Delineation of capture zones for municipal wells in fractured dolomite, Sturgeon Bay, Wisconsin, USA

A wellhead protection study for the city of Sturgeon Bay, Wisconsin, USA, demonstrates the necessity of combining detailed hydrostratigraphic analysis with groundwater modeling to delineate zones of contribution for municipal wells in a fractured dolomite aquifer. A numerical model (MODFLOW) was combined with a particle tracking code (MODPATH) to simulate the regional groundwater system and to delineate capture zones for municipal wells. The hydrostratigraphic model included vertical and horizontal fractures and high-permeability zones. Correlating stratigraphic interpretations with field data such as geophysical logs, packer tests, and fracture mapping resulted in the construction of a numerical model with five high-permeability zones related to bedding planes or facies changes. These zones serve as major conduits for horizontal groundwater flow. Dipping fracture zones were simulated as thin high-permeability layers. The locations of exposed bedrock and surficial karst features were used to identify areas of enhanced recharge. Model results show the vulnerability of the municipal wells to pollution. Capture zones for the wells extend several kilometers north and south from the city. Travel times from recharge areas to all wells were generally less than one year. The high seasonal variability of recharge in the study area made the use of a transient model necessary. Electronic Publication     

Detailed hydraulic head profiles as essential data for defining hydrogeologic units in layered fractured sedimentary rock

This paper describes a study in southern Wisconsin where vertical hydraulic head profiles measured in exceptional detail provided the key data for defining hydrogeologic units (HGUs) in a layered sequence of sandstone, siltstone, shale, and dolostone. The most important data were obtained from corehole MP-6 which was cored 131 m into bedrock and instrumented using a Westbay^® multilevel system with 36 depth discrete monitoring intervals. The resulting head profile is consistant over time and shows eight distinct inflections in hydraulic head. Several of the inflections occur between adjacent permeable units and are likely due to poor vertical connectivity of fracture sets rather than distinct lower permeability layers or aquitards in the conventional sense. No other type of data was capable of identifying the position of such distinct hydrogeologic features. These zones of abrupt head loss provide the primary dataset for delineation of eleven HGUs at MP-6 and are supported by less detailed head profiles at other locations. Although the detailed head profiles are essential, core logs and geophysical logs from other boreholes are nessessary to fully establish the lateral continuity of the HGUs.     

Fractional flow dimensions and hydraulic properties of a fracture-zone aquifer, Leppävirta, Finland

 In order to characterize the hydraulic properties of an aquifer in Finland comprising two subvertical fracture zones, observation-well responses were matched with generalized radial flow (GRF) type curves. The responses in six wells out of seven are consistent with the GRF model. The fractional flow dimensions (1–1.2 and 1.5) were determined by regression analysis of straight-line slopes and type-curve matching. In each test, the flow dimensions in the neighboring fracture zone range from 2–2.25. Comparisons of the late-time responses with the asymptotic GRF solution and the flow dimensions obtained by reversing the pumping and observation points suggest homogeneous hydraulic properties. Deviations in responses can be explained by flow-path tortuosity. After assessments of the extent of the flow and radial distances along the fracture system, hydraulic conductivities and storativities were determined from the results of the type-curve matching procedure. The obtained hydraulic conductivities are 1.3×10^–5 to 7.9×10^–5 m/s and 5.0×10^–6 to 2.5×10^–5 m/s for the western and the eastern fracture zones, respectively. The results were verified by applying them to analytical solutions for pumping wells. The calculated pumping-well responses are consistent with the observations. The analysis of flow dimension also enhances qualitative interpretations on the hydrogeology of fracture zones. Received, April 1997 · Revised, September 1997 · Accepted, May 1998     

Characterising the present-day stress regime of the Georgina Basin

The onshore Georgina Basin in northern Australia is prospective for unconventional hydrocarbons; however, like many frontier basins, it is underexplored. A well-connected hydraulic fracture network has been shown to be essential for the extraction of resources from the tight reservoirs that categorise unconventional plays, as they allow for economic flows of fluid from the reservoir to the well. One of the fundamental scientific questions regarding hydraulic stimulation within the sub-surface of sedimentary basins is the degree to which local and regional tectonic stresses act as a primary control on fracture propagation. As such, an understanding of present-day stresses has become increasingly important to modern petroleum exploration and production, particularly when considering unconventional hydrocarbon reservoirs. This study characterises the regional stress regime in the Georgina Basin using existing well data. Wellbore geophysical logs, including electrical resistivity image logs, and well tests from 31 petroleum and stratigraphic wells have been used to derive stress magnitudes and constrain horizontal stress orientations. Borehole failure features interpreted from wellbore image and caliper logs yield a maximum horizontal stress orientation of 044°N. Integration of density log data results in a vertical stress gradient of 24.6 MPa km^–1. Leak-off and mini-fracture tests suggest that this is the minimum principal stress, as leak-off values are generally shown to be at or above the magnitude of vertical stress. The maximum horizontal stress gradient is calculated to be in the range of 31.3–53.9 MPa km^–1. As such, a compressional stress regime favouring reverse/reverse–strike-slip faulting is interpreted for the Georgina Basin.     

The role of buried bedrock valleys on the development of karstic aquifers in flat-lying carbonate bedrock: insights from Guelph, Ontario, Canada

A spatial relationship between high capacity municipal production wells (>5,000 m³/day), completed in a deep bedrock aquifer, and a buried bedrock valley was recognized in the city of Guelph, southwestern Ontario, Canada. Most production wells are completed in a discrete zone, ～60 m below ground surface, within flat-lying dolostones of the Silurian Amabel Formation. Thick overburden and limited subsurface data make characterization of the karstic aquifer difficult. This study integrates hydrogeologic data with models of karst formation, deriving a conceptual model of porosity development as it relates to valley incision. Bedrock valley incision likely occurred prior to the early Wisconsinan age (>60–75 ka). Incision created steep hydraulic gradients within the flat-lying bedrock, and provided the driving force required to integrate regional groundwater flow into karst conduits that drained at the base of the valley. Dissolution in production zone dolostones was favoured over dissolution in shallower bedrock due to abundant bedding plane partings and fossiliferous facies with high intercrystalline porosity. Burial of the valley during subsequent ice advances reduced the valley’s hydraulic influence and the efficacy of the flow system to cause dissolution. The high capacity municipal wells near the buried bedrock valley tap into the now dormant karst aquifer system.     

Hydrogeologic framework of Pico Island, Azores, Portugal

Pico, the youngest island of the Azores archipelago, is composed of basaltic volcanic deposits less than 300,000 years old. The principal aquifer system consists mainly of recent lava flows that are very permeable and whose head is influenced by tidal fluctuations. Groundwater abstraction is almost entirely by drilled wells. The hydraulic gradient is very low, about 10^–4, which agrees with observations made on similar volcanic islands. Groundwater also occurs in perched-water bodies, but the spring discharge from them is very low, about 10^–3 L/s. The transmissivity of the volcanic rocks ranges from 9.44×10^–3 to 3.05×10^–1 m²/s, indicating the heterogeneity of the aquifers. The hydraulic diffusivity, estimated from observations of the effects of tidal fluctuations, also confirms the high permeability of the aquifer system; the average value is higher than published values for other volcanic islands. A mixing process for fresh water and seawater, often coupled with ion-exchange mechanisms, explains the groundwater composition, which is mainly of the sodium-chloride type. The water salinity influences the groundwater quality, resulting in a chloride content that exceeds the recommended chloride limit in 91% of the wells . Water–rock interactions are dominant in the chemical evolution of the perched water, which is characterized by bicarbonate-anion type water. Electronic Publication     

Groundwater flow and capture zone analysis of the Central Passaic River Basin, New Jersey

Delineating capture zones of pumping wells is an important part of safe drinking water and well protection programs. Capture zones or contributing areas of a groundwater extraction well are the parts of the aquifer recharge areas from which the wells draw their water. Their extent and location depend on the hydrogeologic conditions such as groundwater recharge, pumping scenario and the aquifer properties such as hydraulic conductivity, porosity, heterogeneity of the medium and hydraulic gradient. Different methods of delineation can be used depending on the complexity of the hydrogeologic conditions. In this study, a 3-dimensional transient numerical MODFLOW model was developed for the Central Passaic River Basin (CPRB), and used with a MODPATH particle tracking code to determine 3-dimensional transient capture zones. Analytically calculated capture zones from previous studies at the site were compared with the new numerically simulated capture zones. The study results revealed that the analytical solution was more conservative, estimating larger capture zones than the numerical models. Of all the parameters that can impact the size, shape and location of a capture zone, the hydraulic conductivity is one of the most critical. Capture zones tend to be smaller in lower hydraulic conductivity areas.     

Hydrogeologic and hydrochemical framework of the shallow groundwater system in the southern Voltaian Sedimentary Basin, Ghana

 The southern Voltaian Sedimentary Basin underlies an area of about 5000 km² in east-central Ghana. Groundwater in the basin occurs in fractures in highly consolidated siliciclastic aquifers overlain by a thin unsaturated zone. Aquifer parameters were evaluated from available aquifer-test data on 28 shallow wells in the basin. Hydraulic-conductivity values range from 0.04–3.6 m/d and are about two orders of magnitude greater than the hydraulic conductivity calculated using Darcy's Law and the average groundwater velocity estimated from carbon-14 dating. Linear-regression analysis of the transmissivity and specific-capacity data allowed the establishment of an empirical relationship between log transmissivity and log specific capacity for the underlying aquifers. Groundwater chemistry in the basin is controlled by the weathering of albitic plagioclase feldspar. The weathering rates of various minerals were estimated using ¹⁴C-derived average velocity in the basin. The weathering rate of albite was calculated to be 2.16 μmol L^–1 yr^–1 with the resulting formation of 3.3 μmol L^–1 yr^–1 of kaolinite and 0.047 μmol L^–1 yr^–1 of calcite. The low porosity and permeability of the aquifers in the basin are attributed to the precipitation of secondary minerals on fracture surfaces and interlayer pore spaces. Received, September 1997 Revised, July 1998, August 1998 Accepted, August 1998     

Classifying zones of suitability for manual drilling using textural and hydraulic parameters of shallow aquifers: a case study in northwestern Senegal

A method is proposed that uses analysis of borehole stratigraphic logs for the characterization of shallow aquifers and for the assessment of areas suitable for manual drilling. The model is based on available borehole-log parameters: depth to hard rock, depth to water, thickness of laterite and hydraulic transmissivity of the shallow aquifer. The model is applied to a study area in northwestern Senegal. A dataset of boreholes logs has been processed using a software package (TANGAFRIC) developed during the research. After a manual procedure to assign a standard category describing the lithological characteristics, the next step is the automated extraction of different textural parameters and the estimation of hydraulic conductivity using reference values available in the literature. The hydraulic conductivity values estimated from stratigraphic data have been partially validated, by comparing them with measured values from a series of pumping tests carried out in large-diameter wells. The results show that this method is able to produce a reliable interpretation of the shallow hydrogeological context using information generally available in the region. The research contributes to improving the identification of areas where conditions are suitable for manual drilling. This is achieved by applying the described method, based on a structured and semi-quantitative approach, to classify the zones of suitability for given manual drilling techniques using data available in most African countries. Ultimately, this work will support proposed international programs aimed at promoting low-cost water supply in Africa and enhancing access to safe drinking water for the population.     

Characterization of hydrogeologic properties for a multi-layered alluvial aquifer using hydraulic and tracer tests and electrical resistivity survey

A multi-layered aquifer, typical of riverbank alluvial deposits in Korea, was studied to determine the hydrologic properties. The geologic logging showed that the subsurface of the study site was comprised of four distinctive hydrogeologic units: silt, sand, highly weathered and fresh bedrock layers. The electrical resistivity survey supplied information on lateral extension of hydrogeologic strata only partially identified by a limited number of the geologic loggings. The laboratory column tracer test for the recovered core of the sand layer resulted in a hydraulic conductivity of 5.00×10⁻² cm/s. The slug tests performed in the weathered rock layer yielded hydraulic conductivities of 4.32–7.72×10⁻⁴ cm/s. Hydraulic conductivities for the sand layer calculated from the breakthrough curves of bromide ranged between 2.08×10⁻³ and 2.44×10⁻² cm/s with a geometric mean of 6.89×10⁻³ cm/s, which is 7 times smaller than that from the laboratory column experiment. The trend of increasing hydraulic conductivity with an increase in tracer travel length is likely a result of the increased likelihood of encountering a high conductivity zone as more of the aquifer is tested. The combined hydrogeologic site characterization using hydraulic tests, tracer tests, and column test with geologic loggings and geophysical survey greatly enhanced the understanding of the hydrologic properties of the multi-layered alluvial aquifer.     

Estimation of regional-scale groundwater flow properties in the Bengal Basin of India and Bangladesh

Quantitative evaluation of management strategies for long-term supply of safe groundwater for drinking from the Bengal Basin aquifer (India and Bangladesh) requires estimation of the large-scale hydrogeologic properties that control flow. The Basin consists of a stratified, heterogeneous sequence of sediments with aquitards that may separate aquifers locally, but evidence does not support existence of regional confining units. Considered at a large scale, the Basin may be aptly described as a single aquifer with higher horizontal than vertical hydraulic conductivity. Though data are sparse, estimation of regional-scale aquifer properties is possible from three existing data types: hydraulic heads, ¹⁴C concentrations, and driller logs. Estimation is carried out with inverse groundwater modeling using measured heads, by model calibration using estimated water ages based on ¹⁴C, and by statistical analysis of driller logs. Similar estimates of hydraulic conductivities result from all three data types; a resulting typical value of vertical anisotropy (ratio of horizontal to vertical conductivity) is 10⁴. The vertical anisotropy estimate is supported by simulation of flow through geostatistical fields consistent with driller log data. The high estimated value of vertical anisotropy in hydraulic conductivity indicates that even disconnected aquitards, if numerous, can strongly control the equivalent hydraulic parameters of an aquifer system.     

Detecting transmissive bedrock fracture zones under cover of glacial formations using residential water-well production data

Tracing fractures under glacial drift commonly involves costly and often unfeasible (in populated areas) geophysical methods or outcrop surveys, often far from the area of interest. A hypothesis is tested, that the specific capacity data for wells penetrating through glacial drift into a bedrock aquifer display two statistical populations: assuming uniform well construction, the wells with high specific capacity penetrate transmissive fracture zones, while those with low specific capacity encounter non-fractured rock characterized by primary porosity. The hypothesis was tested on 617 wells drilled into the Pennsylvanian Sharon Sandstone, Geauga County, Ohio (USA). Hydraulic conductivity, calculated using the Cooper and Jacob (1946) approximation to Theis’ non-equilibrium radial flow equation, followed quasi-log-normal distribution (geometric mean 9.88?×?10^?6 m/s). The lower values presumably correspond to primary porosity, and higher values correspond to bedrock fracture zones. The higher hydraulic conductivity followed two distinct orientations (N34°E, N44°W), corresponding with the regional fracture pattern of the Allegheny Plateau. A variogram showed that the wells within a kilometer of each other correlate and that wells penetrating the thicker glacial blanket have lower hydraulic conductivity and larger drawdown. Cooper and Jacob (1946) A generalized graphical method for evaluating formation constants and summarizing well-field history, Am. Geoph. Union Trans. 27/4:526–534.     

储集层非均质性分析的计算机方法:以西弗吉尼亚州下密西西比系油田为例

本文使用几种计算法和统计法研究美国西弗吉尼亚洲中部格兰尼克里克油田大Injun砂岩储集层的结构及其与采油量的关系。计算机程序根据已出版的资料编写,以适合(1)计算渗透率对孔隙度的回归;(2)标绘三维孔隙度;(3)确定和标绘根据地球物理测井记录推断的相;(4)估算原始储量、累积产量和采油率。因为回归分析显示出测井记录和岩心孔隙度及岩心渗透率间微弱但重要的相关,所以,孔隙度可用于地层渗透率中以构成储集层的模式。也使用定量地层对比和多维定算法来估算在缺乏可用数据的情况下构造的影响。于井间使用克里格法内插绘制的剖面,突出了孔隙度较高的地带。使用地球物理测井资料和岩心描述,用聚类分析确定导电相。原始储量的估算结果与累计产量数据相结合,得出采油率的估算值。这些采油量变量图通常呈现在岩相图上观察到的相同的南北走向。     

Interpretation of tracer tests performed in fractured rock of the Lange Bramke basin, Germany

 Two multitracer tests performed in one of the major cross-fault zones of the Lange Bramke basin (Harz Mountains, Germany) confirm the dominant role of the fault zone in groundwater flow and solute transport. Tracers having different coefficients of molecular diffusion (deuterium, bromide, uranine, and eosine) yielded breakthrough curves that can only be explained by a model that couples the advective–dispersive transport in the fractures with the molecular diffusion exchange in the matrix. For the scale of the tests (maximum distance of 225 m), an approximation was used in which the influence of adjacent fractures is neglected. That model yielded nearly the same rock and transport parameters for each tracer, which means that the single-fracture approximation is acceptable and that matrix diffusion plays an important role. The hydraulic conductivity of the fault zone obtained from the tracer tests is about 1.5×10^–2 m/s, whereas the regional hydraulic conductivity of the fractured rock mass is about 3×10^–7 m/s, as estimated from the tritium age and the matrix porosity of about 2%. These values show that the hydraulic conductivity along the fault is several orders of magnitude larger than that of the remaining fractured part of the aquifer, which confirms the dominant role of the fault zones as collectors of water and conductors of fast flow. Received, April 1997 Revised, January 1998, August 1998 Accepted, August 1998     

Numerical analysis of the hydrogeologic controls in a layered coastal aquifer system, Oahu, Hawaii, USA

 The coastal aquifer system of southern Oahu, Hawaii, USA, consists of highly permeable volcanic aquifers overlain by weathered volcanic rocks and interbedded marine and terrestrial sediments of both high and low permeability. The weathered volcanic rocks and sediments are collectively known as caprock, because they impede the free discharge of groundwater from the underlying volcanic aquifers. A cross-sectional groundwater flow and transport model was used to evaluate the hydrogeologic controls on the regional flow system in southwestern Oahu. Controls considered were: (a) overall caprock hydraulic conductivity; and (b) stratigraphic variations of hydraulic conductivity in the caprock. Within the caprock, variations in hydraulic conductivity, caused by stratigraphy or discontinuities of the stratigraphic units, are a major control on the direction of groundwater flow and the distribution of water levels and salinity. Results of cross-sectional modeling confirm the general groundwater flow pattern that would be expected in a layered coastal system. Groundwater flow is: (a) predominantly upward in the low-permeability sedimentary units; and (b) predominantly horizontal in the high-permeability sedimentary units. Received, October 1996 Revised, August 1997 Accepted, September 1997     

四川盐边地区志留纪牙形石生物地层及年代地层

首次提供了四川盐边地区从下志留统至下泥盆统连续的志留系年代地层系统的实际证据和原始材料,由上而下建立8个牙形石带,2个牙形石组合带和1个笔石带,以此为依据建立了攀枝花阶、永兴阶、箐河阶、叉河阶、透底河阶和支六阶,分别归入下泥盆统、顶志留统、上志留统、中志留统和下志留统,为我国志留系安康阶以上地层的建阶研究工作提供了实际的基础资料。四川盐边地区志留系龙马溪组为笔石相地层,其上全为碳酸盐岩相,其中牙形石发育良好,形成连续完整的牙形石带序列,这对我国和更大的范围壳相志留系年代地层系统对比提供了非常有价值的资料。     

Natural arsenic in Triassic rocks: A source of drinking-water contamination in Bavaria, Germany

 The aquifer system of the Upper Triassic Keuper Sandstone, an important source of drinking water in northern Bavaria, is affected by elevated arsenic concentrations. Within the study area of 8000 km², no evidence exists for any artificial source of arsenic. Data from about 500 deep water wells show that in approximately 160 wells arsenic concentrations are 10–150 μg/L. The regional distribution of arsenic in the groundwater shows that elevated arsenic concentrations are probably related to specific lithofacies of the aquifers that contain more sediments of terrestrial origin. Geochemical measurements on samples from four selected well cores show that arsenic has accumulated in the rocks. This indigenous arsenic is the source of arsenic in the groundwater of certain facies of the middle unit of the Keuper Sandstone. Received, June 1998 / Revised, January 1999, May 1999 / Accepted, June 1999