Subsidence: The Problem of Sinking Land
Groundwater supplies 38% of the drinking water in the United States, and nearly half of the drinking water in the world. Groundwater is a freshwater source contained in aquifers that is extracted for human consumption by wells that are drilled into the aquifer. Aquifers are underground formations of rock and sediment, where water can move easily. Most rock and sediment have spaces between their grains called pores, which is the open space that water fills up. This is the porosity of an aquifer. An aquifer is depleted when more water is pumped out than is recharged. An aquifer is naturally recharged when precipitation infiltrates the soils and moves back into the pore spaces of the aquifer.
Land subsidence is an anthropogenic phenomenon that occurs when large quantities of groundwater are depleted from an aquifer without being recharged. Since the water has been removed from the pores between the sediments, the sediments compact and the land sinks. This can result in drastic changes like a sinkhole, or can result in the gradual sinking of land. The image above shows the undulating buildings in Mexico City, Mexico that were once on flat land. Land subsidence is widespread around the globe, and is the most threatening consequence of over depletion of groundwater. The maximum recorded land subsidence was 14 meters in New Zealand. In the United States the land has subsided 10 meters in the San Joaquin Valley, California. Large areas of the globe are impacted by subsidence and a new study released in 2020 funded by UNESCO, estimates that 19% of the global population lives in areas that are directly impacted by land subsidence.
Subsidence causes loss of integrity and functionality of human infrastructure. Foundations, bridges, and roads crack when the land shifts below, leading to expensive repairs or abandonment of structures. In Mexico City many buildings have been deemed unsafe or have been closed because of the impacts to their foundations. Subsidence damage to California infrastructure including canals and bridges has cost the state billions. Subsidence can also damage underground infrastructure such as water, sewer, and drain pipes. The change in relative surface and subsurface water levels also costs cities millions by requiring changes to existing pumping techniques and depths of wells.
Subsidence also leads to adverse impacts on environmental processes. Saltwater intrusion occurs when coastal land sinks, and saltwater moves in to fill the space that freshwater previously filled in an aquifer. This can adversely impact coastal farms and drinking water supplies. Subsidence can also lead to increased flooding and inundation from rainfall due to the compaction of the soils.
Solutions for groundwater subsidence are complicated because many impacts are here to stay. Additionally, land subsidence can go unnoticed until negative impacts occur. Preventative solutions are the best, and ensuring that the rate of recharge in aquifers is equivalent to the rate of depletion prevents land subsidence. Recharge can be left to natural processes, or water can be pumped into the aquifer through artificial aquifer recharge. A downside of artificial aquifer recharge can be anthropogenic land rebound or uplift, where infrastructure is negatively impacted from upward hydrologic forces if the aquifer is re-filled too quickly.
The best solution for large cities over depleting their aquifers is to find water sources elsewhere, although that is a solution that is easier said than done. Advancing technology has given engineers the tools to aid politicians and decision makers in prioritizing subsidence mitigation and prevention. Additionally, societal concern has been growing as the issue of subsidence has impacted the lives of millions and made the front page of newspapers, such as with the sinking of Venice and Mexico City.
Federal legal response in the United States has mostly addressed preventing or repairing effects of land subsidence from mineral extraction. The Code of Federal Regulations (CFR) in the United States addresses subsidence resulting from mineral extraction in 30 CFR § 817.121 by requiring that extraction technologies prevent subsidence and requiring permit holders to repair any subsidence damage on the permittees lands. Many states, including Colorado, have adopted their own version of the federal regulation to remediate and limit land subsidence from mineral extraction. Land subsidence from groundwater pumping has been addressed on the state level, especially in states where subsidence is a major issue. In 2014 the California legislature passed the Sustainable Groundwater Management Act (SGMA), which requires local agencies to develop and implement groundwater sustainability plans to mitigate overdraft of the aquifers in the next 20 years.