GEOCHEMISTRY OF THE HYPORHEIC ZONE IN THE LOWER FRASER RIVER

High resolution geochemistry data was obtained from the hyporheic zone of the lower Fraser River using groundwater chemical profiling and sediment cores. The Fraser River drainage basin covers an area of 238,000 km² in central and southwestern British Columbia with average annual flows of 3500 m³/s at the mouth, peaking at approximately 10,000 m³/s during freshet. Samples were collected adjacent to a PAH-contaminated site in the tidally influenced region of the river, 100 m offshore in 10-12 m of water. The objective of the study is to examine the behavior of iron in the hyporheic zone of a major river. Sediment cores were split into 10 cm sections and subjected to the following analyses: 1) Wet chemical extractions, 24-h CaCl₂, 24-h 0.5M HCl and 21-d 5M HCl, 2) Sediment surface area, 3) Rietveld XRD analyses for mineralogical composition.

Our geochemical results suggest samples from the PAH-contaminated aquifer and samples from the hyporheic zone show no significant difference in the amount of sediment-bound iron. This indicates no accumulation of iron precipitates on the sediments along the flowpath as might be expected when iron-rich (10-90 mg/L) anoxic groundwater mixes through tidal smearing with oxic river water in the hyporheic zone. Other research at the site shows that the hyporheic zone is approximately one meter in vertical extent. The amount of total extractable iron in both zones (15-30 mg/g) is considered relatively high. The lack of noticeable iron accumulation in the hyporheic zone may indicate that iron precipitates on shallow sediments that are subsequently scoured from the river bed during freshet.