Middle Aquifer Research Articles

Localized bedrock aquifer distribution explains discharge from a headwater catchment

Understanding a discharge hydrograph is one of the leading interests in catchment hydrology. Recent research has provided credible information on the importance of bedrock groundwater on discharge hydrographs from headwater catchments. However, intensive monitoring of bedrock groundwater is rare in mountains with steep topography. Hence, how bedrock groundwater controls discharge from a steep headwater catchment is in dispute. In this study, we conducted long‐term hydrological observations using densely located bedrock wells in a headwater catchment underlain by granitic bedrock. The catchment has steep topography affected by diastrophic activities. Results showed a fairly regionalized distribution of bedrock aquifers within a scale of tens of meters, consisting of upper, middle, and lower aquifers, instead of a gradual and continuous decline in water level from ridge to valley bottom. This was presumably attributable to the unique bedrock structure; fault lines developed in the watershed worked to form divides between the bedrock aquifers. Spatial expanse of each aquifer and the interaction among aquifers were key factors to explain gentle and considerable variations in the base flow discharge and triple‐peak discharge responses of the observed hydrograph. A simple model was developed to simulate the discharge hydrograph, which computed each of the contributions from the soil mantle groundwater, from the lower aquifer, and from the middle aquifer to the discharge. The modeling results generally succeeded in reproducing the observed hydrograph. Thus, this study demonstrated that understanding regionalized bedrock aquifer distribution is pivotal for explaining discharge hydrograph from headwater catchments that have been affected by diastrophic activities.

Extent and Distribution of Groundwater Resources in Parts of Anambra State, Southeastern, Nigeria

The extent and distribution of groundwater resources in parts of Anambra State, Nigeria has been investigated. The results show that the study area is directly underlain by four different geological formations including, Alluvial Plain Sands, Ogwashi-Asaba Formation, Ameki/Nanka Sands and Imo Shale, with varying water storage and yielding capacities. Borehole depths within the Alluvial Plain Sands are shallow (5-30m) yet the sands are excellent aquifers with high yields (3-5 litres/sec) especially along the Anambra West – Onitsha - Ogbaru L.G.A. axis. Elsewhere the yield is low (about 0.5litres/sec) and may dry up at peak dry season periods. The Ogwashi-Asaba Formation occurs in a north-southerly trend and underlies Ekwusigo, Nnewi North and South and Ihiala Local Government Areas. This formation consists of multiple aquifers and a depth to water table ranging from 50 to 110m. Within it, transmissivity values of 37.54 to 95.5m2/day and a yield of up to 5litres/sec were recorded. The Ameki/Nanka Sands is a prolific water producer and underlies Aguata, Anaocha, Njikoka, Dunukofia, Oyi and Anambra East Local Government Areas. Four aquifer horizons were identified within this formation, designated; shallow, upper, middle and deep aquifers. The most exploited are the upper and middle aquifers, while the least, but most prolific is the deep aquifer with an average yield of 5litres/sec. Imo Shale, because of its sedimentological nature is a poor aquifer. The gravelly intercalations within this formation are usually too thin to sustain continuous water pumping. This study indicates that the extent and distribution of groundwater within the study area is controlled predominantly by lithology and other secondary factors including topography and nearness to source of recharge.

Hydrogeochemical and Flow Modeling of Aquitard Percolation in the Cariri Valley-Northeast Brazil

Flow modeling and hydrogeochemical inversion calculations were performed on the aquifer systems of the Crato-Juazeiro Graben in the Araripe Sedimentary Basin of the Cariri Valley, Northeast Brazil, in order to assess aquifer interconnections through an aquitard of the Araripe Plateau. Steady state hydraulic head numeric calculations with the finite difference method used Modflow software with hydrologically based boundary conditions and a piezometric surface as initial condition. Hydrogeochemical inversion with PHREEQC was performed for two water samples representing typical initial and final waters along the flow path. Flow simulation shows that approximately 20% of all recharge water from the superior aquifer system passes through the Santana aquitard reaching the middle aquifer system. Quantification of mineral dissolution/precipitation processes, ion exchange and microbiological redox reactions taking place in the aquifer systems also indicates hydraulic interconnection of the superior and middle aquifer systems through the Santana aquitard.

Groundwater Deterioration of Shallow Groundwater Aquifers Due to Overexploitation in Northeast Jordan

AbstractGroundwater is the major water resource in Jordan and most of the groundwater basins are already exploited beyond their estimated safe yield. Azraq basin is one of the most important groundwater basins in Jordan, which supplies Amman with drinking water. However, due to overpumping from the shallow groundwater aquifers, the water level dropped dramatically and signs of salinization and depletion are starting to occur. The severe drawdown in the Azraq well‐field caused a reverse in the hydraulic gradient and consequently, the saltwater in the center of the basin (Qa‐Azraq) started to move in the direction of the well‐field. The salinization in the shallow aquifer (basalt/B5/B4) is believed to result from one of the following scenarios: (i) a reverse flow from Sabkha to the AWSA well field, (ii) an upward leakage from the middle aquifer system (B2/A7) and the combined B3 Aquitard‐B2/A7 aquifer, (iii) a dissolution process between the water and rock matrix due to lowering of the dynamic water levels during pumping which reached the mineralized formations underlying the Basalt. The salinization trend of some AWSA wells represented by the gradual increase of major ions is associated with rather constant stable isotopic contents. This indicates that these constituents originate from the main minerals existing in the matrix of the aquifers and thus this scenario is the most likely to occur.

Recharge origin, overexploitation, and sustainability of water resources in an arid area from Azraq basin, Jordan: case study

The Azraq groundwater basin, found in Jordan, is an important resource for available water to the public. Its magnitude and value has been decreasing for the last two decades. The groundwater level has been declining on a yearly basis since the 1980s, and over-pumping, which has tripled since 1983, has led to the drying up of major springs in the 1990s. Overexploitation is considered as the adverse effect on groundwater mass balance, where the abstraction increases yearly and beyond the safe yield. Over-pumping and irrigation activities cause groundwater to become more saline. The isotopic composition of groundwater is divided into two groups. The first group is associated with EMWL, and the second with GMWL. The recharge origin of the first group, which includes the upper and middle aquifers, originate from outside the Azraq basin with a land elevation higher than 1150 m asl. The groundwater of the second group is considered palaeowater. No major recharge component is identified in the basin. Accurate and reliable scientific approaches are indispensable to better understand, plan and manage the groundwater resources.

Ground water discharge and nitrate flux to the Gulf of Mexico.

Ground water samples (37 to 186 m depth) from Baldwin County, Alabama, are used to define the hydrogeology of Gulf coastal aquifers and calculate the subsurface discharge of nutrients to the Gulf of Mexico. The ground water flow and nitrate flux have been determined by linking ground water concentrations to 3H/3He and 4He age dates. The middle aquifer (A2) is an active flow system characterized by postnuclear tritium levels, moderate vertical velocities, and high nitrate concentrations. Ground water discharge could be an unaccounted source for nutrients in the coastal oceans. The aquifers annually discharge 1.1 +/- 0.01 x 10(8) moles of nitrate to the Gulf of Mexico, or 50% and 0.8% of the annual contributions from the Mobile-Alabama River System and the Mississippi River System, respectively. In southern Baldwin County, south of Loxley, increasing reliance on ground water in the deeper A3 aquifer requires accurate estimates of safe ground water withdrawal. This aquifer, partially confined by Pliocene clay above and Pensacola Clay below, is tritium dead and contains elevated 4He concentrations with no nitrate and estimated ground water ages from 100 to 7000 years. The isotopic composition and concentration of natural gas diffusing from the Pensacola Clay into the A3 aquifer aids in defining the deep ground water discharge. The highest 4He and CH4 concentrations are found only in the deepest sample (Gulf State Park), indicating that ground water flow into the Gulf of Mexico suppresses the natural gas plume. Using the shape of the CH4-He plume and the accumulation of 4He rate (2.2 +/- 0.8 microcc/kg/1000 years), we estimate the natural submarine discharge and the replenishment rate for the A3 aquifer.

Assessment of Groundwater Vulnerability to Contamination by Organochlorine Pesticides in the Area of Caucasian Mineral Water

The procedure of evaluating pesticide travel time through the aeration zone to the groundwater table is presented. It is found that the time of pollutant travel through rocks of the aeration zone depends on the thickness and composition of rocks of this zone, the porosity of rocks, distribution factor, volumetric humidity, and the infiltration recharge. As shown, the most vulnerable are the valleys of principal rivers of the area of Caucasian Mineral Water, where the Recent and Middle Quaternary aquifers confined to these valleys are intensely used for domestic and drinking needs.

Two exact solutions for a cylindrical inhomogeneity in a multi-aquifer system

Two new exact solutions are presented for steady-state, uniform flow through a cylindrical inhomogeneity in a multi-aquifer system. The solutions are valid for an arbitrary number of aquifers, and all aquifer properties may change from outside to inside the cylinder. The first problem concerns flow in a confined aquifer. The second problem is similar to the first problem, but now the cylinder is bounded on top by a lake with a semi-permeable bottom and a fixed water level; a constant amount of water may be extracted from the lake. The equations are derived in a radial coordinate system and make use of the known solution for uniform flow through a cylindrical inhomogeneity in a single aquifer, and the general theory for flow in leaky, multi-aquifer systems. Separate equations are presented for the head, flow, and leakage inside and outside the cylinder; a complete set of linear equations is presented to solve for the coefficients in the solutions. For each solution a specific example is presented for flow in a system with three aquifers. Rather complicated leakage patterns are obtained when inside the cylinder the transmissivity of the top aquifer is increased, the transimissivity of the middle aquifer is decreased, and the transmissivity of the bottom aquifer is kept constant.

Development of groundwater modeling for the Azraq Basin, Jordan

The three-dimensional groundwater flow model MODFLOW was applied to simulate water level change in the complex multi-aquifer systems (the Upper and Middle Aquifers) of the Azraq basin. The model was calibrated by matching observed and simulated drawdown for steady and transient states over the period 1970–1992. Drawdown data for the period 1993–1997 were used to test the model's ability to predict the response of the aquifers. The model performed well in representing the water level contours of the Upper and Middle Aquifers for steady state calibration. Agreement between the observed and simulated drawdowns was obtained for transient state calibration. To predict the aquifer system responses for the period of 1997–2025, four different pumping schemes (scenarios) have been investigated. The first scenario (present pumping rate) reveals that there will be approximately a 25 m drop in the water level at the well-field area in 2025. However, the worst scenario (pumping rate at 1.5 times the present rate) reveals an approximate 39 m drop in the water level at the well-field area in 2025. The safe yield for the Upper Aquifer System was found to be about 25 million cubic meters (MCM) yearly.

DC resistivity and induced polarisation investigations at a waste disposal site and its environments

We report the results from some geoelectrical surveys carried out to monitor the spread of contamination in underlying aquifers due to a landfill site. The geophysically determined electrical properties of the aquifers have subsequently been used to estimate hydraulic conductivities which are required for modelling contaminant transport. The type of waste deposited and the influence of the geological environment were the crucial factors investigated, employing the DC resistivity and time domain induced polarisation (IP) methods. The landfill was mainly a liquid disposal site with existing borehole information showing that the waste contained high concentrations of not only inorganic material (chlorides, sulphates) but also organic matter (indicated by high values of chemical oxygen demand (COD) and total organic carbon (TOC)). The measured fluid resistivities ρ w from the surrounding boreholes showed values as low as 0.25 Ω m, leading us to expect low bulk resistivities ρ o. The aquifer system in the study area, with an eastwards regional groundwater flow, consists of three sand aquifers with intervening semi-pervious clay aquitards. Clay particles are also present in the sand formations and expected to influence the overall bulk resistivity and chargeability. The presence of organic waste is another factor suggesting that the IP method could be employed as a diagnostic tool. Our resistivity measurements along the survey lines perpendicular to the groundwater flow show systematic reductions of resistivities relative to the control line, the effect decreasing progressively eastwards from the landfill. The resistivities of these contaminated sections were higher than expected and one possible explanation for this could be the presence of the organic waste. However, an alternative explanation could be the low porosities in the sand formations. Low porosity may imply reduced fluid content, and therefore increased bulk resistivities, as well as lower hydraulic conductivity values. Field and core hydraulic conductivity measurements in the area from previous investigations had indicated the relatively low hydraulic conductivity values of 9.3×10 −7 and 6.2×10 −6 m/s for the top and middle aquifers respectively, thus, favouring the low porosity hypothesis. The IP measurements showed high apparent chargeability values (80–120 ms) on top of the landfill, possibly due to the presence of disseminated solid metallic waste or the high organic load of the liquid waste disposed. The IP line parallel to the groundwater flow direction, and close to the landfill, produced chargeability anomalies which may be associated with a plume of organic waste. No chargeability anomalies are observed on the second IP line, further away from the landfill and in the SE direction. The bulk resistivities ρ o obtained from the resistivity inversions and the fluid resistivities ρ w, from adjacent boreholes, allowed hydraulic conductivities to be estimated. The intrinsic formation factor was first determined from ρ o and ρ w and was then used in conjunction with a range of Archie's parameters appropriate for sands to evaluate porosity and its likely bounds of error. The hydraulic conductivities obtained through the Kozeny–Carmen–Bear equation, for the geophysically determined range of porosities, give plausible values agreeing within an order of magnitude with each other and with reported values for the formation.

Simulated effects of development on regional ground-water/surface-water interactions in the northern Coastal Plain of New Jersey

Stream flow in the Coastal Plain of New Jersey is primarily controlled by ground-water discharge. Ground-water flow in a 400 square mile area (1035 km 2) of the Potomac-Raritan-Magothy aquifer system (PRMA) in the northern Coastal Plain of New Jersey was simulated to examine development effects on water resources. Simulations showed that historical development caused significant capture of regional ground-water discharge to streams and wetlands. The Cretaceous PRMA primarily is composed of fine to coarse sand, clays and silts which form the Upper and Middle aquifers and their confining units. The aquifer outcrops are the principal areas of recharge and discharge for the regional flow system and have many traversing streams and surface-water bodies. A quasi-three-dimensional numerical model that incorporated ground-water/surface-water interactions and boundary flows from a larger regional model was used to represent the PRMA. To evaluate the influence of ground-water development on interactions in different areas, hydrogeologically similar and contiguous model stream cells were aggregated as ‘stream zones’. The model representation of surface-water and ground-water interaction was limited in the areas of confining unit outcrops and because of this, simulated ground-water discharge could not be directly compared with base flow. Significant differences in simulated ground-water and surface-water interactions between the predevelopment and developed system, include; (1) redistribution of recharge and discharge areas; (2) reduced ground-water discharge to streams. In predevelopment, the primary discharge for the Upper and Middle aquifers is to low-lying streams and wetlands; in the developed system, the primary discharge is to ground-water withdrawals. Development reduces simulated ground-water discharge to streams in the Upper Aquifer from 61.4 to 10% of the Upper Aquifer hydrologic budget (28.9%, if impounded stream flow is included). Ground-water discharge to streams in the Middle Aquifer decreases from 80.0 to 22% of the Middle Aquifer hydrologic budget. The utility of assessing ground-water/surface-water interaction in a regional hydrogeologic system by simulation responses to development is demonstrated and which can compensate for lack of long-term stream-gaging data in determining management decisions.

Identifying Flow Intervals by Pore Geometry and Petrophysics: Middle Trinity Aquifer (Lower Cretaceous Limestone)

A core from the Lower Cretaceous middle Trinity aquifer of southern Kendall County, Texas, was analyzed to compare its petrographic and petrophysical properties to its hydraulic flow characteristics. The analyses helped determine flow intervals in the core. Flow intervals are defined as vertical zones having similar geologic (depositional and diagenetic) history, petrophysical properties, and hydraulic conductivity characteristics. The information gathered from the study of a single core may be integrated with similar data from other cores or well logs to define 'flow units.' Methodology involved visual and image analysis examination of thin sections and epoxy pore casts of samples to evaluate pore geometry (pore size, pore throat size, pore interconnectivity, and pore throat size heterogeneity). Samples were thus grouped into pore-type families. Representative samples from each pore-type family were tested for air and brine permeability, porosity, and mercury injection capillary pressure. Pore throat size distributions obtained from the capillary pressure analyses were compared to the data derived from the pore casts and thin sections. Pore geometry evaluated correlated well with mercury capillary pressure data for the most permeable zone of the core. Geologically, this flow interval consists of a porous, medium-grained peloid grainstone that is characterized by large pores connectedmore » by large pore throats. Wherever present, such pore throat to pore body size ratios enhance flow through the rock. Correlation between pore geometry and capillary pressure data deteriorate in packstones with increasing micrite mud content and wackestones with increasing dolomitization. These lower correlations are due to decreasing pore throat size and increasing pore throat size heterogeneity in these rocks.« less

LEAD AND CADMIUM ASSOCIATED WITH SALTWATER INTRUSION IN A NEW JERSEY AQUIFER SYSTEM1

ABSTRACT: The U.S. Geological Survey collected ground‐water samples from the upper and middle aquifers of the Potomac‐Raritan‐Magothy aquifer system in a 400‐square‐mile area of New Jersey from 1984 through 1986. Concentrations of lead were greater than the U.S. Environmental Protection Agency maximum contaminant level (MCL) of 50 micrograms per liter in water from 16 of 239 wells. The cencentrations of cadmium were greater than the MCL of 10 micrograms per liter in water from 10 to 241 wells. One‐half of the wells that exceeded the lead MCL were in known areas of saltwater intrusion, as were all 10 wells that exceeded the cadmium MCL. The association of elevated concentrations of these metals with elevated concentrations of chloride indicates a mochanism related to saltwater intrusion.

Potential use of the deep aquifers in the Negev Desert, Israel — A conceptual model

The deep aquifers (Jurassic-Paleozoic, 1–5 km deep) underlying the Negev Desert, Israel, contain brackish to saline water (800–120,000 ppm Cl −) and have been penetrated by only 47 wildcat oil drillings. The objective of this study was to evaluate the water potential of these aquifers for irrigation and for industrial consumption as well as their potential for serving as host rocks for hazardous wastes. We developed a special methodology for constructing a conceptual model for the thick sequence of layers in which these aquifers are found. It seems that, although these aquifers have different lithologies, they may be hydraulically connected. Statistics of the distribution of the aquifer system's parameters of state and analysis of their variance are used to aggregate individual formations into a few large hydrologic units. We employed regression analysis to supplement missing data and to evaluate estimation errors. Based on this methodology, a model consisting of three aquifers was defined. The consistency of the conclusions of the model is confirmed by our studies of pressure, permeability, salinity, temperature, and isotopic distribution. Wherever data were scarce, we estimated hydraulic parameters by linear regression. According to our study, the lower aquifer in the northern Negev contains only brines and is hydrologically separated from the two aquifers above it. The lower aquifer could, therefore, be considered as a possible site for toxic waste disposal or, at suitable locations, for hydrocarbon exploration. Hydrological continuity probably exists between the upper and middle aquifers in the eastern and central Negev, where brackish water is found in both aquifers. The methodology used here is suitable for constructing a conceptual model of any complex geohydrologic system in an area with scarce data.

Diagenetic Modification of Knox Evaporative-Dolomite Geochemistry by Middle Ordovician Paleoaquifer/Burial Fluids: ABSTRACT

Early Ordovician upper Knox cyclic carbonates (100,000-year periods) were deposited on a ramp in a semiarid setting. Inner ramp facies are completely dolomitized, whereas only cycle caps on the outer ramp are dolomite. Laminite caps on leading edges of prograding flats were dolomitized by evaporative, storm-recharged marine waters that precipitated CoCO/sub 3/ and minor amounts of CaSO/sub 4/. Subtidal facies beneath interiors of supratidal flats (up to 200 km wide)were dolomitized by shallow subsurface brines as the flats prograded. Inner ramp areas existed as supretidal flats for as long as 100,000 years, hence only there are sequences completely dolomitized. Tidal-flat dolomites consist of euhedral to subhedral fabric preservation mosaics (crystals 5-50 ..mu..m, and up to 300 ..mu..m in replaced pelletal layers). Dolomitized subtidal sediments are composed of 20-200 ..mu..m dolomite crystals. Most of the early dolomites are fabric retentive initially, becoming more fabric destructive with increasing exposure to dolomitizing fluids. Fe values are up to 2000 ppm, and Mn values are up to 500 ppm. delta/sup 18/O values show strong geographic control, with samples depleted (up to - 7 per thousand) relative to pristine evaporative dolomites derived from concentrated Ordovician seawater. Burial dolomites are enriched in Fe (up to 14,000more » ppm) and depleted in delta/sup 18/O, suggesting precipitation from basinal brines at elevated temperatures. Geochemical data suggest that evaporative dolomites have undergone modification by isotopically lighter fluids. Alteration most likely occurred within a Middle Ordovician meteoric aquifer system or less likely by later Paleozoic basinal brines. Regional geologic data for Knox Group carbonates throughout the eastern US are vital in constraining dolomitization models.« less

Distribution of Volatile Organic Compounds in a New Jersey Coastal Plain Aquifer System

ABSTRACTSamples for analysis of volatile organic compounds were collected from 315 wells in the Potomac‐Raritan‐Magothy aquifer system in southwestern New Jersey and a small adjacent area in Pennsylvania during 1980–82. Volatile organic compounds were detected in all three aquifer units of the Potomac‐Raritan‐Magothy aquifer system in the study area. Most of the contamination appears to be confined to the outcrop area at present. Low levels of contamination, however, were found downdip of the outcrop area in the upper and middle aquifers.Trichloroethylene, tetrachloroethylene, and benzene were the most frequently detected compounds. Differences in the areal distributions of light chlorinated hydrocarbons, such as trichloroethylene, and aromatic hydrocarbons, such as benzene, were noted and are probably due to differences in the uses of the compounds and the distribution patterns of potential contamination sources.The distribution patterns of volatile organic compounds differed greatly among the three aquifer units. The upper aquifer, which crops out mostly in less‐developed areas, had the lowest percentage of wells with volatile organic compounds detected (10 percent of wells sampled). The concentrations in most wells in the upper aquifer which had detectable levels were less than 10 /μg/1. In the middle aquifer, which crops out beneath much of the urban and industrial area adjacent to the Delaware River, detectable levels of volatile organic compounds were found in 22 percent of wells sampled, and several wells contained concentrations above 100 μ/1. The lower aquifer, which is confined beneath much of the outcrop area of the aquifer system, had the highest percentage of wells (28 percent) with detectable levels. This is probably due to (1) vertical leakage of contamination from the middle aquifer, and (2) the high percentage of wells tapping the lower aquifer in the most heavily developed areas of the outcrop.

Simulation of Ground‐Water Flow in a Mined Watershed in Eastern Ohioa

ABSTRACTA 43‐acre watershed in Muskingum County, Ohio, was studied to determine the hydrologic consequences of strip mining for coal. A quantitative description of the effects on the ground‐water flow components of the hydrologic system has been obtained using digital models.The premining watershed was characterized by nearly flat‐lying sedimentary rocks of the Pennsylvanian System. Underclay beneath the two major coal beds formed bases for perched zones, creating three separate aquifers. Recharge to the ground‐water system occurred mainly by percolation of precipitation. Most of the discharge from the top and middle aquifers occurred by downward leakage to the underlying aquifers. A smaller amount of discharge occurred as springflow or streamflow near the intersections of the underclays and land surface.Mining has destroyed the top aquifer, and has replaced the bedrock by spoil material. Water levels in the spoils are at a much lower altitude than existed in the premining top aquifer because of a combination of (1) a larger hydraulic conductivity, (2) areal variations of the hydraulic characteristics of the confining bed, and (3) a slower rate of recharge from precipitation caused by removal of vegetation and soil compaction. Covering of previously exposed portions of the middle aquifer and a lower hydraulic head in the spoils has allowed less recharge to the middle aquifer. Additional flow is induced across the western boundary of the watershed and has reduced the outflow across the eastern boundary. Discharge from the middle aquifer as downward leakage and streamflow is less than before mining.

An investigation of recharge to the Namoi Valley aquifers using environmental isotopes

Environmental tritium and radiocarbon were used to study aspects of groundwater origin, transport and mixing in the complex aquifer system underlying the Namoi Valley, N.S.W. A substantial proportion of the water in the upper aquifer (depth less than 25 m) percolated underground in the post-nuclear period (after 1955). The observed relationship between �13C value and distance is evidence for riverwater recharge in the upper aquifer. There is evidence that water in the middle aquifer system (depth 50-75 m) percolated underground about 600 years ago. Finally, it is suggested that water from the Great Artesian Basin leaks into the deeper water of the lower aquifer system.

Electric Analog Computers and Hydrogeologic System Analysis in Illinoisa

AbstractElectric analog computers are playing an important role in the forecast of consequences of developing nonhomogeneous aquifers in Illinois having highly irregular shapes and boundaries and a wide variety of head and discharge controls. Analog computers are versatile and simple equipment, of low to moderate cost, with which ground‐water development schemes can be rapidly and accurately tested and the relative merits of alternate choices of development can be appraised.The electric analog computer used by the Illinois State Water Survey consists of an analog model and excitation‐response apparatus, i.e., waveform generator, pulse generator and oscilloscope. Analog models are regular arrays of resistors and capacitors and are scaled‐down versions of aquifers and confining beds where present. Resistors are inversely proportional to the coefficients of transmissibility and vertical permeability aquifers and the coefficients of leakage of confining beds. Capacitors store electrostatic energy in a manner analogous to the storage of water in the aquifer. The behavior of the electrical network is described by an equation which has the same form as the finite‐difference equation for nonsteady state two or three‐dimensional flow of ground water. Electrical units (voltage, coulombs, amperes and seconds) and corresponding hydraulic units (feet, gallons, gallons per day and days) are connected by 4 scale factors.Excitation‐response equipment force electrical energy in the proper time phase into the analog model and measure energy levels within the energy‐dissipative resistor‐capacitor network. Oscilloscope traces, i.e., time‐voltage graphs, are analogous to time‐drawdown or time‐recovery graphs and describe drawdown or recovery conditions after a step function‐type change in discharge. A catalog of time‐voltage graphs provides data for construction of a series of water‐level change maps. Close agreement of water‐level declines determined with an analog computer and analytical methods for 3 selected idealized aquifer situations is noted with satisfaction.An electric analog model for the East St. Louis area was assembled. The sand and gravel aquifer in the East St. Louis area has an average thickness of 120 feet, an average width of 7 miles and an average length of 30 miles. The analog model for the aquifer consists of a regular array of 2800 resistors and 1350 capacitors. The scale of the model is 1 inch equals 2000 feet. Values of resistors range in magnitude from 2.2 megohms to 33,000 ohms; capacitors are 2500 micromicrofarads. The effects of the Mississippi River were simulated by terminating the portion of the electrical network along the river in an open circuit. Resistors large in magnitude were connected to terminals along the edge of the aquifer and to ground to simulate small amounts of sub‐surface flow through bluffs. The accuracy and reliability of the electric analog computer was established by comparing past records of water‐level declines and values of water‐level declines determined with the analog computer. The electric analog computer was used to evaluate the practical sustained yields of existing pumping centers, to predict the effects of a selected scheme of development and to deter‐mine the potential yield of the aquifer under assumed pump‐ing conditions.An electric analog model for the Champaign‐Urbana area was assembled. The sand and gravel aquifers in the Champaign‐Urbana area are contained in the Mahomet buried bedrock valley which extends across the central part of Illinois from the Indiana border to the Illinois River Valley. The Mahomet buried bedrock valley averages about 12 miles wide in the Champaign‐Urbana area and is largely filled with glacial drift ranging in thickness from 50 to 440 feet. Sand and gravel are encountered within the glac‐ial drift at depths between 60 and 120 feet (upper aquifer), 140 and 170 feet (middle aquifer) and below a depth of 200 feet (lower aquifer). The upper aquifer is thin and discontinuous; the middle aquifer has an average thickness of 43 feet; and the lower aquifer often exceeds 100 feet thick. Till averaging 120 feet thick (upper confining bed) overlies the middle aquifer; about 30 feet of till (lower confining bed) separates the middle and lower aquifers. The analog model for the complex aquifer system consists of a regular array of 7500 resistors and capacitors. The scale of the model is 1 inch equals 1 mile. Values of resistors range from 1 to 68,000 ohms; values of capacitors range from 10e6 to lo‐' farads. The analog model consists of 2 horizontal arrays of resis‐tors and capacitors (lower and middle aquifers) and 1 hori‐zontal ground wire array (water table) interconnected by two vertical arrays of resistors (lower and upper confining beds). The accuracy and reliability of the electric analog computer are being assessed. The computer will be used to predict the effects of future ground‐water development and the practical sustained yields of existing pumping centers in the Champaign‐Urbana area.

GROUND WATER IN PALEOZOIC FORMATIONS OF THE NORTHERN SHILOVO-VLADIMIR DEPRESSION

Drilling in the Shilovo-Vladimir depression has demonstrated that Schwagerina dolomites and limestones are overlain by Tatarian marls; therefore, Kazanian sediments are absent in this region and cannot adversely affect ground water from upper Carboniferous aquifers. In rocks of the Kazanian stage, occurring in the northeastern part of the depression, ground water is highly mineralized. Ground water in upper Paleozoic formations flows from the eastern and western flanks of the northern part of the depression; and, from the Gus-Khrustalnyy region flows south to north. Ground water mineralization in the Schwagerina dolomites and limestones is 2.8 grams per liter; from middle Carboniferous aquifers, 16.5 grams per liter.--Research International.