Saline water intrusion from the Fraser River estuary : a hydrogeological investigation using field chemical data and a density-dependent groundwater flow model
Creators: name: Neilson-Welch, Laurie A.
Publisher: name: University of British Columbia
Year: 2009

description: Field data and a density-dependent groundwater flow model were used to understand the processes affecting saltwater intrusion and saline water distribution at a field site located adjacent to the Fraser River Estuary in Richmond, BC. At the field site saline water from the Fraser River intrudes a confined sand aquifer forming a saline wedge which extends laterally approximately 500 m into the aquifer. Chemical analysis of groundwater samples from three multi-level sampling wells (installed to obtain groundwater samples at closely spaced depth intervals), zone-specific piezometers, a West Bay well, and cone penetrometer tests was completed. The groundwater chemical data were used to delineate salinity contours defining the saline wedge. The chemical data were also analyzed using trilinear diagrams. Four chemically different groundwater facies were identified; shallow fresh groundwater above the saline wedge, brackish groundwater at the transition zone, saline water within the core of the saline wedge, and deep fresh groundwater beneath the saline wedge. Water level data from the zone-specific piezometers at the site were analyzed using a tidal filtering technique to determine average groundwater levels. The average water levels were then converted to equivalent freshwater head and compared for wells screened at the same elevation. The results indicated non-uniform flow conditions within the aquifer. Field data were not sufficient to thoroughly define the flow system. The numerical density-dependent groundwater flow and solute transport model Fracdens (Shikaze et al., 1996) was used simulate site conditions. The model was calibrated to field salinity data and a good match was obtained. The results indicated that the average groundwater flow regime at the site is typical for an area influenced by saltwater intrusion (circulation of dense saline water into the aquifer and then flow of saline, fresh, and mixed water toward the river under a regional freshwater gradient). Calibration of the model indicated that a shallow gradient toward the river exists and that a lower permeability zone (lower than that initially estimated based on available field data) may exist in the area of the saline wedge toe. A simulation to investigate possible effects of seasonal changes in river discharge rate on the river-side model boundary conditions indicated that the groundwater flow regime at the site likely varies due to transient conditions at the river. Seasonal influences caused the dilution of the groundwater salinity near the river-side model boundary due to the migration of fresh river water into the aquifer during high river discharge conditions. The regional groundwater flow regime for the Fraser River delta, developed by Ricketts (1998), was modified to account for the effects of saltwater intrusion from the Fraser River and Georgia Strait. A review of available geochemical data and electrical conductivity measurements indicated that saltwater intrusion into the sand aquifer from the Fraser River and Georgia Strait occurs across the delta. Identification of the different groundwater geochemical facies at the field site and comparison to other groundwater data from the Fraser delta supports the conceptual model for regional groundwater flow. There has been significant industrial development adjacent to the Fraser River Estuary and there is some potential for contamination of soil or groundwater at these industrial sites. For sites influenced by saltwater intrusion, this thesis indicates that a unique densitydependent groundwater flow regime develops that could affect the migration of contaminants. Inadequate site characterization plans, inaccurate contaminant transport predictions, and/or ineffective groundwater remediation designs could result if the effects of saltwater intrusion on the groundwater flow regime are not considered. Characterization of the groundwater flow regime in areas adjacent to the Fraser River is complicated by shallow gradients, tidal effects, and the variable density system. Densitydependent groundwater flow modelling can provide a valuable tool to assess the flow regime.