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 m(2)/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 m(2)/d in unconfined aquifers.
Slug tests performed in high transmissivity aquifers may exhibit underdamped inertial oscillatory behavior. Analytical methods for oscillatory data have been developed, but are mathematically intimidating. Spreadsheet modeling is helpful because ready adjustments to the equations required to match the well‐response data are quickly applied. A simplified presentation of the Kipp method for the practicing hydrogeologist is presented along with a spreadsheet model. A file containing the type curves is available over the Internet at http://www.mtech.edu from the Geological Engineering home page. Examples from southwestern Montana showing a variety of damping responses and well completions are presented to show how to perform the analysis. Sixteen well responses were modeled using the van der Kamp and Kipp methods, and the outcomes were compared using the two methods with oscillatory responses. Generally, there is good agreement between the methods. Screened wells tend to show more uniform oscillatory behavior, although open‐hole completions can be analyzed with success. Other practical field applications and observations are presented. The theory describing inertial effects has been constrained to confined aquifers, although all aquifers exhibit elastic behavior during the initial disturbance from pumping or instantaneous slug removal.
Metal-laden acid drainage from the reclaimed Landusky open-pit mine is thought to be the major source of environmental degradation to Swift Gulch, a small stream in the Little Rocky Mountains of north-central Montana. Ground water enters Swift Gulch through a series of Fe-stained springs located in a fault zone that extends to the southwest into the mine complex. The contamination threatens the nearby Fort Belknap Indian Reservation downstream. In this study, a detailed evaluation of the water chemistry of Swift Gulch during baseflow conditions was performed to assist in development of remedial plans. A continuous tracer injection test was performed in October 2007 using a combination of LiBr and Rhodamine WT dye. The tracer results were combined with field synoptic sampling of Swift Gulch to provide metal loading profiles for major and trace solutes, including Al, As, Fe(II), Fe(III), Mn, Ni, and Zn, along the entire 1,500 m length of the stream. Evidence from the tracer test indicates that nearly all of the metal contaminants in Swift Gulch enter the stream in a 500 m reach that is coincident with the fault zone. Iron enters the stream chiefly in its reduced Fe(II) state but is quickly oxidized to Fe(III) and then precipitated as ferric oxy-hydroxide. This reaction releases protons, and contributes to a drop in pH of Swift Gulch from values [6 to values \4 downstream. Several problems were encountered during the course of the tracer investigation. The rhodamine dye was unstable at the low pH of the stream and consequently was useless as a conservative tracer. Although Li ? was conservative, high background concentrations of this compound compromised its use as a tracer. Brproved to be the best tracer, and was used to quantify solute loads in the top 500 m of the drainage. However, even after 48 h of continuous injection, the concentrations of the Brtracer did not reach steady-state in the lower reach of the stream. These problems are good examples of the types of complications that arise during tracer tests of this type, and possible solutions are discussed.Keywords Bromide Á Geochemistry Á Montana Á Rhodamine Á Stream tracer test Á Trace metals Electronic supplementary material The online version of this article (
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