Reconstruction of urban stormwater contamination of an estuary using catchment history and sediment profile dating

Cores were collected from the length of Pakuranga estuary, a small urban estuary in Auckland, New Zealand, to determine sedimentation and contaminant history, and in particular the impact of urbanization. Catchment sediment loads for the most recent history (1953–1995), including urbanization since 1960, were reconstructed using the landcover history and soil erosion modeling. Pollen and¹⁴C dating and pre-urban landcover history were used to reconstruct early estuary sedimentation (i.e., post-3000 yr BP to 1960). Heavy metal concentrations, particle size,¹³⁷Cs, pollen, and catchment sediment loads were all needed to disentangle the complex estuarine response to urbanization.¹³⁷Cs profiles did not reflect the historical fallout pattern, but deposition of¹³⁷Cs-labelled eroded catchment soil, coinciding with peaks in urban construction. Temporal variations in stormwater¹³⁷Cs concentrations are likely due to varying contributions from¹³⁷Cs-rich topsoil and¹³⁷Cs-poor subsoils. A similar pattern was observed in heavy-metal concentrations and attributed to street runoff rather than topsoil being diluted by metal-poor subsoils. Dating of the sediment profiles showed that during urbanization sedimentation rates in the tidal creek and estuary were higher than sedimentation rates associated with past agricultural landuse and the original forest landcover. Urbanization has brought about substantial environmental changes in the upper estuary through continued infilling of shallow, intertidal areas, contamination by heavy metals to levels of ecological concern, sediment textural changes, and rapid mangrove colonization of formerly bare intertidal sediments.