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UBC Theses and Dissertations

Vulnerability to phosphorus loss : subtitle identifying sites and their characteristics in the Elk Creek watershed, Chilliwack, B.C. Schendel, Emily Kate

Abstract

A growing concern over the quality of water in agricultural areas has increased the interest in interactions between land use and aquatic ecosystems. Of particular interest is the role phosphorus (P) has in increasing eutrophication and degrading water quality. In freshwater systems, P is usually the limiting nutrient in the cycle of events leading to the increase in plant productivity, the depletion of dissolved oxygen, and the destruction of fish habitat. This thesis attempts to address the source of these problems by documenting the P status and the P dynamics in the Elk Creek watershed. Linkages are made between land use, fertilizer and manure applications, and inherent soil characteristics with sediment and water quality analyses. Several different P measurements, including soil test P (STP) using both Bray PI and P2 extractions, organic P, total P, and P adsorption, are used to determine the current status of P in the watershed. Base saturation, manganese, carbon content, pH, and P inputs have the greatest influence on the amount of available P in soils, while nitrogen, carbon, and base saturation have the strongest relationships with total P. The adsorption of P in the Elk Creek soils is influenced by the carbon content, texture, pH, and the cation exchange capacity. The land cover also influences adsorption capacity. Organic soils in this study appeared to have unlimited adsorption capacity, while the naturally occurring sesquioxides in the forest soils allowed for very high sorption capacities. In contrast, the sandy soils of the golf course located in the eastern hillside area of the watershed had extremely low adsorption capacity. The STP measurements as well as two indexes, the P Index and the Degree of P Sorption (DPS) index, were used to identify areas within the watershed that are vulnerable to P loss. Spatially, assessments of available P show an increase in concentration with distance downstream. The sites located near the mouth of the watershed are described as having excessive available P and are vulnerable to P loss. This assessment agrees well with the findings of the P Index, which also shows an increase in vulnerability for P loss, particulady in the mouth area. The P Index, which incorporates both input and transport variables, is strongly influenced by the degree of land cultivation. The DPS index did not exhibit spatial trends. However, the estimation of the Langmuir adsorption maxima (which is the basis of the DPS index) did show a general decline in the sorption capacity with distance downstream. Again, this would agree with an increase in vulnerability. The phosphorus content in sediments and streamwater correlated well with the soil vulnerability rating. The sediments are depleted of nutrients with respect to the corresponding soils, but do show and increase in concentration with distance downstream. Of the water quality indicators, ammonia, total P, fecal coliform, and dissolved oxygen concentrations show strong spatial relationships and agree with the vulnerability assessments. The DPS index did not relate well to the water quality indicators. This model is effective at indicating the degree of saturation, but does not take into account the amount of organic P in soil or the transport factors that make the P Index the more successful model. Accounting for organic P in soils is particularly important, as it has been shown within the literature to be less readily adsorbed, highly mobile, and a major component of the P contributions to runoff and leachate. The possible impacts of future land use changes, such as further agricultural intensification or urban expansion into the forest soils, are now better understood to be actions that would increase the vulnerability for phosphorus loss. Mitigation options to reduce P loss are currently needed, and will be necessary in the future to protect the balance maintained in the Elk Creek watershed.

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