HYDROGEOLOGICAL MAPPING AND NUMERICAL MODELING OF A TCE PLUME IN GRANULAR AQUIFER, VALCARTIER, QUEBEC, CANADA

The Valcartier Military Base and the Defence R&D Canada alcartier (DRDC) are located about 35 km North of Quebec City, in Canada. In 1997, trichloroethene (TCE) was found in groundwater at the site. Following the discovery of TCE in private wells in Shannon (a neighbor municipality), a characterization program was carried in 2001. This program involved the drilling of 205 boreholes, 65 of which with a Rotasonic drill and continuous soil sampling, the installation of 387 observation wells and 183 direct-push TCE profiles with Geoprobe.

The total TCE plume length is 4.5 km and TCE is detected over an area of about 2 800 000 m2. The plume is 650 m wide at the Western limit of the Valcartier Base.. There are three main potential source areas up-gradient of the plume. The maximum TCE concentrations in these areas are about 1000 ppb on the North side and 4500 ppb on the South side of the potential source areas. The dissolved TCE plume migrates along the complex groundwater flow path which is strongly influenced by the presence of a buried valley.

The hydrostratigraphic setting consists of a paleo-delta composed of sand lying on a permeable fluvio-glacial unit. The main aquifer unit in thickness is the deltaic sands. In the Western part of the site, a semi-confining silt unit splits the deltaic sands in two separate aquifers. Unconfined conditions exist above the silt unit and where this unit is absent. Semi-confined conditions occur under the silt unit. In the unconfined aquifer, the groundwater flow pattern at the site consists of converging flow from the North and South to the center of the site, and then a groundwater divide splits this flow to the West and East in the buried.

A modeling study was done by using the finite element groundwater modeling code FRAC3DVS which simulates 3D groundwater flow under saturated and unsaturated conditions. The model was built using the characterization data that comprised detailed stratigraphic descriptions, 750 monitoring wells, 700 slug tests for hydraulic conductivities. TCE transport was reproduced by using particle tracking from the different potential source areas. Inverse particle tracking was performed to define the 3D well capture zone of the drinking water wells in the area. Advective-dispersive TCE transport was also done to broadly reproduce the geometry and concentrations of the TCE plume.