Rate of occurrence of wood degradation in foundations and archaeological sites when groundwater levels are too low

Abstract

In many places around the world, soils preserve ancient wooden elements. These are related to archaeological objects such as shipwrecks, water wells or tools, but also to piled constructions supporting buildings and waterworks. In countries like the Netherlands, a significant part of the built environment still stands on wooden piles and some have been in function for more than 600 years. For the preservation of this wood, it is essential that it remains waterlogged. Due to climate change, more and longer periods of drought are expected during which groundwater levels drop. Previously waterlogged wood that finds itself above groundwater loses its water protection as oxygen becomes available and wood degrading fungi become active. It is unclear how much time it takes before these detrimental processes start to have an effect. Therefore, to better protect buried wood, an understanding of the degradation processes under dynamic groundwater levels is needed. This includes the conditions under which the fungi become active and when these conditions occur in and around the wood. To address this complex issue, two experiments were conducted: Firstly, under optimal laboratory conditions, the rate of degradation over time by different fungal species in different wood species was determined. Subsequently, the changing conditions in and around wooden piles after lowering the groundwater level were examined in a field experiment. The latter experiment showed that the wood-soil matrix provides long-term protection against degradation. The soil types appear to be an important factor as well as the moisture content of the wood, both determining for how long the protection can be maintained. Together with the degradation speed by fungi, a risk assessment can be made for specific conditions when drought leads to loss of archaeological cultural heritage or to instability of our (monumental) buildings and structures.