Foreword
Fresh groundwater resources provide many benefits to humanity and ecological systems, but pumping water from aquifers often has problematic consequences. One such consequence is subsidence of the land surface in urban areas caused by compaction of the low-permeability clay or silt beds that overly, underly or occur within the pumped aquifer, which is the subject of this book. The compaction occurs because the porosity of clay and silt beds decreases as the pore-water pressure declines. The aquifer provides the water while the low permeability beds cause the problem of compaction-induced subsidence of the land surface.
People living on the subsiding land do not sense that their land surface is changing position as when earthquakes occur. Substantial subsidence occurs over a period of years or decades, but the municipal engineers see subsidence effects early because slight subsidence begins to cause water problems due to changes in the slope of streets, ditches and sewer lines such that rainfall cannot escape as before. Areas where substantial subsidence causes problems are a small fraction of the global area in which aquifers have been heavily pumped because most low-permeability beds associated with aquifers are not porous enough to undergo large decreases in porosity. However, a substantial fraction of the global population lives in cities where subsidence is problematic and increasing. This is because the geology of aquifer systems most prone to compaction and subsidence are situated along coasts, especially where rivers discharge to the ocean. This geographical setting typically has thick soft, exceptionally porous deposits of young geological age so that minimal compaction has occurred over geological time, which makes human induced compaction likely. This geological setting exists where most of the world’s expanding megacities are located including Alexandria, Bangkok, Dhaka, Ho Chi Minh City, Houston, Jakarta, Lagos, New Orléans, Rotterdam and Shanghai. Land subsidence is one of the world’s underrated problems with consequences exacerbated by sea level rise as the climate changes.
The two authors of this book, Beppe Gambolati and Pietro Teatini, are distinguished professors in Italy and it is fitting that this book comes from Italy where the magnificent City of Venice has been recognized globally for its land subsidence problem for centuries. Although subsidence is primarily a result of groundwater pumping, the quantitative study of subsidence now resides in the discipline of geotechnical engineering and soil mechanics, which is where these book authors are founded and have global experience concerning subsidence problems.
John Cherry, The Groundwater Project Leader
Guelph, Ontario, Canada, June 2021