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Denitrification in the Abbotsford acquifer and the influence of a stream environment

University of British Columbia

The Abbotsford aquifer has an area of approximately 200 km², half of which lies in British Columbia. The general groundwater flow is southerly, and thus, issues surrounding the aquifer, such as nitrate contamination have gained interest due to their transboundary nature. The protection of groundwater as a resource is a concern in both Canada and the United States. This study investigated the nitrate reduction occurring in an area just north of the border. The study site was chosen due to the presence of Fishtrap Creek and it's unique surrounding geology. Due to a highly variable water table in the aquifer, an intimate relationship exists between the groundwater and surface water in Fishtrap Creek. The conditions which support denitrification were found to be present at the site during the 11-month sampling period. Nitrate concentrations have been monitored over the entire aquifer for decades, and were found to exceed safe water drinking levels (10 mg N/l) in several areas. In general, nitrates are present over the entire aquifer. Within the study area, nitrate-N was found to exceed the safe limit in only three wells. Several wells contained trace amounts of nitrate, as well as low dissolved oxygen levels. Thus, in these areas, which are along Fishtrap Creek, nitrate reduction has occurred. The water chemistry of both Fishtrap Creek and the surrounding groundwater was monitored bi-weekly over the 11-month period. Stiff diagrams and piper plots were employed to group different water types present within the aquifer. Results from this study were similar to those found from studies performed over the entire aquifer. Water chemistry at Zero Avenue is representative of a mixture of water from Huntingdon Avenue, the culvert, and infiltrating groundwater. The comparison of ratios of nitrate and chloride present at various locations provided insight to the amount of dilution occurring. These results suggested that nitrate reduction was occurring. A flow balance performed on Fishtrap Creek revealed that flow was typically lost between Huntingdon Avenue and the bridge at FT5, but gained over the entire reach. During lower flows, groundwater seeps were visible along the stream, especially in the lower section of the reach. The flow in Fishtrap Creek was calculated to be approximately 25 percent of the flow through the aquifer. Thus, the potential for Fishtrap Creek to play a significant role in denitrification exists. The amount of uncertainty associated with flow measurements is dependent on the accuracy of the equipment used. Flow measurements were taken accurately to 0.005 m3/s, and thus mass balance results should be adequately reliable. Local groundwater flow is influenced by Fishtrap Creek since this is an area of significant discharge. Nitrate fluctuations coincided with fluctuations of the amount of flow gained by the creek. As well, the amount of flow gained by the creek was influenced by the gradient between the ground and surface water. An average annual loss of 1.06 mg N/l occurred between upstream and downstream locations. Comparing the loss of nitrate to that of chloride, a conservative tracer, the nitrate reduction occurring due to dilution can be observed. On several occasions, the proportionate amount of nitrate reduction exceeded that of chloride reduction, and thus means of reduction besides dilution exist. By considering water levels, along with water chemistry, it is apparent that an intimate relationship exists between the surface and ground water. Nitrate reduction is occurring along Fishtrap Creek and this area serves as a significant nitrogen sink. However, these results are unique to this study area, and may not be applicable south of the border. Thus, further studies are required to better understand the application of these results to basin management.

Thesis/Dissertation

application/pdf

2009-05-26

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http://hdl.handle.net/2429/8181

Civil Engineering

University of British Columbia

UBCV

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