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An evaluation of water quality and land use in the Salmon River watershed, Langley, B.C, using GIS techniques Cook, Kathryn Emily


The Salmon River watershed is at the rural-urban fringe of the rapidly developing Greater Vancouver Regional District (GVRD) and has undergone substantial land-use change in the last 20 years. This study provides information on spatial and temporal patterns of water quality within the Salmon River, using trace metals in sediments, and nitrate and total phosphorus in water as indicators of the quality of the aquatic environment. Relationships between water quality, surficial materials, and land use within the watershed are examined using a watershed approach combined with GIS techniques. Approximately 80 km² in size, the watershed areas’s land is 50% agricultural, 7% residential, and 25% undeveloped. The glacial outwash deposits in the middle reaches of the river make up the Hopington Aquifer, which is an important source of ground-water supply and baseflow during the low flow period of July through September. The trace-metal concentrations in fine-fraction (<63 μm) sediments from 19 sites within the stream system were compared with the background metal concentration in surficial materials from within the watershed. No evidence was found of elevated total metal concentrations in sediments for the metals Zn, Cr, Cu, Co, Ni, and Mn. At the stations on the glacial outwash parent materials, NO₃-N, specific conductance, and chloride increase in the downstream direction, and are likely due to anthropogenic additions. The difference in specific conductance, chloride, NO₃-N, and total phosphorus from the early 1970s to 1991-93 is small, but localized increases in NO₃-N were observed in Coghian Creek at the confluence with the Salmon River and in Davidson Creek. Seven percent of the over 400 wells within the Salmon River watershed that have been tested during the past 20 years have NO₃-N concentrations above the Canadian drinking water guideline of 10 mg L⁻¹. A spatial lag occurs between the location of wells with elevated NO₃-N and the increase in NO₃-N in the surface water of the Salmon River mainstem. This is attributed to the northward flow of ground water from the Hopington Aquifer into the Salmon River. Residential land use, (1600 septic systems installed since 1970) and agricultural landuse were characterized by: i) a density index, an estimate of the land-use activity immediately upstream of the water sampling station, and ii) a cumulative density index, the density of the land-use activity in the entire watershed upstream of the water sampling station. High density indices of both agricultural and residential activities are present in the areas of high NO₃-N in surface and ground water, which suggests that both of these land-use types are sources of NO₃-N. The pattern of the cumulative density index matches the pattern ofNO₃-N in surface water more closely than the density index, and illustrates that the water quality at a point in the river can reflect the land-use activities of the entire watershed upstream. A spatial lag is observed between the higher density of land use activities and the increase in NO₃-N in surface water. Examining relationships between land use and water quality in the Salmon River watershed is complicated by multiple sources ofNO₃-N in close proximity, changes in the relative importance of sources within different locations in the watershed, and the temporal and spatial lags between NO₃-N leaching from the land surface and its detection in the ground and surface water. Using Spearman rank correlation, both agricultural activities and septic systems are correlated to NO₃-N contamination of ground water. However, the approach used in this study does not allow for the determination of the relative importance of these two sources.

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