Description
Manager of the Indian site: Jean-Christophe Maréchal
Two experimental sites located into the same geological unit constituted by Archean granite of Telangana state (South India) near the city of Hyderabad have been investigated. The research activities on the first one, Maheshwaram watershed, allowed dealing with crystalline rock hydrogeology at the catchment scale with the impact of high rural anthropic activities. On the second one, the Experimental Hydrogeological Park of Choutuppal, the main scientific questions were tackled at local scale (bore well scale) to enhance the understanding of local hydrodynamic and transport processes (including artificial recharge).
Scientific Goals
The Hyderabad Site comprises an aquifer in crystalline rocks in a weathering profile characteristic of a tropical environment. The intense anthropic pressures on groundwater resources have led to a need to develop tools for managing the resource.
The principal scientific objectives of current experiments and monitoring studies are the following:
- To characterize the hydrodynamic properties of the weathering profile and to estimate the environment’s level of hydraulic continuity and compartmentation.
- To acquire the data required for testing and validating hydrogeologic modeling methods designed for fractured environments. This includes describing the aquifer’s geometry and quantifying the flows in this highly disturbed natural environment.
- To characterize the transport parameters and their relationships with the hydrodynamic properties
- To study the environment’s chemical reactivity, and especially the changes in the chemical quality of the waters as extraction proceeds
- To identify the spatial and temporal origins of the sources of contaminants in the waters, in particular contaminants of geogenic origin.
- To investigate the aquifer’s vulnerability to climatic variations and agricultural practices
Monitoring and principal experiments conducted on the site
Under this heading a number of monitoring and experimental programs are being conducted on the Choutuppal and Maheswaram sites in order to provide pertinent data – including time series and long-term experiments – for the characterization and modeling of the transfers of water and chemical elements in this complex, heterogeneous aquifer. The main monitoring campaigns involve:
Hydrologic and climatic monitoring of the site: at Maheshwaram, the groundwater level has been monitored since 2003 by three piezometers at 15-minute intervals. This system is supplemented by a meteorological station and by half-yearly monitoring of the water levels in some 200 abandoned boreholes. An annual hydrologic balance is calculated. At Choutuppal the groundwater level has been monitored every 15 minutes since 2010 by 15 piezometers.
Hydrochemical monitoring of the site: A set of piezometers is sampled one to three times a year, at low water and after recharge. Major elements are measured, along with certain trace elements and isotopes of the H20 molecule. For certain points other isotopic analyses are available: isotopes of nitrogen in nitrates, sulfur and oxygen in sulfates, strontium, and lead. Analyses of CFCs and SF6 enable studies of the residence times of groundwaters. Isotopic and other monitoring of rainwater at the meteorological station allows monitoring of the recharge. In 2008 a rice paddy was equipped with ceramic candle filters for the sampling and analysis of waters at various depths, during infiltration. Analyses were carried out on the solid phases: the aquifer rock and the fertilizers employed by the farmers.
Exploitation-related monitoring: changes in the discharges being sampled are measured on the basis of borehole inventories and satellite imagery.
Advanced projects
Hydrodynamics of the fissured, weathered horizon
A series of hydraulic tests at various scales led to a characterization of the hydrodynamic properties of the aquifer’s fissured-weathered zone, and the formulation of a hydrodynamic model (Figure 1).
Based on pumping tests performed on this fissured horizon, and interpreted using methods that take into account the fractured nature of the formation – a dual-porosity model with vertically anisotropic permeability, a single horizontal fracture, and a fractionally-dimensioned flow model – two systems of conductive fissures were identified. The first, PFN, is the primary fissure system which affects only the matrix, at a sub-meter scale. The permeability of this system, and thus of the blocks, is low: Kblock 10-8 m/s. The second system SFN, intersects the blocks at a scale of several meters. SFN is itself composed of two systems of conductive fissures, one subhorizontal system (HSFN) and one subvertical system (VSFN), which appears to be consistent with geologic observations. The vertical density of the horizontal fissures ranges from 0.15 to 0.24 m-1 for fissures up to several tens of meters in length (10 to 30 meters). This fissure density represents blocks ranging from 4 to 7 meters in size. The strong dependence of the horizons’ permeabilities on the degree of fissuring indicates that the permeability of the fissured zones is more or less the same over the whole profile, and close to KCFZ = 10-4 m/s (Dewandel et al., 2006). The overall horizontal permeability of the fissured zone is estimated at: Kr = 10-5 m/s. The sub-vertical fissure network provides the connection between the horizontal fissures. However, the sub-vertical is less permeable than the horizontal system, Kz = 10-6 m/s, thereby introducing an anisotropy factor for the permeability of about 10, which is not unlike what is observed on the ground. The overall permeability of the fissured horizon, i.e., incorporating the two systems of fissures, is about 10-5 m/s (Maréchal et al., 2004; Dewandel et al., 2006). The effective porosity of the fissured horizon is generally low, from 10-3 to 10-2 (Maréchal et al., 2004, 2006; Dewandel et al., 2010) and is mainly (90%) due to the low-permeability blocks and weathered zones that form the walls of the fissures. The fissure system accounts for only about 10% of the effective porosity (Maréchal et al., 2004). As for permeability, the fissure density plays a very significant role in the figure for effective porosity, implying that the zones of high effective porosity are associated with an increased density of fissuring because of (a) an increase in the number of fissures, and (b) an increase in the frequency of weathered facies in the walls of the fissures.
Geometry of a polyphase weathering
The combined use of a large number of geoelectrical surveys together with drilling has allowed the thickness of the weathering profile to be mapped. These results show that this Indian profile results from polyphase weathering. Although monophase weathering usually creates thick weathering profiles in the bedrock formations (> 100 m), polyphase weathering interrupted by erosional phases gives rise to a substantially different geometry. Under such conditions, the profiles result from the partial erosion of older profiles, which are then re-weathered. In southern India, for example, the current weathering profile has been inherited from an earlier profile dating at least from the Jurassic-Cretaceous, which has been eroded and re-weathered several times during the geodynamic history of the region: the India-Gondwana separation, passage over the Reunion hot spot, and the India-Asia collision. During each of these tectonic episodes the profile was incised as a result of an uplift of the land, and then developed again during periods of tectonic calm. We surmise that today all that remains of the Jurassic-Cretaceous profile is a small portion of the fissured zone.
In terms of geometry the polyphase weathering profile is distinctly thinner and its structure is no longer characterized by broad flat or tabular surfaces, but by a profile sub-parallel to the current topography (Figure 2). The profile thus forms a ‘topping” on the current surface. In contrast, the weathered horizons are always sub-parallel to each other. As compared to the monophase profile the saprolites are abnormally thick in relation to the fissured zones (ratio of 1:2 instead of 1:3). From a hydrodynamic viewpoint the permeability and density of productive fissure zones are not significantly different from those seen in monophase profiles. The main difference arises primarily in terms of transmissivity, a combination of the permeability of the productive fissure zones and their number, owing to the large variations in the profile’s total thickness.
Effects of geologic heterogeneities on hydrogeology
The effect of a “quartz vein” type geologic heterogeneity was studied by geoelectrical profiling and hydraulic testing conducted on both sides of the heterogeneity. Geometrically, the weathering profile is significantly thicker in the vicinity of the heterogeneity: by a factor of 1.5 to 3 for the thickness of saprolite, and 3 to 5 for the fissured zone. Near the contact the profile in the granite is everywhere characterized by sub-parallel layers of saprolite and fissured zone. However, they are no longer sub-parallel to today’s weathering surface, but sub-parallel to the walls of the discontinuity (Figure 3). Consequently the geometric structure of the profile has a “U” shape. The quartz vein is mainly characterized by sub-parallel fissures that are sub-perpendicular to the strike of the vein. This “U” shape, which indicates a local deepening of the weathering profile, results from the same weathering processes as those observed in horizontal and stratiform profiles. During weathering of the granitic host rock the vein becomes permeable and promotes a local deepening of the profile, over several tens of meters laterally in the example discussed. This thickening is the cause of a local increase in the aquifer’s transmissive zone. In addition, the vein itself is subjected to the effects of weathering, which mainly lead to small-scale fissuring and the production of weathering materials, e.g., saccharoidal quartz. The hydrodynamic properties of the vertical fissured zone adjoining the vein are very similar to those encountered in the horizontal and stratiform fissured zones (Figure 3). The permeability is about 10-5 m/s, and the storage capacity 10-3. Its hydrodynamic operation is characterized by double-porosity behavior, 10-7 m/s, in which the flow is channeled (Figure 3). However, unlike the horizontal fissured zone, characterized by a horizontal anisotropy of permeability (Kx/Kz = 10, Kx/Ky = 1), the vertical fissured zone is marked by the dominance of sub-vertical fissure systems sub-parallel to the vein (Ky>Kx, Ky/Kx = 2-3; Figure 3). The quartz vein is typically characterized by double-porosity behavior (Figure 3), the permeability in the fissure system is about 4-6×10-4 m/s, and its storage capacity is about 3-5×10-4 and comes mainly from impermeable blocks (Km = 2×10-9 m/s). Nevertheless, within the vein these hydrodynamic properties are very variable and depend primarily on the degree of weathering and fissuring of the rock: fissure-free and impermeable in the core of the quartz vein (K = 10-7 m/s), but highly fissured and permeable at the walls (K = 5×10-6 m/s). The aquifer in the vein is moreover characterized by a permeability anisotropy in the vertical plane (Kx/Ky = 3) which is interpreted as the consequence of a decrease in the density of fissuring with depth. Like the vertical fissured zone in the granite, the vein aquifer is characterized by a dominant system of sub-vertical fissures leading to an anisotropy in the horizontal plane (Ky>Kx; Ky/Kx = 3.0).
Groundwater geochemistry
The groundwater composition exhibits strong spatial variability, particularly connected to the impact of anthropic activites. The quality of the groundwaters has become degraded by over-exploitation of the resource for agricultural production, especially the development of rice paddies. Among the factors of degradation we note an increase in the content of fluorine, causing the recent appearance of dental fluorosis, diagnosed in 84% of the children examined. A geogenic source of fluorine has been identified, but anthropic activity has heavily promoted its accumulation in the groundwaters (Pauwels et al., 2010). The isotopes of lead, which also has a geogenic origin, have led to a better understanding of these water-rock interaction processes, in particular by identifying a two-stage weathering process: the Pb isotope ratio is initially controlled by the weathering of accessory minerals, but later by the granite’s major phases (Negrel et al., 2010). Agricultural activities, especially rice farming, promote the re-infiltration of water concentrated by evapotranspiration, thereby contributing to the salinization of the groundwater and the accumulation of undesirable or toxic elements. This process was modeled on the basis of a hydrodynamic balance and knowledge of the composition of the groundwater and the contributions of fertilizers. The model explains the development of salinity over the last decade and allows the portrayal of future situations according to various scenarios of basin development (Perrin et al., 2011). This process was studied via the isotopes of the H2O molecule. Analysis of rainwater samples provided a means for identifying the isotopic signatures of the two (Southwest and Northeast) Monsoons. The groundwaters show the varying contributions of these monsoons, and their composition moves away from the straight meteoric line, signaling an impact from evaporation related to anthropic activities (Negrel et al., 2011).
Joint programs and participating researchers
The Hyderabad site offers an infrastructure equipped to handle a variety of national and international projects on widely diversified topics, particularly related to the management of water resources. In all, more than twenty researchers, engineers, and technicians have participated in investigations on the site in recent years, or worked on the data. To these must be added a score of masters or doctorate-level students. The Hyderabad site has been involved in a number of Franco-Indian (CEFIPRA), European (Asia Pro Eco Program, SUSTWATER), and national (ACI EAU, ANR Shiva, ANR Mohini) projects and research groups. The main joint programs involved the National Geophysical Research Institute (NGRI, India), the universities of Paris VI and Ecole des Mines de Paris, the International Water Management Institute (IWMI, India), the Charles University of Prague, and the universities of Rennes and Neuchâtel (Switzerland). Close collaboration is maintained with the governmental organizations responsible for the monitoring and management of groundwater at the local (Andhra Pradesh Ground Water Department) and federal (Central Ground Water Board) levels.
Application of the acquired data
The monitoring program at Choutuppal is devoted to observation of fluctuations in the level of the groundwater aquifer in response to stresses of low frequency (annual monsoon cycles), medium frequency (hydraulic tests), and high frequency (earth tides, pumping by farmers). Interpretation of the system’s reaction to these stresses leads to an understanding of its hydrodynamic functioning. The piezometers in fifteen or even more boreholes will be continuously monitored. In order to appreciate the impact of high-frequency phenomena (tides, pumping), the acquisition interval is 15 minutes. Processing of signals from these time series will enable identification of the various processes involved. Boreholes close to agricultural operations will be equipped with thermal probes with the same sampling interval, so as to identify the pumping cycles.
At Maheshwaram, the piezometer is measured twice a year (pre- and post-monsoon) in a series of some 200 abandoned boreholes undisturbed by pumping, and the continuous recording of the levels in two piezometers will be maintained. The existing meteorological station will be partially automatized. The watershed outlet will be equipped with a probe to measure level and electrical conductivity during the monsoon months, to quantify the emerging flows. Changes in land use will be monitored by the acquisition of high-resolution satellite imagery, and the impact in terms of withdrawals of groundwater will be established by carrying out measurements in the field.
Pre- and post-monsoon piezometric maps are generated in the form of GIS grids.
From a geochemical viewpoint the data acquisition includes borehole logging and water sampling at various levels in boreholes not utilized by farmers. Studies of spatial and temporal change are based on a network of some forty wells. Apart from the usual physico-chemical parameters (pH, conductivity, dissolved O2, redox potential, and temperature), the chemical logs already cover solutes such as nitrates and fluorides. The results have been confirmed by laboratory analyses of water samples. In the future other tools may be tested. The analyses carried out on groundwater samples comprise at minimum the major elements, a few trace elements, and the isotopes of water. Analyses of other isotopes are added as required. A rain gauge enables monitoring of the isotopic signal of recharge in the aquifer.
The BRGM has received ISO 9001:2008 certification for its management system, specifically covering its research activities in the field of water. Its piezometric monitoring is performed by technicians accredited under this quality-management system. All analyses are carried out in BRGM laboratories (work conducted under quality assurance and partially under COFRAC accreditation: Reference NF EN ISO/CEI 17025). Moreover, the Hyderabad site is managed under the aegis of the MOHINI and CEFIRES projects, each of which has a Quality Assurance Plan and a risk assessment.
Short- and medium-term programs
Hydrodynamic properties
The first pumping tests carried out on the Choutuppal Site showed the presence of a certain level of heterogeneity in the hydrodynamic functioning of the site, which can be divided into several compartments. The reasons for their contrasting behaviors in terms of compartmentation or heterogeneity of hydrodynamic parameters need to be identified. The very dense piezometric network at Maheshwaram enables a visualization of the groundwater aquifer’s response to withdrawals by farmers and to recharge during the monsoon. A geostatistical analysis of the piezometer fluctuations together with a modeling will be conducted in order to characterize the compartmentation of the reservoir.
Transport properties
The hydrodynamic characterization conducted on the Hyderabad site must be supplemented by a study of transport properties. These data are useful in themselves for constraining the range of travel times, but also for defining appropriate transport models and testing the adequacy of conceptual models. The siting of boreholes at Choutuppal was designed for inter-borehole tracing tests in various directions and at various depths. Combined interpretation of the hydraulic and tracing tests will enable identification of the impacts of anisotropy, double porosity, and scale effects on the transport of solutes.
Geochemical monitoring
The processes of water-rock interaction, the residence times of waters in the aquifer, and anthropic activities all affect the chemical composition of the waters. The chemical and isotopic analyses performed show very strong spatial and temporal heterogeneity. The Choutuppal Site provides a means of investigating the physical and biogeochemical processes which control the composition of the waters and the transport of solutes at a scale of a few meters, and free of anthropic impacts. The Maheshwaram Site enables investigation of the impact of evaporation caused by certain agricultural practices, as well as that of various contributions related to anthropic activities, while also taking into account disturbances affecting the processes of water-rock interaction.
Hydrologic monitoring
monitoring of pre- and post-monsoon water levels, combined with geochemical monitoring, will enable continued investigation of the impact of the monsoon and anthropic activities on the levels and quality of the water table. These latter parameters will be constrained by consulting land-use maps.