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Land use and water quality dynamics on the urban-rural fringe : a GIS evaluation of the Salmon River watershed, Langley, B.C. Wernick, Barbara Gail


The Salmon River Watershed, Langley, B.C., is on the urban-rural fringe of the Greater Vancouver Regional District. A major aquifer within the Salmon River Watershed provides rural residents with drinking water and maintains stream flow during the summer. The highly mixed land use activities in the watershed, consisting of residential development, commercial agriculture and hobby farming, are resulting in non-point source nitrogen pollution of stream and groundwater. The purpose of this study was to determine how the type, intensity and changes in land use activities have affected water quality. Indicators such as nitrate-N, ammonia-N, orthophosphate, and faecal coliforms and streptococci were used to characterize water quality. Animal unit and septic system densities and nitrogen loading were used as land use indicators. Land use/water quality relationships were analyzed with a Geographic Information System (GIS). The Salmon River and its tributaries are relatively healthy. Most of the water quality indicators met the appropriate criteria for drinking water and aquatic life. Nitrate-N concentrations and microbial counts, however, have been and continue to be a concern. While nitrate-N was below the maximum drinking water quality criterion of 10 mg-N L'1 at all stations there are localized areas where nitrate-N concentrations are above background and reaching levels of concern (5 mg-N L"1). The highest nitrate-N concentrations were measured during low-flow conditions. This suggests that the nitrogen-polluted groundwater is affecting the stream during the summer. In contrast, faecal coliforms and streptocci counts were higher during high-flow conditions suggesting runoff from agricultural fields on which manure is spread in the late fall. More than 3,200 septic systems have been installed in the Salmon River Watershed between 1930 and 1994, a large number of which are located on the Hopington Aquifer. The pattern of increasing septic system densities closely matched the increase in streamwater nitrate-N from up to downstream in both the Salmon River mainstem and Coghlan Creek upstream of their confluence. Agricultural activities are concentrated on large commercial operations. However, hobby farms are becoming a more important component of the agricultural sector in the urban-rural fringe environment. There has been an overall decrease in animal numbers, mostly due to fewer cattle, poultry and pigs between 1986 and 1991. In contrast, horses and sheep, often associated with small farms, increased in number over the same time period. Animal unit densities increased from up to downstream in the Salmon River mainstem to its confluence with Coghlan Creek as does the streamwater nitrate-N concentration. In the Coghlan, however, animal unit densities did not vary, yet the nitrate-N values in this section of the stream increased the most. These results suggest that residential and agricultural uses are both sources of nitrogen in the Salmon mainstem, while septic systems are the primary source in Coghlan Creek. A nitrogen mass balance was used to quantify the sources (manure, fertilizers, the atmosphere and septic systems) and sinks (crop uptake, management losses, dentrification) of nitrogen in the watershed in order to determine the amount of surplus nitrogen being applied. The contribution of septic systems accounted for about 20 % of the surplus loading in the watershed, while large farms contributed about 68 % and small farms 12 % of the surplus loading. There is a poor linear relationship between high nitrate-N values in the stream and corresponding spatial inputs of nitrogen from manure, fertilizers and septic systems. This is due to the highly variable surficial geology, the complexity of groundwater hydrology and the spatial lag between areas of high nitrogen surplus applications and water quality sampling stations. The area near the Salmon River-Coghlan Creek confluence is the most affected section of streams in the watershed and should be used as the key site to monitor environmental quality in the watershed.

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