The most important of these units is 2 m thick and forms a low-permeability clay layer at about 15 m depth. Organic-rich soil horizons also occur in the sequence. The region has been slowly deformed over the last several million years and this has resulted in local fracturing of consolidated sediments of one part of the site. The subsurface hydrology of the site is governed by recharge in the higher terrain adjacent to the site. Groundwater then travels through the site in heterogeneous porous media and fractures, eventually discharging to a small river about 10 km from the site.
Flow velocities vary widely due to the heterogeneous nature of the of the sequence.
Depth to water table is approximately 20 m with little seasonal variation. During operations, a localized groundwater mound several meters above the natural water table occurred beneath the drainfield. Groundwater is relatively alkaline and ferrous iron increases down-gradient. The site has been characterized by 10 boreholes. Because of the limited number of data points, the contaminant plume in the groundwater has been mis-located by 0.5km.