Gary S. Johnson

Evaluation of drawdown curves derived from multiple well aquifer tests in heterogeneous environments

Aquifer coefficients derived from nonsteady-state, multiple well, aquifer tests in laterally heterogeneous environments often have uncertain meaning. Drawdown at observation wells reflects the removal of water from storage in the aquifer and transient refraction of ground water pathlines during the evolution of a non-symmetrical cone of depression. These effects are masked within observation well drawdown data such that “good” Theis (1935) type curve matches often result. Transmissivity and storativity values derived from independent drawdown curves plotted as drawdown versus time (t) or drawdown versus time/distance2 (t/r2) usually differ from observation well to observation well. These aquifer coefficients often are considered to represent some type of average of the materials between and/or about the pumping well and the observation wells. Simulations of two multiple well aquifer tests with simple, arbitrary distributions of block heterogeneities suggest that transmissivity (T) and storativity values derived from independent drawdown curves by the Theis (1935) method generally increase with distance from the pumping well. This apparent scale effect is related to the force-fitting of earlytime drawdown data to the steep portion of the Theis type curve without sufficient late-time drawdown data to constrain vertical shifting of the drawdown data relative to the type curve.Log-log plots of drawdown versus t/r2 for multiple well aquifer tests form families of curves that are characteristic of the distribution of observation wells and the degree of heterogeneity within the cone of depression. Separation between discrete drawdown curves within a family provides a qualitative measure of the degree of heterogeneity within the cone of depression. All of the drawdown curves within a family converge on a single curve at large values of t/r2. A composite analysis of all of the drawdown data within the family yields an estimate of the average T within the cone of depression. Analysis of discrete drawdown curves as integral members of the family of curves provides a means to constrain type curve matches and minimizes force-fitting if drawdown data are defined for large values of t/r2 for at least one well. The constrained type curve matches provide more reasonable estimates for T near individual observation wells than analysis of drawdown curves independently.

Forecast of Natural Aquifer Discharge Using a Data-Driven, Statistical Approach

In the Western United States, demand for water is often out of balance with limited water supplies. This has led to extensive water rights conflict and litigation. A tool that can reliably forecast natural aquifer discharge months ahead of peak water demand could help water practitioners and managers by providing advanced knowledge of potential water-right mitigation requirements. The timing and magnitude of natural aquifer discharge from the Eastern Snake Plain Aquifer (ESPA) in southern Idaho is accurately forecast 4 months ahead of the peak water demand, which occurs annually in July. An ARIMA time-series model with exogenous predictors (ARIMAX model) was used to develop the forecast. The ARIMAX model fit to a set of training data was assessed using Akaikes information criterion to select the optimal model that forecasts aquifer discharge, given the previous years discharge and values of the predictor variables. Model performance was assessed by application of the model to a validation subset of data. The Nash-Sutcliffe efficiency for model predictions made on the validation set was 0.57. The predictor variables used in our forecast represent the major recharge and discharge components of the ESPA water budget, including variables that reflect overall water supply and important aspects of water administration and management. Coefficients of variation on the regression coefficients for streamflow and irrigation diversions were all much less than 0.5, indicating that these variables are strong predictors. The model with the highest AIC weight included streamflow, two irrigation diversion variables, and storage.

SIMULATION OF ELECTRICAL POTENTIAL DIFFERENCES NEAR A CONTAMINANT PLUME EXCITED BY A POINT SOURCE OF CURRENT

Finite-difference simulations of electrical excitation of conductive contaminant plumes indicated that approximate dimensions of a plume and the approximate location of its center of mass can be derived, under specified circumstances, from the resulting electrical potential fields. Direct electrical excitation of a contaminant plume by a point current source was simulated for homogenous and isotropic conditions as well as in the presence of conductive clay layers and lenses. When a very shallow water table was assumed, changes in the electrical potential field between baseline (preplume) conditions and conditions that included a developing plume graphically formed a difference dipole. Simulations suggested that electrical flow is channeled preferentially through the negative difference pole at the approximate location of the center of mass in a dispersive contaminant plume. Electrical flow was channeled directly through the negative difference pole at the terminal end of a conductive clay lens. Simulations showed that even in the presence of conductive clays, the approximate location of the center of mass of an evolving contaminant plume could be delineated. This illustrates the potential future value of this approach, assuming continued technological advances in the field.

Forecasting Natural Aquifer Discharge Using a Numerical Model and Convolution

If the nature of groundwater sources and sinks can be determined or predicted, the data can be used to forecast natural aquifer discharge. We present a procedure to forecast the relative contribution of individual aquifer sources and sinks to natural aquifer discharge. Using these individual aquifer recharge components, along with observed aquifer heads for each January, we generate a 1-year, monthly spring discharge forecast for the upcoming year with an existing numerical model and convolution. The results indicate that a forecast of natural aquifer discharge can be developed using only the dominant aquifer recharge sources combined with the effects of aquifer heads (initial conditions) at the time the forecast is generated. We also estimate how our forecast will perform in the future using a jackknife procedure, which indicates that the future performance of the forecast is good (Nash-Sutcliffe efficiency of 0.81). We develop a forecast and demonstrate important features of the procedure by presenting an application to the Eastern Snake Plain Aquifer in southern Idaho.

Quantifying Ground-Water Pumping Impacts on Surface Water in Idaho

Idaho has embraced the concept of conjunctive management and the administration of surface and ground water rights in a common priority system. The means for quantifying impacts of ground water use on surface water resources is currently being developed. The process is complicated by the widespread effects of ground water use and the high degree of temporal attenuation of effects. One possible quantification method relies on the development and application of response functions. Response functions describing the temporal variation in river gains and losses resulting from ground water use have been developed from a numerical ground water flow model of the Snake River Plain aquifer. Response functions describing impact on each of several reaches of the Snake River are being aggregated to form zones throughout the aquifer. The response functions are proving to be useful educational tools for water users and managers and will be instrumental in the development of plans for mitigating ground-water pumping impacts on senior surface water users.