Relative Effects of Land Use and Near-Stream Chemistry on Phosphorus in an Urban Stream

Elevated levels of P in urban streams can pose significant water quality problems. Sources of P in urban streams, however, are difficult to identify. It is important to recognize both natural and anthropo-genic sources of P. We investigated near-stream chemistry and land use factors on streamwater P in the urbanizing JohnsonCreek watershed in Portland,OR,USA.We sampled streamwater and shallow groundwater soluble reactive P (SRP) and total P (TP) andestimatedP flux at 13 sites along the main stem of Johnson Creek, with eight sites in urban land use areas and five sites in nonurban land use areas. At each site, we sampled theA andBhorizons, measuring soil pH, water-soluble P, acid-soluble P, base-soluble P, total P, Fe, andAl.We found continuous input of P to the stream water via shallow groundwater throughout the Johnson Creek watershed. The shallow groundwater P concentrations were correlated with streamwater P within the nonurban area; however, this correlatio