1.4 Major Environmental Impacts
Some major impacts of anthropogenic land subsidence include:
- increased flood risk (frequency, depth and duration of flooding events) and more frequent inundation induced by rainfall because of the reduced effectiveness of the drainage systems;
- damages to buildings, foundations, infrastructures (roads, bridges, dikes) and underground structures (drainage, sewerage, pipes); and,
- disruption of water management and related effects (change of gradient of streams, canals, drains, increased seawater intrusion, increased pump power).
Moreover, as a result of limited available space, housing, industrial buildings and infrastructures are increasingly located in land settlement‑prone areas, including flood plains and coastal marshes. These conditions may be aggravated in the long term by future climate changes climate causing sea level rise, stronger storm surges and increased precipitation.
Land subsidence causes direct and indirect damages. Direct damages include the loss of functionality and/or integrity of the structures such as buildings, roads, subways and underground utility networks (infrastructures). Indirect damages also occur such as a decrease of farmland productivity in deltaic areas because freshwater availability has been limited by an increase in saltwater intrusion (resulting from a decreased land elevation). The most common indirect effects are related to changes in relative surface and subsurface water levels. The estimation of the associated cost is quite complex. In practice operational and maintenance costs are addressed in several short‑ and long‑term policies and budgeting. In China the average total economic loss due to anthropogenic land subsidence is estimated around 1.5 billion dollars per year 80‑90 percent of which are indirect costs. In Shanghai, over the decade 2001‑2010, the total cumulative loss approached two billion dollars. In Bangkok, Thailand, many private and public buildings, roads, pavements, levees and subsurface structures (sewage, drains) have been severely damaged by land subsidence although reliable estimate of costs are not available. The total cost of damage referred to subsidence in The Netherlands was estimated at over 3.5 billion euro per year.
Unexpected environmental problems can also occur after the cessation of land subsidence. When pumping regulation allows water levels to begin to recover water may begin to appear in unexpected areas. For example, in Tokyo a fast recovery of the piezometric head caused infrastructure damages by buoyant forces acting on the building foundations and groundwater seeped into the basement floor of buildings and tunnels (Tokunga, 2008). In the industrial zone on the Venice mainland, Italy, a significant re‑pressurization of the deep confined aquifers occurred once the pumping was shut down in the early 1970s. More than 400 deep abandoned boreholes, improperly plugged, acted as preferential conduits that supplied water to recharge the phreatic aquifer requiring large water draining and treatment costs for the factories established in the area (Paris et al., 2010).
Prior to development, evaluation of the prospective impacts that groundwater/hydrocarbon production may have on the local environment and a set of guidelines describing steps needed to assess the potential environmental risk and implement a strategy for a “sustainable” development are needed. Three basic major steps can be envisaged in a control program to be set up in advance of the withdrawal inception:
- Prediction of the expected land settlement in the area using the state‑of‑the‑art models. These should rely on the available information supplied by the project related exploratory boreholes and the previous general knowledge of the subsurface basin of interest.
- Continuous monitoring and measuring of the subsidence where environmental, economic and social vulnerability is high. Monitoring should start well before the inception of production so as to identify, with reasonable certainty, the actual consequences of the planned development. Land surface monitoring using methods such as spirit leveling, DGPS, Differential Global Positioning System, InSAR, and Interferometric Synthetic Aperture Radar should be conducted. Monitoring at the depth of the depleted formations should also be conducted using tools like extensometers. A network for measuring micro‑seismicity should also be installed.
- Prevention of the expected anthropogenic land subsidence or mitigation of the settlement experienced during aquifer/field development. Sensitive spots (subsidence values larger than defined sustainable) should be identified and mitigation proposed. A pressure maintenance program including options for recharging the formation with properly treated surface water should be considered.
The activities described above are obviously interconnected and data acquired in one step may be used in the others. For a recent thorough review of the major issues associated with anthropogenic land subsidence due to fluid withdrawal the reader is referred to Gambolati et al. (2005) and Gambolati and Teatini (2015). A discussion that integrates the technical, social, economic, legal, and political conflicts arising from land subsidence is provided by Freeze (2000).