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Groundwater availability studies in the arid southwestern United States traditionally have assumed that groundwater discharge by evapotranspiration (ETg) from desert playas is a significant component of the groundwater budget. However, desert playa ETg rates are poorly constrained by Bowen ratio energy budget (BREB) and eddy‐covariance (EC) micrometeorological measurement approaches. Best attempts by previous studies to constrain ETg from desert playas have resulted in ETg rates that are within the measurement error of micrometeorological approaches. This study uses numerical models to further constrain desert playa ETg rates that are within the measurement error of BREB and EC approaches, and to evaluate the effect of hydraulic properties and salinity‐based groundwater density contrasts on desert playa ETg rates. Numerical models simulated ETg rates from desert playas in Death Valley, California and Dixie Valley, Nevada. Results indicate that actual ETg rates from desert playas are significantly below the uncertainty thresholds of BREB‐ and EC‐based micrometeorological measurements. Discharge from desert playas likely contributes less than 2% of total groundwater discharge from Dixie and Death Valleys, which suggests discharge from desert playas also is negligible in other basins. Simulation results also show that ETg from desert playas primarily is limited by differences in hydraulic properties between alluvial fan and playa sediments and, to a lesser extent, by salinity‐based groundwater density contrasts. Article impact statement: Groundwater discharge from Great Basin desert playas is negligible. [ABSTRACT FROM AUTHOR]

A new computer program, FLASH (Flow-Log Analysis of Single Holes), is presented for the analysis of borehole vertical flow logs. The code is based on an analytical solution for steady-state multilayer radial flow to a borehole. The code includes options for (1) discrete fractures and (2) multilayer aquifers. Given vertical flow profiles collected under both ambient and stressed (pumping or injection) conditions, the user can estimate fracture (or layer) transmissivities and far-field hydraulic heads. FLASH is coded in Microsoft Excel with Visual Basic for Applications routines. The code supports manual and automated model calibration. [ABSTRACT FROM AUTHOR]

Arsenic concentrations can be managed with a relatively simple strategy of grouting instead of completely destroying a selected interval of well. The strategy of selective grouting was investigated in Antelope Valley, California, where groundwater supplies most of the water demand. Naturally occurring arsenic typically exceeds concentrations of 10 µg/L in the water produced from these long-screened wells. The vertical distributions of arsenic concentrations in intervals of the aquifer contributing water to selected supply wells were characterized with depth-dependent water-quality sampling and flow logs. Arsenic primarily entered the lower half of the wells where lacustrine clay deposits and a deeper aquifer occurred. Five wells were modified by grouting from below the top of the lacustrine clay deposits to the bottom of the well, which reduced produced arsenic concentrations to less than 2 µg/L in four of the five wells. Long-term viability of well modification and reduction of specific capacity was assessed for well 4-54 with AnalyzeHOLE, which creates and uses axisymmetric, radial MODFLOW models. Two radial models were calibrated to observed borehole flows, drawdowns, and transmissivity by estimating hydraulic-conductivity values in the aquifer system and gravel packs of the original and modified wells. Lithology also constrained hydraulic-conductivity estimates as regularization observations. Well encrustations caused as much as 2 µg/L increase in simulated arsenic concentration by reducing the contribution of flow from the aquifer system above the lacustrine clay deposits. Simulated arsenic concentrations in the modified well remained less than 3 µg/L over a 20-year period. [ABSTRACT FROM AUTHOR]

A sample of Key Largo Limestone from southern Florida exhibited turbulent flow behavior along three orthogonal axes as reported in recently published permeameter experiments. The limestone sample was a cube measuring 0.2 m on edge. The published nonlinear relation between hydraulic gradient and discharge was simulated using the turbulent flow approximation applied in the Conduit Flow Process (CFP) for MODFLOW-2005 mode 2, CFPM2. The good agreement between the experimental data and the simulated results verifies the utility of the approach used to simulate the effects of turbulent flow on head distributions and flux in the CFPM2 module of MODFLOW-2005. [ABSTRACT FROM AUTHOR]

Interpretation of single-well tests with the Cooper-Jacob method remains more reasonable than most alternatives. Drawdowns from 628 simulated single-well tests where transmissivity was specified were interpreted with the Cooper-Jacob straight-line method to estimate transmissivity. Error and bias as a function of vertical anisotropy, partial penetration, specific yield, and interpretive technique were investigated for transmissivities that ranged from 10 to 10,000 m2/d. Cooper-Jacob transmissivity estimates in confined aquifers were affected minimally by partial penetration, vertical anisotropy, or analyst. Cooper-Jacob transmissivity estimates of simulated unconfined aquifers averaged twice the known values. Transmissivity estimates of unconfined aquifers were not improved by interpreting results with an unconfined aquifer solution. Judicious interpretation of late-time data consistently improved estimates where transmissivity exceeded 250 m2/d in unconfined aquifers. [ABSTRACT FROM AUTHOR]

A new method was developed for characterizing geohydrologic columns that extended >600 m deep at sites with as many as six discrete aquifers. This method was applied at 12 sites within the Southwest Florida Water Management District. Sites typically were equipped with multiple production wells, one for each aquifer and one or more observation wells per aquifer. The average hydraulic properties of the aquifers and confining units within radii of 30 to >300 m were characterized at each site. Aquifers were pumped individually and water levels were monitored in stressed and adjacent aquifers during each pumping event. Drawdowns at a site were interpreted using a radial numerical model that extended from land surface to the base of the geohydrologic column and simulated all pumping events. Conceptually, the radial model moves between stress periods and recenters on the production well during each test. Hydraulic conductivity was assumed homogeneous and isotropic within each aquifer and confining unit. Hydraulic property estimates for all of the aquifers and confining units were consistent and reasonable because results from multiple aquifers and pumping events were analyzed simultaneously. [ABSTRACT FROM AUTHOR]

Demonstrates that the recession-curve-displacement method and other hydrograph-separation techniques are dubious tools for estimating ground water recharge except under limited circumstances. Why the estimates of the recession index for a basin from ground water hydrographs and stream-discharge hydrographs should be similar; Expected ratio of ground water discharge to a stream.

Acid mine drainage is an intensifying problem facing mining companies in the Northern Territory. Increasingly, more and more mining practitioners and researchers are studying ways to control the potential environmental impact of acid rock drainage. To date, conventional engineering methods are failing against the extremes of the Top End wet/dry climate. Because of this defalcation, a method of abatement will be required for the long-term protection of aquatic ecosystems. The purpose of this paper was to identify a "whole mine" method of abatement, which could remain at the waste site, generally, maintenance free and provide a significant buffering affect for receiving water bodies. Although wetland treatment has been proven in the treatment of heavy metals, little evidence exists on their viability once subjected to extended periods of low pH water. This paper proposes a design incorporating both control and abatement features. The key to the design is the sequence in which the effluent is received, as well as the ability of the structure to act as a trickle system and retain the wastewater for long periods of time. Exposure to physical, chemical and biological mechanisms also aid in the purification process. Wetland treatment should not be considered a "stand alone" method of waste management. It forms only a part of a strategy that should be designed, constructed and implemented in the primary phase. Wetlands should be incorporated along with appropriate characterisation and monitoring, legitimate waste handling procedures, adequate drainage systems and effective waste rock dump and tailings dam designs. [ABSTRACT FROM AUTHOR]

The rationale for this paper was to examine the beneficial or detrimental effects of treating and disposing of process water from the production plant and wastewater from Rustler's Roost Gold Mine through a modified natural wetland system. Rustler's Roost is located in the wet/dry tropics, 90 km southeast of Darwin at the northern end of the Pine Creek Geosyncline in an area distant from the main centres of historic gold production. At the time of the trial, data on water quality indicated that elevated level's of Cu, Al, and NaCN (Total and WAD) Cr, Se and Zn were present in Rustler's Roost plant water. Wetland treatment was being examined as an alternative to traditional wastewater treatment options. With an increase in production wastewater expected for 1996/1997 wet season, disposal of wastewater would be essential to insure integrity of the hydraulic circuit. Release of waste was dependent on meeting release criteria set by the Controller of Water Resources in a site-specific discharge licence. A 3-month dry season trial was conducted by modifying a section of a relict creek system. Wastewater was introduced by pumping waste directly from retention ponds. Results of the trial indicated dramatic removal in metals from solution, in particular, Cu and cyanide species. [ABSTRACT FROM AUTHOR]

Provides a study on the effects of steady-state assumption on recharge estimates and hydraulic conductivity in a surficial aquifer system. Methodology; Optimization routine; Presence of impervious land cover.

Focuses on the effects of the unsaturated zone when analyzing unconfined aquifer tests in a shallow-aquifer system at Cecil Field Naval Air Station, Jacksonville, Florida. Description of numerical models; Field procedures; Testing effects of unsaturated zone with a hypothetical aquifer; Parameter estimation applied to a hypothetical aquifer; Analysis of aquifer tests at field site.