MICRO-SINKHOLE DEVELOPMENT FROM GROUNDWATER PUMPING IN A NON-KARST SYSTEM

Groundwater pumping from a single well in a sand aquifer in Magee, Mississippi generated hundreds of small sinkholes, generally less than 35 cm in diameter and up to a meter in depth. Sinkholes first began appearing soon after heavy pumping began from an industrial well located approximately 100 m north of the property, and continued to increase in number and size over the next 7 years. Sinkholes were only identified on land that had been cleared of trees at an unspecified date. Several sinkholes were found forming around highly decayed tree stumps that had been cut off at ground level. A trench dug adjacent to a larger sinkhole with no visible stump had vertical sides nearly a meter in depth, and decayed roots were found angling downward at the base of the hole. Water levels monitored beneath the property over a period of several months documented drawdown of the water table by the industrial well, with clear responses to initiation and cessation of pumping each day.

The apparent cause of sinkhole formation is a decrease in water saturation surrounding old tree stumps that allowed accelerated decomposition. The sand aquifer is overlain by approximately 6 m of sandy clay that extends to the surface. Water levels during the period of monitoring were all below the base of the sandy clay. Prior to heavy pumping, the water table was likely within the sandy clay, allowing capillary action to keep saturation levels high around the stumps and minimize the rate of decomposition. After lowering the water table below the sandy clay, desaturation and increased exposure to oxygen resulted in rapid decomposition of the partially decayed stumps, and the formation of micro sinkholes.