Spatially-detailed assessment of phosphorus discharge with groundwater to an urban stream

Alexander Fitzgerald, James W. Roy, James E. Smith

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Groundwater-Surface Water Interactions I

ABSTRACT: Urban streams commonly suffer from elevated nutrients, including phosphorus (P), which can lead to eutrophication problems. These nutrients are generally attributed to runoff (e.g. combined sewer overflow) and wastewater treatment plants. It is commonly assumed that P is not readily transported with groundwater; however, there are some indications of elevated dissolved phosphorus detected in groundwater near streams and lakes. The objective of this study was to assess the potential contribution of P from groundwater to a small urban stream impacted by relic landfill disposal. Spatially-detailed measurements of shallow pore water soluble reactive phosphorus (SRP) concentrations and groundwater fluxes derived from continuous temperature profile measurements were combined to derive a SRP mass flux plot for the streambed interface of a 28-m long reach. Such fine-scale measurements of nutrient discharge using shallow streambed data like these have not previously been reported. Both concentrations and groundwater flux were spatially variable, and these tended to be inversely related. Zones of higher SRP were associated with iron and sulphate-reducing conditions, which may have been promoted by the presence of the landfill. However, the SRP concentrations were not that well correlated with those of other contaminants likely sourced from the landfill (e.g. ammonium, saccharin). These findings suggest that the SRP was sourced from local sediments, with release to groundwater under reducing conditions. Integration of the SRP mass flux plot gave a total mass discharge of about 8-10 g/day for this small reach, which compares to a total stream SRP discharge at this location of 10-120 g/day under base flow conditions. This study did not assess the fate of the SRP at the streambed interface or in the stream, where it might adsorb or be taken up for biological needs; however, the findings suggest that groundwater can be an important source of P to urban streams.

Please include this code when submitting a data update: GEO2013_190

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