Preferential Groundwater Flow Paths Research Articles

Estimation of groundwater recharge from groundwater level fluctuations and baseflow rates around Mount Meru, Tanzania

Estimating groundwater recharge, direct runoff and baseflow is essential for understanding groundwater resource availability and managing groundwater systems. This study estimates groundwater recharge, direct runoff and baseflow on two slopes of Mount Meru: the northern and southern slopes using the water-table fluctuation (WTF) method and baseflow separation technique. High-frequency groundwater level measurements in five shallow wells over three hydrological years from 2018 to 2021 were analysed, while streamflow data in four gauging stations over nine hydrological years from 2010 to 2019 were used. The results of the WTF method show that the aquifer undergoes an average recharge of 544 mm/year and 90 mm/year on the south-western and north-eastern slopes, respectively. On average, this recharge is about 53% and 13% of the annual rainfall on each slope. The baseflow results show that the aquifer on the south-eastern and north-western slopes recharges an average of 88 mm/year and 54 mm/year, respectively, which is on average about 12% and 7% of annual rainfall, respectively. In general, the high recharge on the south-western slope is attributed to the high rainfall, and the high hydraulic conductivity and high hydraulic diffusivity of the pyroclastic deposits compared to the debris avalanche deposits on the north-eastern slope. In addition, debris avalanche deposits show homogeneous recharge conditions, while pyroclastic deposits show heterogeneous recharge conditions. The WTF method can be useful to identify areas of preferential recharge so that preferential groundwater flow paths can be mapped for focused recharge of surface runoff during the rainy season.

Study of the groundwater regime in unsaturated slopes prone to landslides by multidisciplinary investigations: Experimental study and numerical modelling

The triggering of shallow landslides in granular deposits is highly controlled by the groundwater regime, namely, the pore water pressure distribution, which directly affects soil shear strength. The slope hydraulic state is usually variable over time and space due to the high variability of atmospheric loads at the ground surface and local geologic conditions, such as stratigraphic irregularities and preferential groundwater flow paths. The unfavourable combination among critical geomorphological and topographical settings with stratigraphic and hydrogeological features are commonly recognized as predisposing factors of flowslides and debris flow occurrences.This paper proposes a multidisciplinary approach that combines geological, geophysical and geotechnical investigations to identify the role of local geological and geotechnical factors on the groundwater regime in slopes prone to flow-like landslides. The study is based on seasonally repeated electrical resistivity tomography measurements integrated with geotechnical numerical modelling of hydraulic phenomena affecting the soil cover. The latter is used to analyse the effects of the stratigraphic variability in terms of the geometry, continuity, and thickness of the soil horizons on the groundwater regime over time. The proposed approach has been applied to a test site located on the northern slope of Faito Mt. in the Lattari/Sorrento Peninsula mountain chain (southern Italy), an area historically affected by many rapid instability phenomena, such as flow-like landslides and flash floods. Both geophysical and geotechnical models obtained for the test site were cross-checked and validated, providing significant insights into the hydraulic response of the soil cover to rainfall and its hydraulic interaction with the underlying bedrock. Specifically, the integrated approach proved that i) the buried paleo-morphology of the bedrock severely affects the pore water distribution in the soil cover and ii) ashy soil fills the upper karst portion of the bedrock, providing a hydraulic connection of the water flow infiltrating from the topsoil downwards.

Groundwater flow paths drive longitudinal patterns of stream dissolved organic carbon (DOC) concentrations in boreal landscapes

Abstract. Preferential groundwater flow paths can influence dissolved organic carbon (DOC) concentration and export in the fluvial network because they facilitate the inflow of terrestrial DOC from large upslope contributing areas to discrete sections of the stream, referred to as discrete riparian inflow points (DRIPs). However, the mechanisms by which DRIPs influence longitudinal patterns of stream DOC concentrations are still poorly understood. In this study, we ask how DRIPs affect longitudinal patterns of stream DOC concentrations under different hydrologic conditions, as they can simultaneously act as major sources of terrestrial DOC and important locations for in-stream processes. To answer this question, we tested four model structures that account for different representations of hydrology (distributed inflows of DRIPs vs. diffuse groundwater inflow) and in-stream processes (no DOC uptake vs. in-stream DOC uptake downstream of DRIPs) to simulate stream DOC concentrations along a 1.5 km headwater reach for 14 sampling campaigns with flow conditions ranging from droughts to floods. Despite the magnitude and longitudinal patterns of stream DOC concentration varying across campaigns, at least one model structure was able to capture longitudinal trends during each campaign. Specifically, our results showed that during snowmelt periods or high-flow conditions (>50 L s−1), accounting for distributed inputs of DRIPs improved simulations of stream DOC concentrations along the reach, because groundwater inputs from DRIPs diluted the DOC in transport. Moreover, accounting for in-stream DOC uptake immediately downstream of DRIPs improved simulations during five sampling campaigns that were performed during spring and summer, indicating that these locations served as a resource of DOC for aquatic biota. These results show that the role of DRIPs in modulating DOC concentration, cycling, and export varies over time and depends strongly on catchment hydrology. Therefore, accounting for DRIPs can improve stream biogeochemistry frameworks and help inform management of riparian areas under current and future climatic conditions.

Hydrogeological Model of the Forefield Drainage System of Werenskioldbreen, Svalbard

The significant recession of Arctic glaciers caused by climate warming is expanding their proglacial zones. Thus, their importance for the hydrology of glacierised basins is growing. In contrast to the surface waters in such areas, the role of groundwater in the hydrological balance of Svalbard catchments is poorly known. This paper presents the hydrogeological conditions and groundwater flow within the permafrost active layer in the forefield of the Werenskioldbreen glacier basin (44.1 km2), 61% of which is glacierised. Based on field studies of groundwater in the 2017 ablation season and laboratory analyses of the hydrogeological properties of proglacial sediments, a three-dimensional groundwater flow model (FEFLOW) for part of the glacier forefield (4.8 km2) was developed. The main results show the components and characteristics of the groundwater balance and indicate the preferential groundwater flow paths. The volume of water retained in the sediments of the marginal zone is 1.0073 mln m3. The maximum potential free pore space that could be filled by water is 2.0689 mln m3. The calculated groundwater discharge for average conditions is 6076.9 m3 d−1, which is about 2% of the total seasonal catchment runoff from the main glacial river. The results of the spatial analysis for the groundwater depth and the groundwater flow directions are also presented. There need to be further detailed studies of hydrogeological processes in glacial basins in Svalbard in order to develop existing knowledge.

Field-scale investigation of nanoscale zero-valent iron (NZVI) injection parameters for enhanced delivery of NZVI particles to groundwater

The effects of the injection parameters on delivery of nanoscale zero-valent iron (NZVI) to contaminated groundwater were investigated. The first two NZVI injections (gravity injection at low flow rates) resulted in NZVI being poorly mobile and gave total cumulative mass recoveries at the monitoring wells of 1.07%–2.43%. NZVI reached some wells (KDMW-3, MW-2, MW-4, and MW-7) earlier than the bromide tracer. The dominant travel directions for NZVI and the bromide tracer were very different. The NZVI transport characteristics suggested that targeted NZVI delivery requires preferential groundwater flow paths and local heterogeneity to be considered. In the gravity injection tests, the maximum NZVI concentrations and cumulative NZVI mass recoveries in the wells decreased markedly as the injected NZVI concentration and dose increased. In the third and fourth tests, in which NZVI was injected under pressure at high flow rates, NZVI was effectively delivered to the wells despite the injected NZVI concentration and dose being high. Relatively high cumulative mass recoveries of 26.0% and 74.5% were found for the third and fourth injections, respectively. Controlling the flow rate (pressure) and NZVI concentration and dose simply and effectively controlled NZVI mobility in the groundwater. The colloidal and electrostatic characteristics of the NZVI particles were monitored and modeled, and the results indicated that NZVI particles without Derjaguin–Landau–Verwey–Overbeek energy barriers were successfully delivered to the target zone and that decreased magnetic attractive forces between NZVI particles caused by iron corrosion probably decreased the degree of NZVI particle aggregation and therefore contributed to NZVI being delivered to the target zone.

Hydrogeological controls on the flow regime of an ephemeral temperate stream flowing across an alluvial fan

Alluvial fans often support ephemeral streams whose flow regimes and sediment dynamics are strongly controlled by fan sedimentary characteristics and interactions with shallow groundwater aquifers. The hydrogeology of such fans has most often been documented for large fans located within arid climate zones. This research focuses on a small (0.075 km2) alluvial fan situated in a temperate, high rainfall climate (the Lake District, North West England). An ephemeral stream flowing across this alluvial fan plays a key role in supplying water and sediment to the River Ehen, which is the focus of a habitat restoration initiative. This study combines high spatial resolution, near surface geophysics and outcrop data with hydrological data to characterise the hydrogeological properties of the alluvial fan. A conceptual hydrogeological model was developed to understand the hydrology across the alluvial fan and how that affects water and sediment supply to the River Ehen. The alluvial fan is composed predominantly of permeable debris flow deposits and numerous palaeochannels which may provide preferential groundwater flow paths. During streamflows, partial to full stream-aquifer connectivity occurs, especially in the distal fan where groundwater discharges back into the stream. Streamflows occur when the fan apex infiltration rate/capacity threshold of approx. 60 l/s−1 is exceeded, typically following rainfall events >9–11 mm. Understanding interactions between ephemeral streams and their underlying aquifers enables better predictions of the timing and magnitude of future flows, and in-turn, their likely impacts on the water courses into which they discharge.

Magnetometric resistivity: a new approach and its application to the detection of preferential flow paths in mine waste rock dumps

The injection of a low frequency electrical current in the ground between two electrodes A and B generates a magnetic field that can be measured at the ground surface with sensitive magnetic sensors. The map of the magnetic field, measured at the frequency of the injected current, can be used to determine the paths of the current through the ground. When the current is channelled along preferential conductive paths, the MagnetoMetric Resistivity (MMR) method can be used to detect these paths. Conductive current paths can be associated with preferential flow paths of groundwater when the two electrodes A and B are in the direction of the flow and when the flow path is highly electrically conductive with respect to the background. We first review the background equations for the magnetic field in MMR. Then, we provide the kernel of the problem using Biot and Savart law to connect the components of the observed magnetic field to the current density distribution. We also develop a simple approach to invert the magnetic field in terms of electrical current paths. To illustrate how the method works, we develop five synthetic models to test the sensitivity of the method to the properties of the conductive targets channelling the electrical current. The targets are characterized by different shapes, sizes, depths, and conductivity contrasts with the background. Then, we proceed with a case study for which the MMR method is used to identify and map preferential groundwater flow paths bypassing a mine waste rock dump drainage collection trench into the tailings pond. In this case, the conductivity of the flow paths is much stronger than the background conductivity due to the high mineralization of the ground water along these paths. The method underlines the 3-D architecture of these flow paths.

Sinkholes, subsidence and subrosion on the eastern shore of the Dead Sea as revealed by a close-range photogrammetric survey

Ground subsidence and sinkhole collapse are phenomena affecting regions of karst geology worldwide. The rapid development of such phenomena around the Dead Sea in the last four decades poses a major geological hazard to the local population, agriculture and industry. Nonetheless many aspects of this hazard are still incompletely described and understood, especially on the eastern Dead Sea shore. In this work, we present a first low altitude (< 150m above ground) aerial photogrammetric survey with a Helikite Balloon at the sinkhole area of Ghor Al-Haditha, Jordan. We provide a detailed qualitative and quantitative analysis of a new, high resolution digital surface model (5cm px−1) and orthophoto of this area (2.1km2). We also outline the factors affecting the quality and accuracy of this approach.Our analysis reveals a kilometer-scale sinuous depression bound partly by flexure and partly by non-tectonic faults. The estimated minimum volume loss of this subsided zone is 1.83 ⋅ 106m3 with an average subsidence rate of 0.21myr−1 over the last 25years. Sinkholes in the surveyed area are localized mainly within this depression. The sinkholes are commonly elliptically shaped (mean eccentricity 1.31) and clustered (nearest neighbor ratio 0.69). Their morphologies and orientations depend on the type of sediment they form in: in mud, sinkholes have a low depth to diameter ratio (0.14) and a long-axis azimuth of NNE–NE. In alluvium, sinkholes have a higher ratio (0.4) and are orientated NNW–N. From field work, we identify actively evolving artesian springs and channelized, sediment-laden groundwater flows that appear locally in the main depression. Consequently, subrosion, i.e. subsurface mechanical erosion, is identified as a key physical process, in addition to dissolution, behind the subsidence and sinkhole hazard. Furthermore, satellite image analysis links the development of the sinuous depression and sinkhole formation at Ghor Al-Haditha to preferential groundwater flow paths along ancient and current wadi riverbeds.

Nitrate-N removal using slowly released molasses barrier in a shallow aquifer: Obstacles from lab/pilot-scale results to field application

This study was conducted to test the field applicability of a well-type barrier system containing Slowly Released Molasses (SRM) as an extra carbon and energy source to promote the indigenous bacterial denitrification activity. A total of 22 wells were placed as a grid system in the 4 m × 4 m square by 1-m interval in one alluvial area. A total of 70 SRM rods were placed in the center of the grid system to consist of a field-scale SRM system. Nitrate-N plume (72 mg N L−1 of nitrate-N) was introduced into the SRM system and change in nitrate-N concentration was monitored for 90 hours. Nitrate-N concentration decreased up to only 39% while 79-85% of removal efficiency was attained from lab/pilot experiments. Such a difference probably resulted from heterogeneity/anisotropy of the aquifer, DO concentration in the shallow groundwater, and a wide SRM rods-containing well distance. Spatial heterogeneity/anisotropy induced the preferential groundwater flow paths in which nitrate plume moved so fast and met the limited quantity of molasses molecules within or near the paths. Relatively aerobic condition of the shallow aquifer had not changed during the test period. Relatively far distance (100 cm) between the wells led to the incomplete mixing of the released molasses and nitrate-N plume. A detailed characterization of a target site should proceed before applying the SRM system for more effective nitrate-N treatment in the field.

Using radon as environmental tracer for the assessment of subsurface Non-Aqueous Phase Liquid (NAPL) contamination – A review

The radioactive noble gas radon has an ambivalent nature: on the one hand is it of main concern with regard to radiation protection, on the other hand can it be applied as powerful tracer tool in various fields of applied geosciences. Due to its omnipresence in nature, its chemical and physical properties, and its uncomplicated detectability radon fulfils all requirements for being used as environmental tracer. This application is discussed in the paper with focus on the use of radon as tracer for subsurface contamination with Non-Aqueous Phase Liquids (NAPL). After a short introduction in the ambivalence and ubiquitous presence of radon in nature, the theoretical background of its suitability as NAPL tracer is summarized. Finally three potential applications are discussed. Background information and practical examples are given for (i) the investigation of residual NAPL contamination in soils, (ii) the investigation of residual NAPL contamination in aquifers and (iii) the monitoring of the remediation of dissolved NAPL contamination in groundwater. The presented information reveals that radon is an ideal tracer for the assessment of a wide range of subsurface NAPL contamination. Still, its application is not without restrictions. Problems may occur due to mineralogical heterogeneity of the soil or aquifer matrix. Furthermore, local changes in the permeability of the subsurface may be associated with preferential groundwater or soil gas flow paths bypassing isolated sub-domains of an investigated NAPL source zone. Moreover, NAPL aging may result in alterations in the composition of a complex NAPL mixture thus giving rise to significant changes of the radon partition coefficient between NAPL and water or soil gas. However, since radon shows a strong affinity to NAPLs in general, semi-quantitative results will always be possible.

Effects of Aquifer Heterogeneity and Geochemical Variation on Petroleum-Hydrocarbon Biodegradation at a Gasoline Spill Site

In subsurface environment, small-scale heterogeneities usually cause the reduction of the applicability of in situ remedial techniques. Biogeochemical heterogeneities and preferential groundwater flow paths create complex hydrogeologic conditions at most contaminated sites. A thorough understanding of the resulting three-dimensional distribution of contaminants is a necessity prior to determining a need for remediation. In this study, a gasoline spill site was selected to examine the effects of aquifer heterogeneities and geochemical variations on petroleum hydrocarbon biodegradation via different oxidation-reduction process. At this site, two multilevel sampling wells were installed to delineate the lateral (5 m) and vertical (0.5 m) distribution of contaminant concentrations and different biogeochemical parameters. Two 5-cm (I.D.) continuous soil cores [from 4 to 8 m below land surface (bls)] were collected within the gasoline plume to evaluate the distribution of the microbial population in soils. Results show that high microbial activities were observed in soil samples based on the following evidences: (1) high petroleum hydrocarbon degradation rate, and (2) high microbial biomass. Each soil section was used for chemical extraction, microbial enumeration, and grain size distribution. Results show that the soil sections with more permeable sediment materials corresponded with higher biomass (total anaerobes &gt; 2 x 106cells/g) and significant contaminant degradation. However, those sections with less permeable sediments contained lower microbial population. Results indicate that the subsurface microorganisms were distributed unevenly in the aquifer, and some regions were devoid of microorganisms and biodegradation activities. Spatial distribution of microorganisms, soil materials, and biogeochemical characteristics in the subsurface soils control the extent and kinetics of contaminant biodegradation. Thus, using blended aquifer materials for measurement of in situ biodegradation rates may not achieve representative results.

Integrated geophysical and morphostratigraphic approach to investigate a coseismic (?) translational slide responsible for the destruction of the Montclús village (Spanish Pyrenees)

A 12 million of m3 translational rockslide developed on a dip slope underlain by limestone with interlayered marls, and responsible for the destruction of the Montclus village in the fourteenth century, has been investigated by means of geomorphological and geophysical surveys. The combination of historical-geoarcheological, geomorphological, seismic refraction and electrical resistivity imaging datasets allowed the (1) reconstruction of the late Quaternary episodic evolution of the landslide, (2) characterization of the geometry and internal structure of the slid mass and (3) identification of preferential groundwater flow paths that favoured slope instability. The development of the landslide involved at least two different displacement episodes controlled by sliding surfaces at successively deeper stratigraphic positions. The first landsliding event, recorded by highly weathered landslide deposits situated above a perched failure plane, occurred approximately during the global Last Glacial Maximum (23–19 ka BP). The most recent event, which destroyed the Montclus village built on already slid rocks, is placed in the fourteenth century. Most probably, this reactivation event was triggered by the 1373 Ribagorza earthquake, with an estimated moment magnitude of M w 6.2. This work illustrates the benefits of combining geomorphological data with complementary geophysical technics in landslide investigation.

Heavy metal pollution in the Quaternary Garza basin: A multidisciplinary study of the environmental risks posed by mining (Linares, southern Spain)

Abstract In the metallogenic district of Linares (Jaen, Spain), intense deep mining associated with vein-type deposits of Pb and Cu has been conducted for centuries and has generated large mine waste accumulations. The objective of this study was to elucidate the influence of these mine wastes on the quality of the soil, the surface water and the groundwater. For this purpose, a sedimentary endorheic basin in the mining district was selected as the study location: the Quaternary Garza basin. The characterisation of the basin geometry and the tailings dams in the area was conducted by electrical resistivity imaging (ERI), which was supported with data provided by mechanical probing. The ERI data indicated that the basin is bounded by fractures that cause sudden changes in the thickness of the sedimentary cover and the wedging of the layers toward the edges of the basin. Furthermore, the resistivity profiles allowed the morphological identification of the deposits associated with the tailings dams. In addition, zones of weakness, preferential groundwater flow paths and the contact between the slimes deposits and the substratum were identified. These results confirmed the absence of impervious barriers at the tailings sites, which is a situation that poses a high pollution risk for soils and waters. A geochemical characterisation of the tailings dams and the surrounding soils indicated the presence of a high concentration of metals and metalloids. Of the 19 elements analysed at the dams, it is important to highlight the following values expressed in mg kg− 1: 19 913 for Pb, 418 for Cu, 4149 for Mn, 43 for As, 190 for Zn and 26 200 for Fe. Furthermore, the hydrochemical data showed the mobilisation through the aqueous media of certain heavy elements (Fe, Mn and Pb) that originate from lixiviation at these ancient tailings impoundments and from the adits that drain mine waters.

Use of the AquaTrack™ technology to identify potential preferential groundwater flow paths in a gypsum-rich bedrock unit

Determining the location of preferential groundwater flow paths is highly challenging in most fractured bedrock settings. AquaTrack™ is a surface geophysical method patented by Willowstick® Technologies, LLC, that can be used to identify potential preferential groundwater flow paths within the saturated zone. A circuit is created within the subsurface by passing an alternating electric current between two strategically placed electrodes. The distribution of electric current is influenced by variations in the porosity and permeability of the subsurface materials and the conductance of the groundwater. The magnetic field produced by the electric current can be measured at the surface and compared to the magnetic field expected for a homogeneous case. Significant deviations suggest the presence of heterogeneities, which can be modeled to gain additional insights. An example is presented, where AquaTrack™ was used to identify potential preferential groundwater flow paths in a gypsum-rich bedrock unit located more than 38 m below an industrial facility in upstate New York. Swallets are common in the area and appear to feed the deep bedrock hydrogeologic system. Data from existing bedrock wells show that the permeable gypsum-rich unit is about 1.9 m thick and is saturated on a year-round basis. The groundwater within the unit is about twice as conductive as in the overlying carbonates. The study was performed over a 39 ha area to help determine the placement of additional bedrock wells. Results of the AquaTrack™ survey and associated modeling indicated that groundwater flow within the gypsum-rich bedrock unit was more homogeneous than expected, but four potential preferential groundwater flow paths were identified. Six additional bedrock wells were subsequently installed; five wells were located within the modeled “channels”, and one well was deliberately located outside of the channels for comparison purposes. Another AquaTrack™ study has been performed to extend the original survey area to the south, again to help determine the placement of additional bedrock wells.

Regionalization based on water chemistry and physicochemical traits in the ring of cenotes, Yucatan, Mexico

Assessing water quality in aquifers has become increasingly important as water demand and pollution concerns rise. In the Yucatan Peninsula, sinkholes, locally known as cenotes, are karst formations that intercept the water table. Cenotes are distributed across the peninsula, but are particularly dense and aligned along a semi- circular formation called the Ring of Cenotes. This area exhibits particular hydrogeological properties because it concentrates, channels, and discharges fresh water toward the coasts. In this study, we identify spatial and temporal variations in chemical and physical variables at twenty-two cenotes to identify groups that share similar characteristics. Water samples from each cenotes were taken at three depths (0.5, 5.5, and 10.5 m) and during three seasons (dry, rainy, and cold-fronts season). Field measurements of pH, temperature, electrical conductivity, and dissolved oxygen were taken, and the concentrations of major ions (K + ,N a + ,M g 2+ ,C a 2+ ,H CO { ,S O 2{ 4 ,C l 2 and NO { ) were quantified. Identifying regions of the cenotes were done by applying multivariate statistical techniques (PCA, PERMANOVA, CAP). The chemical variables revealed spatial trends among the cenotes. We identified three main regions. Region 1 is associated with sea-water encroachment and high levels of sulfate that travel through preferential groundwater flowpaths from evaporites in the southern Yucatan Peninsula; Region 2 is a recharge zone, and Region 3 is characterized by sea water encroachment and by the high chemical and physical variability associated with groundwater flow from the east.

Hydrogeological definition and applicability of abandoned coal mines as water reservoirs

Hydrogeologically, the Central Coal Basin (Asturias, Spain) is characterized by predominantly low-permeability materials that make up a multilayer aquifer with very low porosity and permeability values, where the sandstones act as limited aquifers, and wackes, mudstones, shales and coal seams act as confining levels. Preferential groundwater flow paths are open fractures and zones of decompression associated with them, so the hydraulic behaviour of the system is more associated with fracturing than lithology. Thus, abandoned and flooded mines in the area acquire an important role in the management of water resources, setting up an artificial "pseudo-karst" aquifer. This paper evaluates the potential application of the abandoned mines as underground reservoirs, both for water supply and energetic use, mainly through heat pumps and small hydropower plants. In particular, the groundwater reservoir shaped by the connected shafts Barredo and Figaredo has been chosen, and a detailed and multifaceted study has been undertaken in the area. The exposed applications fit with an integrated management of water resources and contribute to improve economic and social conditions of a traditional mining area in gradual decline due to the cessation of such activity.

Unique applications of MMR to track preferential groundwater flow paths in dams, mines, environmental sites, and leach fields

Groundwater systems have been notoriously difficult to map with high degrees of accuracy. As a result, not only have traditional geophysical methods proven inaccurate for groundwater characterization work, but they are often costly in terms of time, money, and environmental trauma. This paper describes a unique application of magnetometric resistivity or MMR (Edwards and Nabighian, 1991) for groundwater mapping and modeling, which is high-speed, accurate, minimally invasive, and cost effective. This method has now been deployed at many different sites all over the United States and in other countries like Canada, England, Peru, Sri Lanka, and Argentina. In 2007, the method was employed at 17 dams; some are large well-known structures in the United States. Through two case histories, this paper will assess the effectiveness of this methodology.

Hydrochemistry and origins of mineralized waters in the Puebla aquifer system, Mexico

Significant upward movement of mineralized water takes place in the Puebla aquifer system. Preferential groundwater flow paths related to the geological structure and the lowering of the potentiometric surface are suspected to be the prime factors for this intrusion. A combined approach of geochemical and isotope analyses was used to assess the sources of salinity and processes that are controlling the changes in groundwater chemical composition in the Puebla aquifer. Geochemical and isotope data indicate that the likely source of increased solutes is mineralized water from the dissolution of evaporites of the Cretaceous age at the base of the Upper deep aquifer, which is deeper than the intakes of the shallow wells. Dedolomitization and cation exchange seems also to occur along flow paths where sulphate concentrations tend to increase. The deep regional flow paths controls the chemical stratification of groundwater in response to decreased heads through interconnecting vertical and horizontal pathways, such as in the Fosa Atlixco. The results also suggest that high sulphate concentrations originating in the Lower deep aquifer are currently affecting shallow production wells. It is concluded that hydrodynamic aspects together with hydrogeochemical characteristics need to be taken into account to correctly explain the hydrochemical evolution in the stratified aquifer.

Some stupid shallow seismic experiments I have done

While near-surface and classical seismic explorations obey the same laws of physics, the relative importance of those laws is different for the two types of surveys. These differences have led to some eccentric experiments with unexpected and occasional serendipitous outcomes. Progress attained by our research group has occurred through a mixture of stupid experiments that turned out to be clever and clever experiments that turned out to be stupid. Shallow seismic methods have matured noticeably since the time 25 years ago when the world’s scientific literature contained few refereed papers on shallow reflection. Much of the maturation is related to the revolution in microelectronics and the associated several orders of magnitude decrease in computational costs, while developments in sources, seismographs, and field methods have all played a role to differing degrees. However, other driving factors in this improvement have included demonstrable attainment of objectives such as providing structural contour maps of bedrock beneath alluvium, delineating shallow faults, evaluating near-surface stratigraphy to detect preferential groundwater flow paths, and detecting underground cavities. By 1999, we had demonstrated seismic reflection images from depths of less than a meter, easily within reach of a marginally competent grave digger. Detecting such shallow reflectors is expensive, however, because of the requirement to plant geophones at intervals of 10 cm or less. The effective resolution potential of classical seismic exploration data recorded on land is often determined by geologic conditions in the upper few tens of meters; in addition, the majority of statics problems commonly occur in the upper 30 meters. We are currently experimenting with methods of making near-surface threedimensional seismic imaging more cost-effective.

Three-dimensional mapping of geomorphic controls on flood-plain hydrology and connectivity from aerial photos

The Nyack flood plain of the Middle Fork Flathead River, MT, USA is a 9-km anastomosed alluvial montane flood plain. Upstream from the flood plain, the river is unregulated and the catchment virtually pristine. A patchy mosaic of vegetation and channels exists on the flood-plain surface. The surface and subsurface geomorphic structures of the flood plain facilitate high hydrologic connectivity (water flux between the channel and flood plain) marked by complex seasonal patterns of flood-plain inundation, extensive penetration of channel water laterally into the alluvial aquifer, and springbrooks formed by ground water erupting onto the flood-plain surface. After delineating and classifying flood-plain “elements” (vegetation patches and channel reaches) on the flood plain, we analyzed field-based elevation survey data to identify expected relationships among flood-plain element type, surface scour frequency, and flood-plain elevation. Data analyses show that scour frequency was inversely proportional to the elevation of the flood plain above river stage, except when localized geomorphic controls such as natural levees prevent normal high flows from inundating and scouring relatively low flood-plain elements. Further, while different flood-plain element types occupy distinct elevation zones on the flood plain, the elevation of each zone above the river channel varies with localized channel entrenchment. We found that topographic variation among flood-plain elements is greater than the variation within elements, suggesting that coarse-scale flood-plain topography can be characterized by delineating flood-plain elements. Field data document strong associations between specific classes of flood-plain elements and preferential ground-water flow paths in the upper alluvial aquifer. Combined with preexisting ground penetrating RADAR (GPR) surveys, these data intimate a sinuous lattice of preferential ground-water flow paths (buried abandoned streambeds) in the upper alluvial aquifer at approximately the same elevation as the main channel's streambed. Using aerial photo interpretation and the identified relationships among element-types, elevation, and preferential ground-water flow paths, we developed a quantitative, three-dimensional characterization of surface and subsurface geomorphology across the entire flood plain to support a heuristic modeling effort investigating the influence of flood-plain geomorphology on spatio-temporal patterns of surface and ground-water flow and exchange under dynamic hydrologic regimes.