Geological controls on submarine groundwater discharge in Long Bay,South Carolina (USA)

A combination of geophysical methods including continuous electrical resistivity and high-resolution Chirp sub-bottom profiling were utilized to characterize geologic controls on pore fluid salinity in the nearshore of Long Bay, SC. Resistivity values ranged from less than 1 Ω m to greater than 40 Ω m throughout the bay. Areas of elevated electrical resistivity suggest the influence of relatively fresher water on pore water composition. Geophysical evidence alone does not eliminate all ambiguity associated with lithological and porosity variations that may also contribute to electrical structure of shallow marine sediments. The anomalous field is of sufficient magnitude that lithological variation alone does not control the spatial distribution of elevated electrical resistivity zones. Geographical distribution of electrical anomalies and structures interpreted from nearby sub-bottom profiles indicates abrupt changes in shallow geologic units control preferential pathways for discharge of fresh water into the marine environment. Shore parallel resistivity profiles show dramatic decreases in magnitude with increasing distance from shore, suggesting a significant portion of the terrestrially driven fresh SGD in Long Bay is occurring via the surficial aquifer within a few hundred meters of shore.     

Resistivity imaging of Pleistocene alluvial aquifers in a contractional tectonic setting: A case history from the Po plain (Northern Italy)

In this work we present the hydrogeophysical imaging of a key sector of the Quaternary Po foreland basin (northern Italy), focussing on the reconstruction of clastic aquifers and aquitards in a complex tectono-sedimentary subsurface architecture. The study area includes the relic reliefs of Casalpusterlengo and Zorlesco, two smooth morphological features involving uplifted and gently folded Pleistocene marine to alluvial sediments, plausibly linked to the buried Northern Apennines thrust and fold belt. The geophysical data include 35 Direct Current Vertical Electrical Soundings collected over a 37 km² wide area, acquired with Schlumberger array and maximum half-spacing of 500 m. 1-D resistivity-depth profiles were computed for each VES. An integrated hydrostratigraphic approach was applied, to constrain the interpretation of the geophysical data along several cross-sections, including the comparison of resistivity soundings to stratigraphic logs, borehole electric logs and the pore-water properties.The resistivity interfaces, traceable with the same laterally continuous vertical polarity, were used to develop an electrostratigraphic model in order to portray the stacking of electrostratigraphic units down to 200 m below ground surface. Their vertical associations show a general upward increase of electrical resistivity. This assemblage mimics the regional coarsening upwards depositional trend, from the conductive units of the Plio-Pleistocene marine-to-transitional depositional systems to the resistive units of the Middle–Late Pleistocene fluvial and alluvial plain depositional systems. Middle Pleistocene depositional systems host an alternation of North-dipping, high-to-intermediate permeability aquifer systems (70–180 Ωm, thickness of 5–70 m) separated by low permeability aquitards (20–50 Ωm, thickness up to 40 m). These units pinch out against the Casalpusterlengo and Zorlesco relic reliefs, where they cover the uplifted and folded regional aquitard (20–50 Ωm) formed by Pliocene-Lower Pleistocene clays to sandy silts with gravel lenses in agreement with borehole data. In the deepest part of the local stratigraphy, a broad low-resistivity anomaly (< 10 Ωm) was clearly mapped through the study area. By comparison with electrical borehole logs in deep oil-wells, it could be interpreted as the fresh–saltwater interface due to the presence of connate waters and brines hosted by the marine-to-transitional shales.     

Investigation of groundwater resources using controlled-source radio magnetotellurics (CSRMT) in glacial deposits in Heby,Sweden

We have combined tensor radio magnetotelluric- (RMT, 15–250 kHz) and controlled source tensor magnetotelluric (CSTMT, 1–12 kHz) data for the mapping of aquifers in gravel formations lying in between crystalline bedrock and clay rich sediments in the Heby area some 40 km west of Uppsala in Sweden. The estimated transfer functions, the impedance tensor and the tipper vector generally satisfy 1D or 2D necessary conditions except for the lowest CSTMT frequencies where near field effects become more dominant.The data measured from 8 profiles were inverted with the Rebocc code of Siripunvaraporn and Egbert (2000) assuming plane wave conditions. This meant that only 12 frequencies in the range of 4–180 kHz could be used. The four lowest frequencies of CSTMT in the range of 1–2.8 kHz were excluded because of source effects. Data from all profiles were inverted with a starting model of 100 Ω-m and a relative error floor of 0.02 on apparent resistivity, corresponding to less than 1^° on phase. Tipper vectors are generally small except when source effects become dominant in the lowest frequencies of CSTMT and were therefore not used for inversion. Comparing with models derived from vertical electrical soundings, refraction and reflection seismic data as well as ground truth from exploration wells assessed the reliability of the deep part of the models. Furthermore we carried out a non-linear resolution analysis to better quantify the depth extent of the aquifers.The inverted models from the Heby area show well the thickness variations of glacial deposits overlying crystalline bedrock. Generally, the upper 20 m of the models have resistivities below 40 Ω-m, taken to represent clay rich formations. Below the clay layer resistivities increase to about 40–400 Ω-m, interpreted to represent sand/gravel formations with a maximum thickness of about 40 m and a width of several hundred metres. This is a potential aquifer that extends in approximately N–S direction for some kilometres.     

Electrical resistivity characterization and defect detection on a geosynthetic clay liner (GCL) on an experimental site

In this paper we analyze the onsite characterization of a geosynthetic clay liner (GCL) that serves to ensure the impermeability of a landfill cap by DC electrical methods. The imaging of the GCL geoelectrical properties is a challenging problem because it is a very thin (between 4 and 7 mm thick) and resistive layer (from 100,000 to 2,000,000 Ω·m) depending on meteorological conditions and aging. We compare results obtained using electrical resistivity tomography (ERT) using two different kinds of arrays (dipole–dipole DD and Wenner–Schlumberger) on an experimental site with engineered defects. To confirm these results and to find the real onsite GCL resistivity we have performed sampling of the posterior distribution of this parameter using vertical electrical sounding (VES) inversions. Different VES methods were extracted from ERT with DD array and converted into a Schlumberger array.As a main conclusion the dipole–dipole array provides a better resistivity resolution of the defects than the Wenner–Schlumberger array. On ERT images, the defect detection seems to be impossible if the GCL has very high resistivity, as it happened when it was put in place. Taking into account the equivalence rules, the inversions are in both cases (ERT and VES) compatible. The GCL resistivity estimated from PSO (particle swarm optimization) varies from 3.0 10⁵ to 1.10⁶ Ω·m depending on saturation conditions during the twenty first months of its placing. Then, the resistivity dropped to 4.10⁴–9.10⁴ Ω·m, indicating a probable chemical damage of the GCL due to aging. Finally the fact that the VES inversions are solved via PSO sampling allows for the detection of a very thin and resistive layer and opens the possibility of performing micro VES surveys along the landfill to detect possible GCL defects.     

Resistivity monitoring of the tephra barrier at Crater Lake,Mount Ruapehu,New Zealand

The eruptions of Mt Ruapehu in the North Island of New Zealand in 1995 and 1996 caused a tephra barrier to be formed across the outlet of Crater Lake. By 2005 seepage from the refilled lake into the barrier raised the possibility of an eventual collapse of the barrier, releasing a catastrophic lahar down the mountain.As part of an extensive monitoring programme of the tephra barrier, direct current (dc) resistivity surveys were carried out on a number of lines along and across it in order to test whether the extent of the seepage could be measured (and monitored) by geophysical means. Two dimensional inversion of measured apparent resistivity data showed that between the initial measurements, made in January 2005, and February 2006, there was a gradual decrease in resistivity above the old outlet from ~ 50–60 Ωm to ~ 30 Ωm. This gave the first indication that lake water was seeping into the barrier. Between October and December 2006 there was a rapid rise in lake level to only 2 m below the top of the barrier, and a further resistivity survey in January 2007 showed that there had been a further decrease in resistivity throughout the entire barrier with values dropping to < 10 Ωm. The extent of this low resistivity indicated that the barrier was now saturated. At this stage lake water was penetrating the barrier and starting to cause erosion on its downstream side. Catastrophic collapse occurred on 18 March 2007, accompanied by a lahar in the Whangaehu river valley.Subsequent forward 3D numerical modelling of the resistivity structure of the barrier has confirmed that the observed changes in measured resistivity were directly related to the progress of seepage of lake water into the barrier.     

Combined electrical and electromagnetic imaging of hot fluids within fractured rock in rugged Himalayan terrain

Integrated electrical resistance tomography (ERT) and short-offset transient electromagnetic (TEM) measurements were carried out to investigate a geothermal area in the Main Central Thrust (MCT) zone of Garhwal Himalayan region, India. The study area is located around Helang on either side of Alaknanda River and it is dotted with hot water springs with water temperature of 45°–55 °C emerging at the surface.To assess the geothermal potential and its lateral and vertical extension in and around the hot water springs in the study area, 7 ERT profiles and 21 TEM stations on 7 profiles were established around the hot water spring and at far distant locations. The 2D inversion of ERT data indicates a low resistivity (< 50 Ωm) zone in the vicinity of hot springs, which appears to be associated with an underground water channel through the fractured rock. The bedrock resistivity is very high (> 1000 Ωm) whereas the resistivity of the weathered near surface soil at a far distant location from the hot spring is low (< 100 Ωm) again. A common feature of all TEM data is the sign reversal observed at roughly 10 μs. The consistent sign reversal in all TEM data indicates the existence of the multi-dimensionality of the geoelectrical structure. Therefore, the TEM data were treated by using the SLDM (Spectral Lanczos Decomposition Method) 2D/3D forward modeling code based on the finite difference algorithm. The resistivity structure obtained from ERT data was used as an input for the modeling of TEM data. Based on the joint analysis of the ERT and TEM data it can be inferred that geothermal anomalies associated with the hot spring in the MCT zone are a local feature appearing as a low resistivity zone (< 50 Ωm) at shallow depth (< 100 m) in the vicinity of the hot spring region.     

The use of MRS in the determination of hydraulic transmissivity: The case of alluvial aquifers

Magnetic Resonance Sounding (MRS) is nowadays accepted as a new geophysical method that can be used for a reliable determination of the ground water content distribution in the top 150 m. A great effort has also been made in MRS development to deduce the hydraulic transmissivity, based on empiric relationships of the permeability with a factor F which is calculated with NMR parameters measured at laboratory scale. To use this relationship under field conditions a calibration coefficient C_T = T_pt / F has to be previously established, which demands the knowledge of the transmissivity T_pt evaluated in the pumping test. The transmissivity can then be calculated at any other site of the same aquifer using the relation T_mrs = C_TF. The C_T values reported suggest a certain relationship with the lithology, but with a great dispersion and contradictory results. MRS surveys carried out in alluvial aquifers in Spain have shown that the value of C_T evaluated at one site may not be valid at another place of the same aquifer, because of the great heterogeneity of this kind of geological environment. The demand of a pumping test at each site where a MRS is measured invalidates the method actually used for MRS transmissivity evaluation. More than 50 MRS have been used to propose a new methodology. The aquifers visited cover a great range of transmissivities (from 2 × 10^− 6 to 9 × 10^− 3 m²/s). The MRS signal amplitude varies between 20 and 1400 nV, the signal/noise ratio is in the range from 0.6 to 42, and the value of the decay time constant varies from 200 to 800 ms. It has been demonstrated that when the transmissivity increases, the value of F decreases, and C_T increases, except for certain groups of MRS taken at the same aquifer or part of one aquifer, for which F increases with T_pt, keeping C_T constant. A function C_T(F) of the type C_T = mF^− n has been obtained that allows the transmissivity evaluation without the need of T_pt. Considering that both values of transmissivity, T_pt and T_mrs, are subjected to deviations due to the experimental errors as well as due to evaluation errors, the prediction achieved by the proposed equation is rather good. To perform a better evaluation of the values of the coefficients m and n it is necessary to have a greater number of MR soundings of good quality and with a trustworthy inversion at locations where a really comparable and good performed pumping test is available, covering a sufficient range of transmissivities. Though the data we have used do not always fulfil these conditions, the result is promising. Once a trustable function is available, the forecast of the transmissivity using MRS will not need the existence of any pumping test in the area. The general extension of this methodology demands the availability of MRS taken at all kinds of geological and hydrogeological environments, which is impossible without the existence of a universal MRS data base.     

Using geophysical methods to define the attitude and extension of water-bearing strata in the Miocene sediments of the Pannonian Basin

In the area near the village of Jazak (southern part of Fruška Gora mountain, Serbia), hydrogeological investigations were carried out for the purpose of finding a water supply source to provide an adequate volume of water for a mineral water bottling plant. The first exploratory borehole (IBJf-1) penetrated a water-bearing layer of Miocene organogenic limestones. This aquifer has a thickness of about 30 m and a yield of only 2.2 l/s, which falls short of the required water volume (5 l/s).The objective of further exploration was to define the attitude and extension of the aquifer and thus select a more favourable site for a new exploratory borehole that would secure the required volume of water. For this purpose, geophysical exploration was carried out in 2003 through vertical electrical sounding (VES) and high-resolution 3D reflection seismic methods. The VES measurements enabled determination of aquifer depth and indicated that the water-bearing strata extend over the entire area studied. However, because of the equivalence problem, it was not possible to determine the thickness of the water-bearing stratum based solely on the VES data. Thus, the 3D seismic method was used in the second stage of investigation. A low-cost 3D seismic survey was carried out with fixed receiver lines, using a vibrator as the source of the seismic waves.From the 3D seismic data it was possible to determine the aquifer thickness. The depth of the aquifer determined by interpretation of the 3D seismic data was in accordance with the depth determined by the VES method. Based on the assumption that the hydraulic conductivity of this formation is identical or similar over the entire area, as well as the fact that the first well showed the presence of a subartesian aquifer, we proposed drilling another borehole (IBJf-2) in the zone where the data indicated that the water-bearing stratum was much thicker. The data obtained by drilling and coring were in agreement with the predicted aquifer thickness. Pumping tests showed that the water discharge in borehole IBJf-2 was 6 l/s.The results show that the objective of delineating the groundwater body by combined application of two geophysical methods (VES and 3D seismic) was successfully performed.     

A 3D resistivity model derived from the transient electromagnetic data observed on the Araba fault,Jordan

72 inloop transient electromagnetic soundings were carried out on two 2 km long profiles perpendicular and two 1 km and two 500 m long profiles parallel to the strike direction of the Araba fault in Jordan which is the southern part of the Dead Sea transform fault indicating the boundary between the African and Arabian continental plates. The distance between the stations was on average 50 m.The late time apparent resistivities derived from the induced voltages show clear differences between the stations located at the eastern and at the western part of the Araba fault. The fault appears as a boundary between the resistive western (ca. 100 Ωm) and the conductive eastern part (ca. 10 Ωm) of the survey area. On profiles parallel to the strike late time apparent resistivities were almost constant as well in the time dependence as in lateral extension at different stations, indicating a 2D resistivity structure of the investigated area.After having been processed, the data were interpreted by conventional 1D Occam and Marquardt inversion. The study using 2D synthetic model data showed, however, that 1D inversions of stations close to the fault resulted in fictitious layers in the subsurface thus producing large interpretation errors. Therefore, the data were interpreted by a 2D forward resistivity modeling which was then extended to a 3D resistivity model. This 3D model explains satisfactorily the time dependences of the observed transients at nearly all stations.     

Large internal waves in Massachusetts Bay transport sediments offshore

A field experiment was carried out in Massachusetts Bay in August 1998 to assess the role of large-amplitude internal waves (LIWs) in resuspending bottom sediments. The field experiment consisted of a four-element moored array extending from just west of Stellwagen Bank (90-m water depth) across Stellwagen Basin (85- and 50-m water depth) to the coast (24-m water depth). The LIWs were observed in packets of 5–10 waves, had periods of 5–10 min and wavelengths of 200–400 m, and caused downward excursions of the thermocline of as much as 30 m. At the 85-m site, the current measured 1 m above bottom (mab) typically increased from near 0 to 0.2 m/s offshore in a few minutes upon arrival of the LIWs. At the 50-m site, the near-bottom offshore flow measured 6 mab increased from about 0.1 to 0.4–0.6 m/s upon arrival of the LIWs and remained offshore in the bottom layer for 1–2 h. The near-bottom currents associated with the LIWs, in concert with the tidal currents, were directed offshore and sufficient to resuspend the bottom sediments at both the 50- and 85-m sites. When LIWs are present, they may resuspend sediments for as long as 5 hours each tidal cycle as they travel westward across Stellwagen Basin. At 85-m water depth, resuspension associated with LIWs is estimated to occur for about 0.4 days each summer, about the same amount of time as caused by surface waves.     

Combining Electrical Resistivity Tomography and Ground Penetrating Radar to study geological structuring of karst Unsaturated Zone

This paper highlights the efficiency and complementarity of a light package of geophysical techniques to study the structure of karst Unsaturated Zone (UZ) in typical Mediterranean environment where soil cover is thin or absent. Both selected techniques, 2D Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT), are widely used in environmental studies and their application is accessible for a lot of scientists/engineers. However, GPR or ERT alone is not able to provide an enhanced characterization of geological features in karst media. In the present study, GPR results supply a near surface high resolution imaging and thus can provide relevant geological information such as stratifications and fractures. Despite the quality of the results GPR's investigation depth remains limited to around 12 m. Apparent and inverted resistivity provided by ERT surveys shows strong lateral and vertical variations. These variations can inform about general geological structuring and feature orientation. ERT is able to prospect down to 40 m but it's a low resolution integrative technique. In the study area the investigated limestone is a commonly electrical resistive formation (more than 2000 Ω.m). However deeper than 5–7 m, the ERT profiles reveal several zones of moderate resistivity (around 900 Ω.m). In these zones a stratification change corresponding to slanted bedding is clearly identified by GPR results. The combination of both GPR and ERT results can allow a well-established geological interpretation. These moderate resistivity zones with slanted beddings can explain the presence of a perennial water flow point 35 m below the surface of the studied site within the underground gallery of the Low-Noise Underground Laboratory (LSBB).     

Inversion of resistivity in Magnetic Resonance Sounding

Magnetic Resonance Sounding (MRS, or Surface Nuclear Magnetic Resonance - SNMR) is used for groundwater exploration and aquifer characterization. Since this is an electromagnetic method, the excitation magnetic field depends on the resistivity of the subsurface. Therefore, the resistivity has to be taken into account in the inversion: either as a priori information or as an inversion parameter during the inversion process, as introduced in the presented paper. Studies with synthetic data show that water content and resistivity can be resolved for a low resistive aquifer even using only the amplitude of the MRS signal. However, the inversion result can be significantly improved using amplitude and phase of the MRS signal. The successful implementation of the inversion for field data shows that the resistivities derived from MRS are comparable to those from conventional geoelectric methods such as DC resistivity and transient electromagnetic. By having information about both the resistivity and the water content, MRS inversions give information about the quality of the water in the aquifer. This is of utmost interest in hydrogeological studies as this specific information cannot be determined solely by geoelectric measurements, due to the nonunique dependence of resistivity on water content and salinity.     

Direct and indirect methods for groundwater investigations: A case-study of MRS and VES in the southern part of Sweden

Magnetic Resonance Sounding (MRS) has been successfully tested for detecting groundwater in two areas in southern Sweden. Measurements of Schlumberger VES have been conducted in the same place as the MRS and the results are generally consistent. Low resistivity layers interpreted as clay are sometimes identified close to the surface. The MRS result in site 2 is a good example of signals penetrating through the clay and deeper aquifer still being detected. The MRS data suggest aquifers that are not only hosted in soft sediment materials (moraine, sand, and mixed materials), but hosted in basement rocks. Based on the MRS and borehole pumping test data, the results agree with yield, average water content and subsurface geological data.     

Volcano-sedimentary stratigraphy in the Valsequillo Basin,Central Mexico inferred from electrical resistivity soundings

Initial results of an electrical resistivity survey of the volcano-sedimentary sequence of the Valsequillo basin in central Mexico are presented. The volcano-sedimentary deposits preserve rich paleontological, paleoclimatic and paleoenvironmental records, which include extinct megafauna remains associated with human artifacts. The report of possible 38 ka old human footprint tracks in the Xalnene tuff attracted renewed interest in the basin stratigraphy. We examine the shallow stratigraphic sequence in the Xalnene tuff outcrop plain northwest of Cerro Toluquilla volcano using vertical resistivity soundings (VES). Inversion models of VES soundings show a layered structure of high and low resistivity units, which characterize the Xalnene tuff, lacustrine and fluvial sediments and volcanic rocks. 2-D resistivity cross sections document three major units corresponding to the Xalnene tuff and sediments filling a <30 m deep basin lying on volcanic rocks. Resistivity models provide further support for the association of Xalnene tuff with the Toluquilla volcano and emplacement of the pyroclastic deposits on a shallow lacustrine environment. The resistivity cross sections constrain the thickness of the tuff layers and underlying lacustrine sediments. Observations during the data acquisition field work provide insight on the possible origin of the apparent tracks, which seem to develop from erosion processes acting on quarrying marks. Further analysis and experimental evidence is required to understand the morphology and weathered patterns. The tuff layers are being removed by quarrying operations and the outcrops significantly altered. Adequate conservation measurements should be implemented to preserve the deposits for scientific research.     

Monitoring of Guadalentín valley (southern Spain) through a fast SAR Interferometry method

Ground subsidence is a critical problem in populated areas, which requires attention and routine monitoring. Specifically, subsidence related to the exploitation of aquifer systems is of particular interest in terms of risk for the possible damage to buildings and availability of water resources. We propose to realise a survey of the aquifer systems near the city of Lorca, southern Spain, using Synthetic Aperture Radar Interferometry (InSAR) with a light method necessitating a small number of interferograms rather than analysing hundreds of radar images. We computed nine interferograms with good coherence over the years 2004 and 2005. We analysed them by a two-step filtering process in order to characterise the atmospheric contribution and the ground deformation. We are then able to identify two deforming areas and quantify subsidence rates at 0.20 ± 0.02 mm/day south of Lorca and southeast to the city of Alhama de Murcia with the same efficiency as the studies using sophisticated InSAR methods. Using continuous records of water levels in boreholes, we deduced a mean storage coefficient (S_k) at 4.1 × 10^− 3 which is very close to the previous published values, and coincides in this case to an inelastic deformation mode of all the system. With this value, we determine the groundwater level variations from the measurements of the ground subsidence.     

Efficiency of joint use of MRS and VES to characterize coastal aquifer in Myanmar

The productivity and the water quality of coastal aquifers can be highly heterogeneous in a complex environment. The characterization of these aquifers can be improved by hydrogeological and complementary geophysical surveys. Such an integrated approach is developed in a non-consolidated coastal aquifer in Myanmar (previously named Burma).A preliminary hydrogeological survey is conducted to know better the targeted aquifers. Then, 25 sites are selected to characterize aquifers through borehole drillings and pumping tests implementation. In the same sites, magnetic resonance soundings (MRS) and vertical electrical soundings (VES) are carried out. Geophysical results are compared to hydrogeological data, and geophysical parameters are used to characterize aquifers using conversion equations. Finally, combining the analysis of technical and economical impacts of geophysics, a methodology is proposed to characterize non-consolidated coastal aquifers.Depth and thickness of saturated zone is determined by means of MRS in 68% of the sites (evaluated with 34 soundings). The average accuracy of confined storativity estimated with MRS is ± 6% (evaluated over 7 pumping tests) whereas the average accuracy of transmissivity estimation with MRS is ± 45% (evaluated using 15 pumping tests). To reduce uncertainty in VES interpretation, the aquifer geometry estimated with MRS is used as a fixed parameter in VES inversion. The accuracy of groundwater electrical conductivity evaluation from 15 VES is enough to estimate the risk of water salinity. In addition, the maximum depth of penetration of the MRS depends on the rocks' electrical resistivity and is between 20 and 80 m at the study area.     

Characterization of recharge through complex vadose zone of a granitic aquifer by time-lapse electrical resistivity tomography

The vadose zone is the main region controlling water movement from the land surface to the aquifer and has a very complex structure. The use of non-invasive or minimally invasive geophysical methods especially electrical resistivity imaging is a cost-effective approach adapted for long-term monitoring of the vadose zone. The main aim of this work is to know the fractures in the vadose zone, of granitic terrene, through which the recharge or preferred path recharge to the aquifer takes place and thus to relate moisture and electrical resistivity. Time lapse electrical resistivity tomography (TLERT) experiment was carried out in the vadose zone of granitic terrene at the Indian Geophysical Research Institute, Hyderabad along two profiles to a depth of 18 m and 13 m each. The profiles are 300 m apart. Piezometric, rainfall and soil moisture data were recorded to correlate with changes in the rainfall recharge. These TLERT difference images showed that the conductivity distribution was consistent with the recharge occurring along the minor fractures. We mapped the fractures in hard rock or granites to see the effect of the recharge on resistivity variation and estimation of moisture content. These fractures act as the preferred pathways for the recharge to take place. A good correlation between the soil moisture and resistivity is established in the vadose zone of granitic aquifer. Since the vadose zone exhibits extremely high variability, both in space and time, the surface geophysical investigations such as TLERT have been a simple and useful method to characterize the vadose zone, which would not have been possible with the point measurements alone. The analyses of the pseudosection with time indicate clearly that the assumption of the piston flow of the moisture front is not valid in hard rocks. The outcome of this study may provide some indirect parameters to the well known Richard's equation in studying the unsaturated zone.     

Nutrient inputs from submarine groundwater discharge on the Santiago reef flat,Bolinao, Northwestern Philippines

Submarine groundwater discharge (SGD) on the reef flat of Bolinao, Pangasinan (Philippines) was mapped using electrical resistivity, ²²²Rn, and nutrient concentration measurements. Nitrate levels as high as 126 μM, or 1–2 orders of magnitude higher than ambient concentrations, were measured in some areas of the reef flat. Nutrient fluxes were higher during the wet season (May–October) than the dry season (November–April). Dissolved inorganic nitrogen (DIN = NO₃ + NO₂ + NH₄) and soluble reactive phosphorus (SRP) fluxes during the wet season were 4.4 and 0.2 mmoles m⁻² d⁻¹, respectively. With the increase population size and anthropogenic activities in Bolinao, an enhancement of SGD-derived nitrogen levels is likely. This could lead to eutrophic conditions in the otherwise oligotrophic waters surrounding the Santiago reef flat.     

Optical dating of the upper 22 m of cored sediments from Daihai Lake,northern China

Daihai Lake is one of the largest lakes in the mid-latitude of northern China. Previous environmental change investigations using the sediments from the lake have been mainly focused on the last 13 ka. In 2006, we drilled an 80-m borehole on the southwest coast of the lake. Here we report the results of initial optical dating of the upper 22-m sediments from the core. Most of the samples allow separation of the 4–11 μm fine-grained and 45–63 μm medium-grained quartz, both of which were used for equivalent dose (D_e) determination. Dose recovery test experiments with the single-aliquot regeneration-dose (SAR) protocol indicate that a preheat at 240 °C for 10 s combined with a cut-heat of 240 °C is suitable for these samples. The D_e values show marked discrepancy between the two grain size fractions with the fine-grained quartz yielding up to over 50% higher D_e values than the medium ones for some samples. We consider the OSL ages for the samples from upper ～10 m to be overestimated. The main cause of the overestimation is attributed to the incomplete bleaching of the sediment grains at deposition. The OSL ages for the lower part of the sequence are considered to be reasonable age estimates which provide useful temporal constrains on the deposition of the sediments for the period of 20–40 ka.     

Importance of reversible attachment in predicting E. coli transport in saturated aquifers from column experiments

Drinking water wells indiscriminatingly placed adjacent to fecal contaminated surface water represents a significant but difficult to quantify health risk. Here we seek to understand mechanisms that limit the contamination extent by scaling up bacterial transport results from the laboratory to the field in a well constrained setting. Three pulses of Escherichia coli originating during the early monsoon from a freshly excavated pond receiving latrine effluent in Bangladesh were monitored in 6 wells and modeled with a two-dimensional (2-D) flow and transport model conditioned with measured hydraulic heads. The modeling was performed assuming three different modes of interaction of E. coli with aquifer sands: (1) irreversible attachment only (best-fit k_i = 7.6 day⁻¹); (2) reversible attachment only (k_a = 10.5 and k_d = 0.2 day⁻¹); and (3) a combination of reversible and irreversible modes of attachment (k_a = 60, k_d = 7.6, k_i = 5.2 day⁻¹). Only the third approach adequately reproduced the observed temporal and spatial distribution of E. coli, including a 4-log₁₀ lateral removal distance of ∼9 m. In saturated column experiments, carried out using aquifer sand from the field site, a combination of reversible and irreversible attachment was also required to reproduce the observed breakthrough curves and E. coli retention profiles within the laboratory columns. Applying the laboratory-measured kinetic parameters to the 2-D calibrated flow model of the field site underestimates the observed 4-log₁₀ lateral removal distance by less than a factor of two. This is promising for predicting field scale transport from laboratory experiments.