Reverse Water Level Fluctuations Research Articles

Characteristics of the soil seed bank of planted and natural restored draw-down zones in the Three Gorges Reservoir Region

The large-scale reverse seasonal water-level fluctuation caused by the Three Gorges Reservoir Project has dramatically changed the ecology of draw-down zones leading to degradation of local riparian vegetation. We studied the Wanzhou draw-down zones in the Three Gorges Reservoir Region via field investigations and soil seed bank (SSB) germination experiments. The composition and spatial distribution of the SSB in the planted and natural restored sites revealed the effects of restoration modes on seed bank development and demonstrated responses of SSB composition and structure to water fluctuation. The SSB of planted sites contained 43 species of plants, with a mean seed bank density of 9418.79 seeds/m2, where annual herb species accounted for 57.34%. The SSB at natural restored sites had 38 species of plants, with a mean seed bank density of 6090.09 seeds/m2, where annual herb species accounted for 52.57% of the species. Species diversity was low and annual plants dominated the two restoration sites. SSB species diversity was the highest in mid to upper levels of the draw-down zones. Plantation has increased co-occurrence species of SSB and standing vegetation, and increased their similarity. The number of co-occurrence dominant species of SSB and standing vegetation in planted areas was greater than in natural restored areas. However, the stability of plant communities naturally restored was greater than artificial plant communities as there was a higher proportion of perennials in the SSB with more dominant species similar to standing plant communities. In conclusion, the reverse water-level fluctuation simplified the species composition and shaped the spatial distribution of the SSB at the two restoration sites, and plantation promotes the development both of SSB and vegetation. However, plant communities of the two restoration areas, influenced by water-level fluctuations, will be mainly grasslands dominated by annual plants.

Joint Estimation of Hydraulic and Poroelastic Parameters from a Pumping Test.

The coupling of hydraulic and poroelastic processes is critical in predicting processes involving the deformation of the geologic medium in response to fluid extraction or injection. Numerical models that consider the coupling of hydraulic and poroelastic processes require the knowledge of relevant parameters for both aquifer and aquitard units. In this study, we jointly estimated hydraulic and poroelastic parameters from pumping test data exhibiting "reverse water level fluctuations," known as the Noordbergum effect, in aquitards adjacent to a pumped aquifer. The joint estimation was performed by coupling BIOT2, a finite element, two-dimensional, axisymmetric, groundwater model that considers poroelastic effects with the parameter estimation code PEST. We first tested our approach using a synthetic data set with known parameters. Results of the synthetic case showed that for a simple layered system, it was possible to reproduce accurately both the hydraulic and poroelastic properties for each layer. We next applied the approach to pumping test data collected at the North Campus Research Site (NCRS) on the University of Waterloo (UW) campus. Based on the detailed knowledge of stratigraphy, a five-layer system was modeled. Parameter estimation was performed by: (1) matching drawdown data individually from each observation port and (2) matching drawdown data from all ports at a single well simultaneously. The estimated hydraulic parameters were compared to those obtained by other means at the site yielding good agreement. However, the estimated shear modulus was higher than the static shear modulus, but was within the range of dynamic shear modulus reported in the literature, potentially suggesting a loading rate effect.

Reverse Water‐Level Fluctuations Associated with Fracture Connectivity

Reverse water-level fluctuations (RWFs), a phenomenon in which water levels rise briefly in response to pumping, were detected in monitoring wells in a fractured siliciclastic aquifer system near a deep public supply well. The magnitude and timing of RWFs provide important information that can help interpret aquifer hydraulics near pumping wells. A RWF in a well is normally attributed to poroelastic coupling between the solid and fluid components in an aquifer system. In addition to revealing classical pumping-induced poroelastic RWFs, data from pressure transducers located at varying depths and distances from the public supply well suggest that the RWFs propagate rapidly through fractures to influence wells hundreds of meters from the pumping well. The rate and cycling frequency of pumping is an important factor in the magnitude of RWFs. The pattern of RWF propagation can be used to better define fracture connectivity in an aquifer system. Rapid, cyclic head changes due to RWFs may also serve as a mechanism for contaminant transport.

Poroelastic Effects on Fracture Characterization

Reverse water-level fluctuations have been widely observed in aquitards or aquifers separated from a pumped confined aquifer (Noordbergum effect) immediately after the initiation of pumping. This same reverse fluctuation has been observed in a fractured crystalline-rock aquifer at the Coles Hill uranium site in Virginia in which the reverse water-level response occurs within a pumped fracture and results from an instantaneous strain response to pumping that precedes the pore-pressure response in observation wells of sufficient distance from the pumped well. This response is referred to as the Mandel-Cryer effect. The unique aspect of this water level rise during a controlled 24 h pumping test was that the reverse water levels lasted for approximately 100 min and reached a magnitude of nearly 1 cm prior to a typical drawdown response. The duration and magnitude of the response reflects the poromechanical properties of the fractured host rock and hydraulic properties of the pumped fracture. An axisymmetric flow and deformation model were developed using Biot2 in an effort to simulate the observed water-level response along an assumed 0.5 to 1.0 cm aperture horizontal fracture 176 m from the pumping well and to identify the importance of the poroelastic effect. Results indicate that traditional aquifer-testing methods that ignore the poromechanical response are not significantly different than results that include the response. However, the poroelastic effect allows for more accurate and efficient parameter calibration.

Hydromechanical characterization of CO2 injection sites

Clear understanding of coupled hydromechanical effects, such as ground deformation, induced microseismicity and fault reactivation, will be crucial to convince the public that geologic carbon storage is secure. These effects depend on hydromechanical properties, which are usually determined at metric scale. However, their value at the field scale may differ in orders of magnitude. To address this shortcoming, we propose a hydromechanical characterization test to estimate the hydromechanical properties of the aquifer and caprock at the field scale. We propose injecting water at high pressure and, possibly, low temperature while monitoring fluid pressure and rock deformation. Here, we analyze the problem and perform numerical simulations and a dimensional analysis of the hydromechanical equations to obtain curves for overpressure and vertical displacement as a function of the volumetric strain term. We find that these curves do not depend much on the Poisson ratio, except for the dimensionless vertical displacement at the top of the caprock, which does. We can then estimate the values of the Young's modulus and the Poisson ratio of the aquifer and the caprock by introducing field measurements in these plots. Hydraulic parameters can be determined from the interpretation of fluid pressure evolution in the aquifer. Reverse-water level fluctuations are observed, i.e. fluid pressure drops in the caprock as a result of the induced deformation that undergoes the aquifer-caprock system when injecting in the aquifer. We find that induced microseismicity is more likely to occur in the aquifer than in the caprock and depends little on their stiffness. Monitoring microseismicity is a useful tool to track the opening of fractures. The propagation pattern depends on the stress regime, i.e. normal, strike slip or reverse faulting. The onset of microseismicity in the caprock can be used to define the maximum sustainable injection pressure to ensure a permanent CO2 storage.

A theoretical model for reverse water-level fluctuations induced by transient permeability in thrust fault zones

A numerical model was applied to simulate the poroelastic response to changes in fault permeability as a result of earthquakes. The ‘fault valve’ model describes faults as impermeable barriers for fluids except immediately after earthquakes, when fault zones are damaged and transient pathways for fluids are created. In this case the fault is viewed as a discharging well, draining fluids from the surrounding rock. The reverse water-level effect is characterized by the increase of water level in adjacent aquifers and aquitards, resulting from withdrawing fluids through a well. Theoretical calculations suggest that the reverse water-level effect exists also in earthquake cycling and is in the same order of magnitude as the co-seismic hydraulic head change. A significant rise of the hydraulic head (>1 m) occurs within the country rock from both sides of the fault. The rise of the water level takes months to years to occur, and perhaps that is why it cannot be easily distinguished from seasonal hydrologic changes observed in the field. The reverse water-level effect also propagates away from the fault at a rate of hundreds of meters per year, depending on the permeability of the country rock. In deep formations where the permeability is low, the propagation takes years. The magnitude of the reverse water-level effect is greater when the fault efficiently drains fluids, when it is highly permeable and slow to reseal.

Numerical simulation of the Noordbergum effect resulting from groundwater pumping in a layered aquifer system

Numerical simulations were carried out to investigate the so-called Noordbergum effect (i.e. reverse water-level fluctuation) resulting from groundwater withdrawal. Two specific cases were analyzed: a three-layer aquifer system composed of two permeable aquifers separated by a semi-impermeable aquitard, and a corresponding single-layer aquifer system composed of an equivalent lumped material. In the numerical simulation of the layered aquifer system, the Noordbergum effect is observed during the early time period of pumping in the overlying aquitard and unpumped aquifer while the underlying aquifer is pumped. In contrast, the numerical simulation results of the lumped aquifer system do not show such a reverse water-level fluctuation throughout the entire domain. These results strongly suggest that the Noordbergum effect can be explained by the difference in poroelastic responses of the layered (heterogeneous) and lumped (homogeneous) aquifer systems to the hydraulic pumping stress. The Noordbergum effect is caused by two mechanisms: a faster mechanical propagation (deformation) of the pumping stress than its hydraulic propagation (drawdown) from the pumped aquifer into the adjacent aquitard and unpumped aquifer due to relatively lower hydraulic conductivity of the aquitard, and a mechanical amplification (excessive compression) in the lower part of the relatively soft aquitard. However, the pumping stress is evenly distributed throughout the entire domain of the lumped aquifer system without such mechanisms since it has homogeneous hydraulic and mechanical properties.

Reverse water level fluctuations in semiconfined aquifer systems — “rhade effect”

Pumping tests in semiconfined aquifer systems near Dorsten (F.R.G.) caused anomalous reverse water level fluctuations in observation wells tapping the overlying confining bed (aquitard) at different depths (multilevel piezometer). Whenever water is pumped from a well that is screened in the aquifer of the “Halterner Sande”, the piezometric surface in the aquitard (“Bottroper Mergel”) rises. After that increase, the water level falls according to the drawdown of head within the pumped aquifer (Noordbergum effect according to Verruijt, 1969). Conversely, the piezometric surface in the aquitard falls whenever the pump is shut off (Rhade effect). The anomalous water level reactions propagate with decreasing amplitude from the aquifer-aquitard boundary to the top of the semipermeable layer. Such water level fluctuations (“swelling effects”) may be explained by the capability of a water-saturated, compacted material to change volume when subjected to sudden pressure changes. On a practical basis, the Noordbergum and Rhade effects must be taken into consideration for evaluating long-term changes in chemical and hydraulic properties in pumped, semiconfined aquifer systems.

Second‐order theory of flow in three‐dimensional deforming media

A basic assumption in groundwater flow theory is that consolidation occurs in the vertical direction only. In real three‐dimensional media this condition is not satisfied a priori. In the present paper a second‐order theory of flow is developed including the consideration of horizontal soil displacements. The ‘strain nucleus’ or ‘tension center’ approach, well‐known in thermoelasticity, is used. The formulation of the theory shows that volume strain in a point P of the system may be viewed as being distinctly contributed by pore pressure variations in and outside P. The latter contribution, which vanishes identically in one‐dimensional media, is called the ‘three‐dimensional effect’. Solutions applied to a semi‐infinite mechanically homogeneous medium show that the three‐dimensional effect is negligible and the diffusion equation is good as long as the ratio W between the average depth and the thickness of aquifer is ≥2, whatever its areal exent. If W < 2 (shallow and thick aquifers), the above effect produces an additional compression of soil during an intermediate stage after pumping has begun and hence a slowing down of the rate of pressure head decline, as has been predicted by standard solutions. The most critical conditions occur for W=1/2, i.e., in unconfined aquifers. The present theory also allows for a short qualitative analysis of the ‘Noordbergum effect’ or reverse water level fluctuation.

Reply [to “Comment on ‘Relationship between Horizontal Strain near a Well and Reverse Water Level Fluctuation’ by Roger G. Wolff”

Reply [to “Comment on ‘Relationship between Horizontal Strain near a Well and Reverse Water Level Fluctuation’ by Roger G. Wolff”

Comment on ‘Relationship between Horizontal Strain near a Well and Reverse Water Level Fluctuation’ by Roger G. Wolff

Comment on ‘Relationship between Horizontal Strain near a Well and Reverse Water Level Fluctuation’ by Roger G. Wolff

Relationship between Horizontal Strain near a Well and Reverse Water Level Fluctuation

Measurements of radial surface strains near a pumping well showed compression near the well that changed to tension farther away from the well. A theoretical model is presented. In general, this model reproduces the measured strains. Observations of anomalous increase in pore pressure in a clay adjacent to the pumped aquifer are related to the development of these strains. These increases in pressure are attributed to distortion of the pore space in the clay resulting from the transference of horizontal strain from the aquifer via shear. Applying a conceptual model that permits the transference of strain across confining layers to unpumped artesian aquifers accounts for reverse water level fluctuations.