Investigating physicochemical sediment conditions at decayed wooden pile foundation sites in Amsterdam

For thousands of years wooden foundation piles have been widely used along coastal areas and river sites to stabilise urban settlements. Recent investigations, however, indicate that colonising bacteria can destroy the wood even below the groundwater level under low-oxygen conditions and cause severe strength loss over unexpectedly short periods. Existing knowledge of the species involved and the conditions required for growth of the bacteria is insufficient. The aim of this study was to characterise physicochemical site conditions by taking oxygen, redox potential, and chemical measurements at wooden foundation sites in Amsterdam, The Netherlands, that showed different decay intensities but had comparable sediment profiles. Oxygen (optode principle), redox potential (platinum electrodes), and groundwater table measurements were conducted biweekly over a year. Sediment solution was sampled every three months and analysed for pH, conductivity, total and dissolved carbon and nitrogen, NH4+, NO3−, SO42−, PO43−, Cl−, and major cations. Oxygen and redox measurements indicated oxygen-free conditions at both sites examined, but the water surrounding piles exhibited significantly higher total nitrogen concentrations at the site showing light decay. As other simultaneous investigations revealed a positive correlation between the nitrogen content in wooden pile samples and the decay intensities, we conclude that nitrogen might be a key factor in the decay process. We hypothesise that concentration gradients between sediment and wood matrix may control the decay intensity.