Reservoir Loading Research Articles

Deciphering the Interplay between Reservoir Loading and Dynamic Stresses at the Pertusillo Reservoir

Deciphering the Interplay between Reservoir Loading and Dynamic Stresses at the Pertusillo Reservoir

Earthquakes Induced by Rapid Loading of Faults During Pulichintala Reservoir Impoundment in the Stable Continental Region of India

AbstractThe first stage impoundment of the Pulichintala reservoir in the Proterozoic Cuddapah basin in the southeastern Indian shield began in August 2014. The second stage of impoundment started in 2019 when the height of the dam was increased and the filling of the reservoir was rapid. Soon after earthquakes started occurring which continued for a few months with a maximum magnitude of ML 4.6 on 25 January 2020. Two distinct clusters within the shallow crust were identified, which follow the faults/shear zones in the region. The estimated focal mechanisms with predominant strike slip motion on steep planes and the derived stress state are consistent with the regional stress and plate motion of the Indian plate. We simulated the influence of reservoir impoundment on the inferred seismogenic faults and found that the stress and pore pressure due to reservoir load indeed promoted failure, thus inducing earthquakes. We ascribe the induced earthquakes to rapid filling, longer and higher retention of high‐water levels during 2019–2020, which caused a stress change of 4–8 kPa. This stress change was at least two orders of magnitude higher than the typical tectonic loading rate in the stable continental region of the Indian Shield.

Fault resonance process and its implications on seismicity modulation on the active fault system

Various periodic external loading processes (e.g., snow, hydrological and atmospheric, earth tide and surface reservoir load, etc.) affect tectonic deformation and sometimes modulate seismicity in diverse tectonic settings. Small stress variations induced by external loads may influence the fault dynamics and destabilize fault zones. In this article, we report five cases of earthquake occurrence in the plate boundary and plate interior regions where such a destabilization seems to be operating. We test the possibility of resonance amplification assuming rate-and-state friction and look for the model parameters that could justify resonance destabilization. We find that frictional resonance destabilization is possible in regions that are near the stability boundary (velocity neutral areas). The critical slip distance for the evolution of friction is similar to laboratory estimates. In some cases, effective normal stress lower than the lithostatic stress have to be assumed for the model to be consistent with the observations, implying high pore pressure. We propose that crustal faults of conditionally stable frictional domains with anomalous pore pressure are sensitive to periodic stress perturbations by the external loading process.

Development of linear-element boundary element method for inverse solution from induced far-field displacements to reservoir loading source

An inverse solution strategy using linear element of Boundary Element Method(BEM)was developed, which will become a novel application base to potentially reflect the equivalent mechanical effect around reservoir corresponding to CO2 spreading during its injection process. Firstly, a linear-element BEM code was studied to improve the accuracy in analyzing the connection between induced ground minute deformations and underground inner loading. Secondly, the space regularization was adopted to deal with the ill-posed problem during the optimization to inverse problem solution. The boundary conditions of a near-field structure as active source were reversely figured out in an efficient numerical way based on its passive deformation that was sampled at a few specific positions at far field. Meanwhile, the effectiveness of linear-element BEM in solving inverse problems was demonstrated. Finally, based on the implementation of hydraulic fracturing in a coal seam in Alberta, Canada, the monitored data of its induced surface displacements was input to invert the equivalent internal pressure onto are-opened fracture within reservoir. The results show that the proposed inverse solution workflow and programmed algorithm are applicable to effectively guiding hydraulic fracturing interpretation so as to further facilitating coal bed methane recovery.

Hydraulic fracture of concrete-rock interface in reservoir-gravity dam-foundation system under seismic conditions

Under strong seismic excitation, hydraulic fractures considering ultrahigh reservoir load at the gravity dam-foundation interface adversely affect the stability of the generalized structure. Based on the reservoir-dam-foundation interaction model, dynamic water pressure model, and contact element, a new numerical model that simulates interfacial hydraulic fracture is established by introducing generalized stress intensity factors and the Hutchinson-Suo propagation criterion. Using a preset crack at the dam heel and tracking its trajectory, this study compares the structural damage and crack propagation process in the interface and homogeneity models, investigates the influence of different water pressure distributions along the crack faces on the seismic response of the structure and fracture characteristics, and finally conducts a sensitivity analysis of the parameters in the dam-foundation system. The numerical results show that the presence of the interface between the dam and foundation significantly alters the damage characteristics of the structure and the propagation path of the cracks. Regarding seismic analysis in actual engineering, it is reasonable to assume that the water pressure is uniformly distributed along the crack faces initiated from the dam heel. When the fracture toughness of the foundation rock is less than or greater than that of concrete by up to 14.3%, and the difference is not more than 0.5 MPa·m1/2, the cracks kink into the foundation; otherwise, the cracks may kink into the dam body.

Research on the Effect of Shale Core Mechanical Behavior on Casing Deformation

As an unconventional, high-quality, efficient, and clean low-carbon energy, shale gas has become a new bright spot in the exploration and development of global oil and gas resources. However, with the increasing development of shale gas in recent years, the anisotropic load of the shale reservoir during the mining process has caused the casing to be deformed or damaged more and more seriously. In this paper, the mechanical behavior of shale core shear, triaxial and radial compression are studied using rock true compression tests, shear tests and nuclear magnetic resonance (NMR) technology. The process of macroscopic and microscopic changes of shale fractures during the tests were analyzed to predict the effect of the fracture-state changes and stress-state changes of different shale reservoirs on the casing deformation. The results show that after the shale core is damaged, the overall pore structure changes, resulting in the decrease or increase in shale porosity. During the process of triaxial pressurization, as the pressure continues to increase, there will be a critical pressure value from elastic deformation to plastic deformation. When the pressure value exceeds the critical pressure value, the shale reservoir will have strong stress sensitivity, which can easily cause wellbore collapse. The research results have important guiding significance for determining the casing deformation under shale reservoir load and preventing casing deformation failure.

Considerations on the deformation of Ridracoli dam foundation based on the analysis of monitoring data

The behaviour of the Ridracoli arch-gravity dam in the 2002-2020 operation years has been analysed, by applying “statistical” models of increasing complexity to the time series of pendulum and long-base extensometer measurements. The modelling approach has proved robust. A satisfactory estimate of the reservoir load, thermal and irreversible drift components has been obtained for most of the instruments. As expected, the influence of temperature is lower for the instruments deeply embedded in the rock mass. The deformations recorded on the left and right bank of the dam have shown a minor influence of the anisotropic proprieties of the rock mass, well characterized by the mechanical tests performed at the time of dam construction.

Surge of Mangla reservoir loading promoted failure on active Décollement of western Himalayas

The extreme rainfall weather during the Indian monsoon season has increased in recent decades changing the hydrological mass distribution that may be responsible for triggering regional earthquakes. In the western Himalayas, a shallow Mw 5.7 earthquake took place right after the withdrawal of the 2019 unusual Indian summer monsoon resulting in a rapid increase of ~4.5 km3 of water storage at the nearby Mangla reservoir within three months. Through a joint inversion of space geodetic and teleseismic data, we found that the mainshock occurred on the décollement structure of the Main Himalayan Thrust in the western Himalayas, which was previously underdetermined. The significant water loading, and pore pressure diffusion led to Coulomb stress increases of ~10 kPa on the maximum coseismic slip zone promoting fault failure. Our findings demonstrate that climate change could influence certain reservoir-associated earthquakes in the Himalayas. We provide recommendations to improve the regulations for the reservoir operations in Mangla and probably other contexts with similar tectonic settings in the Himalayas during the monsoon season.

Vertical Accuracy Assessment of Open Access Digital Elevation Models Using GPS

In this study, the vertical accuracy of the Shuttle Radar Topography Mission Digital Elevation Model Version 2.0 (SRTM30), the Advanced Spaceborne Thermal Emission and Reflection Radiometer Global DEM Version 2.0 (ASTER GDEM2), and Advanced Land Observing Satellite World 3D Digital Surface Model Version 2.1 (ALOS AW3D30) was statistically assessed using GPS data. The Tashkent geodynamical polygon was chosen as a study region, where the land surface can reflect as both natural (tectonic) and man-made (Charvak reservoir load effect) processes. The geometric approach using GPS/leveling stations and EGM96 GGM-based geoid undulations was applied for local geoid modeling. RMSE of ~7.48 m, ~6.90 m, and ~8.24 m was obtained for SRTM30, ASTER GDEM2, and ALOS AW3D30, respectively. ASTER GDEM2 proved to be the most accurate DEM with absolute vertical accuracy LE90 of 11.35 m and ME of 5.25 m and it is very suitable for geodetic and hydrological studies in this mountainous area.

Spatial database and website for reservoir-triggered seismicity in Brazil

Abstract. After confirming that impoundment of large reservoirs could cause earthquakes, studies on reservoir-triggered seismicity (RTS) have had a considerable scientific incentive. Most of the studies determined that the vertical load increase due to reservoir load, and the reduction of effective force due to the increase in pore pressure, can modify the stress field in the reservoir region, possibly triggering earthquakes. In addition, the RTS is conditioned by several factors such as pre-existing tectonic stresses, reservoir height/weight, area-specific geological and hydromechanical conditions, constructive interaction between the orientation of seismotectonic forces, and additional load caused by the reservoir. One of the major challenges in studying RTS is to identify and correlate the factors in the area of influence of the reservoir, capable of influencing the RTS process itself. A spatial seismicity-triggered reservoir database was created to facilitate the research in this field, based on the specifications of the national spatial data infrastructure (INDE), and to assemble data pertinent to the RTS study in the area of reservoirs. In this context, this work presents the procedures and results found in the data processing of seismotectonic factors (dam height, reservoir capacity, lithology, and seismicity) and compared first to the dams that triggered earthquakes and secondly to the Brazilian dam list. The list has been updated with four more cases, making a total of 30 cases. The results indicate that the occurrence of RTS increases significantly with dam height since dams less than 50 m high cause only 2 % of earthquakes while those higher than 100 m cause about 54 %. The reservoir volume also plays a role, and it was estimated that RTS occurrence requires a limiting minimum value of 1×10-4 km3. There was no clear correlation between the geology and geological provinces with RTS. The delayed response time of the reservoirs represents 43 % of the total; that is, almost half of them have hydraulic behavior. The highest magnitude, 4.2, was observed at a reservoir with a volume greater than 10−3 km3. As a practical outcome, to assist the analysis by the general community, the web viewer RISBRA (Reservoir Induced Seismicity in Brazil) was developed to serve as an interactive platform for Reservoir-Triggered Seismicity Database (BDSDR) data.

Dynamic Characterization of an Alkali-Ion Battery as a Source for Laser-Cooled Atoms

We investigate a solid-state, reversible, alkali-ion battery (AIB) capable of regulating the density of alkali atoms in a vacuum system used for the production of laser-cooled atoms. The cold-atom sample can be used with in-vacuum chronoamperometry as a diagnostic for the voltage-controlled electrochemical reaction that sources or sinks alkali atoms into the vapor. In a combined reaction-diffusion-limited regime, we show that the number of laser-cooled atoms in a magneto-optical trap can be increased both by initially loading the AIB from the vapor for longer, and by using higher voltages across the AIB when atoms are subsequently sourced back into the vapor. The time constants associated with the change in atom number in response to a change in AIB voltage are in the range of 0.5 s - 40 s. The AIB alkali reservoir is demonstrated to survive oxidization during atmospheric exposure, simplifying reservoir loading prior to vacuum implementation as a replacement for traditional resistively-heated dispensers. The AIB capabilities may provide an improved atom number stability in next-generation atomic clocks and sensors, while also facilitating fast loading and increased interrogation times.

Numerical modelling of cracking in gravity dam under static and seismic loadings with multiple pre-embedded discontinuity FEM

This paper presents a numerical study on the earthquake analysis of a concrete gravity dam. The numerical approach is based on the multiple pre-embedded discontinuity finite element method. The equations of motion with the ground motion excitation, static self-weight, hydrostatic reservoir load and the hydrodynamic reservoir loads as the boundary conditions are solved with explicit time stepping. Mass proportional damping is used instead of the usual stiffness proportional damping since the latter drastically decreases the critical time step of explicit time integration. The Koyna dam with the genuine accelerograms recorded during the 1967 earthquake is analyzed as the numerical example. The simulation results agree with the previously reported results by other studies, i.e. similar crack propagation in the monolith is predicted here.

Liquid-Loading-Mitigation Strategies Maximize Recovery From Gas Reservoirs

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 194600, “Revisiting Old Sands With a Different Perspective: A Pragmatic Approach for Maximizing Recovery From Gas Reservoirs,” by Sagun Devshali, SPE, Vinod Manchalwar, SPE, Budhin Deuri, Sanjay Kumar Malhotra, Bulusu V.R.V. Prasad, Mahendra Yadav, SPE, Avinav Kumar, and Rishabh Uniyal, ONGC, prepared for the 2019 SPE Oil and Gas India Conference and Exhibition, 9–11 April, Mumbai. The paper has not been peer reviewed. In two gas fields in India, many sands had to be isolated after the wells ceased to flow because of liquid loading in the absence of continuous deliquification. To predict liquid-loading tendencies and to identify opportunities for production enhancement, the performance of 150 gas wells was analyzed. All technically feasible methods of deliquification were evaluated and compared to achieve maximum ultimate gas recovery. Deliquification Techniques ­­in Well-Life Extension A properly designed tubing string can solve the problem of liquid loading to some extent. With a smaller tubing size, gas velocity can be exceeded and can be kept greater than the critical velocity, thus preventing loading in the beginning phase. However, this is not a viable long-term solution. During later stages, the energy of the gas is not sufficient to lift the liquid droplets with even the smallest tubing. At that point, the use of artificial-lift modes becomes necessary. Case Studies Field B. This onshore field in western India produces primarily oil with only 60 gas wells. During the time of study, 17 out of 60 wells in the field were producing gas; the remaining 43 were nonflowing either because of depletion in reservoir pressure or loading. Analysis determined a total of 31 gas wells for deliquification. Of these, in the first phase, two ceased wells (B1 and B2) were selected. Sucker-rod pumps (SRPs) were selected as the most suitable mode of lift in both wells. These wells were then worked over and SRPs were installed in the wells for continuous deliquification. Well B1. This well was completed in the 1507.5- to 1511-m interval, with 2⅞-in. tubing and 5½-in. casing, in 2011. During the time of study in 2017, the well was not producing. The last production re-ported was 3900 m3/d of gas and 1.4 m3/d of liquid. The critical rate to prevent loading in the well has been calculated as 14 000 m3/d. Analysis indicated that the well has been flowing below the critical rate since 2011. A gradient survey performed in October of 2016 indicated that the liquid level was below 370 m. Action Plan and Implementation. The plan included recompleting the well with an SRP unit with the pump placed at 1520 m and tailpipe run to a depth of 1570 m to prevent gas interference. The design included approximately 10 sinker bars to prevent buckling. Well B1 has been completed with the SRP method. A 1½-in. subsurface pump has been placed at 1520 m and tailpipe has been run to a depth of 1570 m. Sucker rods of ⅞ in. have been used as sinker bars. The well has been completed with guides and at the time of writing is producing 6500 m3/d of gas and 6 m3/d of liquid. Well B2. This well was completed with 2⅞-in. tubing in 2002. The well has not produced since August 2015. The last production reported was 8000 m3/d, less than the critical rate of 11 000 m3/d. A gradient survey performed in December 2016 indicated that the liquid level was below 800 m.

Pollutant load capacity of Situ Parigi, Banten Province

Situ Parigi is located in Parigi Lama Village, Pondok Aren District, South Tangerang City. Based on the water quality measurements of Situ Parigi, BOD parameter has the highest concentration. This indicates that the type of waste that enters this water body is dominated by organic waste produced from domestic activities surrounding. The purpose of this study is to determine the pollutant load capacity of BOD in Situ Parigi. Pollutant load capacity is calculated using formulas in Minister of Environment Regulation No. 28 of 2009 concerning Capacity of Lake Water Pollution and/or Reservoir Load. Situ Parigi has an area of 4 Ha, an average depth of 0.98 m, a debit of 0.023 m3/sec and a hydraulic residence time of 18.25 days. Based on the calculations results, the pollutant load capacity of Situ Parigi is 3,169.81 kg/year but must accommodate a load of 20,553.15 kg/year. These results show that the pollution that occurs in Situ Parigi has exceeded its capacity. Control strategies such as dredging work and the construction of communal wastewater treatment plants could be carried out in an effort to reduce the occurring pollution levels.

Environmental Study of Water Reservoirs for the Watershed Management in Pakistan

Water reservoirs in most parts of the world face the problem of sedimentation. Environmental impacts and land-use changes/landslide have enormous potential to generate more sediment load in water reservoirs. Hence, the capacity of water reservoirs in Pakistan has been constantly reducing due to sedimentation. In this study, we have computed the environmental impacts on sedimentation load in Tarbela and Mangla water reservoirs using multi-satellite and ground-based data. The total drainage area of watershed is calculated using Shuttle Radar Topographic Mission data. Satellite data from NOAA-AVHRR are used to monitor urban development and changes in vegetation area, while MODIS data are used to estimate the snow area in the watershed region. Normalized Difference Vegetation Index is used to estimate the temporal changes in vegetation cover, whilst principle component analysis is applied to monitor urban changes in watershed region. Similarly, snow cover area in the study region is estimated using Normalized Difference Snow Index algorithm. Rainfall data obtained from Pakistan Meteorological Department are used to evaluate the relationship between sedimentation deposited in water reservoirs and annual rainfall amount. Results of the current study reveal that increase in urban development and high amount of precipitation has direct relationship with the load of sedimentation, while amount of vegetation has inverse relationship with the load of sedimentation in water reservoirs of the country.

TGF-\u03b21 increases viral burden and promotes HIV-1 latency in primary differentiated human bronchial epithelial cells

Combination antiretroviral therapy (cART) has increased the life expectancy of HIV patients. However, the incidence of non-AIDS associated lung comorbidities, such as COPD and asthma, and that of opportunistic lung infections have become more common among this population. HIV proteins secreted by the anatomical HIV reservoirs can have both autocrine and paracrine effects contributing to the HIV-associated comorbidities. HIV has been recovered from cell-free bronchoalveolar lavage fluid, alveolar macrophages, and intrapulmonary lymphocytes. We have recently shown that ex-vivo cultured primary bronchial epithelial cells and the bronchial brushings from human subjects express canonical HIV receptors CD4, CCR5 and CXCR4 and can be infected with HIV. Together these studies suggest that the lung tissue can serve as an important reservoir for HIV. In this report, we show that TGF-β1 promotes HIV latency by upregulating a transcriptional repressor BLIMP-1. Furthermore, we identify miR-9-5p as an important intermediate in TGF-β-mediated BLIMP-1 upregulation and consequent HIV latency. The transcriptionally suppressed HIV can be reactivated by common latency reactivating agents. Together our data suggest that in patients with chronic airway diseases, TGF-β can elevate the HIV viral reservoir load that could further exacerbate the HIV associated lung comorbidities.

EVALUATION OF VIRTUAL EFFICIENCY APPARATUS AND WELFARE PROCESSING IN RESERVOIRS WITH MULTENERGY IMPACT AND DIFFERENT FORM OF TRANSFER PERIPHERALS

The nature and effectiveness of the process of multi-energy vibration finishing and grinding treatment is given. The assumption is made that the effectiveness of vibration processing depends on the intensity of the movement of the reservoir loading, relative to its working surfaces. It has been established that the factors of this movement are the cross-sectional shape of the reservoir and the field of its oscillations. A description and selection of the parameters of the physical and dynamic models of the process for determining the cross-section of the reservoir is given. The study of the dynamic model allows us to conclude that the shape of the transverse cut and the oscillation fields of the reservoir affect the speed of the circulating movement of its load and the magnitude of the forces acting in the load, which directly affects the efficiency of the vibrating process. It has been established that the greatest importance is the speed in the tank “rectangular” with rounded transitions from the bottom to the walls of the overcut shape, which do not change the cut geometry, but only exclude the stagnation of the mobile load. In a reservoir of this cross-cut shape, the forces acting in the load take the least value. Comparison of the intensity of changes in the roughness of the surface of parts treated in tanks of the studied cross-cut shapes with the parameters of the load movement calculated according to the dynamic model shows that the intensity of the surface roughness changes the higher the higher the circulation circulation rate and the smaller the effective forces acting in the load. Such modes are implemented in tanks that have a "rectangular" and "ellipsoid" cut. For finishing operations in vibratory finishing of parts, it is necessary to use tanks that have a "rectangular" with rounded transitions from the bottom to the walls and an "ellipsoid" transverse overcut.

Utilizing Fiber Reinforced Concrete as the Concrete Face of Rockfill Dams

Construction of concrete faced rockfill dams (CFRD) includes pouring rock materials as the main body of the dam and compacting the rockfill to an acceptable compaction level. Then, a reinforced concrete slab is constructed on the upstream surface of dam to transfer water pressure to the rockfill materials. Considering the high permeability of rockfill materials, the concrete slab serves as a sealing element on the upstream side. Indeed, the required characteristics of concrete face are adequate flexibility in case of long-term settlement of dam, durability and its sufficiency in waterproofing the dam. In this study, to improve the quality of concrete face, increase its flexibility, optimize the construction cost and enhance the bearing capacity of concrete in tension, developed by bending moment in the face slab, three types of prevalent fibers (polypropylene, MEXO200 and steel) are utilized. The performance of each fiber is compared with the conventional reinforced concrete specimens by conducting some bending tests. Furthermore, carrying out numerical simulation, the efficacy of each fiber on performance of face slab is investigated. Based on the results, the fibers, especially the steel fibers, are suitable to substitute rebars of the conventional reinforced concrete employed as the concrete face of CFRDs. The steel fiber reinforced concrete has adequate capacity and ductility to mitigate the expected deformations from reservoir loading on the upstream face of CFRDs.

A review of mechanisms of induced earthquakes: from a view of rock mechanics

The induced earthquake recently has gained an increasing public awareness of environmental and safety issue. The earthquakes associated with fluid injection and extraction, reservoir impoundment and mining/rock removal have been extensively reported. Here, we reviewed injection induced earthquakes and their mechanisms from a view of rock mechanics. This review begins by briefly introducing the classification and the state-of-the-art research of induced earthquakes. From a view of rock mechanics, three fundamental mechanisms of induced earthquakes, i.e., pore pressure increase, stress change, and change in coefficient of friction, are introduced in details. Firstly, we discussed pore pressure increase due to fluid injection and reservoir impoundment, and explained earthquakes caused by fluid injection and related to reservoirs according to the Mohr–Coulomb failure criterion and effective stress law in the saturated rock. Secondly, we discussed stress change resulting from fluid extraction, temperature change, reservoir loading and quarry unloading. Thirdly, we investigated factors determining coefficient of friction, i.e., mineralogy, fluid pressure and temperature. Moreover, it is a remarkable fact that additional physical or chemical effects of fluids may lead to weakening of materials in fault zones owing to stress corrosion and stable slip, according to the rate and state friction law. Finally, we summarized and compared mechanisms of induced earthquakes that occurred in a variety of past human activities and projects, and recommended future potential means and scopes to investigate the mechanism of induced earthquakes.

Interpretations of Reservoir‐Induced Seismicity May Not Always Be Valid: The Case of Seismicity during the Impoundment of the Kremasta Dam (Greece, 1965–1966)

The ‘Kremasta seismic sequence’ in western Greece is one of the most commonly cited examples of Reservoir Induced Seismicity (RIS). Here, we show that this ‘sequence’ is a result of normal tectonic activity and that only some small, unrelated microseismic events are reservoir induced. Shortly after the beginning of the impoundment of the Kremasta Dam in 1965, the then newly established seismic monitoring network in Greece recorded two Ms ≥ 6.0 events and numerous small shocks spread over a 120 km wide region. These were interpreted as a single seismic sequence (namely the Kremasta seismic sequence), and assumed to be reservoir induced. We revisit the epicenter locations of these events and interpret them in the framework of the regional tectonic context and the local hydrogeology. Placing these events into the local context shows that they represent an amalgamation of separate, ordinary (tectonic) seismic sequences. Further, the regional rocks are highly fragmented by small faults and the spatial distribution of seismic events is not consistent with a model of stress transfer from reservoir loading. In addition, it is not likely that events at such long (> 20-30 km) distances from the reservoir could be induced by an initial reservoir load head of 30 m. Whilst the larger magnitude events are tectonic, after impoundment local residents reported an unusual frequency of small microseismic events felt only within 10 km of the dam. We provide evidence that these are a result of the collapse of numerous shallow karstic cavities adjacent and beneath the reservoir due to increased water load (locally 100-150 m depth). This study has significant implications for interpretation of seismic triggering mechanisms in other regions: earthquake occurrence within the proximity of reservoirs during and after impoundment time cannot be assumed to be RIS unless supported by seismological, geological and hydrogeological evidence.