H42B-01
Aquatic electrical resistivity imaging of rainfall-driven solute transport in contaminated wetlands
A continuous aquatic electrical resistivity imaging (ERI) technique, using floating electrodes and a shallow-draft paddleboat, was employed to predict spatial and temporal patterns of pore-fluid conductivity in wetland soils of a contaminated, shallow-water wetland. ERI measurements were obtained with marine-acquisition software and a multi-channel resistivity instrument at six times over a four month period, covering a 10 square kilometer grid. A set of 10 simultaneous reception channel measurements were continuously recorded every two seconds yielding an average of 13,000 measurements per survey. Three dimensional inversion was carried out to determine the conductivity distribution of the subsurface using the smoothness-constrained least-squares optimization method. The continuously recorded surface water depth and conductivity were entered as known information in the inversion and measurement error (further constraining the inversion) estimated using a tie point technique. Pore- fluid conductivity estimates were constrained using surface conduction measurements obtained from laboratory experiments on soils extracted from the wetland, as well as a correction for temporal and spatial temperature variations based on direct surface water temperature measurements and existing data on the thermal characteristics of peat soils. The study demonstrated that: (1) continuous aquatic ERI is an ideal method for resolving the resistivity structure of wetland sediments covered by a shallow (less than 1 m) surface water layer, (2) temperature variations must be considered in such shallow monitoring studies as they may otherwise have the most significant influence on the results, and (3) surface conduction is significant in marsh soils and must be accounted for if subsurface conductivity models are to be reliably interpreted in terms of pore-fluid chemistry. In the field example presented here, changes in pore-water conductivity estimated from inverted models suggest that migration of contaminants from marginal landfills into the wetland soils accompanies major rainfall events.
H42B-02
Characterization of saltwater intrusion using electrical imaging: numerical simulation and field study
The understanding of subsurface seawater intrusion dynamics is an important requirement for the sustainable management of water resources in coastal areas. Such dynamics is strongly dependent on the given hydrogeological and geochemical conditions and often involves high spatio-temporal variability which may be difficult to characterize using local monitoring wells. In this context, geophysical imaging can help unveiling the dynamics of such systems continuously on a larger scale. In particular electrical imaging has proven to be an efficient non- to minimally invasive tool to characterize subsurface hydrogeological structures as well as to monitor water salinity changes with relatively high spatial resolution. We here investigate the potential of electrical imaging for the characterization of saltwater intrusion in a numerical (two-dimensional) simulation study, where density-dependent flow and transport is modelled for a typical hydrogeological setting. Tomographic acquisition of electrical data is simulated employing surface and borehole electrode arrays, and assuming petrophysical relations between water salinity and electrical conductivity for different lithological units. In a first step, the ability of electrical imaging is studied to reconstruct the spatial electrical conductivity variations associated with both lithology and the salinity distribution corresponding to a steady state scenario for a given set of boundary and initial hydrological conditions. The different investigated scenarios demonstrate the potential of the method for saltwater intrusion characterization, but also show its limitations, for example regarding resolution at depth and for given uncertainty in the underlying petrophysical models or regarding the overlapping effects due to variations in lithology and salinity. Preliminary field results from the site of Almeria, SE Spain, further demonstrates the approach by identifying a saltwater intrusion. Surface and surface to borehole field electrical imaging are compared and highlights inherent limitations of electrical tomography. However, it seems practical that electrical imaging results can be used to constrain seawater intrusion models. This is of importance for improved model predictions for a sustainable management of coastal regions. This work is part of the EU project ALERT (GOCE-CT-2004-505329).
H42B-03
Using Geophysics to Define Hydrostratigraphic Units in the Edwards and Trinity Aquifers, Texas
Airborne and ground geophysical surveys conducted in Uvalde, Medina, and northern Bexar counties, Texas, can
be used to define and characterize hydrostratigraphic units of the Edwards and Trinity aquifers. Airborne magnetic
surveys have defined numerous Cretaceous intrusive stocks and laccoliths, mainly in Uvalde County, that
influence local hydrology and perhaps regional ground-water flow paths. Depositional environments in the
aquifers can be classified as shallow water platforms (San Marcos Platform, Edwards Group), shoal and reef
facies (Devils River Trend, Devils River Formation), and deeper water basins (Maverick Basin, West Nueces,
McKnight, and Salmon Peak Formations). Detailed airborne and ground electromagnetic surveys have been
conducted over the Edwards aquifer catchment zone (exposed Trinity aquifer rocks), recharge zone (exposed
Edwards aquifer rocks), and artesian zone (confined Edwards) in the Seco Creek area (northeast Uvalde and
Medina Counties; Devils River Trend). These geophysical survey data have been used to divide the Edwards
exposed within the Balcones fault zone into upper and lower hydrostratigraphic units. Although both units are high
electrical resistivity, the upper unit has slightly lower resistivity than the lower unit. The Georgetown Formation, at
the top of the Edwards Group has a moderate resistivity. The formations that comprise the upper confining units
to the Edwards aquifer rocks have varying resistivities. The Eagleford and Del Rio Groups (mainly clays) have very
low resistivities and are excellent electrical marker beds in the Seco Creek area. The Buda Limestone is
characterized by high resistivities. Moderate resistivities characterize the Austin Group rocks (mainly chalk). The
older Trinity aquifer, underlying the Edwards aquifer rocks, is characterized by less limestone (electrically resistive
or low conductivity units) and greater quantities of mudstones (electrically conductive or low resistivity units). In
the western area (Devils River Trend and Maverick Basin) of the Trinity aquifer system there are well-defined
collapse units and features that are marked by moderate resistivities bracketed by resistive limestone and
conductive mudstone of the Glen Rose Limestone. In the central part of the aquifer (San Marcos Platform) the
Trinity's lithologies are divided into upper and lower units with further subdivisions into hydrostratigraphic units.
These hydrostratigraphic units are well mapped by an airborne electromagnetic survey in Bexar County. Electrical
properties of the Edwards aquifer also vary across the fresh-saline water interface where ground and borehole
electrical surveys have been conducted. The saline- saturated Edwards is predictably more conductive than the
fresh-water saturated rocks. Similar fresh-saline water interfaces exist within the upper confining units of the
Edwards aquifer (Carrizo-Wilcox aquifer) and the Trinity aquifer rocks.
http:esp.cr.usgs.gov/info/edwards/
H42B-04
Hydrogeophysics and Water Balance of Cerro Prieto Dam, NE Mexico
The geographical location of the State of Nuevo Leon, due to its physiographic features, has temperate and arid climate; undeveloped drainage, low precipitations, and high evapotranspiration rates, as well as rapid demographic growth. The hydrological data of the Pablillo basin, registered in the hydrometric station Cerro Prieto, showed an annual precipitation from 415 up to 1130 mm/a , the mean evaporation of 705 mm/a (up to 2460 mm/a in 1996). The maximum water storage of the Cerro Prieto reservoir is 395 millions m3 which corresponds to a water level of 295 meters. However, this level was reached only three times after the dam's construction. By the end of June 2006 the water level was at 276.2 m which corresponds to a water volume of about 127,806,300 m3 which is less than a third of maximum storage. Analysis of hydrological data showed sufficient misbalance between water recharge (by rain, river flow) and loss due to evaporation, filtration, extraction, discharge, etc. 160 gravity reading points, 400 onshore magnetic field readings as well as about 250 offshore magnetic points were carried out. The standard corrections as instrumental drift, latitude, elevation, IGRF, etc. were applied to obtained data. Data procession includes Fourier transformation, wavelength filters, upward continuation, vertical and horizontal derivates, etc. As a result a 2D geological-geophysical models and 3D maps were elaborated. The general trend of the magnetic field reduced to a pole is NW - SE on which background anomalies of northeast trend are obviously traced. The general trend of the gravity field received as a result of our works is the same. However, local magnetic and residual gravity anomalies have mosaic character and, being morphologically extended in a NE direction, grouped in chains of northwest trend. Potential data interpretation allows assuming a series of the superficial fractures focused in a NE direction, perpendicular (NW-SE) to the general deep fault. The analysis of the received data indicates a sharp variability of volume of fresh water in the basin, due to several complex factors. Major factors are: hydrogeological and climatic conditions of the region (1), intensive extraction of fresh water from surrounding areas (urbanization and agricultural activity of the city of Linares), which is reflected in strong fracturing of the subsurface layers and lowering of aquifer depth (2), presence of a deep fault trending northwest (3), and a high velocity of recent sedimentation (4).
H42B-05
Groundwater Resources and Land Subsidence investigations in the Toluca Valley, Mexico
The sustained growth in population in the Toluca Valley and neighboring Mexico City has primarily depended on the continuous development of both local and regional water resources for industrial, agricultural and domestic uses. The Toluca Valley Basin, covering an area of approximately 2000 Km2, is the focus of this study. Currently, there is a significant net loss of water within the basin primarily due to groundwater pumping, and the loss is increasing with time. These stresses on the aquifer have caused significant changes on the water flow patterns, a reversal in the direction of hydraulic gradients, the disappearance of artesian springs and wetlands and noticeable land subsidence within the basin. Neighboring Mexico City's land subsidence problems have been well documented, however, no comprehensive studies exist for the Toluca Basin. This study is divided into two parts: 1) investigation of groundwater depletion in the Toluca Valley; and 2) assessment of land subsidence in the Toluca Valley. We examine various changes in regional flow patterns, and groundwater levels decline throughout the valley and 3D numerical flow simulations are run to predict the ever decreasing level of the piezometric surface. Currently there is a net loss (recharge - extraction) of 142 Mm3 per year of groundwater within the Toluca Basin aquifers. We have documented a decrease in groundwater levels with a rate of up to 1.4 m/year between 1970 and 2006 in the central part of the valley. At the current rate of consumption, groundwater resources will not be sustainable for the population of the valley. Directly related to the drawdown in groundwater levels is the occurrence of land subsidence throughout the valley. Neighboring Mexico City, where total subsidence of up to 9 meters has been observed, has a similar geology as the one in the Toluca valley. We have documented several sites in the Toluca Valley where land subsidence is occurring. Ongoing work includes the mapping of regional land subsidence with the use of InSAR (Synthetic Aperture Radar Interferometry) images obtained from the European Space agency's ERS-1, ERS-2 and Envisat Satellites and the Canadian Space Agency's RADARSAT satellite. Data from years 1995 to 2007 are used to produce subsidence maps of the Toluca Valley. Our findings are verified with in-situ extensometers installed around the city of Toluca and the industrial corridor where the most significant decline in groundwater levels are found. Future land subsidence occurrences will be predicted with the use of a numerical model calibrated with remote-sensing images and on-site surveys.
H42B-06 INVITED
Large-scale hydrogeophysical characterization for sustainable groundwater development in a semi-arid terrain in NE Brazil using a combined TEM-AMT approach
Accurate identification of groundwater sources in granular deposits and crystalline basement and preventative mapping of their recharge zones to aid sustainable development are major issues in semi-arid terrains. We propose that combined TEM-AMT surveying has the potential to contribute towards finding solutions to both problems and demonstrate this using a major case study in semiarid northeast Brazil. Combined TEM-AMT and TEM-CSAMT surveys were carried out along several profiles (totaling around 220 line km) at the southeastern margin of Parnaiba basin in NE Brazil to determine structural controls on groundwater distribution, pin-point zones for drilling and delineate potential recharge zones. The profiles transect sedimentary and bordering Precambrian crystalline terrains and in particular, cut across linear basin-border grabeform structures suggested by previous aeromagnetic surveys. Localised studies in wholly crystalline terrains were also conducted near several small communities. The AMT-CSAMT data were corrected for static shift using collocated TEM soundings. Two-dimensional regularized inversion of AMT-CSAMT data was carried out for all profiles. For the basin-margin studies, the contact between the sedimentary and crystalline basement rocks was well imaged on all profiles and graben-like structures mapped near the basin borders on some of the transects are suggested for deep drilling for groundwater. 2D inversion results from TEM-CSAMT measurements on 6 profiles were able to map important fault structures cutting across the basin which appear to connect sedimentary aquifers with fracture-zone basement aquifers in the area and are interpreted as important recharge zones. This finding is supported by evidence from very productive wells located along these fault structures. We suggest that the combined TEM- AMT approach is the way forward for such semi-arid environments.
H42B-07 INVITED
Hydrogeophysics in an Alpine Watershed: Lake O'Hara, Canadian Rocky Mountains
Alpine watersheds are the headwaters of major rivers in western Canada and many other regions in the world. Consequently, understanding the storage, flow paths and timing of the release of water is an important aspect of water resources management. Recent studies have shown that groundwater stored in moraines, talus slopes and alpine meadows are important water reservoirs in the Canadian Rocky Mountains. A research program in the Lake O'Hara Research Basin, Yoho National Park is characterizing the hydrologic cycle within this alpine watershed. Bedrock topography, preferential flow paths and the internal structure of the subsurface play a fundamental role in determining the volume and timing of groundwater discharge. Geophysical methods are needed to help delineate the subsurface, because drilling is not an option. A preliminary survey using electrical resistivity imaging (ERI) was conducted in the summer of 2006. ERI profiles were run across a small wetland in an alpine meadow and over the nose of a glacial moraine. Much of the surfaces consisted of dry rock rubble and boulders, so contact resistance was at times as high as several hundred thousand ohms. To establish electrical contact, we drove spikes into sponges saturated with salt water that were forced into boulder contacts. Another enhanced contact strategy was to smear medical contact jell over boulder surfaces and connect an electrode to aluminum foil plastered onto the jell. The survey was conducted with an 800 V transmitter, a Wenner array and an electrode spacing of 2 to 5 m. Due to the resistive environment, currents as small as 1 mA produced measured potentials in the volt range. Due to the contact difficulties, some electrode locations were offset one or more meters from the preferred array location and this offset may cause some distortion in the inverted images. The data produced inversions with resistivity values ranging from 500 ohm-m to 100,000 ohm-m. The lowest resistivity values correspond to wetlands and the saturated outflow at the toe of the moraine. The highest resistivity values correspond to dry bolder fields at the surface of the moraine. Bedrock appears to be in the 10,000 to 40,0000 ohm-m range. Although interpretation ambiguities exist in some portion of the images, the wetland is seen to be on the order of 2 m deep and overlies bedrock. In the moraine, discrete zones of groundwater flow have been identified. These results are consistent with water chemistry results that indicate that groundwater discharging from the toe of the moraine comes from more than one source.