Abstract:Short-term changes in the hydraulic head of surface water bodies are known to influence the shallow response of hydraulically connected groundwaters. Associated with these fluctuations is the physical increase in stream water creating a mechanical load on the ground surface. This load is supported by the geologic materials (sediment or rock) and the pore fluid contained within the pores. Changes in this surface load have a direct effect on the total stress of the aquifer causing either a change in effective st… Show more
“…For the BAS, a value of 1.5 × 10 −4 m 2 /s for D derived from K
v and S
s determined by inverse modelling 20 , suggests that hydraulic penetration of the diurnal tidal signal would be less than 10 m; the spring-neap tidal signal should not penetrate beyond a few tens of metres. Therefore we interpret the undamped periodic oscillations at up to 212 m depth as the fluid pressure changes in this coastal, confined and unconsolidated aquifer due to mechanical loading and unloading of the aquifer by tidal water movements above the aquifer surface, commensurate with analytical treatments 21–23 .Figure 2Amplitudes of the tidal and atmospheric signals and groundwater responses at Gabura and Laksmipur. Top: amplitude versus frequency for ( a ) the Chittagong tide and ( b – d ) the groundwater heads at GbPZ67, GbPZ116 and GbPZ212 respectively, 1 st June 2013 to 31 st May 2014, showing the principal solar-lunar tidal components 17 O 1 , K 1 , N 2 , M 2 and S 2 , and additional minor components (see text).…”
Groundwater-level fluctuations represent hydraulic responses to changes in groundwater storage due to aquifer recharge and drainage as well as to changes in stress that include water mass loading and unloading above the aquifer surface. The latter ‘poroelastic’ response of confined aquifers is a well-established phenomenon which has been demonstrated in diverse hydrogeological environments but is frequently ignored in assessments of groundwater resources. Here we present high-frequency groundwater measurements over a twelve-month period from the tropical, fluvio-deltaic Bengal Aquifer System (BAS), the largest aquifer in south Asia. The groundwater level fluctuations are dominated by the aquifer poroelastic response to changes in terrestrial water loading by processes acting over periods ranging from hours to months; the effects of groundwater flow are subordinate. Our measurements represent the first direct, quantitative identification of loading effects on groundwater levels in the BAS. Our analysis highlights the potential limitations of hydrogeological analyses which ignore loading effects in this environment. We also demonstrate the potential for employing poroelastic responses in the BAS and across other tropical fluvio-deltaic regions as a direct, in-situ measure of changes in terrestrial water storage to complement analyses from the Gravity and Climate Experiment (GRACE) mission but at much higher resolution.
“…For the BAS, a value of 1.5 × 10 −4 m 2 /s for D derived from K
v and S
s determined by inverse modelling 20 , suggests that hydraulic penetration of the diurnal tidal signal would be less than 10 m; the spring-neap tidal signal should not penetrate beyond a few tens of metres. Therefore we interpret the undamped periodic oscillations at up to 212 m depth as the fluid pressure changes in this coastal, confined and unconsolidated aquifer due to mechanical loading and unloading of the aquifer by tidal water movements above the aquifer surface, commensurate with analytical treatments 21–23 .Figure 2Amplitudes of the tidal and atmospheric signals and groundwater responses at Gabura and Laksmipur. Top: amplitude versus frequency for ( a ) the Chittagong tide and ( b – d ) the groundwater heads at GbPZ67, GbPZ116 and GbPZ212 respectively, 1 st June 2013 to 31 st May 2014, showing the principal solar-lunar tidal components 17 O 1 , K 1 , N 2 , M 2 and S 2 , and additional minor components (see text).…”
Groundwater-level fluctuations represent hydraulic responses to changes in groundwater storage due to aquifer recharge and drainage as well as to changes in stress that include water mass loading and unloading above the aquifer surface. The latter ‘poroelastic’ response of confined aquifers is a well-established phenomenon which has been demonstrated in diverse hydrogeological environments but is frequently ignored in assessments of groundwater resources. Here we present high-frequency groundwater measurements over a twelve-month period from the tropical, fluvio-deltaic Bengal Aquifer System (BAS), the largest aquifer in south Asia. The groundwater level fluctuations are dominated by the aquifer poroelastic response to changes in terrestrial water loading by processes acting over periods ranging from hours to months; the effects of groundwater flow are subordinate. Our measurements represent the first direct, quantitative identification of loading effects on groundwater levels in the BAS. Our analysis highlights the potential limitations of hydrogeological analyses which ignore loading effects in this environment. We also demonstrate the potential for employing poroelastic responses in the BAS and across other tropical fluvio-deltaic regions as a direct, in-situ measure of changes in terrestrial water storage to complement analyses from the Gravity and Climate Experiment (GRACE) mission but at much higher resolution.
“…Small responses (a few mm or less) to precipitation loading (e.g., Rasmussen and Mote 2007) have been observed, but had no influence on the analyses. Water‐level responses to stream‐stage loading (e.g., Boutt 2010) have also been observed, but were negligible during the period of the analyses.…”
Hydrologists have long recognized that changes in barometric pressure can produce changes in water levels in wells. The barometric response function (BRF) has proven to be an effective means to characterize this relationship; we show here how it can also be utilized to glean valuable insights into semi-confined aquifer systems. The form of the BRF indicates the degree of aquifer confinement, while a comparison of BRFs between wells sheds light on hydrostratigraphic continuity. A new approach for estimating hydraulic properties of aquitards from BRFs has been developed and verified. The BRF is not an invariant characteristic of a well; in unconfined or semi-confined aquifers, it can change with conditions in the vadose zone. Field data from a long-term research site demonstrate the hydrostratigraphic insights that can be gained from monitoring water levels and barometric pressure. Such insights should be of value for a wide range of practical applications.
“…Deformation and flow induced by the total stress change is purely vertical, occurring only in the lower part of the aquifer. The total stress change in this study is referred to the temporal change in mechanical load on the aquifer induced by, for example, stream stage fluctuations [23,24] , seasonal changes of soil moisture storage [25,26] , sedimentation [10] or erosion [8] of the aquifer surface etc.…”
a b s t r a c tThe direct effect of changes in applied total stresses on fluid-saturated geologic materials is reflected in fluctuations in pore pressure. The quantification of these pressure changes has traditionally been based on the classical poroelasticity theory, developed ignoring the natural variability in aquifer properties (parameters). Uncertainty of excess pore pressure prediction is therefore expected to be large when applying the classical model to field aquifer systems which generally have high degree of heterogeneity. Very limited attention to this issue has been paid. The motivation for this work is to quantify the uncertainty associated with the classical model, the spatial variability of the predicted excess pressure head in the field, where the groundwater flow is produced by the changes in total stress. The stochastic methodology used to develop the results of this study is based on the nonstationary spectral approach. It is found that the heterogeneity and correlation length scale of the log hydraulic conductivity process, and the pore compressibility parameter play essential roles in enhancing the variability in excess pressure head. In addition, the pore pressure head distribution predicted using the classical poroelasticity theory is subject to large uncertainty at a large depth in heterogeneous aquifers.
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