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publications > water resources investigations > report 87-4034 > estimates of transmissivity, hydraulic conductivity and storage coefficient > tests conducted in this study
Estimates of Transmissivity, Hydraulic Conductivity and Storage CoefficientTests Conducted in this Studyfig. 11). For various pumping tests, 23 wells were installed: 22 were 6-inch wells (with 6-inch open interval) and 1 was a 5-inch well (with 8-inch open interval). At many sites, separate wells were placed in the upper and lower parts of the Biscayne aquifer. At six western sites, wells were also installed in the gray limestone or in a shell sand of the Tamiami Formation. One pumping test was performed in a deep zone in east Broward County where the only data available in deep zones (below 150 feet) were two specific capacity tests of production wells. All of the production wells have open-hole construction, except for two, which are screened, located at the Twenty-Six Mile Bend site (fig. 11, G-2312). Because the wells were installed using an air-circulation method (no drilling mud), clogging of pore spaces or cavities in the aquifer was minimized. Several small-diameter (1 1/2- or 2-inch) wells were also installed for monitoring water levels, primarily in zones other than the pumped zone and for slug testing. Most of these wells gave oscillatory responses to slug tests, which indicate moderate to high permeability; therefore, those results are used only as a general guide for the permeability framework. The field tests may be divided into three types: single-well pumping tests using only the pumped wells for observations of response in the production zone, multiple-well pumping tests where a separate observation well in the pumped zone was also motored, and slug tests. The single- well pumping tests included step-drawdown tests, drawdown-recovery tests, and specific capacity tests. By use of a 4-inch suction pump, discharges up to 530 gal/min were obtained. Transmissivities and hydraulic conductivities obtained from the pumping tests and well construction data are listed in table 4. Step-drawdown tests were run at sites with moderate to high transmissivities. Pumping rates typically were about 160, 260, 370, and 500 gal/min, but for comparatively low transmissivity zones, the pumping rates were reduced as needed. For the lowest transmissivity zones, only specific capacity tests were run at low pumping rates to minimize well losses. The step-drawdown tests were usually run as independent cycles, each cycle consisting of 30 minutes of pumping followed by a recovery period. Whenever recovery was sufficiently slow, the highest discharge step was run for 60 to 100 minutes followed by recovery measurements. The Biscayne aquifer recovered far too quickly, within 1 or 2 minutes, to make satisfactory measurements, but recovery tests were sometimes possible in the gray limestone aquifer, and transmissivity values calculated from recovery data were similar to the step-drawdown test results. Jacob (1947) expressed drawdown in a pumped well by the relation:
where sw is drawdown in pumped well, The step drawdown test is a method of evaluating the aquifer loss, B, and well loss, C, constant and assigning relative amount of drawdown (head loss) to the aquifer and to the well. Bierschenk (1963) used a plot of sw /Q versus Q to determine B (the Y-intercept) and C (the slope of the line). Once B is determined, the specific capacity of an ideal well (one that measures only aquifer losses can be calculated from Q/S = 1/B, and a transmissivity can be estimated from this specific capacity. The average constant 270 was used, as before, for the Biscayne aquifer, The multiplier of 300 was used for the gray limestone aquifer, which is semiconfined and has a much narrower range of hydraulic characteristics (small storage coefficient and limited ranges of transmissivity, on the basis of results shown in table 4 and other tests recently completed in Dade County).
The relation between specific capacity and pumping rate for selected wells is shown in figure 13. As in figure 12, the wells may be divided into three groups. In the upper group are wells that tap high transmissivity zones of the Biscayne aquifer at depths generally ranging from 40 to 80 feet. Specific capacities for this group increase rapidly with decreasing pumping rate, and the curvature indicates that well losses are greater than aquifer losses in the drawdown relation (eq. 6) over most of the range of pumping rate. Drawdown at the lowest pumping rates (about 160 gal/min) ranged from 0.16 to 0.30 foot. A second group of three wells are open to most or all of the gray limestone aquifer at the respective sites. Results from these wells from straight or slightly curved lines at much lower specific capacities. For these wells, aquifer and well losses are more nearly equal for the range of pumping rate shown. Three shallow wells open to less cavernous limestone and muddy sand in the upper part of the Biscayne aquifer plot in the range similar to wells finished in the gray limestone. The plots for these wells are slightly more curved than those of the gray limestone aquifer. This variation could be caused by different hydraulic effects of flow in cavities, variable effective radius as a function of pumping rate, or by some influence of nearby canals. Aquifer tests or recovery tests were performed at four sites, of which three had one or more observation wells in the production zone. The observation wells were placed near the pumped wells to minimize the effects of leakage and to obtain measurable drawdowns. A transducer and high- speed chart recorder were used on one of the tests (well G-2312J) to obtain accurate early time water-level measurements during pumping and recovery periods. Data from the aquifer tests were analyzed by a nonleaky semilog drawdown method described by Cooper and Jacob (1946) and the recovery method (Theis, 1935; Todd, 1980, p. 131-135), or by leaky aquifer methods described by Cooper (1963) and Hantush and Jacob (1955). The results, listed in table 4, include storage coefficients that are accurate within about one-half order of magnitude. Examples of recovery tests are shown in figure 14. An estimate of average horizontal hydraulic conductivity may be calculated from the transmissivities obtained from the tests (table 4) using equation 3. For specific capacity or step-drawdown test, the length of open hole or screen is a reasonable estimate of the thickness of the aquifer that contributes most of the flow to the well, if the aquifer has significantly greater horizontal hydraulic conductivity than vertical hydraulic conductivity (as shown by layering) and if the open interval is relatively long (McClymonds and Franke, 1972, p. E11). For wells having short open intervals (less than about 20 to 25 feet) in the Biscayne aquifer, an estimate of the principal aquifer intervals supplying water to the wells, on the basis of drilling observations, lithologies, and wells in other intervals, was used to calculate the hydraulic conductivity. For the multiwell aquifer tests and the single-well recovery tests, the aquifer thickness (excluding sand beds in permeable limestone) rather than the open interval was used in the calculations. Slug tests were performed on five small-diameter wells open to sand or semiconfining materials to obtain hydraulic conductivities for some of the less-permeable materials in the surficial aquifer system. For three tests, air pressure was used to displace the water level, and a transducer and high-speed chart recorder were used to monitor water-level recovery (method described by Prosser, 1981). For two tests where water-level responses were slow, a bailer and chalked tape were used. Data from these tests were analyzed by the Hvorslev method (Hvorslev, 1951), which is suitable for water-table conditions (E.P. Weeks, U.S. Geological Survey, oral commun., 1984). Results of these tests are shown in table 5. Laboratory tests for hydraulic conductivity and porosity were performed on nine rock samples obtained from various zones during normal drilling operations by the dual-tube reverse-air method. (Coring and testing were performed by Core Laboratories, Inc., Dallas Texas.) Horizontal and vertical directions were determined for the samples from bedding features and shape. Horizontally oriented cores of 1-inch diameter were cut to obtain horizontal hydraulic conductivities, Kh, except for one sample which probably was oriented so as to yield a vertical hydraulic conductivity, Kv. Water similar in composition to native ground water was uses for the tests. Porosity was determined using a helium–Boyle's law technique to measure pore volume and then crushing the sample to grain size to measure grain volume. Results of these tests are shown in table 6.
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U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology This page is: http://sofia.usgs.gov /publications/wri/87-4034/testconducted.html Comments and suggestions? Contact: Heather Henkel - Webmaster Last updated: 10 November, 2008 @ 01:12 PM (KP) |
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1x10-5 and S/S Ss