NEGATIVE EFFECT OF DEPTH ON SATURATED HYDRAULIC CONDUCTIVITY OF HISTOSOLS

Abstract

We examined the current concept that deeper layers of organic soils have lower saturated hydraulic conductivities (Ksat) because they are more humified, and that such degraded and humus-rich layers deviate from Darcian behavior so that Ksat changes with both time and driving force. All visually discernible layers in 0− to 125-cm depths of eight virgin peatlands were measured and characterized for their degree of decomposition or extent of humification, using eight criteria. The auger hole method was used to measure Ksat of the 0− to 60-, 0− to 100-, and 0− to 125-cm depth layers. Statistical analyses revealed that Ksat decreased sharply as thickness of the soil increased, although the degree of decomposition or extent of humification did not change significantly with the soil thickness. It was thus newly established that Ksat decreases with depth even without a significant increase in the degree of decomposition. It was proposed that the methane, produced and present at lower depths of organic soils, was subjected to pressure, concentration, temperature, and density gradients when an air-filled cavity was created to make the Ksat measurements. The evolved gas bubbles would have blocked some pore spaces due to the Jamin effect. This proposed explanation is in accord with our observations, and it resolves other anomalies reported in the literature.