In Situ Hydraulic Testing

Soon after saturation of the pilot-test tank and establishment of the flow regime, slug tests were performed to estimate the in situ hydraulic conductivities of the aquifer sand and the SMZ. The slug tests were performed using piezometers (2.5-cm ID schedule 40 PVC, 1 deep, with a 0.17-m screened interval at the bottom) installed during tank filling. Hydraulic conductivities were determined from the slug test data using the Hvorslev method (Freeze and Cherry 1979). Tests were performed at different locations within the sand and the SMZ. The average results from these tests, along with the laboratory-measured hydraulic conductivity values, are shown in Table 1. Whereas the laboratory tests showed a desired hydraulic conductivity contrast of about 5:1, the in situ tests showed essentially identical hydraulic conductivities for the SMZ and the sand. The major change was a much lower conductivity of the SMZ in the barrier than what had been determined in the laboratory. The explanation for this decreased conductivity is not certain. At the time, it was thought that entrapped air, which would slowly dissolve under sustained water flow, might be responsible for the decreased conductivity of the SMZ. Another potential explanation was that some compaction of the SMZ had occurred during barrier filling. The decision was made to proceed with the pilot test even though the sand/SMZ permeability contrast was lower than desired.

Table 1. Hydraulic conductivity of the 14-40 SMZ measured in the laboratory and after installation in the pilot-test tank.

Hydraulic Conductivity (m/sec * 10"3)

Material

Constant-Head Permeameter Falling-Head Permeameter

Slug Test

Sand

0.5 0.2

0.2

SMZ

2.0 1.0

0.2

0 0

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