Abstract
Abstract. The combination of groundwater withdrawal, hydrocarbon extraction, salt-dome movement and faulting have caused widespread subsidence in the Houston-Galveston region (HGR). Subsidence results from primary consolidation consisting of inelastic (nonrecoverable) and elastic (recoverable) compaction caused by subsurface fluid withdrawal and secondary consolidation (creep) over time caused by overburden pressure. Subsidence in the HGR is monitored using borehole extensometers that were installed at 13 locations across Harris and Galveston counties between 1962 and 1980. By 1977, withdrawals from the Chicot and Evangeline aquifers resulted in groundwater-level declines of about 114 and 115 m relative to predevelopment water levels, respectively in parts of Harris County. By 1979, as much as 3 m of land subsidence was estimated to have occurred in localized areas of the HGR. Land subsidence can be hazardous in populated areas because it exacerbates the effects of storm surge and impedes storm-water runoff by decreasing land-surface elevations in areas where water accumulates. To assess aquifer compaction in response to changes in groundwater levels, a bulk land-surface subsidence rate is assumed to be the sum of the primary consolidation rate and the negligibly variable component of overburden pressure referred to as the “pseudo-constant secondary consolidation rate.” From 1931 to 1976, groundwater levels decreased as groundwater withdrawal rates increased from 0.57 to 4.3 million m3 d−1, causing pressure heads in aquitards the Chicot and Evangeline aquifers to continually decline. In response to reductions in groundwater withdrawal rates from 4.3 to 3.0 million m3 d−1 between 1976 and 2001, groundwater levels rebounded, decreasing inelastic compaction rates in some parts of the HGR from as much as about 40 mm yr−1 in the early 1980s to negligible amounts by 2000. Inelastic consolidation from about 1937 to 2000 contributed to land-surface subsidence and its associated effects. Land-surfaces have rebounded in localized areas of the HGR where groundwater levels rebounded significantly. Pseudo-constant secondary consolidation rates were computed at each of the 13 extensometers and ranged from 0.48 to 8.49 mm yr−1 in areas where groundwater levels in the two aquifers were stabilizing. This secondary consolidation subsidence is beyond the control of any groundwater-level management schemes because it is caused by geohistorical overburden pressure on the two aquifers.
Highlights
Land subsidence (LS) can be a gradual settling or sudden sinking of the Earth’s surface owing to subsurface movement of earth materials (Galloway et al, 1999)
From 1931 to 1976, groundwater levels decreased as groundwater withdrawal rates increased from 0.57 to 4.3 million m3 d−1, causing pressure heads in aquitards of the Chicot and Evangeline aquifers to continually decline (Kasmarek, 2013)
A total of 19 groundwater level wells in the two aquifers at or near the 11 extensometer station locations were employed to provide supporting evidence to be used in conjunction with the extensometer data in order to analyze changes in inelastic and elastic compaction and secondary consolidation in response to trends in groundwater-level fluctuations based on the methodology (Eq 4)
Summary
Land subsidence (LS) can be a gradual settling or sudden sinking of the Earth’s surface owing to subsurface movement of earth materials (Galloway et al, 1999). More than 80 % of the subsidence in the nation has been identified as a consequence of human impact on subsurface water (Galloway et al, 1999). Aquifer system compaction due to subsurface-fluid withdrawal was recognized in the Houston region in the early 1920’s. The theory of aquifer system compaction due to subsurface-fluid withdrawal along with the more than 40 years of recorded compaction in the Houston region can be employed to analyze secondary consolidation due to geohistorical overburden pressure. It is assumed in this paper that a bulk land subsidence rate from aquifer-system compaction is the sum of inelastic and elastic compaction rates due to groundwater withdrawal as well as secondary consolidation rate due to geohistorical overburden pressure
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