Sinkholes are caused by erosion of rocks beneath soil’s surface. Groundwater dissolves soft rocks such as gypsum, salt and limestone, leaving gaps in the originally solid structure. This is exacerbated when water is acidic from contact with carbon dioxide or acid rain. Even humidity can play a major role in destabilizing water underground.
The land remains intact until the spaces become too big to support the surface above – called “overburden” – causing it to collapse and create a “sinkhole.” They can be undetectably small or hundreds of miles in diameter.
The prevalence of limestone and history of mining in California makes it vulnerable – though less-so than many other states – to sinkholes.
In addition to manmade pollution contributing to acid rain, other human activities increase the possibility of sinkholes. They can be formed from overdrafting groundwater, which sometimes supports overhangs of land. When the water table falls, that support is lost, causing a sinkhole to form. New water drainage patterns introduce water to new areas not accustomed to accommodating erosion. Additionally, artificial water storage ponds add excessive weight to the supporting bedrock, triggering collapse.
Open sinkholes connect surface and groundwater, introducing any contaminants applied at the site directly to some water supplies. In the past, sinkholes were popular sites to deposit trash, including rusty metals and pesticide containers. These led to highly polluted wells until eventually the problem was identified and the practice mostly stopped.