Investigation of Land Subsidence due to Fluid Withdrawal

Land subsidence can occur on local and regional scales worldwide and is often discovered only after damage to buildings and important infrastructure occurs. Many mechanisms cause land subsidence; much known subsidence occurs along coasts where subsidence contributes to relatively higher sea levels and coastal flooding, which may be exacerbated by global warming. This book explores problems associated with subsidence caused by aquifer system compaction accompanying subsurface fluid withdrawal, including changes in the natural gradients of streams, as well as the as-built gradients of engineered water conveyance structures, increased riverine and coastal flooding, and damage to wells penetrating the affected lithologic units.

Subsidence from groundwater withdrawal develops principally under two types of contrasting environments and mechanical processes. The first consists of aquifer system compaction in unconsolidated deposits, which comprise a significant fraction of fine-grained compressible material. The second consists of collapsing cavities in areas with underground voids such as those that occur naturally in karst terrains. This chapter focuses on the development of groundwater resources in aquifer systems; most subsidence attributed to subsurface fluid withdrawal is caused by groundwater extractions. The similarities of the subsidence processes in hydrocarbon and geothermal resource development are detailed. Compaction or land subsidence associated with extracting fluids such as water, crude oil, and natural gas from subsurface formations is also analyzed.

This chapter focuses on aquifer mechanics and land subsidence resulting from aquifer system compaction caused by groundwater withdrawals. It provides details on how the various approaches to analyzing and modeling deformation of aquifer systems are derived from the basic relations between head, stress, compressibility, and groundwater flow. Specific emphasis is on the hydrogeologic approach and regional aquifer system compaction, but many links to the soils engineering and geotechnical approaches are included. In addition, emphasis is given to the hydrogeologic approach that is associated with the regional nature of groundwater extraction processes. The chapter also includes an assessment of the use of stress–strain relations to calibrate storage and compressibility parameters of aquifer systems with certain assumptions.

In the absence of known historical subsidence, the identification of areas that are susceptible to subsidence accompanying the withdrawal of groundwater may be difficult. Characteristics of general geology and hydrogeology can be evaluated for the presence of unconsolidated alluvial or basin fill deposits that comprise a significant fraction of susceptible fine-grained deposits within an aquifer system. This chapter describes the methods used for measuring vertical and horizontal displacement of the land surface and subsurface deposits. Error in the Global Positioning System (GPS) position of the aircraft is propagated to the land surface measurements. Increasing the number of ground-based GPS control base stations minimizes the aircraft-position error and favors a higher-quality airborne Light Detection and Ranging (LiDAR) dataset. GPS surveys are therefore evaluated in the context of the reliability of GPS measurements, which generally have a smaller uncertainty than airborne LiDAR measurements.

The analysis of subsidence-prone areas generally involves evaluation of the material surface and subsurface geologic properties, as well as the potential physical and chemical processes that cause land subsidence. Empirical methods use the available data to derive relations between some forcing function, such as head, and subsidence or compaction. These are input–output models that typically involve time series of the measured inputs and outputs. This chapter discusses the simulation technique, which is based on the conceptual aquitard-drainage model. The quasi-theoretical approach utilizes the relation between subsidence and related phenomena. Some principal theoretical bases of several aquifer system compaction models are presented. Several types of subsidence models are examined, with principal focus on the more widely applied aquitard-drainage model and the emerging poroelasticity model.

Methods used to mitigate subsidence in subsidence-affected areas may be categorized as reduced groundwater withdrawal (conjunctive use, conservation, and regulation) and artificial aquifer recharge. The implementation of both methods is facilitated by the availability of local supplemental surface water or imported water supplies. The principal goal has been to manage the overall water supply and distribution so that the water levels in wells from the susceptible hydrogeologic units of the aquifer system, or systems, are maintained above critical thresholds. An approach is presented for retarding or arresting subsidence through repressuring of aquifer systems by artificial recharge directly through wells. Most problems with well recharge are related to clogging of the well and aquifer, and it is concluded that wells should be recharged with treated water to avoid the clogging problems.