Pore-water chemistry and its influence on rock mechanical properties and hydrogeophysical processes in a mudstone slope in the southwestern Taiwan badlands

Abstract

Bedrock characteristics and its influence on hydrogeophysical processes in a Plio-Pleistocene mudstone slope were examined in southwestern Taiwan. Previous works carried out in this area indicated seasonal patterns of the bedrock physical properties and deterioration of the mechanical strength during weathering to generate extremely high rates of erosion during heavy rainfall, thereby forming badland topography. This area has a humid subtropical climate that is characterised by distinct dry and rainy seasons. During the rainy season, high rates of erosion of the weathered mudstone occur during rainfall events. To investigate impact of characteristics of bedrock physical properties on hillslope weathering and erosion processes in this area, we analysed the pore-water chemistry, specific surface area, and swelling of the mudstone of the studied slope and performed in situ monitoring of water content and pore-water electrical conductivity at 2.5–40 cm depths near the slope surface. The results indicate that the mudstone contains saline pore water, is categorized as dispersive, and has a dense fabric, thereby favouring the occurrence of rock mechanical breakdown due to chemical osmosis. The influence of rock mechanical properties and bedrock behaviour on the hydrogeophysical processes near the slope surface varies with the season. During the dry season, the water content of the mudstone near the slope surface decreases, causing desiccation cracks to form and concentrating salt on rock surfaces. Water contents at 40 cm depths increased within 1 h after rainfalls during the monitoring intervals of the dry season, suggesting that surface water infiltrates rapidly by gravity into networks of cracks during infrequent rain events. During the following rainy season, intense rainfall events moisten the slope surface layer and increase the volume of the mudstone by swelling caused by chemical osmosis. The swelling is associated with an enlargement of the pore size of the mudstone during the dilution of saline water by fresh rainwater, with the greatest amount of swelling occurring at or near the surface where salt is most concentrated. Water contents at 40 cm depths scarcely responded to rainfalls during the monitoring intervals of the rainy season, suggested that this swelling closes desiccation cracks and prevents surface water from migrating downwards into the slope. The resulting seasonal erosion occurs by slope surface wash caused by intense rain during the late rainy season. The newly exposed mudstone on the slope starts to become desiccated during the subsequent dry season, and a new cycle of weathering commences.