Rapid Development of Small Steep-Walled Sinkholes Associated with Forest Clearing and Later Water-Table Drawdown

More than 100 small sinkholes, 12 to 50 cm wide and 10 to 80 cm deep, developed rapidly in sandy-clay soils in two adjacent plots in south-central Mississippi, USA, following the installation and operation of a nearby high-capacity water well. The local area is not known for karst features, evaporite dissolution cavities, underground mining, soil erosion around manmade structures, or desiccation fractures of shallow clays. When studied several years after first appearing, the sinkholes had nearly vertical walls, with some cavities widening with depth. Some had tree stumps visible in the center. Further investigation found remnants of stumps or roots in and radiating outward from multiple sinkholes. Installation of piezometers along a transect revealed a continuous hydraulic gradient under the impacted property toward the production well and evidence of reduced overall water levels. The most plausible causes of rapid sinkhole formation are (1) initial partial preservation of tree stumps in soils saturated by a high water table and capillary action in fine-grained surface sediments, (2) lowering of the water table and aeration of near-surface sediments, and (3) accelerated decomposition of stumps faster than soil could creep or cave into the cavities. We propose designating the resulting features as “decomposition sinkholes.”