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An automated base flow separation technique has been developed and tested. Base flow is considered to be the ground-water contribution to stream flow. Estimates of the amount of base flow can be derived from stream flow records. Such estimates are critical in the assessment of low flow characteristics of streams for use in water supply, water management, and pollution assessment. An automated base flow separation technique using a digital filter has been tested against three other automated techniques and manual separation methods. The filter appears to be comparable to other automated techniques in its ability to reproduce the results produced from graphical separation techniques. The filter technique is easy to use and has the added advantage in that it can be adjusted by the user to take into account personnel preferences in separation of stream flow into surface flow and base flow. The slope of the base flow recession has been used to estimate the volume of water in storage in the basin above the level of the stream channel, the amount of recharge to the shallow aquifer, and as an input into water budget models. A second automated technique was developed to calculate the slope of the base flow recession curve from stream flow record. This technique is an adaptation of the Master Recession Curve procedure. The results of this method were compared to manual estimates with an efficiency of 74 percent

Inverse models using, for example, nonlinear least-squares regression, provide capabilities that help modelers take full advantage of the insight available from ground-water models. However, lack of information about the requirements and benefits of inverse models is an obstacle to their widespread use. This paper presents a simple ground-water flow problem to illustrate the requirements and benefits of the nonlinear least-squares regression method of inverse modeling and discusses how these attributes apply to field problems. The benefits of inverse modeling include: (1) expedited determination of best fit parameter values: (2) quantification of the (a) quality of calibration, (b) data shortcomings and needs, and confidence limits on parameter estimates and predictions; and (3) identification of issues that are easily overlooked during nonautomated calibration

The Regional Municipality of Waterloo in Ontario, Canada (population 250,000) depends on ground water for most of its water supply. The ground water is extracted from the Waterloo Moraine, an extensive and complex glacial aquifer system extending over a 400 km2 area. A methodology is being developed to inventory the ground water resource, to define its susceptibility to contamination, and to create the basis for optimal management and protection strategies. A key component of this methodology is a three-dimensional conceptual hydrogeologic model based on the geologic characteristics of the multiple-aquifer Moraine system. The steps in the development of the model include screening of the large database, interpretation and interpolation of the data to define the variable hydrostatigraphy and to generate consistent hydraulic conductivity functions, and model calibration. The numerical basis is a fully three-dimensional finite element model that provides flexibility and adaptability in representing the natural boundaries, the highly irregular stratigraphy, and the numerous wellfields. The model has the capability to automatically find the location of the water table consistent with given recharge and pumping conditions, and to direct recharge from low-permeability areas to higher-permeability areas. Capture zones generated by the model are found to he highly sensitive with respect to the geologic structure, in particular the presence or absence of windows in the aquitard units. Professional judgment is found to be an essential component of the modeling process

Phosphate distribution was reviewed in 10 mature, highly monitored septic system ground water plumes in central Canada. It was shown that six plumes (primarily those on calcareous sands) of enriched P concentrations (0.5 to 5 mg/L P) exceeding 10 m in length are present. In each case, phosphate migration velocity is highly retarded (retardation factor, 20 to 100) compared to the ground water velocity, but migration rates remain sufficiently fast (approximately 1 m/a) to be of concern when considering long-term operation and the normal setback distance of septic systems from adjacent surface water bodies (approximately 15 m). Much smaller scale phosphate plumes ( 3 m in length) are present at the acidic sites on noncalcareous sands and on silt- and clay-rich sediments. At all of the sites, ground water concentrations are lower than effluent values by amounts ranging from 23 to 99%, suggesting that P accumulation has occurred in toe vadose zone. This was confirmed by sediment analyses at four of the sites which, in each case, showed that zones of P enrichment were present within 1 m of the infiltration pipes (Wood 1993; Zanini et al. 1998). Also, observed phosphate concentrations are generally consistent with values expected based on the solubility constraints of the minerals vivianite in reducing zones (including the septic tank), and strengite and variscite in oxidizing zones, providing further evidence that mineral precipitation reactions play a role in limiting P concentrations. Strengite and variscite have the potential to limit P to low concentrations (0.1 mg/L) under acidic conditions, but oxidation of sewage effluent leads to acidic conditions only in noncalcareous terrain or beneath old septic systems where calcium carbonate has been depleted. Overall, phosphate plume migration velocities in ground water appear to be controlled by sorption processes, but the phosphate concentrations that are present in the plumes appear to be strongly controlled by mineral

The presence of dense nonaqueous phase liquids is one of the principal problems associated with current ground-water remediation efforts. Standard pump-and-treat methods are ineffective largely because of the low aqueous solubilities of DNAPL components. Surfactants can increase DNAPL solubility and hence have the potential for increasing the rate of DNAPL dissolution in pump-and-treat systems. To test the effectiveness of surfactants under field conditions, a controlled field test at Canadian Forces Base Borden was undertaken. Results indicate surfactant-enhanced aquifer remediation can rapidly remove the majority of DNAPL using simple modifications of a pump-and-treat system. As in all pump-and-treat systems, the efficiency is a function of the hydraulic conductivity. The persistence of high DNAPL concentrations at specific elevations within the aquifer throughout the test indicates that little vertical movement of DNAPL occurred as a result of the introduction of the surfactant. Since the test was stopped when small amounts of DNAPL still remained, the limit of removal was not investigated

Ground-water withdrawal from a confined or semiconfined aquifer causes three-dimensional deformation in the pumped aquifer and in adjacent layers (overlying and underlying aquifers and aquitards). In response to the deformation, hydraulic head in the adjacent layers could rise or fall almost immediately after the start of pumping. This deformation-induced effect is analyzed by a linear poroelasticity model using properties typical of unconsolidated sedimentary materials. Model simulations suggest that an adjacent layer undergoes horizontal compression and vertical extension when pumping begins. Hydraulic head initially drops in a region near the well and close to the pumped aquifer, but rises outside this region. Magnitude of head change varies from a few centimeters to more than 10 centimeters. Factors that influence the development of deformation-induced effects includes matrix rigidity (shear modulus), the arrangement of aquifer and aquitards, their thicknesses, and proximity to land surface. Induced rise in hydraulic head is prominent in an aquitard that extends from land surface to a shallow pumped aquifer. Induced drop in hydraulic head is likely observed close to the well in an aquifer that is separated from the pumped aquifer by a relatively thin aquitard. Induced effects might last for hours in an aquifer, but could persist for many days in an aquitard. Induced effects are eventually dissipated by fluid flow from regions of higher head to regions of lower head, and by propagation of drawdown from the pumped aquifer into adjacent layers

We assess the predictive accuracy of Glover's (1974) stream-aquifer analytical solutions, which are commonly used in administering water rights, and evaluate the impact of the assumed idealizations on administrative and management decisions. To achieve these objectives, we evaluate the predictive capabilities of the Glover stream-aquifer depletion model against the MODFLOW numerical standard, which, unlike the analytical model, can handle increasing hydrogeologic complexity. We rank-order and quantify the relative importance of the various assumptions on which the analytical model is based, the three most important being: (1) streambed clogging as quantified by streambed-aquifer hydraulic conductivity contrast; (2) degree of stream partial penetration; and (3) aquifer heterogeneity. These three factors relate directly to the multidimensional nature of the aquifer flow conditions. From these considerations, future efforts to reduce the uncertainty in stream depletion-related administrative decisions should primarily address these three factors in characterizing the stream-aquifer process. We also investigate the impact of progressively coarser model grid size on numerically estimating stream leakage and conclude that grid size effects are relatively minor. Therefore, when modeling is required, coarser model grids could be used thus minimizing the input data requirements.

Seven hundred seventy liters of a dense nonaqueous phase liquid (DNAPL), tetrachloroethylene (PCE), were released into an isolated volume of a completely saturated natural sandy aquifer. The release was monitored over a period of 984 hours with a variety of geophysical methods including ground penetrating radar, time domain reflectometry, in situ resistivity, and a neutron soil moisture probe. The PCE formed a pool on a low permeability layer at approximately 1 m depth and spread over an area exceeding 32 m2. In its course of downward migration, the PCE subsequently formed eight smaller pools. At the end of the experiment an estimated 41 percent of the total PCE volume remained trapped in the upper pool. The PCE mass and its spatial moments were calculated from radar reflection amplitudes. Between 48 and 100 percent of the PCE mass was accounted for by radar measurements. The center of mass moved a total of 0.5 m south southeast and 1.3 m downward. Spatial variances showed that the greatest lateral spreading occurred in the east-west direction. The results demonstrate that natural heterogeneities, even in a relatively homogeneous aquifer, can cause DNAPLs to spread laterally over large areas in the subsurface. This experiment also demonstrated that while the ability of geophysics to uniquely measure the presence of DNAPL is limited, certain techniques are well-suited to monitoring changes in DNAPL saturation

Pingtung Plain is formed by Quaternary alluvial fan material from the three main rivers: Kaoping, Tungkang and Linpien. Ground water is the major water supply source on the plain. This is principally extracted from two aquifers. The natural ground water source is derived mainly from direct rainfall percolation and infiltration from the three rivers, with their catchments lying partly outside the plain. Rainfall characteristics are therefore the main factors controlling water resources availability. Pingtung Plain is an important primary production area for southern Taiwan, the comparatively warm climate allowing a long growing season, diversified cropping and the rearing of aquacultural produces. Approximately 75 percent of irrigation and domestic water supplies are derived from ground water. A water balance for the entire plain indicates that ground water resources, under optimized management, are sufficient to meet the existing multi-purpose uses. Development of a hydrogeological conceptual model is the first phase of a numerical ground water flow simulation. Preliminary results are encouraging, with the final simulations affording better insight to the hydraulic behavior of the aquifer system. Data input requirements for model operation fall into three categories: hydrological stresses, hydrogeological parameters and boundary conditions. After the model is built, the normal numerical modeling process requires significant calibration and sensitivity analyses for the hydrogeological parameters and stresses which are the most sensitive, but the least well defined. A well-calibrated simulation model can lead to a reliable and realistic management model. With this in mind, the calibration processes detailed are presented, and these data are introduced as initial values in the calibration process

Rates of biodegradation were measured in a petroleum hydrocarbon-contaminated aquifer using a combination of field and laboratory methods. These methods are based on tracking concentration changes of substrates (both electron donors and acceptors) or final products of microbial metabolism over time. Ground water at the study site (Hanahan, South Carolina) is anoxic, and sulfate reduction is the predominant terminal electron accepting process. Laboratory studies conducted with sediment cored from the site showed that 14C-toluene was mineralized to 14CO2 with a first-order degradation rate constant (k(tol) of -0.01 d-1 under sulfate-reducing conditions. Under nitrate-amended, Fe(III)-amended, or nonamended (methanogenic) conditions, toluene was not significantly mineralized. 14C-Benzene was degraded at low but measurable rates (k(ben) = -0.003 d-1) under sulfate-reducing conditions whereas degradation under methanogenic conditions was negligible. These results illustrate the extreme sensitivity of laboratory-measured biodegradation rates to terminal electron-accepting conditions, and show the necessity of carefully matching experimental conditions to in situ conditions. Concentration decreases of toluene along aquifer flowpaths, when the uncertainty of ground-water flow velocities was considered, indicated k(tol) values ranging from -0.0075 to -0.03 d-1. Concentration decreases of sulfate and concentration increases of dissolved inorganic carbon (DIC), when normalized for assumed stoichiometric oxidation of toluene coupled to sulfate reduction, yielded a k(SO4) range of -0.005 to -0.02 d-1, and a k(DIC) value range of +0.00075 to +0.003 d-1. Because both laboratory and field methods have numerous sources of uncertainty, a combination of these methods is the most appropriate procedure for evaluating biodegradation rate constants in contaminated ground-water systems