Groundwater overexploitation and soil subsidence monitoring on Recife plain (Brazil)

The city of Recife, in the northeast of Brazil, is formed by a coastal plain, which is surrounded by several hills, the Atlantic ocean and a number of rivers that cross the city. The plain was formed by fluviomarine sediment, which was produced by marine transgressions and regressions. Its hydrogeological characteristics and geographical position, located slightly above sea level, lead to water-related problems, such as coastal erosion and frequent flooding. In the last 50 years, an increase in the exploitation of groundwater has caused a lowering of the piezometric surface (up to 100 m at certain points). In porous sedimentary aquifers, pumping fluid decreases pore pressure and reduces the support provided by the overlying layers of soil. This reduction in pressure is caused by the lowering of the piezometric surface and leads to soil deformation, usually called subsidence. As a result of the excessive exploitation of groundwater in Recife, and the consequent decrease in groundwater levels, soil subsidence has become a great concern and requires careful investigation. Geodesic methods of monitoring and quantifying the vertical deformation of soil, caused by the removal of groundwater, have been used around the world. The present study describes a method of assessing the occurrence of soil subsidence in an area where excessive exploitation of groundwater has taken place. High-precision geometric leveling was used to quantify the phenomenon and to perform comparative analysis of altitude values from the year 1958 with more recent altitude values (from 2012 and 2015). The experiments confirmed a difference of 3.86 cm for one of the reference levels, located within the subsidence monitoring area. Thus, the leveling analysis provided estimates of a vertical displacement of approximately 0.68 mm/year in this area.