Potentiometric Surface Map Research Articles

Estimate of groundwater flow and salinity contribution to the Great Salt Lake using groundwater levels and spatial analysis

Groundwater discharge to Great Salt Lake (GSL) is difficult to quantify but represents a potentially significant source of water and salinity to the lake’s overall water budget and chemistry, respectively. Understanding groundwater and its role in the overall health of GSL is critical due to the current and historically low lake levels. We compiled existing groundwater level data in basin-fill wells around GSL and used spatial analysis methods to 1) create potentiometric-surface maps in the areas adjoining GSL, 2) calculate groundwater contributions to GSL, and 3) estimate salinity inputs from groundwater to GSL. We observed groundwater-level declines in most of the basin-fill wells from the 1980s to 2010s. These declines are consistent with historical groundwater-level trends in the Salt Lake, Tooele, Curlew, and Weber Valleys and are a consequence of aquifer overdraft associated with less than average precipitation in the basin and increased groundwater withdrawals in the GSL watershed. Using the Darcy flux equation, we calculated a groundwater flux to GSL of 313,500 acre-feet per year, substantially greater than previous estimates derived from water balance studies but consistent with estimates derived from geochemical modeling of GSL water chemistry. We calculated a salt contribution from groundwater to GSL of 1.18 million metric tons per year, which represents about 10% of the solutes derived from surface flows to GSL in 2013.

Minimizing Errors in the Prediction of Water Levels Using Kriging Technique in Residuals of the Groundwater Model

Groundwater monitoring and water level predictions have been a challenging issue due to the complexity of groundwater movement. Simplified numerical simulation models have been used to represent the groundwater system; these models however only provide the conservative approximation of the system and may not always capture the local variations. Several other efforts such as coupling groundwater models with hydrological models and using geostatistical methods are being practiced to accurately predict the groundwater levels. In this study, we present a novel application of a geostatistical tool on residuals of the groundwater model. The kriging method was applied on the residuals of the numerical model (MODFLOW) generated by the TWDB (Texas Water Development Board) for the Edwards–Trinity (Plateau) aquifer. The study was done for the years 1995 through 2000 where 90% of the observation data was used for model simulation followed by cross-validation with the remaining 10% of the observations. The kriging method reduced the average absolute error of approximately 31 m (for MODFLOW simulation) to less than 5 m. Furthermore, the residuals’ average standard error was reduced from 9.7 to 4.7. This implies that the mean value of residuals over the entire period can be a good estimation for each year separately. The use of the kriging technique thus can provide improved monitoring of groundwater levels resulting in more accurate potentiometric surface maps.

New insights into the hydrogeology and groundwater flow in the Great Barrier Reef catchment, Australia, revealed through 3D modelling

Study regionThe Great Barrier Reef catchment is located adjacent to the world's largest coral reef system, the Great Barrier Reef, in eastern Queensland, Australia. Study focusThis study characterized the geologic and hydrogeologic settings and evaluated the influence of regional faults on groundwater flow. 3D geological models of six regions within the catchment were constructed using drill-log data from >49,000 wells, digital elevation models and surface geological maps. The 3D models were then integrated with potentiometric surface maps and faults data to conceptualize the hydraulic relationships of aquifer units and estimate groundwater development potentials. Potentiometric surfaces and fault orientations were used to conceptualize groundwater flow directions. New hydrological insights for the regionThe 3D geological and hydrogeological characterizations revealed previously unknown faults and aquifer units in the study area. The study found that the central regions consisted of fractured and porous-unconfined aquifers, while confined aquifers, which extend to the coast and likely beyond, were also found in the northern and southern most regions. The orientations of the faults trended in NW-SE directions and could form conduits for south-easterly groundwater flow as opposed to the predominate easterly flow in the porous-unconfined and confined aquifers. The 3D models, aquifer connectivities and geometries provided crucial information to determine groundwater development potentials and offer a first step in developing local and regional groundwater flow and contaminant transport models.

Estimating groundwater resources of the Içá-Solimões Aquifer System in the Urucu Oil Province Central Amazon Region, Brazil, focused on a balance between availability and water demand

ABSTRACT Hydrogeologic information is still scarce in the Amazon region being required more studies for improving the understanding of local hydrogeologic contexts. The goal of this paper, focused on the Urucu Oil Province, Brazilian Central Amazon, is to quantitatively estimate groundwater resources of the Içá-Solimões Aquifer System (ISAS) in the region. The work focuses on a balance between availability and water demand, considering current and future uses, contributing to a sustainable and integrated use of groundwater. Cartographic, lithologic and geophysical logs were analyzed coupled with ISAS hydraulic data (transmissivity, hydraulic conductivity, and storativity) and its local aquifer geometries, as well as the potentiometric surface map to estimate values of total and renewable groundwater resources, sustainable yields, and well-field facility capacities. Results showed the portion of the Içá-Solimões Aquifer System has a total groundwater resource of 0.45 km3. As there is a groundwater consumptive projection to 2050 close to 2.7 x 10-3 km3 yr–1 and the aquifer system has a sustainable yield of 3 x 10-3 km3 yr–1, the current resource allocation for future consumption rate appears to be sustainable.

Impact of land cover on groundwater quality in the Upper Floridan Aquifer in Florida, United States

Although agricultural lands are generally assumed to correlate negatively with groundwater quality, the intricate relationship between general land cover and contaminant concentrations present in an aquifer may vary substantially; contingent upon the land type, interacting factors, and scale considered. The Upper Floridan Aquifer (UFA) is a primary source of potable water supply for the state of Florida. The Suwannee River Water Management District (SRWMD), located in northcentral Florida, relies exclusively on the UFA for water supplies. Over much of the SRWMD in the UFA is unconfined, rendering it vulnerable to contamination from surface sources. This study analyses groundwater concentrations of Nitrate-Nitrogen (NO3–N) and Potassium (K) from shallow wells across the SRWMD for assessing the effect of different land covers on groundwater quality over time. Annual potentiometric surface maps were used to delineate semicircular recharge zones of 500 m, 1000 m, and 2000 m radii upstream of sampled well stations. Proportions of agriculture, forest, and urban lands were identified for each buffer zone using USDA Cropland Data Layer. Multivariate regression models were developed to infer relationships between land cover and NO3–N and K concentrations. Results show significant associations among land cover type, water table height, and groundwater quality parameters. Specifically, we find a large proportion of agricultural cover consistently associated with larger increases in groundwater pollutant loads relative to urban or forest cover across all models, after controlling for depth to water table. Our study suggests a need for widespread adoption of cost-effective agricultural best management practices (BMPs) that could help in securing regional water supply.

Relations of the groundwater quality and disorderly occupation in an Amazon low-income neighborhood developed over a former dump area, Santarém/PA, Brazil

Groundwaters are better protected than surface waters. However, in unconfined conditions, groundwater can be easily contaminated when the land use and occupation are considered as determining factor. In the city of Santarem/PA, Brazil, the district of Santo Andre has experienced a population growth until reaching areas close to a former dump, and this may have caused an impact on the quality of the Alter do Chao Aquifer. In order to analyze these impacts generated, seven ground-penetrating radar sections to set the dump boundaries, physicochemical and bacteriological analyses in six pumping wells to set the drinkability values of the water, and potentiometric surface map in order to understand groundwater flow directions were carried out. Geophysical data indicated the presence of lateritic crusts, buried pipelines, as well as faults and fractures, which may serve as preferred pathways for contaminant infiltrations up to the aquifer. The geophysical data also indicated the dump boundaries are larger than expected, where the population has built their residences over the compacted garbage. Consequently, every pumping well presented total amounts of coliforms higher than the allowed values, and three of these wells have above 10 mg/L values of nitrate, indicating the region has waters in disagreement to the drinkability standards set, directly influenced by the contamination of the former dump under the Alter do Chao Aquifer.

Transmissivity of Aquifer by Capture Zone Method: An Application in the Sete Lagoas Karst Aquifer, MG, Brazil.

Transmissivity is an important hydraulic parameter to determine the amount of water passed horizontally across a given saturated thickness of an aquifer. The techniques to quantify this parameter, such as grain size analyses or pumping tests, can have limitations of time/spatial scale, viability, or economically. One technique that can be used, but little adopted, is the capture zone analysis. In this paper, capture zone analytical equations were used to estimate transmissivity values in order to verify the effectiveness of this methodology as alternative in situations where other traditional methods present implementation difficulties. The results were compared with field data estimated by aquifer tests conducted in the same region. A sensitivity analysis was also performed to identify possible discrepancies between the analytical and field data results. The aquifer studied was the Sete Lagoas Karst Aquifer in the urban region of the municipality of Sete Lagoas, Brazil. The method proved to be a viable and economical tool, where the analytical values compared to the aquifer tests showed similarities, being confirmed by a sensitivity analysis. However, a reliable potentiometric surface map, which enables the identification of the parameters for analytical capture zone equations, is needed.

Superfície potenciométrica e possíveis fontes de contaminação do aquífero parecis no município de Vilhena - RO, BR

O controle e a preservação da qualidade das águas subterrâneas são as principais tarefas a serem realizadas quando se trata de planejamento ambiental desse recurso. Desta forma, o presente estudo tem como objetivo determinar as linhas de fluxo das águas subterrâneas no município de Vilhena-RO, Região Amazônica. Foram levantados dados de poços e mapeadas as fontes potenciais de contaminação incluindo-se os postos de combustíveis, cemitério municipal, poços de abastecimento contaminados e lixões desativados. Por meio do mapa de superfície potenciométrica foi possível constatar alguns pontos vulneráveis, como o poço número 27, de responsabilidade do Serviço Autônomo de Águas e Esgoto, o qual está localizado em uma área de convergência de fluxos, podendo assim receber cargas contaminantes oriundas do cemitério municipal e de dois lixões desativados. Sendo assim, o estudo da superfície potenciométrica do Aquífero Parecis é uma valiosa ferramenta de espacialização em Sistema de Informações Geográficas que permite, através da observação do mapa gerado, localizar os melhores pontos para instalação de novos poços de abastecimento, postos de combustíveis, entre outros, fornecendo subsídios para o processo de gestão, ocupação dos solos e controle da explotação das águas subterrâneas em escala municipal.

MAPEAMENTO AMBIENTAL DAS ZONAS VULNERÁVEIS À CONTAMINAÇÃO DO AQUÍFERO GUARANI NO MUNICÍPIO DE FAXINAL DO SOTURNO - RS, BRASIL

The increasing demand for natural resources, for agricultural use and urban development, could advance towards areas with possible risk of groundwater contamination, because they are not well planned and/or inadequate environmental studies appropriate to a particular condition of use. This situation is related to the natural vulnerability that evaluates soil type, geology, slope and permanent preservation area; identified as environmental conflicts, due to inappropriate activities, in areas with potential risk of groundwater contamination. Thus, the objectives of this study were to conduct a survey of potentially contaminating activities and to relate their areas of influence with the map of natural vulnerability, which defines the interrelationship among soil maps, geology, slope the municipality of Faxinal do Soturno – RS, Brazil to check possible improper soil use. For this purpose, data were collected from wells and a potentiometric surface map was generated from the static level of wells, which allowed the estimation of groundwater flow. In the municipality, potential activities, including cemeteries, hospitals, fuel station, brick kilns and sawmills with potential risk of contamination were identified. With available technical procedures, potentially contaminating activities were identified in areas of medium and high vulnerability, which characterizes the possible risk of contaminating the groundwater.

APLICAÇÃO DO SISTEMA GOD PARA DETERMINAR A VULNERABILIDADE NATURAL DAS ÁGUAS SUBTERRÂNEAS DO MUNICÍPIO DE TEUTÔNIA (RS)

Nowadays, knowledge of groundwater resources is extremely important, since industrial, agricultural and mining activities are expanding constantly, impacting in several ways on the quality of groundwater. The Teutonia municipality that is located in the countryside of Rio Grande do Sul state southern Brazil, isn’t different. Thus this study aims to identify the natural vulnerability of the aquifer to contaminatinon, calculate the potentiometric surface and spatialize these results. Wells that are registered at Companhia de Pesquisas de Recursos Minerais in Groundwater Information System (SIAGAS/CPRM) for the Teutonia mu nicipality was be used. Using the GOD system, there were analyzed 40 tubulado wells. They penetrate Serra Geral Aquifer System/SASG and Guarani Aquifer System/GAS. Were generated the vulnerability map and the potentiometric surface map, and there was identified that most of the wells presented a low level of vulnerability and areas of low potentiometric level are also more vulnerable to contamination. As a preliminary study, the results are important to help in the ground water resources management, as well as to open new work alternatives linked to groundwater of the section ion. The results are also useful for master plans and land use at local scale.

Regional hydraulic behavior of structural zones and sedimentological heterogeneities in an overpressured sedimentary basin

The paper presents the results of a regional scale hydrogeological study conducted in two areas of the Pannonian Basin in Eastern and Southeastern Hungary. The purpose of the study was to investigate the role of fault zones and sedimentological heterogeneities in areas of overpressure dissipation and hydrocarbon entrapment. The Study Areas (∼10,400 km2 in total) were chosen so as to see the effects of their differing geological characteristics, specifically that of a regional scale Pre-Neogene basement high (Battonya High), and two regional scale depressions (Derecske Trough, Békés Basin). Groundwater flow patterns were inferred for both areas from distributions of fluid dynamic parameters presented on potentiometric surface maps, vertical hydraulic cross sections and pressure-vs.-elevation [p(z)] profiles. The data suggest that both fluid-flow patterns and the potential for hydrocarbon entrapment are predominantly controlled by the dissipation paths of overpressure. In turn, the routes of overpressure dissipation are controlled by the structural and sedimentological heterogeneities of the aquitards. Diagnostic relationships were found between the heterogeneities and fluid-potential anomalies, as well as hydrocarbon occurrences. The observed pattern of overpressure dissipation has allowed the identification of seven regions by seven basic types of pressure–elevation profiles, p(z). As well, a new concept, “the upper boundary of hydrocarbon migration” was proposed. Finally, based on the established diagnostic relationships and the deduced type-profiles of pressure–elevation, a hydrogeological methodology was developed which could be used in other areas and for other purposes, e.g., exploration for groundwater and geothermal resources, petroleum and in preparation for numerical modeling.

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.

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.

Using Multiple Chemical Indicators to Assess Sources of Nitrate and Age of Groundwater in a Karstic Spring Basin

Research Article| April 01, 2005 Using Multiple Chemical Indicators to Assess Sources of Nitrate and Age of Groundwater in a Karstic Spring Basin Brian Katz; Brian Katz 1U.S. Geological Survey, 2010 Levy Ave., Tallahassee, FL 32310 Search for other works by this author on: GSW Google Scholar Rick Copeland; Rick Copeland 2Florida Geological Survey, 903 W. Tennessee St., Tallahassee, FL 32304 Search for other works by this author on: GSW Google Scholar Tom Greenhalgh; Tom Greenhalgh 2Florida Geological Survey, 903 W. Tennessee St., Tallahassee, FL 32304 Search for other works by this author on: GSW Google Scholar Ron Ceryak; Ron Ceryak 3Suwannee River Water Management District, 9225 CR49, Live Oak, FL 32060 Search for other works by this author on: GSW Google Scholar Warren Zwanka Warren Zwanka 3Suwannee River Water Management District, 9225 CR49, Live Oak, FL 32060 Search for other works by this author on: GSW Google Scholar Environmental & Engineering Geoscience (2005) 11 (4): 333–346. https://doi.org/10.2113/11.4.333 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Brian Katz, Rick Copeland, Tom Greenhalgh, Ron Ceryak, Warren Zwanka; Using Multiple Chemical Indicators to Assess Sources of Nitrate and Age of Groundwater in a Karstic Spring Basin. Environmental & Engineering Geoscience 2005;; 11 (4): 333–346. doi: https://doi.org/10.2113/11.4.333 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract Human health and ecological concerns have arisen due to a steady increase in nitrate-N concentrations during the past 40 years in Fannin Springs (0.3–4.7 mg/L), a regional discharge point with an average flow of >2.8 m3/second (>100 ft3/second) for water from the karstic Upper Floridan aquifer (UFA). Multiple chemical indicators (major dissolved species, 15N and 18O of nitrate, dissolved gases, 78 pesticides and degradates, and 67 organic compounds typically found in domestic and industrial wastewater) and transient tracers (3H/3He, chlorofluorocarbons [CFCs], sulfur hexafluoride [SF6]) were analyzed in water samples from nine wells along three transects and in spring water to assess groundwater age and potential contaminant sources. Land use is predominantly agricultural (52 percent) and forest (31 percent) in the 320 km2 (124 mi2) spring basin, which was delineated from a potentiometric-surface map of the UFA using high-resolution water-level data. Nitrate-N concentrations were highly variable in the oxic UFA and ranged from <0.02 to 4.7 mg/L. δ15N-NO3 values (3.4–9.9 per mil) indicated that nitrate contamination originated from inorganic sources (synthetic fertilizer) and organic sources (manure spreading or waste disposal). Higher nitrate concentrations and the younger age of spring water relative to water from upgradient wells indicate better communication with N sources at the surface. Apparent ages of groundwater correlated positively with well depth (P < 0.05) and were younger in water from wells nearer to the spring (<8 years) compared with other wells (10–50 years). Most transient tracer concentrations were consistent with binary mixing curves representing mixtures of water recharged during the past 10 years and older water (recharged before 1940). Young water mixing fractions ranged from 0.07 to 0.90. Trace levels of herbicides found in groundwater and spring water were indicative of applications for vegetative control in agricultural and other land-use types. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Linking hyporheic flow and nitrogen cycling near the Willamette River — a large river in Oregon, USA

Several approaches were used to characterize ground water/surface water interactions near the Willamette River — a large (ninth order) river in Oregon, USA. A series of potentiometric surface maps demonstrated the presence of highly dynamic hydraulic gradients between rivers and the adjacent aquifer. Hyporheic zone gradients extended on the order of hundreds of meters. River gains and losses at the river stretch scale (tens of kilometers) were consistent with fluxes implied by the potentiometric surface maps, and apparently reflect regional ground water/surface water interactions. Gains and losses of up to 5–10% of streamflow were observed at this scale. On the river reach scale (1–2 km), gains and losses on the order of 5% of streamflow were interpreted as representing primarily local hyporheic exchange. Isotopic and chemical data collected from shallow hyporheic zone wells demonstrated interaction between regional ground water and river water. The origin of sampled hyporheic zone water ranged from a mixture dominated by regional ground water to water containing 100% river water. The common assumption that ground and river water mix primarily in the river channel is not applicable in this system. Isotopic and chemical data also indicated that significant (nearly complete) vegetative nitrate uptake and/or nitrate reduction occurred in water from 4 of 12 hyporheic zone sites. In these cases, it was primarily nitrate transported to the hyporheic zone in regional ground water that was removed from solution. Isotopes of water and nitrate indicated that hyporheic zone water sampled at two sites was composed of water originating as river water and demonstrated that significant vegetative nitrate uptake and nitrate reduction occurred along these hyporheic zone flowpaths. Thus, the hyporheic zone may, in some instances, serve to remove nitrate from river water. Additional investigations with chemical tools and microbial enzyme assays were conducted at one hyporheic site. A strong vertical redox gradient was observed, with nitrate-limited denitrification potential in deeper sediment and both nitrification and denitrification potential in shallower sediment. Since nitrogen cycling is strongly affected by redox conditions, nitrogen cycling in the hyporheic zone of this large-river system likely is affected by dynamics of ground water/surface water interactions that control fluxes of nitrogen and other redox species to hyporheic zone sediment.

Regional groundwater flow model construction and wellfield site selection in a karst area, Lake City, Florida

The city of Lake City is in the process of expanding their water supply facilities by 45 420 m 3 day −1 (12 MGD) to meet future demands. One portion of wellfield site selection addressed here includes analysis of ambient groundwater quality and its potential for contamination. This study also addresses the potential impacts of groundwater withdrawals to existing legal users, wetlands, surface waters and spring flows. A regional groundwater flow model (MODFLOW) was constructed using existing hydrogeologic data from state and federal agencies in order to simulate the existing hydrologic conditions of this karst area and to predict withdrawal impacts. The model was calibrated by matching potentiometric surface maps and spring flows to within reasonable ranges. Drawdowns in the Floridan and surficial aquifers predicted by the model show minimal impacts to existing legal users and only a 5% reduction in the flow at Ichetucknee Springs ca 21 km (13 miles) away. Due to the karstic nature of the Floridan aquifer here, the equivalent-porous-medium flow model constructed would not be appropriate for contaminant transport modeling. The groundwater flow model is, however, appropriate to represent hydraulic heads and recharge/discharge relationships on a regional scale.

Ground-water contribution from the Silurian/Devonian carbonate aquifer to the Mad River Valley, southwestern Ohio : R. A. Sheets & W. P. Yost, Ohio Journal of Science, 94(5), 1994, pp 138–146

The U.S. Geological Survey Regional Aquifer-System Analysis is examining the Silurian and Devonian carbonate aquifer and overlying glacial deposits of western Ohio and eastern Indiana in terms of the hydrogeologic framework, ground-water-flow system and water chemistry, and response to withdrawals and development. This paper describes the hydrogeology of an area in southwestern Ohio that contains the Mad River Valley, an outwash valley that is incised through the relatively permeable Silurian bedrock into the relatively Ordovician bedrock at the southern boundary of Silurian rocks. Ground-water-flow patterns based on a synoptic water-level survey of 80 bedrock wells in the Silurian and Devonian carbonate aquifer indicate that flow is toward the valley. A ground-water basin was delineated from the potentiometricsurface map and used in conjunction with a low-flow gain-loss study to derive a generalized ground-water/ surface-water budget for the area. Results of this budget indicate that ground-water recharge rates through the ground moraine to the carbonate aquifer range from 4 to 6 in/yr (10-15 cm/yr) and that approximately 60 to 80% of the base flow in the Mad River may ultimately consist of ground water derived from the Silurian and Devonian carbonate aquifer. Previous studies of surfaceand ground-water chemistry in the area also support the conclusion that the Mad River Valley functions as a discharge area for the surrounding carbonate aquifer. Results of this study can be useful in characterizing ground-water flow and recharge rates for the Silurian and Devonian carbonate and valley aquifers along the southern boundary of this extensive regional aquifer system. OHIO J. SCI. 94 (5): 138-146, 1994 INTRODUCTION In 1988, the U.S. Geological Survey's Regional AquiferSystem Analysis (RASA) program began an investigation of the hydrogeologic framework, the present ground-water-flow system and water chemistry, and effects of ground-water development in the Midwestern Basin and Arches Region (Fig. 1). This RASA investigation has focused primarily on the Silurian and Devonian carbonates and surficial unconsolidated deposits of western Ohio and eastern Indiana (Bugliosi 1990). A major objective of this RASA is characterization of the aquifer system, in terms of regional ground-water-flow patterns. A system of river valleys filled with various lacustrine, alluvial, and glacial deposits is present throughout the Silurian and Devonian carbonate aquifer system. These valleys were formed predominantly as a result of several Pleistocene continental glaciations. As the ice retreated northward, sediment-laden rivers incised the bedrock of western Ohio and eastern Indiana, creating the valleys and later depositing valley-fill material. The hydrologic characteristics of these valleys vary with the type of materials deposited in them and the depth of incision into the bedrock. In some valleys, unconsolidated deposits yield more ground water than the bedrock that they incise; in other valleys, usable ground-water resources are negligible. The hydrologic interaction of these valleys with the ground-water-flow regime of the permeable bedrock is important to the understanding of boundary conditions for the regional ground-water analysis. 'Manuscript received 8 April 1994 and in revised form 12 September 1994 (#94-07). Previous Investigations Orton (1874) and Leverett (1897) were probably among the first to describe the water-bearing potential of bedrock and unconsolidated deposits of southwestern Ohio. Fuller and Clapp (1912) wrote an overview of the area's water resources, which includes a county-bycounty description of the physiography, springs, wells, and the water-bearing potential of lithologic units. The large water-bearing potential of the river valleys in this part of Ohio has led many authors to publish waterresource information for these valleys (e.g., Norris and Spieker 1966, Norris and Eagon 1971). In contrast, few publications describe the ground-water resources of the Ordovician shale and limestone subcrops in this area, probably because of the scarcity of potable ground water, which would limit the amount of subsurface data available from well drillers. In fact, most authors refer to the Ordovician bedrock as impermeable or impervious to ground-water flow (Orton 1874; Fuller and Clapp 1912; Harker and Bernhagen 1943; and Norris et al. 1947, 1949, 1952). Works that describe the Silurian and Devonian carbonate aquifer system in this area are more numerous than those that describe the Ordovician bedrock, but are less numerous than those that describe ground-water resources of the valleys. Purpose and Scope The purpose of this paper is to describe the hydrogeology of the Mad River Valley aquifer system in southwestern Ohio and its hydrologic relation to the surrounding Silurian and Devonian carbonate aquifer that it incises. Ground-water flow based on measured water levels in the Silurian and Devonian carbonate aquifer, stream-discharge measurements of the Mad River OHIO JOURNAL OF SCIENCE R. A. SHEETS AND W. P. YOST 139

The Use of Soil-Gas Sampling in the Study of Groundwater Pollution by Volatile Solvents (VOC): The Example of the Porto Feliz (São Paulo, Brazil) Case

In 1983, a serious accident occurred in Porto Feliz, São Paulo, Brazil when the rupture of a storage tank released 400,000 litres of volatile organic solvents. A large portion of the underlying aquifer was contaminated including some deep wells and shallow hand-dug wells. Soil-gas sampling was the technique used to evaluate the degree and extent of contamination. The method is based on the analysis of small quantities of soil vapors extracted at shallow depths. The origin of these vapors is the underlying contaminated groundwater and the vapor concentration distribution reflects the degree and extent of aquifer contamination. Other techniques were rejected since the physico-chemical characteristics of the pollutants did not allow their detection by traditional geophysical methods and the thick unsaturated zone and difficult geology would have resulted in exorbitant drilling costs for monitoring wells. The use of soil-gas sampling allowed mapping of the pollution plume, defining its two-dimensional shape and location. Vapor analyses at different depths permitted the establishment of concentration gradients and an estimation of vertical vapor flow behavior in the unsaturated zone as well as surface losses by volatilization. Water levels in existing wells made it possible to construct potentiometric surface maps, defining the groundwater flow pattern. Periodic groundwater quality monitoring furnished data for the evaluation of the plume migration velocity and the influence of rainfall on the increase of pollutant concentrations in the aquifer. The groundwater pollutant concentrations obtained gave a good correlation between the two means.