The importance of ‘Spekkoek’ for the hydrological functioning of wetland ecosystems in Pleistocene aeolian sand landscapes A case from northwestern Europe

In recent studies in the south of the Netherlands, a specific sediment type consisting of an alternation of loamy and sandy laminae, known as ‘spekkoek’, was identified as a key factor in the formation of peatlands in these otherwise well-draining Pleistocene sand landscapes. The areas are situated in the transitional zone between cover sands and loesses in the North European Plain, in which such spekkoek is seemingly quite common. We studied its hydraulic conductivity, bulk density, texture and chemical composition, and identified the loam as reworked (sandy) loess. The spekkoek is very compact and – for sandy sediments – has a high bulk density and a low hydraulic conductivity. It is usually present at shallow depth and impedes the vertical flow of water. Spekkoek forms a substrate upon which peatlands developed and can (re)develop, because it allows for minimal infiltration and subsequent stable water tables. Our results underline the importance of understanding soil properties and respecting the soil-archive in conservation or rehabilitation projects. To date, incorporating the properties of such spekkoek in hydrological models underlying policy-decisions remains a serious challenge. This calls for action to overcome the limitations of such models and to develop models that can cope with the complexity of hydrological systems with such surficial spekkoek strata and associated perched water table.