AQUIFER PARAMETERS FROM CONSTANT DRAWDOWN NONSTEADY-LEAKY TYPE CURVES1

ABSTRACT: A drain function and set of type curves were defined for the mathematical solution that represents one-dimensional flow under nonsteady conditions in a leaky aquifer for the constant drawdown boundary condition. A match point procedure was developed for determining the aquifer parameters transmissivity, storage coefficient, and leakance based on the drain function and type curves. Use of the procedure is illustrated by an example that utilizes simulated aquifer drawdowns and flowrate data. The drain function and type curves developed in this investigation include the effects of leakage for the constant drawdown boundary condition, which is not included in the existing drain function and type curve found in the literature. Thus, a new set of type curves was developed that can be used to analyze drawdowns for one-dimensional flow in a leaky aquifer with constant drawdown at a line sink. Applications would include flow to a canal or river, drainage of agricultural lands, and dewatering associated with strip mining operations.     

OBSERVATION WELL NETWORK DESIGN FOR PUMPING TESTS IN UNCONFINED AQUIFERS1

ABSTRACT: This paper presents a method for determining locations of observation wells to be used in conducting pumping tests in unconfined aquifers. Sensitivity coefficients, the distribution of relative errors, and the correlation coefficients between four aquifer parameters (horizontal and vertical hydraulic conductivities K_r and K_z, storage coefficient S, and specific yield S_y) are used as the criteria for the design of observation well networks and the interpretation of pumping tests. The contours of the relative errors over a vertical profile are very useful in selecting the “best” location of an observation well. Results from theoretical analyses suggest that a wide range of locations is suitable for the determination of K_r and that good locations for the determination of K_z and S may be poorly suited for the determination of S_y. Consideration must be given to the position and lengths of the pumping well screen in the selection of observation well locations. For a given location, the quality of test data can be improved by using high pumping rates and frequent sampling of drawdowns. We found that a minimum of two and preferably three observation locations are needed along a given transect. Results of the four parameters from a single well analysis may contain higher uncertainties. However, composite analyses of multiple observation wells can reduce the correlation between the four aquifer parameters, particularly between K_r and S_y, thus, improving the quality of parameter estimation. Results from two pumping tests conducted at sites located in Nebraska were examined with regard to the proposed methodology.     

A HYBRID OPTIMIZATION APPROACH TO THE ESTIMATION OF DISTRIBUTED PARAMETERS IN TWO-DIMENSIONAL CONFINED AQUIFERS1

ABSTRACT: In using non-linear optimization techniques for estimation of parameters in a distributed ground water model, the initial values of the parameters and prior information about them play important roles. In this paper, the genetic algorithm (GA) is combined with the truncated-Newton search technique to estimate groundwater parameters for a confined steady-state ground water model. Use of prior information about the parameters is shown to be important in estimating correct or near-correct values of parameters on a regional scale. The amount of prior information needed for an accurate solution is estimated by evaluation of the sensitivity of the performance function to the parameters. For the example presented here, it is experimentally demonstrated that only one piece of prior information of the least sensitive parameter is sufficient to arrive at the global or near-global optimum solution. For hydraulic head data with measurement errors, the error in the estimation of parameters increases as the standard deviation of the errors increases. Results from our experiments show that, in general, the accuracy of the estimated parameters depends on the level of noise in the hydraulic head data and the initial values used in the truncated-Newton search technique.     

DETERMINATION OF AQUIFER PARAMETERS USING REGRESSION ANALYSIS1

ABSTRACT: Transmissivity and storativity of an aquifer are usually determined by analysis of steady or nonsteady pumping test data. The classical methods of nonsteady pumping test analysis are mostly graphical in nature and are, therefore, subject to errors of judgment in curve fitting, interpolating, and reading graphs and charts. A method is described here which does not require construction of graphs or use of charts and tables. The transmissivity and the storativity are calculated using regression analysis of the nonsteady time drawdown field data. The calculations can readily be performed on a hand held calculator. The procedure is described using four examples, and the results are compared with those obtained from graphical techniques. It is shown that the method is a viable alternative to the type curve solution of Theis or Straight line solution of Jacob for nonleaky artesian aquifers. However, the regression method poses problem in the cases of leaky artesian and water table aquifers.     

BANKFULL DISCHARGE RECURRENCE INTERVALS AND REGIONAL HYDRAULIC GEOMETRY RELATIONSHIPS: PATTERNS IN THE PACIFIC NORTHWEST,USA1

ABSTRACT: The model bankfull discharge recurrence interval (annual series) (T_a) in streams has been approximated at a 1.5‐year flow event. This study tests the linkage between regional factors (climate, physiography, and ecoregion) and the frequency of bank‐full discharge events in the Pacific Northwest (PNW). Patterns of Ta were found to be significant when stratified by EPA Ecoregion. The mean value for T_a in the PNW is 1.4 years; however, when the data is stratified by ecoregion, the humid areas of western Oregon and Washington have a mean value of 1.2 years, while the dryer areas of Idaho and eastern Oregon and Washington have a mean value of 1.4 to 1.5 years. Among the four factors evaluated, vegetation association and average annual precipitation are the primary factors related to channel form and T_a. Based on the results of the T_a analyses, regional hydraulic geometry relationships of streams were developed for the PNW, which relate variables, such as bank‐full cross‐sectional area, width, depth, and velocity, to bankfull discharge and drainage area. The verification of T_a values, combined with the development of regional hydraulic geometry relationships, provides geographically relevant information that will result in more accurate estimates of hydraulic geometry variables in the PNW.     

EVALUATION OF A STREAM AQUIFER ANALYSIS TEST USING ANALYTICAL SOLUTIONS AND FIELD DATA1

ABSTRACT: Considerable advancements have been made in the development of analytical solutions for predicting the effects of pumping wells on adjacent streams and rivers. However, these solutions have not been sufficiently evaluated against field data. The objective of this research is to evaluate the predictive performance of recently proposed analytical solutions for unsteady stream depletion using field data collected during a stream/aquifer analysis test at the Tamarack State Wildlife Area in eastern Colorado. Two primary stream/aquifer interactions exist at the Tamarack site: (1) between the South Platte River and the alluvial aquifer and (2) between a backwater stream and the alluvial aquifer. A pumping test is performed next to the backwater stream channel. Drawdown measured in observation wells is matched to predictions by recently proposed analytical solutions to derive estimates of aquifer and streambed parameters. These estimates are compared to documented aquifer properties and field measured streambed conductivity. The analytical solutions are capable of estimating reasonable values of both aquifer and streambed parameters with one solution capable of simultaneously estimating delayed aquifer yield and stream flow recharge. However, for long term water management, it is reasonable to use simplified analytical solutions not concerned with early‐time delayed yield effects. For this site, changes in the water level in the stream during the test and a varying water level profile at the beginning of the pumping test influence the application of the analytical solutions.     

ASSESSMENT OF HYDRAULIC PROPERTIES IN AN UNCONFINED ALLUVIAL AQUIFER NEAR GRAND ISLAND,NEBRASKA1

ABSTRACT: A method is presented to analyze time-drawdown data from one or more observation wells for the calculation of four hydraulic parameters for unconfined aquifers: vertical hydraulic conductivity, horizontal hydraulic conductivity, storage coefficient, and specific yield. The hydraulic parameter results are further analyzed for reliability and the possible ranges of the actual parameter values. After verification using a theoretical example, the method was used to analyze pumping test data from 22 observation wells in an unconfined alluvial aquifer near Grand Island, Nebraska. Results indicate that this method can be used to efficiently calculate the four hydraulic parameters in this type of aquifer. The method can also identify the impact of measurement errors on the parameter estimates, and provide ranges of the actual parameter values. The parameter values calculated using this method were compared to those determined using other theories. It is found that this method is very useful for calculating the hydraulic properties from pumping test data and for analyzing the parameter reliability.     

ANALYSIS OF PUMPING TESTS: SIGNIFICANCE OF WELL DIAMETER,PARTIAL PENETRATION,AND NOISE1

ABSTRACT: The nonlinear least squares (NLS) method was applied to pumping and recovery aquifer test data in confined and unconfined aquifers with finite diameter and partially penetrating pumping wells, and with partially penetrating piezometers or observation wells. It was demonstrated that noiseless and moderately noisy drawdown data from observation points located less than two saturated thicknesses of the aquifer from the pumping well produced an exact or acceptable set of parameters when the diameter of the pumping well was included in the analysis. The accuracy of the estimated parameters, particularly that of specific storage, decreased with increases in the noise level in the observed drawdown data. With consideration of the well radii, the noiseless drawdown data from the pumping well in an unconfined aquifer produced good estimates of horizontal and vertical hydraulic conductivities and specific yield, but the estimated specific storage was unacceptable. When noisy data from the pumping well were used, an acceptable set of parameters was not obtained. Further experiments with noisy drawdown data in an unconfined aquifer revealed that when the well diameter was included in the analysis, hydraulic conductivity, specific yield and vertical hydraulic conductivity may be estimated rather effectively from piezometers located over a range of distances from the pumping well. Estimation of specific storage became less reliable for piezometers located at distances greater than the initial saturated thickness of the aquifer. Application of the NLS to field pumping and recovery data from a confined aquifer showed that the estimated parameters from the two tests were in good agreement only when the well diameter was included in the analysis. Without consideration of well radii, the estimated values of hydraulic conductivity from the pumping and recovery tests were off by a factor of four.     

FLOOD FREQUENCY ANALYSIS FOR SMALL WATERSHEDS IN SOUTHERN ILLINOIS1

ABSTRACT: Twenty-two gaging stations were selected for developing a regional flood frequency curve for small (area less than 2 square miles) watersheds in southern Illinois. Five probability functions were compared, and the extreme value type I function was selected to develop the regional flood curve. The curve was generated with the index flood method and also another empirical method that related the function parameters to the watershed area. Estimated peak discharges with various return periods were compared with the results obtained from multiple regression analysis.     

DEVELOPMENT OF A FRACTURED MEDIUM FLOW MODEL FOR A WASTE SITE1

ABSTRACT: A three‐dimensional fractured medium flow model was developed for the Bear Creek Valley (BCV) S‐3 site of the Oak Ridge Reservation (ORR) using SWIFT III. The numerical modeling for this site focused on a conceptual model established through the analysis of heterogeneous geologic units and matrix fracture properties of the subsurface in the BCV area. The SWIFT III modeling analysis was based on the previous modeling studies that used MODFLOW and MODPATH. A rigorous calibration was obtained first by comparing simulated results with the existing data on ground water levels and then by comparing pumping test results with the simulated ground water levels. A satisfactory agreement between observed and simulated results was obtained. The calibrated model was used to determine sustained yield from a ground water interceptor trench. Different withdrawal rates were used to simulate the performance of the trench for the sustained withdrawal of ground water.