Network Of Piezometers Research Articles

Patterns of remotely sensed floodplain saturation and its use in runoff predictions

Principal components analysis (PCA) is applied to a time series of European Remote Sensing (ERS) synthetic aperture radar (SAR) scenes of the Alzette River floodplain (Grand-Duchy of Luxembourg). These images cover markedly different hydrological conditions during several winter seasons in order to enable the examination of the decrease of the radar backscattering signal during drying-up phases following important flood events. At the floodplain scale, with homogeneous land use and constant topography, the first principal components (PCs) are mainly dominated by the variance related to the changing areas. The PCs are thus mainly controlled by subsurface and surface water dynamics. The field observations of a densely equipped piezometric network in the floodplain are used to calculate a mean soil saturation index (SSI) continuously. A classification scheme, based on the PCs and k-means algorithm, leads to the segmentation of the floodplain into several hydrological behaviour classes with distinctive responses versus changing moisture conditions. To validate this classification method with ground-based estimations, the relation between the mean backscattering values of microplots within each PCA-derived hydrological class and the water table measurements, expressed by means of the SSI, is evaluated. Results show that each class of microplots is characterized by the slope of the ‘backscattering–SSI’ function and by the SSI threshold value at which groundwater resurgence appears. The water ponding implies very low signal return due to the specular backscattering effect on the water surface. Based on established relationships between measured initial water table depths, runoff coefficients and rainfall-induced water table rises, these results are used to discuss the potential of SAR-derived information in flood management applications. Copyright © 2005 John Wiley & Sons, Ltd.

Estimation of ground-water exchange with semi-arid playa lakes (Antequera region, southern Spain)

Playa lakes occur in arid and semi-arid climates and have a significant economic, ecological and cultural value. They are particularly vulnerable to changing hydrologic regimes due to climate change or human activities. We determined the hydrological regime of 10 playa lakes in southern Spain based on water budgets for a 5-year period (1997–2001). Surface runoff was measured in the watersheds, precipitation and evaporation/evapo-transpiration estimated using local meteorological data and ground-water flows assessed using a network of wells and piezometers. The examination of seasonal variations in the lakes’ water levels and the water budgets indicated that ground-water discharge plays a key role in the hydrological dynamics and the maintenance of the playa ecosystems. Major inorganic ions and stable hydrogen and oxygen isotopes in lake water, ground-water and springs were measured. Water samples from the lakes and ground-water were a mixture of cations −SO 4 2− and −Cl − type due to the aridity of the climate and lithologies of the basin. The high TDS (63.4 g/l) content of some ground-water samples indicates regional geologic control on water quality and the dissolution of gypsum and halite. Stable oxygen isotope ratios for lake water (−3.6 to 9.8‰ δ 18O SMOW) were different from those of ground-water and springs (−6.8 to 1.1‰ δ 18O SMOW) suggesting different evolutional patterns. The results of this study provides evidence that the combination of water budgets, hydrochemistry and stable isotope hydrology can lead to a better understanding of wet playa's hydrology. This approach could, thus, prove helpful to establish hydrological regimes in this type of semi-arid environments.

Denitrification capacity in a subterranean estuary below a Rhode Island fringing salt marsh

Coastal waters are severely threatened by nitrogen (N) loading from direct groundwater discharge. The subterranean estuary, the mixing zone of fresh groundwater and sea water in a coastal aquifer, has a high potential to remove substantial N. A network of piezometers was used to characterize the denitrification capacity and groundwater flow paths in the subterranean estuary below a Rhode Island fringing salt marsh.15N-enriched nitrate was injected into the subterranean estuary (in situ push-pull method) to evaluate the denitrification capacity of the saturated zone at multiple depths (125–300 cm) below different zones (upland-marsh transition zone, high marsh, and low marsh). From the upland to low marsh, the water table became shallower, groundwater dissolved oxygen decreased, and groundwater pH, soil organic carbon, and total root biomass increased. As groundwater approached the high and low marsh, the hydraulic gradient increased and deep groundwater upwelled. In the warm season (groundwater temperature >12 °C), elevated groundwater denitrification capacity within each zone was observed. The warm season low marsh groundwater denitrification capacity was significantly higher than all other zones and depths. In the cool season (groundwater temperature <10.5 °C), elevated groundwater denitrification capacity was only found in the low marsh. Additions of dissolved organic carbon did not alter groundwater denitrification capacity suggesting that an alternative electron donor, possibly transported by tidal inundation from the root zone, may be limiting. Combining flow paths with denitrification capacity and saturated porewater residence time, we estimated that as much as 29–60 mg N could be removed from 11 of water flowing through the subterranean estuary below the low marsh, arguing for the significance of subterranean estuaries in annual watershed scale N budgets.

A water-budget approach to restoring a sedge fen affected by diking and ditching

A vast, ground-water-supported sedge fen in the Upper Peninsula of Michigan, USA was ditched in the early 1900 s in a failed attempt to promote agriculture. Dikes were later constructed to impound seasonal sheet surface flows for waterfowl management. The US Fish and Wildlife Service, which now manages the wetland as part of Seney National Wildlife Refuge, sought to redirect water flows from impounded C-3 Pool to reduce erosion in downstream Walsh Ditch, reduce ground-water losses into the ditch, and restore sheet flows of surface water to the peatland. A water budget was developed for C-3 Pool, which serves as the central receiving and distribution body for water in the affected wetland. Surface-water inflows and outflows were measured in associated ditches and natural creeks, ground-water flows were estimated using a network of wells and piezometers, and precipitation and evaporation/evapotranspiration components were estimated using local meteorological data. Water budgets for the 1999 springtime peak flow period and the 1999 water year were used to estimate required releases of water from C-3 Pool via outlets other than Walsh Ditch and to guide other restoration activities. Refuge managers subsequently used these results to guide restoration efforts, including construction of earthen dams in Walsh Ditch upslope from the pool to stop surface flow, installation of new water-control structures to redirect surface water to sheet flow and natural creek channels, planning seasonal releases from C-3 Pool to avoid erosion in natural channels, stopping flow in downslope Walsh Ditch to reduce erosion, and using constructed earthen dams and natural beaver dams to flood the ditch channel below C-3 Pool. Interactions between ground water and surface water are critical for maintaining ecosystem processes in many wetlands, and management actions directed at restoring either ground- or surface-water flow patterns often affect both of these components of the water budget. This approach could thus prove useful in guiding restoration efforts in many hydrologically altered and managed wetlands worldwide.

Dynamics of floodplain-island groundwater flow in the Okavango Delta, Botswana

Surface water-groundwater interactions play a crucial role in the hydrology and ecology of the Okavango Delta. The hydrology of the Delta is dominated by the annual arrival of a flood wave which is distributed over an number of branches. Subsequently, the flood water feeds the phreatic aquifers underlying the Delta islands. In order to evaluate the seasonal and long-term dynamics of the surface water-groundwater interactions between the floodplains and the islands, a network of piezometers located in various locations of the Delta was monitored. Groundwater table fluctuations observed for up to 6 years were analysed and modelled using groundwater flow models. The floodplain-island groundwater flow is in general very dynamic and driven by island evaporation and transpiration. A typical small to medium sized island (width <500 m), appear not to be influenced by long-term antecedent conditions. Only on large islands (width >500 m) and at the perimeter of the flooded area is the influence of long-term antecedent conditions apparent. The knowledge gained during this study will be used for the improvement of the hydrological and hydro-ecological model of the Delta, and can be useful for the description of floodplain dynamics in semi-arid regions in general.

Vegetation patterns resulting from spatial and temporal variability in hydrology, soils, and trampling in an isolated basin marsh, New Hampshire, USA

This study investigated the role of hydrology, soils, and trampling in determining the distribution of vegetation in an isolated wetland from 1996 to 1999. Grassy Pond, in Litchfield, New Hampshire, is a seasonally flooded basin marsh situated in sandy soils. It is a depressional wetland consisting of three connected basins; it has no surface-water inlets or outlets. This acidic, low-nutrient wetlands contains several rare species and represents an uncommon ecosystem type in New England. A network of wells and piezometers monitored from 1996 to 1999 established that the wetland receives an average of 95.4% of its growing season inputs from precipitation and the rest from shallow ground water flowing through the wetland; as a result, it experiences large fluctuations in water levels. Vegetation in the wetland fell into five major elevation zones. Variation in plant diversity within each zone, and differences between adjacent zones, result in part from differences in depth of organic layer, trampling by hikers, all-terrain vehicles, and native wildlife, and the extent of water-level fluctuations. Several of the low basin species are more likely to be found in trampled areas. Shrub invasion of the open basin areas is prevented by trampling in some areas and by high water levels in others. Variability in hydrology resulted in temporal, as well as spatial, variability in the plant community, as dry years yielded significantly greater diversity, including a large increase in tree seedlings. Both hydrologic variability and trampling are external factors that explain a significant portion of the variation in vegetation on a large scale and connect this geographically isolated wetland to the surrounding landscape. On a smaller scale, however, autogenic forces related to soil formation and plant species interactions may be more important in explaining the plant diversity in the wetland.

How to model shallow water-table depth variations: the case of the Kervidy-Naizin catchment, France

The aim of this work is threefold: (1) to identify the main characteristics of water-table variations from observations in the Kervidy-Naizin catchment, a small catchment located in western France; (2) to confront these characteristics with the assumptions of the Topmodel concepts; and (3) to analyse how relaxation of the assumptions could improve the simulation of distributed water-table depth. A network of piezometers was installed in the Kervidy-Naizin catchment and the water-table depth was recorded every 15 min in each piezometer from 1997 to 2000. From these observations, the Kervidy-Naizin groundwater appears to be characteristic of shallow groundwaters of catchments underlain by crystalline bedrock, in view of the strong relation between water distribution and topography in the bottom land of the hillslopes. However, from midslope to summit, the water table can attain a depth of many metres, it does not parallel the topographic surface and it remains very responsive to rainfall. In particular, hydraulic gradients vary with time and are not equivalent to the soil surface slope. These characteristics call into question some assumptions that are used to model shallow lateral subsurface flow in saturated conditions. We investigate the performance of three models (Topmodel, a kinematic model and a diffusive model) in simulating the hourly distributed water-table depths along one of the hillslope transects, as well as the hourly stream discharge. For each model, two sets of parameters are identified following a Monte Carlo procedure applied to a simulation period of 2649 h. The performance of each model with each of the two parameter sets is evaluated over a test period of 2158 h. All three models, and hence their underlying assumptions, appear to reproduce adequately the stream discharge variations and water-table depths in bottom lands at the foot of the hillslope. To simulate the groundwater depth distribution over the whole hillslope, the steady-state assumption (Topmodel) is quite constraining and leads to unacceptable water-table depths in midslope and summit areas. Once this assumption is relaxed (kinematic model), the water-table simulation is improved. A subsequent relaxation of the hydraulic gradient (diffusive model) further improves water-table simulations in the summit area, while still yielding realistic water-table depths in the bottom land.

Monitoring of landfill influences on groundwater

Landfills of waste present serious threat to groundwater. To prevent groundwater pollution from landfill monitoring is performed. Rule of groundwater pollution monitoring from dangerous substances implements principles in Slovene legislation. In everyday practice certain questions arose since validity of the rule. These questions are about responsible parties in monitoring, groundwater distribution in space, target groundwater units, characterization level of the landfill and its surroundings, background values in groundwater, table of content of groundwater monitoring plan, quality of groundwater monitoring network, phases of monitoring, maintenance of monitoring network and activation of piezometers.

Geostatistical Ground Water Monitoring of a Point Source Plume Entering a Restored Riparian Zone

concentrations associated with a point-source ground water plume were monitored as the plume entered a restored riparian zone (RRZ) on the University of Idaho campus in Moscow, Idaho. Seasonal data collected for a network of piezometers installed in the RRZ were analyzed geostatistically and then modeled spatially using the conditional simulation method of sequential Gaussian simulation (SGS). SGS was utilized to predict the spatial distribution of ground water concentrations at unsampled locations in the RRZ and to quantify the uncertainty associated with the prediction. Maps prepared using SGS results illustrate the short-scale variability, or patchiness, expected of concentration distributions in a riparian zone. Manipulation of SGS output provided graphical and quantitative estimates of the likelihood of exceeding a specified concentration threshold at a given confidence level for any location within the RRZ. Geostatistical simulation tools for quantifying uncertainty also provide a potential risk assessment methodology for making remediation decisions and reducing remediation costs.

Impacts on stream and groundwater quality during the inter-rotation phase of a Pinus plantation in the coastal lowlands of south-east Queensland

Summary The impacts of exotic Pinus plantation management on water quality in the coastal lowlands of south-east Queensland were studied for 6 years in a large catchment. The study utilised nested catchments with a 395 ha upper ‘control’ sub-catchment which drained to a lower 904 ha sub-catchment. Stream monitoring stations were constructed at the outlet of each sub-catchment. A combination of regular grab samples, automated sampling (during flood events) and real time sampling with in situ instruments was carried out to monitor trends in the key water quality parameters of nitrogen, phosphorus and suspended solids. Water quality in surrounding regional streams was also monitored for a 12-month period. A network of piezometers within the study area was used to monitor groundwater depth and quality. Cross-sectional surveys of watercourses and adjoining riparian zones were carried out before and after a harvest treatment to assess the stability of these important areas of the drainage basin. After a 4-year calibration, part of the lower sub-catchment was harvested by clearfelling 80 ha of a mature 36-year-old stand and thinning 250 ha of a 24-year-old stand. The clearfelled area was subsequently re-established with Pinus plantation. Both streamflow and export loads of nitrogen, phosphorus and suspended solids were highly episodic in keeping with the ephemeral nature of streams originating in the coastal lowlands region. Annual export loads for nitrogen, phosphorus and suspended solids ranged from 0 to 8.4, 0 to 0.5 and 0 to 606 kg ha-1 respectively. These loads were within the range reported for forest-dominated catchments both in Australia and overseas and are recognised as the lowest ranked land-use with respect to impacts on water quality per se. Despite a major rainfall and flood event shortly after the clearfelling and thinning operations, harvest did not affect key water quality parameters measured, nor groundwater quality. Cross-sectional surveys of the watercourse protection zones found these areas to be stable in the harvested area, but subject to a degree of erosion in the upper ‘control’ sub-catchment; this was thought to be due to a combination of the inverse relationship between sediment yield and catchment size as well as poor management practices in the 1970s. The results of this study should serve as a benchmark for important ‘best management practices’ in relation to: observance of watercourse protection zones; harvesting techniques based on processing at stump and retention of in situ harvest debris; and appropriate site preparation practices with regard to erosion hazard and maintenance of in situ harvest debris.

Effects on catchment water balance from the management of Pinus plantations on the coastal lowlands of south‐east Queensland, Australia

AbstractA paired catchment study was conducted over a 10‐year period on the hydrology of an exotic Pinus plantation in the coastal lowlands of south‐east Queensland, Australia. Each catchment was instrumented with a stream monitoring station, tipping bucket rain gauge, and a network of piezometers to monitor the shallow perched water table. After a 6‐year calibration period a harvest treatment was imposed on one catchment (Review): clearfelling approximately 90% of the catchment area, which contained a mature (44‐year‐old) Pinus elliottii plantation. This subsequently was re‐established with a second rotation plantation of a hybrid of P.elliottii × P.caribaea var. hondurensis. The control catchment (Crayfish) contained a P. elliottii plantation similar to that clearfelled at Review. The post‐harvest period was monitored for a further 4 years.Evapotranspiration was found to be the major output flux, with stream flow only a minor component of the study catchments' water budget. Areas with gleyed podzolic soils were found to remain waterlogged for periods up to 7 months following the seasonally high summer rainfall period, with other soils having water logging periods of only a few months. Little change was observed in waterlogging characteristics following harvesting, in contrast to stream flow which increased for 3 years but with evidence of a decline after 5 years.The perched‐watertable piezometric surface indicated that its lateral drainage differs from that of surface flows; the perched‐watertable drainage fluxes from the study catchments seem minor and off‐site movement of solutes (e.g. nutrients and contaminants) via them would be limited. It was identified that the interaction between the perched and deep aquifers was poorly understood, as well as the relative importance of the soil moisture storage of the aquitard clay layer between them. It was concluded that there is a need to undertake more detailed analysis using modelling, and to obtain additional field data on soil‐layer properties and piezometric levels of the deep aquifer. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Ground Response during Pile Driving

The paper presents a synthesis of the field measurements obtained in a series of large-scale pile-driving tests. The field test was performed at the Chiayi-Taipo area of the high speed rail system currently under construction in Taiwan. The test project included a complete instrumentation of three precast concrete piles with a network of piezometers, inclinometers, level posts, and velocity sensors. The instruments were used to monitor the variations of the dynamic soil pore water pressure, lateral ground deformations, vertical surface displacements, and ground vibrations induced by the pile on the surrounding soil medium during the driving process. Based on the test results, a correlation between the pile-driving resistance and the geological condition of the test site is identified.

Uranium depositional controls at the Prairie Flats surficial uranium deposit, Summerland, British Columbia

A hydrological and geochemical investigation of the Prairie Flats surficial uranium deposit in Summerland, BC was undertaken to identify the principal controls on uranium deposition. A network of piezometers was installed and used to measure the hydraulic conductivities of the host sediments as well as the general flow direction and aqueous geochemistry of the resident groundwaters. Two hydrostratigraphic units were identified: a peat and clay unit overlying a sand and gravel unit. Measured hydraulic conductivities were on the order of 10–7 and 10–5 m/s, respectively, and the vertical hydraulic gradients indicate significant groundwater discharge upward into the peat and clay unit. Prairie Flats groundwaters are neutral to alkaline in pH, enriched in Ca2+ and HCO3 –, and have dissolved uranium concentrations ranging from 10 to nearly 1,000 µg/l. Groundwater flow and geochemistry data were used to estimate the flux of uranium in groundwater at the site. A major fraction of the uranium is taken up by adsorption to organics. There is also evidence for subsequent desorption by the formation of soluble complexes with bicarbonate. Uranium that is not held by adsorption is most likely precipitated as uraninite, UO2(c). Reducing conditions in the peat and clay unit (Eh 0.2 V) may explain the high concentrations of uranium nearer ground surface. The current flux of uranium into the flats is significantly smaller than that calculated from the size and age of the deposit, which may be an indication of changing rates of deposition in response to varying climatic and hydrogeologic conditions over time.

Topographic controls on the chemistry of subsurface stormflow

AbstractModels are needed that describe how topography and other watershed characteristics affect the chemical composition of runoff waters, yet little spatially distributed data exist to develop such models. A topographically driven flushing mechanism for nitrate (NO3−) and dissolved organic carbon has been described in recent literature; however, this mechanism has not yet been thoroughly tested. A 24 ha catchment in the Catskill Mountains of New York was clearcut in the winter of 1996–97, resulting in elevated NO3− concentrations in soil water, groundwater and streamflow. We sampled shallow subsurface stormflow (SSSF) and streamflow six times during the spring and summer of 1998, 1 year after the harvest. We used a spatially distributed network of piezometers to investigate the relationship between topography and SSSF chemistry. Several indices of topography were computed, including the commonly employed topographic index of Beven and Kirkby (1979; Hydrological Sciences Bulletin 24: 43–69). Topographic index was positively correlated with NO3− concentrations in SSSF. The strength of the NO3−–topography relationship was best explained by antecedent soil temperature and antecedent precipitation conditions. Results suggest a topographically driven flushing of high NO3− shallow soil at the site during storm events. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Effect of the 1997-1998 ENSO-Related Drought on Hydrology and Salinity in a Micronesian Wetland Complex

The potential effects of global climate change on coastal ecosystems have attracted considerable attention, but the impacts of shorter-term climate perturbations such as ENSO (El Nino-Southern Oscillation) are lesser known. In this study, we determined the effects of the 1997–1998 ENSO-related drought on the hydrology and salinity of a Micronesian mangrove ecosystem and an adjacent freshwater swamp. A network of 9 piezometer clusters installed at the study site served as sampling points for continuous and manual measurements of salinity and water level. During the drought period from January through April 1998, mean water table levels in the mangroves and freshwater swamp were approximately 12 and 54 cm lower, respectively, than during May through December when precipitation returned to near normal levels. At the peak of the drought (February 1998), the most dramatic result was a reversal in groundwater flow that sent groundwater from the mangroves upstream toward the freshwater swamp. Flow nets constructed for this period and immediately after illustrate the strong hydrological linkage between the two systems. This linkage was also illustrated by measurements of groundwater salinity in the piezometer network. Ninety-six percent of the salinity measurements taken in the mangroves during the study were at least 10‰ less than the salinity of sea water, indicating that the mangroves were consistently receiving freshwater flows. An analysis of variance of groundwater salinity measurements during and after the drought showed that salinity levels in the 0.5 and 1.0 m depth piezometers were greater during than after the drought. In a comparison of salinity values in 0.5-m wells during low tide, mean salinity was approximately twice as high during the drought than after (14.7‰ versus 6.2‰, respectively). This study demonstrates that short-term climate perturbations such as ENSO can disrupt important coastal processes. Over repeated drought cycles, such perturbations have the potential to affect the structure and function of mangrove forests and upstream ecosystems.

Use of spatially distributed water table observations to constrain uncertainty in a rainfall–runoff model

The Generalised Likelihood Uncertainty Estimation (GLUE) methodology is used to investigate how distributed water table observations modify simulation and parameter uncertainty for the hydrological model TOPMODEL, applied to the Sæternbekken Minifelt catchment in Norway. Errors in simulating observed flows, continuously-logged borehole water levels and more extensive, spatially distributed water table depths are combined using Bayes' equation within a `likelihood measure' L. It is shown how the distributions of L for the TOPMODEL parameters change as the different types of observed data are considered. These distributions are also used to construct corresponding simulation uncertainty bounds for flows, borehole water levels, and water table depths within the spatially-extensive piezometer network. Qualitatively wide uncertainty bounds for water table simulations are thought to be consistent with the simplified nature of the distributed model.

Design of a Piezometer Network for estimation of the sample variogram of the hydraulic gradient: The role of the instrument

The process of designing a piezometer network for estimating the sample variogram of the hydraulic gradient is used to illustrate one strategy for designing a sampling network which is both appropriate for the estimation of spatial statistics and efficient in terms of the measurement instrument. Constraints which impacted this design can be broken into three categories: (1) constraints discussed by previous authors and related to increasing confidence in the estimate of the sample variogram, (2) new constraints related to the spatial distribution of sample location for the purpose of estimating the sample variogram, and (3) constraints related to the measurement instrument. After assessing a number of sampling strategies, including uniform grids, randomized grids, various geometric patterns, and scaled sampling patterns, an acceptable design for the present application was developed from a scaled sampling pattern which was modified to utilize the instrument more efficiently. In this situation the final design could not have been established except through consideration of both the statistics to be derived from the sampling effort and the instrument used to obtain the measurements.

The Embarras River : L. Durham, Biological Report - US Fish & Wildlife Service, 19, 1993, pp 101–122

A vast, ground-water-supported sedge fen in the Upper Peninsula of Michigan, USA was ditched in the early 1900 s in a failed attempt to promote agriculture. Dikes were later constructed to impound seasonal sheet surface flows for waterfowl management. The US Fish and Wildlife Service, which now manages the wetland as part of Seney National Wildlife Refuge, sought to redirect water flows from impounded C-3 Pool to reduce erosion in downstream Walsh Ditch, reduce ground-water losses into the ditch, and restore sheet flows of surface water to the peatland. A water budget was developed for C-3 Pool, which serves as the central receiving and distribution body for water in the affected wetland. Surface-water inflows and outflows were measured in associated ditches and natural creeks, ground-water flows were estimated using a network of wells and piezometers, and precipitation and evaporation/evapotranspiration components were estimated using local meteorological data. Water budgets for the 1999 springtime peak flow period and the 1999 water year were used to estimate required releases of water from C-3 Pool via outlets other than Walsh Ditch and to guide other restoration activities. Refuge managers subsequently used these results to guide restoration efforts, including construction of earthen dams in Walsh Ditch upslope from the pool to stop surface flow, installation of new water-control structures to redirect surface water to sheet flow and natural creek channels, planning seasonal releases from C-3 Pool to avoid erosion in natural channels, stopping flow in downslope Walsh Ditch to reduce erosion, and using constructed earthen dams and natural beaver dams to flood the ditch channel below C-3 Pool. Interactions between ground water and surface water are critical for maintaining ecosystem processes in many wetlands, and management actions directed at restoring either ground- or surface-water flow patterns often affect both of these components of the water budget. This approach could thus prove useful in guiding restoration efforts in many hydrologically altered and managed wetlands worldwide.

Field Study of a Long and Very Narrow Contaminant Plume

AbstractA contaminant plume more than 7,700 meters long and a few hundred meters wide has been delineated using a network of bundle piezometers. The location, concentration, and starting time of the source are well‐defined. The plume occurs in a stratified, highly permeable, sand and gravel aquifer in which the ground‐water flow is unusually steady in direction and magnitude. A ground‐water velocity of 380 meters per annum and a longitudinal dispersivity of 60 to 120 meters are obtained by simulating chloride breakthrough data at 7,700 meters from the source. Simulation of the cross‐sectional distribution of chloride concentrations at 5,500 meters shows the horizontal transverse dispersivity to be at most 0.10 meters. This very small maximum value for horizontal transverse dispersivity is two to four orders of magnitude smaller than values reported in the literature for plumes of similar lengths. It is postulated that the very weak lateral dispersion is due primarily to the steadiness of the ground‐water flow.An ICP‐MS, equipped with an ultrasonic nebulizer and active‐film multiplier detector, is used to attempt to determine 54 trace elements directly in ground water. Lithium, arsenic, rubidium, strontium, barium, and antimony are found in the microgram‐per‐liter (part‐per‐billion = ppb) range. Most of the other elements are present at nanogram‐per‐liter (part‐per‐trillion = ppt) concentrations. Ion exchange preconcentration is utilized in order to improve the sensitivity for measuring the rare earth elements that exist at concentrations as low as 0.05 ppt for lutetium, thulium, and terbium. The formation of molecular species in the plasma produces false positive results for some of the elements. The presence of silicon or carbon dioxide interferes with the measurement of scandium, strontium interferes with rhodium and palladium, and barium interferes with europium. Correction procedures for these interferences are discussed. All together, the concentrations of the 54 elements in water from four Nevada springs span almost seven orders of magnitude.

Groundwater flow and solute transport in fractured lacustrine clay near Mexico City : Water Resour ResV27, N9, Sept 1991, P2187–2201

A network of piezometers was installed in a surficial lacustrine clay aquitard overlying a thin saline water aquifer of volcanoclastic origin at a study site near Mexico City in the Basin of Mexico. The aquifer is underlain by additional lacustrine sediments which in turn overlie a thick regional freshwater aquifer. The regional aquifer provides approximately 70% of the water supply for 20 million people in the Basin of Mexico. In the study area, major ions, oxygen 18, and deuterium in the pore water of the surficial aquitard exhibit large variations with depth. The nature of these variations suggests that the saline pore water is being displaced downward by infiltrating meteoric water. The infiltration has been induced by strong downward hydraulic gradients imposed two to three decades ago when heavy aquifer pumping of the thin saline water aquifer began. One-dimensional analytical models representing solute transport in both fractured and unfractured porous media were used to simulate the geochemical profiles in the surficial aquitard. The fractured porous medium model, using a realistic mean hydraulic gradient and fracture spacing (1.5 m) and small but significant fracture aperture (30 μm) provide nearly an exact match to the field data. From this we infer that, because of vertical fractures, there is a much greater potential for downward leakage of water and contaminants through the Mexico City clay into underlying aquifers than has been previously thought.