Potentiometric Map Research Articles

Hydrogeochemical and isotope evidence of groundwater evolution in El Guettar oasis area, Southwest Tunisia

In southern Tunisia, the major aquifer system is characterized by the superposition of three principal levels; the upper unconfined Quaternary alluvial aquifer; the intermediate semi-confined/Confined Complex Terminal (Segui, Beglia and Zebbag Formations) and the Continental Intercalaire (Bou Dinar and Sidi Aïch Formations) which constitutes the deeper confined aquifer. The hydrodynamic functioning of this aquifer system is largely influenced by tectonics (Gafsa and Sehib faults), lithologic variation and recharge conditions. The tectonics has contributed to the discontinuity of the aquifer levels by creating some barriers which play the role of hydraulic sills. A study of the potentiometric maps shows four principal flow directions. These flow directions converge at the Sill Zones (Chott El Guettar and Chott Djerid depressions). The total mineralization evolution shows an increase from the periphery of the basin to the discharge area (1.75 ≤ TDS ≤ 14.4 g/l). Dissolution of halite, gypsum and/or anhydrite-bearing rocks is the principal source of the salinity of the groundwater. The isotopic signatures permit to classify the studied groundwaters into two different groups. Non-evaporated groundwaters that are characterized by depleted δ18O and δ2H contents highlighting the importance of modern recharge at higher altitude. Evaporated groundwaters with enriched contents reflecting the significance infiltration of return flow irrigation waters and the influence of the water dam. Tritium data in the studied groundwaters lend support to the existence of pre-1950 and post-1960 recharge. According to radiocarbon residence time estimates, the deep groundwater is approximately 40 ka, and was recharged during a period when the climate was wetter and colder. Based on radiocarbon content, the shallow groundwater is older than 0.5 ka, and represents modern waters mixed with a limited quality of palaeo-recharge. Moreover, the insulation of certain compartments of the reservoir and inter-communications between the aquifer levels seem to play a principal role in the degradation of quality of groundwater.

An assessment of hydrogeologic characteristics of Bamikemos hard rock terrain using geophysical techniques

Resistivity depth soundings integrated with the very low-frequency electromagnetic (VLF-EM) profiling and some hydrogeologic measurements were conducted at Bamikemo, Nigeria. The exercise attempted to evaluate the groundwater potential of the main water supply area. The integrated approach enabled the delineation of the regolith/weathered layer containing the overburden aquifers from the hard rock housing the bedrock aquifers. The study reveals that the thickness of the overburden/weathered layer ranges from 2.4 to 24.5 m, with average value of 12.6 m. Static water level also varies from 0.2 to 14 m in the area. The potentiometric surface map constitutes groundwater occurrence guide for the area. In all the fifteen electromagnetic (EM) profiles established, prominent fractures in varying magnitudes were only in two profiles. A comparative correlation with the appropriate geoelectric sections show that resistivity parameters around the EM-delineated bedrock fractures within the granite/granite-gneiss bedrock in the area which reveals that the bedrock is not considerably decomposed or fractured to hold significant quantities of groundwater. The prominent EM anomalies at these locations are apparently attributable to fairly thick unconsolidated weathered mantle. Consequently, the weathered overburden in the area, especially the central/western segment is considered groundwater prospect specific area. Beyond the results concerning the main objectives of the survey, the study also justified the reliability in integrated application of hydrogeologic, VLF-EM and depth sounding data in locating groundwater prospect specific areas in hard rock terrains. Key words: Electromagnetic profiling, depth sounding, overburden/weathered layer aquifers, bedrock aquifers, potentiometric surface.

Nitrate dynamics in the soil and unconfined aquifer in arid groundwater coupled ecosystems of the Monte desert, Argentina

[1] In arid ecosystems, vegetation controls water and nitrate movement in the soil, reducing solute transport to aquifers. Here we analyzed nitrate distribution and transport throughout the soil profile and to the groundwater under different ecologic (vegetation type) and topographic (upland/lowland) situations across sand dune ecosystems with shallow water tables, subject to domestic grazing in the Monte desert. Based on vertical nitrate distributions in deep soil profiles we found that dune uplands (deep groundwater, low productivity) lost relatively more nitrogen than lowlands (shallow groundwater, high productivity), likely reinforcing productivity contrasts along these topographic positions. The traditional practice of nighttime animal concentration in corrals may affect nitrogen transport, with poorly vegetated interdunes at livestock posts showing higher subsoil nitrate concentrations than a well-vegetated nonsettled interdune. Vegetation left its imprint on the vertical distribution of nitrate, as suggested by the presence of a depletion zone that matched the depth of maximum root densities, followed by an underlying zone of accumulation. To explore how nitrogen exports to groundwater could affect water quality and nutrient supply to phreatophyte plants, we characterized groundwater flow patterns based on a potentiometric map and sediment characteristics, and measured groundwater electric conductivity, nitrate and arsenic concentration, and stable isotopes across 29 wells (5.8–12 m deep). Under the present land use and climate conditions, nitrate leaching does not seem to have an important and widespread effect on water quality. Nitrate concentration exceeded established limits for human consumption (45 mg L−1) in only one well, while arsenic concentration exceeded the established limits (10 μg L−1) in all but one well, reaching extreme values of 629 μg L−1. Yet, our analysis suggests that nitrate exports from corrals can reach the aquifer in localized areas and be transported to the surrounding vegetation in a relatively short time. Vegetation access to groundwater could allow ecosystems to recover part of this nutrient loss, buffering the effects of land use.

Interactions between the surface water and groundwater in the western shoreline of Lake Nasser, Upper Egypt

The present study indicates that the factors controlling the hydraulic relation between surface water and groundwater at the western lake shoreline change from one locality to another. This depends upon the lithological characteristics and the major structures. In the southern sectors, sedimentation at the bottom and sides of the lake prevents the water movement to the Nubian sandstone aquifer. The potentiometric map reveals that the water level altitudes range between 170 m in the vicinity of the lakeshore line and 110 m west of the lake. The groundwater flow lines show that the main recharge to the aquifer comes from the southwest direction, as well as from the lake inland to variable distances (about 30 Km). During the present study, Darcy’s law was applied to calculate the recharge from the western shoreline of Lake Nasser to the adjacent Nubian aquifer. The maximum value of seepage was at Garf Hussein (27.71 × 106 m³/year), which may be related to high permeability and hydraulic gradient. Also, it may be related to the N–S strike faults that cut the area on both sides of the Lake, and the groundwater is expected to have free circulation through the faults of this trend. The minimum value was recorded in Adindan section (0.61 × 106 m³/year). This may be related to the limited recharge from the lake to the aquifer, due to the sedimentation that dislocates this recharge.

Análise dos sistemas hidroquímicos dos campos de captação de abastecimento público, para identificação dos padrões de qualidade da água e definição das massas de água subterrânea: Holanda

Hydrochemical system analysis (HCSA) is used to better understand the individual state of and spatial patterns in groundwater quality, by addressing the spatial distribution of groundwater bodies with specific origins (hydrosomes) and characteristic hydrochemical zones within each hydrosome (facies). The origin is determined by environmental tracers or geomorphological and potentiometric maps, the facies by combining age, redox and alkalinity indices. The HCSA method is applied to all 206 active public supply well fields (PSWFs) in The Netherlands, resulting in the distinction of nine hydrosomes and eleven facies parameters—age (young, intermediate, old), redox ((sub)oxic, anoxic, deep anoxic, mixed) and alkalinity (very low, low, intermediate and high). The resulting classification of PSWFs provides a means to (1) predict their vulnerability; (2) optimize groundwater-quality monitoring programs; and (3) better delineate groundwater bodies, by considering groundwater origin and flow. The HCSA translates complex hydrochemical patterns into easily interpretable maps by showing PSWFs, groundwater bodies and hydrochemical facies. Such maps facilitate communication between researchers, water resources managers and policy makers and can help to solve complex groundwater resources management problems at different scales, ranging from a single well(field) or region to the national or European scale.

Identification and characterisation of hydrogeological relays of continental intercalaire aquifer of southern Tunisia

In southern Tunisia, the major aquifer system is characterised by the superposition of two principal levels; the “complex terminal” (CT) which forms the surface aquifer and the “continental intercalaire” (CI) which constitutes the deep aquifer. The hydrodynamic functioning of this aquifer system is largely influenced by tectonics, lithologic variation and recharge conditions. The tectonics has contributed to the discontinuity of the aquifer levels by creating some barriers which play the role of hydraulic sills. A study of the CI potentiometric map shows three principal flow directions. These flow directions converge at the Sill Zone. The total mineralisation evolution shows an increase from the periphery of the basin to the discharge area. Dissolution of halite, gypsum and/or anhydrite-bearing rocks is the principal source of the salinity of the groundwater. The dissolution of these evaporitic rocks is confirmed by mineral saturation indices, which show an under-saturation of water samples with respect to the mentioned minerals. Moreover, the insulation of certain compartments of the reservoir and intercommunications between the aquifer levels seem to play a principal role in the deterioration of chemical quality of water. In this study, the isotopic tools have been applied to understand the hydrodynamic functioning in the region. These techniques have confirmed the results of the hydrogeologic and hydrochemical studies concerning the interconnection between the different aquifers. They indicate also the recent local recharge of the CI reservoir throughout Cretaceous outcrops, and the older origin of all groundwater in the aquifer system.

Cokrigagem Ordinária versus Krigagem com Deriva Externa: Aplicações para a Avaliação do Nível Potenciométrico em um Aquífero Livre

Equipotential maps are obtained from monitoring wells distributed over the study area. These maps can be built using several interpolation methods, nevertheless the available potentiometric information is not sufficient to ensure good estimation for unsampled locations. In these cases one can use more complex interpolators such as normal cokriging and external drift kriging, with and without correction of the smoothing effect. These methods consider not only the variable to be estimated but also another correlated variable. Thus, we can use information coming from a secondary source to estimate the main variable. For instance, elevation is well correlated with potentiometric data and it is very easy to acquire over the study area. In this paper cokriging, kriging with external drift, and kriging with external drift after correcting the smoothing effect have been used for the preparation of the potentiometric map of a free aquifer in the NE part of Fortaleza City, Ceara State. Among the three tested methods cokriging gave the worst results because of the instability of the system of cokriging equations. Kriging with an external drift seems to be appropriate since it is simpler to implement than cokriging, and it depends on correlation of the secondary information with the variable to be estimated. Furthermore, kriging with external drift after correcting the smoothing effect did not improve the results because available information is not uniformly distributed over the study area. Actually, errors coming from portions with few data are scattered out over other better sampled portions resulting in poorer estimates.

Using Synthetic Data and Artificial Wells to Teach the Construction and Use of Water Level Contour Maps

Contour maps that depict groundwater levels (e.g., water table maps and potentiometric surface maps) are essential to the practice of hydrogeology. However, there are significant barriers to effectively teaching students how to create and interpret such maps. Barriers to instruction include the logistics of accessing real wells, assuring that students are provided with a challenging problem, and the lack of a unique solution. We present a new approach that overcomes these barriers through the use of artificial wells and synthetic data. Our approach provides students with a challenging problem that takes them through the whole process, from data collection to interpretation of the resulting maps. In the end, students are able to see how their efforts compare to a known solution, rather than another estimate. Students are also able to choose locations for additional wells that they believe will enhance their ability to create an accurate map. These qualities lead to a substantial improvement in student comprehension over instructional approaches that are based on existing data or limited field measurements.

Groundwater Pollution of the Quaternary Aquifer in Northern United Arab Emirates

Natural conditions and human activities have caused serious quality degradation of the Quaternary aquifer in the north of the United Arab Emirates (UAE). The aquifer within Ajman City is unconfined, receiving limited recharge (12 542 m3/day) from the east and large pollutants flux (4,800 m3/day) from land surface. Field survey and laboratory analyses revealed anomalies in groundwater salinity (TDS), total hardness (TH), dissolved oxygen (DO), cations (Ca2+, Mg2+, Na+ and K+), anions (\({\text{HCO}}_3^ - \), \({\text{SO}}_4^{2 - } \), Cl− and \({\text{NO}}_3^ - \)) and trace elements (Fe, Pb, Cd and Cr), which can be correlated to point and non-point pollution sources. Concentrations of trace elements are more responsive to anthropogenic sources than natural ones. High Fe and Pb levels were measured close to the untreated sewage disposal site, while high Cd and Cr contents were observed near hospitals and clinics. Iso-concentration maps of salinity and major ions, in addition to hydrochemical profiles were used to define the seawater–groundwater interface in Ajman City. The potentiometric surface map of the Quaternary aquifer within the study area shows that groundwater flows from the east towards the Arabian Gulf in the west. The proposed landfill site is suitable because it lies within a topographic low, receiving groundwater flow from all directions.

Testing of stack-unit/aquifer sensitivity analysis using contaminant plume distribution in the subsurface of Savannah River Site, South Carolina, USA

Published information on the correlation and field-testing of the technique of stack-unit/aquifer sensi- tivity mapping with documented subsurface contaminant plumes is rare. The inherent characteristic of stack-unit mapping, which makes it a superior technique to other analyses that amalgamate data, is the ability to deconstruct the sensitivity analysis on a unit-by-unit basis. An aquifer sensitivity map, delineating the relative sensitivity of the Crouch Branch aquifer of the Administrative/Manufactur- ing Area (A/M) at the Savannah River Site (SRS) in South Carolina, USA, incorporates six hydrostratigraphic units, surface soil units, and relevant hydrologic data. When this sensitivity map is compared with the distribution of the contaminant tetrachloroethylene (PCE), PCE is present within the Crouch Branch aquifer within an area classified as highly sensitive, even though the PCE was primarily released on the ground surface within areas classified with low aquifer sensitivity. This phenomenon is explained through analysis of the aquifer sensitivity map, the groundwater potentiometric surface maps, and the plume distributions within the area on a unit-by- unit basis. The results of this correlation show how the paths of the PCE plume are influenced by both the geology and the groundwater flow.

Potenciometria e fluxo subterrâneo no aqüífero aluvionar do baixo curso do Rio Macaé, município de Macaé-RJ

Macaé city is inserted in the coastal portion of the Northern region of Rio de Janeiro State, Brazil. The study accomplished in the alluvial aquifer of the Macaé river lower course, whose waters fulfill the domestic needs of a significant part of the rural and peripheral population of the city, comprised the neighborhoods of Córrego do Ouro, Fazenda Severina, Virgem Santa and Aterrado do Imburo. Results comprise the hydrodynamic studies that lead to the elaboration of a potentiometric map in that alluvial aquifer. The potentiometry shows the relationship between the surface and subsurface water. The Macaé River has an effluent character in the studied area. Aquifer recharge is associated to the topographical adjacent higher grounds and discharge is towards the Macaé river channel and river mouth. The hydrographs of water level temporal variation suggest a possible hydraulic connection among the monitored water points, evidencing a lateral continuity in the area.

Enhancement of aquifer vulnerability indexing using the analytic-element method

Indexing methods are used for the evaluation of aquifer vulnerability and establishing guidelines for the protection of ground-water resources. The principle of the indexing method is to rank influences on groundwater to determine overall vulnerability of an aquifer to contamination. The analytic element method (AEM) of ground-water flow modeling is used to enhance indexing methods by rapidly calculating a potentiometric surface based primarily on surface-water features. This potentiometric map is combined with a digital-elevation model to produce a map of water-table depth. This is an improvement over simple water-table interpolation methods. It is physically based, properly representing surface-water features, hydraulic boundaries, and changes in hydraulic conductivity. The AEM software, SPLIT, is used to improve an aquifer vulnerability assessment for a valley-fill aquifer in western New York State. A GIS-based graphical user interface allows automated conversion of hydrography vector data into analytic elements.

Developing Analytical and Communication Skills in a Mock-Trial Course Based on the Famous Woburn, Massachusetts Case

A mock trial in which undergraduates serve as expert witnesses and law students serve as their attorneys is an effective vehicle for developing quantitative skills and enhancing written and oral communication skills. I have developed an interdisciplinary course based on the book A Civil Action. The book deals with the legal struggle of families in Woburn, Massachusetts, who sued two corporations alleging that improperly handled industrial chemicals entered the groundwater system, were captured by two municipal wells, and prolonged ingestion of the contaminated water caused leukemias and other health disorders. Students analyze aerial photographs, well logs, streamflow records, permeability tests, and water-level and water-quality data from the trial to complete assignments that become exhibits in the mock trial. Assignments include construction of geologic cross sections, potentiometric maps, hydrographs, flood recurrence graphs, and calculation of hydraulic gradients, groundwater velocities, and contaminant travel times. Trial transcripts and newspaper articles serve as background materials for a term paper. Based on the computational assignments, background readings, and a discussion of professional ethics, students compose an expert opinion from the viewpoint of their client and are deposed by opposing counsel. A jury of undergraduates is impaneled for the one-day mock trial in which the law students make opening statements and closing arguments, and conduct direct examinations and cross examinations of the scientific experts. The course teaches students how to develop and defend their opinions, how to question the opinions of others, the limitations of data collection and analysis, and the importance of integrating computational and communication skills.

Hydrogeology of the Mercosul aquifer system in the Paraná and Chaco-Paraná Basins, South America, and comparison with the Navajo-Nugget aquifer system, USA

The giant Mercosul aquifer system consists of Triassic-Jurassic eolian-fluvio-lacustrine sandstones confined by Cretaceous basalt flows, and it covers about 1,195,500 km2 (461,583 miles2) in South America. The aquifer system encompasses all of the Parana Basin and part of the Chaco-Parana Basin and is one of the world's largest. The eolian Botucatu Sandstone and its equivalents form an important part of this system. Maps of structure, thickness of overlying rocks, and water temperature, and a potentiometric map, all based on 322 wells, define hydrogeologic characteristics and provide the basis for establishing guidelines for the long-term equilibrium use of this important multinational aquifer system. The Mercosul aquifer system is divided into two domains – the larger and better understood Parana Basin and the smaller and less well understood Chaco-Parana Basin. Most of the northern part of the Parana Basin has axially-directed groundwater flow, whereas the southern part of the aquifer discharges mostly to the southwest into the Corrientes Province of Argentina, with negligible discharge into the Atlantic Ocean. The Mercosul aquifer system is conservatively estimated to have been flushed at least 180 times since deposition. Various factors are responsible for this flushing, including appreciable rainfall since the end of the Cretaceous Period, probable uplift of the basins' borders in Late Cretaceous time, simple basin geometry, long-term riverine and groundwater flow to the southwest (ancestral and present Parana River Systems), and stable cratonic setting.

Source-identification investigations of petroleum contaminated groundwater in the Missouri Ozarks

The growing population in the Missouri Ozarks rely on the abundant supply of potable groundwater found in the fractured bedrock underlying the region. Protection of this valuable resource from surface, or near-surface contamination is essential to the health and safety of the citizens of Missouri. Petroleum products stored in underground storage tanks pose a threat to groundwater if a release occurs. “Pools” of free product of petroleum can become trapped in voids commonly found on the karst bedrock of the Ozarks. These pools become underground sources of contamination by slowly dissolving into the groundwater. Although the free product rarely travels far from the surface source, dissolved-phase contamination can migrate thousands of meters through underground conduits. Dissolved phase petroleum degasses and partitions out of the groundwater in these conduits. Consequently, contaminated groundwater is not commonly found more than a kilometer or two (few thousand feet) from the source. The Missouri Department of Natural Resources, Division of Geology and Land Survey's Leaking Underground Storage Tank Unit (DGLS/LUST) investigates sites where petroleum-contaminated groundwater has been found in springs or private wells but a source of the contamination is unknown. These investigations start at the regional scale by determining the recharge area of the contaminated groundwater and by identifying the potentially responsible parties (PRPs). A review of DGLS documents and well logs supplemented by field measurements of water levels in private wells throughout the area is used to construct a potentiometric map and determine regional groundwater flow direction. Water tracing is used to confirm or deny the validity/accuracy of the potentiometric map and to establish hydrogeologic connections between the potential surface source of the contamination and the affected groundwater. Fluorescent dyes are injected into the subsurface, recovered in packets containing activated carbon and analyzed by a synchronously scanning spectrofluorometer. These results are compared to background fluorescence conditions established at the site prior to injection. The background conditions and dye recovery results must be submitted to the Missouri Water Trace Committee for approval as an independent quality assurance/quality control measure. Each of the PRPs found to be within the recharge area of the contaminated groundwater are investigated to determine the source, or sources, of the contamination. Initial field screening techniques are followed up by confirmatory sampling of soil and groundwater. Geophysical methods such as resistivity and electromagnetic conductivity are used to locate the bedrock surface, fracture patterns and areas of extensive petroleum contamination. On-site drilling activities are performed at each of the PRPs with on-site contamination screening methods such as analyzing soil gas within the borehole with a photoionization detector (PID) and colorimetric tubes. Soil samples are collected and can be analyzed on-site with immunoassay kits with confirmatory samples sent to the lab. Temporary monitoring wells are installed in boreholes to collect on-site groundwater data. Depth-to-water levels are measured, and the relative elevations of the temporary wells are surveyed so that a more detailed potentiometric map can be constructed. Water samples are collected from the temporary wells, and the boreholes are abandoned per Missouri Law. Petroleum fingerprinting by fluorescence is another tool used to investigate PRPs and determine the source of contamination. Petroleum is extracted from soil or water samples collected during on-site drilling and compared to the petroleum which is extracted from a sample collected at the contaminated spring or well.

Use of 3H/3He Ages to evaluate and improve groundwater flow models in a complex buried‐valley aquifer

Combined use of the tritium/helium 3 (3H/3He) dating technique and particle‐tracking analysis can improve flow‐model calibration. As shown at two sites in the Great Miami buried‐valley aquifer in southwestern Ohio, the combined use of 3H/3He age dating and particle tracking led to a lower mean absolute error between measured heads and simulated heads than in the original calibrated models and/or between simulated travel times and 3H/3He ages. Apparent groundwater ages were obtained for water samples collected from 44 wells at two locations where previously constructed finite difference models of groundwater flow were available (Mound Plant and Wright‐Patterson Air Force Base (WPAFB)). The two‐layer Mound Plant model covers 11 km2 within the buried‐valley aquifer. The WPAFB model has three layers and covers 262 km2 within the buried‐valley aquifer and adjacent bedrock uplands. Sampled wells were chosen along flow paths determined from potentiometric maps or particle‐tracking analyses. Water samples were collected at various depths within the aquifer. In the Mound Plant area, samples used for comparison of 3H/3He ages with simulated travel times were from wells completed in the uppermost model layer. Simulated travel times agreed well with 3H/3He ages. The mean absolute error (MAE) was 3.5 years. Agreement in ages at WPAFB decreased with increasing depth in the system. The MAEs were 1.63, 17.2, and 255 years for model layers 1, 2, and 3, respectively. Discrepancies between the simulated travel times and 3H/3He ages were assumed to be due to improper conceptualization or incorrect parameterization of the flow models. Selected conceptual and parameter modifications to the models resulted in improved agreement between 3H/3He ages and simulated travel times and between measured and simulated heads and flows.

HYDRODYNAMICS AND FAULT SEAL ASSESSMENT IN THE VULCAN SUB-BASIN, TIMOR SEA

The study of hydrodynamics sheds light on the current day fluid flow regime of traps of different integrity and on the mechanisms leading to seal breach. It also provides insights into potential migration pathways and zones of secondary hydrocarbon accumulation. This is a preliminary hydrodynamic investigation carried out on a few selected wells in the Vulcan Sub-basin from high, moderate and low integrity traps, referred to here as HIT, MIT and LIT respectively (after O'Brien et al, 1996). The fields sampled include East Swan (LIT), Skua and Jabiru (MIT) and Montara and Oliver (HIT). The findings of this study suggest a trend between trap type and current day fluid flow. The best result was obtained for Skua-2, where pressure testing was conducted over the bounding fault for the field, indicating high integrity. Some wells showed upward head gradients, such as East Swan-2; others showed a low or no vertical gradient, such as Jabiru-1 A,−2, −3, −10 and Oliver-1. There was general agreement between the flow direction and the conductivity of the faults, as indicated by mud loss data. Preliminary potentiometric surface mapping of Jabiru indicated lateral flow, controlled by production, and hydraulic communication within the faulted reservoir. Wellbore hydrodynamics and potentiometric surface mapping of the reservoir heads on a regional scale promise to give indications of major structural controls on flow as well as secondary migration pathways.

The hyporheic zone: Hydrogeological and geochemical study of a stream in the pyrenees mountains

AbstractThe hydrogeological and geochemical characteristics of the hyporheic zone were studied for a year on a 30 m2 site within a stream in the Pyrenees Mountains. This included the drawing of potentiometric maps, measuring hydraulic conductivity and porosity, and the monthly analysis of major chemical elements in water samples.Use of the combined data led to the clarification of certain aspects of the characteristics, functioning and development of the hyporheic zone.Even though the area studied lies in a zone of groundwater recharge, the direct vertical exchange between surface‐ and groundwater does not seem to be determinant in the potentiometric and chemical development of the area. However, the state of reserves, which can be reduced or reconstituted, seems to play an essential role in flow dynamics, and thus of the chemistry of this area. Flow dynamics in the hyporheic zone apparently are of the longitudinal‐transversal type, related to the local stream morphology.The results show how difficult it is to define the hyporheic zone in a structural sense, even though the zone represents an entity that is clearly characterized by specific and identifiable dynamics.

Groundwater analysis using a Geographic Information System following finite-difference and -element techniques

Groundwater analysis involves understanding both spatial and temporal changes. Changes in many different types of media have been modeled in the past with finite-difference and finite-element meshes. In the same way, groundwater can be represented as a finite-element analysis problem. This document describes the approach in which a Geographic Information System (GIS) was used to evaluate changes in groundwater levels resulting from a dam impoundment. This study focuses on areas of landuse impact caused by changes in groundwater conditions. A finite-element-type model created potentiometric steady-state surfaces. The computer software converted the finite elements to finite differences for raster based GIS multiconditional analysis. The raster GIs divided the study area into 50 m grid cells, approximately equal to the area of one acre. The study was limited to the alluvial valley boundary. Pre-dam and post-dam data sets were compared to evaluate causes and effects of the dam, and on landuse. Data sets were obtained for record lows and highs both before and after the dam was constructed. Analysis revealed the dam caused changes in the low groundwater regime but not in the high water levels. Once effects of the dam to the groundwater regime were understood, grid cells were compared against landuse critical threshold values to determine areas of probable impact. Graphic results included a series of potentiometric maps, pre-dam and post-dam depth to groundwater maps, cross valley groundwater profiles, effected areas maps, and possible impact maps. The GIS provided a management tool to select areas for future site specific assessment.

Nanoscale Modification Of Amorphous Diamond-Like Carbon Film Surfaces

ABSTRACTScanning Tunneling and Atomic Force Microscopy (STM, AFM) of amorphous tetrahedral carbon films (a-tC) film surfaces grown by pulsed laser deposition indicate extreme flatness and uniform electronic properties.1 This is consistent with the observed predominance of sp3 (diamond-like) bonding in these materials.2 Potential applications of a-tC films may be enhanced with the ability to modify their surfaces on the nanoscale. Exploiting the metastable nature of the sp3 hybridized state of carbon, the high flux electron beam from an STM tip was used to modify ˜100 nm regions; processing in air or in vacuum produces similar results. STM and AFM maps of the modified regions indicate stable morphologic and electronic structures consistent with a local transformation to sp2 (graphitic) hybridization. Surface potentiometric mapping of these regions indicates a correlation between electron dose and a lowering of the surface potential. In addition, spatially-resolved electron energy loss spectroscopy of the modified areas performed with a Scanning Auger Microscope shows plasmon energy shifts that confirm an elevated sp2 content in the modified regions.3