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USGS2010

Large-scale natural gradient tracer test in sand and gravel, Cape Cod, Massachusetts: 3. Hydraulic conductivity variability and calculated macrodispersivities

Hydraulic conductivity ( K ) variability in a sand and gravel aquifer on Cape Cod, Massachusetts, was measured and subsequently used in stochastic transport theories to estimate macrodispersivities. Nearly 1500 K measurements were obtained by borehole flowmeter tests and permeameter analyses of cores. The geometric mean for the flowmeter tests (0.11 cm/s) is similar to that estimated from other field tests. The mean for the permeameter tests (0.035 cm/s) is significantly lower, possibly because of compaction of the cores. The variance for the flowmeter (0.24) is also greater than that for the permeameter (0.14). Geostatistical analyses applying negative exponential models with and without nuggets reveal similar spatial correlation structures for the two data sets. Estimated correlation scales range from 2.9 to 8 m in the horizontal and from 0.18 to 0.38 m in the vertical. Estimates of asymptotic longitudinal dispersivity (b.35–0.78 m) are similar in magnitude to that observed in the natural gradient tracer test (0.96 m) previously conducted at this site.

Massachusetts↗

USGS2005

Evaluation of hydraulic conductivities calculated from multi-port permeameter measurements

A multiport permeameter was developed for use in estimating hydraulic conductivity over intact sections of aquifer core using the core liner as the permeameter body. Six cores obtained from one borehole through the upper 9 m of a stratified glacial-outwash aquifer were used to evaluate the reliability of the permeameter. Radiographs of the cores were used to assess core integrity and to locate 5- to 10-cm sections of similar grain size for estimation of hydraulic conductivity. After extensive testing of the permeameter, hydraulic conductivities were determined for 83 sections of the six cores. Other measurement techniques included permeameter measurements on repacked sections of core, estimates based on grain-size analyses, and estimates based on borehole flowmeter measurements. Permeameter measurements of 33 sections of core that had been extruded, homogenized, and repacked did not differ significantly from the original measurements. Hydraulic conductivities estimated from grain-size distributions were slightly higher than those calculated from permeameter measurements; the significance of the difference depended on the estimating equation used. Hydraulic conductivities calculated from field measurements, using a borehole flowmeter in the borehole from which the cores were extracted, were significantly higher than those calculated from laboratory measurements and more closely agreed with independent estimates of hydraulic conductivity based on tracer movement near the borehole. This indicates that hydraulic conductivities based on laboratory measurements of core samples may underestimate actual field hydraulic conductivities in this type of stratified glacial-outwash aquifer.

Groundwater↗