Geophysical Logging Studies in the Snake River Plain Aquifer at the Idaho National Engineering Laboratory: Wells 44, 45, and 46

A geophysical logging program was undertaken to vertically profile changes in the hydrology and hydrochemistry of the Snake River Plain aquifer that underlies the Idaho National Engineering Laboratory (INEL). Field investigations were concentrated within an area west of the Idaho Chemical Processing Plant (ICPP) in three wells that penetrated the upper 190 feet of the aquifer. The logs obtained in these wells consisted of temperature, caliper, nuclear (neutron porosity and gamma-gamma density), natural gamma, borehole televiewer, gamma spectral, and thermal flowmeter (with and without pumping).

The nuclear, caliper, and televiewer logs are used to delineate individual basalt flows or flow units and to recognize breaks between flows or flow units at interflow contact zones and sedimentary interbeds. The temperature logs and flowmeter measurements obtained under ambient hydraulic head conditions identified upward fluid-circulation patterns in the three wells. Reversal of in-hole fluid-flow direction indicated hydraulic communication between well 46 and the supply well CPP2 at the ICPP. The vertical distributions of hydraulic conductivity in wells 44 and 45 were determined by measuring fluid flow in the wells concurrently with pumping. The large variations in the vertical distributions and magnitudes of hydraulic conductivity determined from these field tests testify to the complex, heterogeneous nature of the hydrogeologic system at the ICPP.

Gamma-spectral analyses performed at several depths in each well showed that the predominant source of gamma radiation in the formation at this site originates mainly from potassium ( 40K). However, the anthropogenic, y-emitting isotope 137 Cesium was detected at 32 feet below land surface in well 45.

An empirical investigation of the effect of source-receiver spacing on the response of the neutron-porosity logging tool was attempted in an effort to understand the conditions under which this tool might be applied to large-diameter boreholes in unsaturated formations. Results indicate that large spacings of 4 ft or more are required in order to effectively use the conventional porosity calibration approach for this purpose.