 31 -