AECOM Report B&W - N - City of Guelph

City of Guelph2008 Annual ReportClosed Eastview Road Landfill Site4.5.2 Predicted Exceedances of Reasonable UseThe leachate in the waste, or in the outwash beneath the waste, can move very slowly downward through the tilland, eventually, into the bedrock aquifer. Although the groundwater flow in the till is very slow (a few centimetresper year) and leachate will require a long time to reach the bedrock (several hundreds of years), it is stillnecessary to demonstrate that the contaminant concentrations in the bedrock will remain within the limits set outin Guideline B-7, the Reasonable Use Guideline (RUG).Two methods of predicting impacts on groundwater quality are used at the site: the leachate hydraulic targetelevations, and monitoring of the water quality in the till above the bedrock. Each is discussed below.4.5.2.1 Target ElevationsAt the Eastview Road Landfill Site, the RUG predictions for the bedrock aquifer have been made using acomputer model called POLLUTE. A series of modelling runs have been made to back-calculate theleachate water level elevations that have to be maintained beneath the landfill, through operation of thePLCCS, to ensure that the RUG is ultimately met in the bedrock (Gartner Lee, 1993c). These are called thetarget elevations. Leachate water level elevations in the outwash beneath the waste are measured monthlyand compared to target elevations. Section 3.6.2 in this report provided a detailed analysis of the targetelevations. In previous years, most areas within the landfill met their target elevations and, therefore, RUGwill be met in the bedrock beneath these areas.A few small, localized areas of the landfill have not met their target elevations over the past few years, eitheroccasionally or consistently (Section 3.6.2), specifically: monitor 51-II, and monitors 62 through 67. Monitor 59technically does not meet its target elevation as well, but field investigations in 1995 determined that its water levelwas anomalous compared to monitors at surrounding locations 62 through 67, which are used in the assessment;see the 1997 revised modelling report for more details. Monitoring has, however, demonstrated that the impactsin the outwash below the waste are significantly lower than were assumed in the original POLLUTE modelling, tothe extent that RUG can be met in the bedrock despite not meeting target elevations at some locations. Based onrecommendations made in the 1995 Annual Report 10 , further POLLUTE modelling was conducted in late 1996and early 1997 with the more up-to-date data to confirm that RUG will be met (Re-Modelling of Leachate Migrationin the Tills - Eastview Road Landfill, GLL 96-362 dated May 1997). A brief discussion of the findings of thismodelling exercise is provided below for the reader’s benefit.The original modelling incorporated conservative contaminant concentrations for boron and chloride with thesmaller database available in 1992. These concentrations were 20 and 45 mg/L for boron and 1,550 mg/L forchloride. However, the addition of several landfill monitoring locations (15) in both the waste and the outwashbeneath the waste that had been sampled over three to five years, up to 1996, showed that the leachate qualityconcentrations were significantly lower than the original contaminant concentrations employed. This is stronglydemonstrated in the groundwater quality in the outwash beneath the waste. Monitoring of the outwash beneaththe waste has shown that on average boron was about 3 mg/L and chloride was about 350 mg/L since 1994,however, locally some monitors had shown boron and chloride highs of 14.7 mg/L and 960 mg/L, respectively.10. 1995 Annual Report - Section 6.2.3.3, Page 110.(111414_2ra_apr29-09_city_of_guelph.doc) - 30 -