DESIGN AND CONSTRUCTION OF INSTRUMENTED BOREHOLES TO MEASURE NATURAL AND ARTIFICIAL RECHARGE IN THICK UNSATURATED ZONES

The U.S. Geological Survey has constructed more than 20 instrumented boreholes in the Mojave Desert to measure the quantity, movement, and quality of natural and artificial recharge, including incidental recharge associated with septic or irrigation return water, through the thick unsaturated zones in the region.

The instrumented boreholes are installed using the Overburden Drilling and Exploration (ODEX) technique because the technique does not use water as a drilling fluid that could affect the moisture content of the unsaturated zone. Cuttings, collected at one-foot intervals, are logged and analyzed in the field for specific conductance as a measure of the accumulation of soluble salts. Cores are collected at selected intervals to determine hydraulic properties and to extract pore fluids for chemical or isotopic analysis. Instruments are placed at selected depths in the borehole on the basis of specific-conductance measurements, geologic logs from cutting descriptions, and geophysical logs. Instruments installed within the boreholes commonly include advanced tensiometers, heat-dissipation probes, suction-cup lysimeters, gas samplers, and piezometers. Advanced tensiometers measure matric potential and saturated pressures between -800 and 800 cm (of water), and are usually installed near the water-table or above clay layers or other materials where water may accumulate. Heat-dissipation probes measure matric potentials from -700 cm to as dry as -106 cm, and are typically installed in drier materials. Suction-cup lysimeters and gas samplers are used to collect unsaturated-zone water and gasses, respectively. Instruments are installed in a manner to promote hydraulic contact with the surrounding formational materials, and are isolated from each other by low-permeability grout. The piezometers are used to monitor changes in water levels and water quality at the water-table and also serve as access tubes for repeated collection of geophysical logs sensitive to changes in water content in the unsaturated zone.