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Assessment of cumulative effects of urban and agricultural land uses on indicators of water and stream quality Wilson, Julie Elizabeth


This study assessed the cumulative effects of intensive urban and agricultural land use in Marshall Creek, B.C. The focus was to document how groundwater from the Abbotsford Aquifer and surface water from urban and agricultural tributaries contribute to cumulative stream quality downstream. Landscape stressors such as livestock numbers, Total Impervious Area (TIA) and human population numbers were related to the quality of water, bed sediment, suspended sediment and biofilms. Biofilms and suspended sediments were selected to provide an integrated indicator of potentially bioavailable contaminants. Nitrate-N concentrations in Marshall Creek are strongly influenced by groundwater discharges from the Abbotsford Aquifer, which has elevated nitrate-N concentrations due to non-point source agricultural activities. Nitrate decreased in the downstream due to significantly lower nitrate-N inputs from the tributaries (M-W U, p<0.015). In contrast, Soluble Reactive Phosphorus (SRP) and TOC increased downstream from agricultural runoff. Cu, Mn and Zn have increased in bed sediments in Marshall Creek since 1993. P concentrations have not changed markedly over time, but are significantly higher at the mouth than in the headwaters. SRP, NH4+-N, Cl-, turbidity and TOC were significantly higher in water in the agricultural tributary than the urban tributary (M-W U, p < α=0.0167). Differences in bed sediments, suspended sediments and biofilms showed that concentrations of P were significantly higher in the agricultural tributary than in the urban tributary (M-W U, p < 0.001), which is the result of nutrient and organic inputs from manure in the agricultural subwatershed. Biofilm chl-a and trace element concentrations were significantly different between sites and seasons. In the summer, chl-a and P were highest in the agricultural tributary, whereas in the winter chl-a was highest in the urban tributary due to warmer water temperatures in urban runoff. Trace element concentrations (e.g. Cu, Ni, P) were significantly higher in suspended sediments than in bed sediments or biofilms at all sites as a result of the higher surface area and greater organic matter associated with fine suspended sediments. Current agricultural activities are key contributors to the cumulative effects observed on Marshall Creek. Thus, improved livestock density and manure management practices are needed.

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