Embedded Sidewall Samplers and Sensors to Monitor the Subsurface

Abstract

AbstractSubsurface properties such as moisture content, hydraulic head, or chemical composition may vary markedly over short vertical distances in soil and ground water systems, but conventional samplers and sensors placed in vertical boreholes are often unable to resolve these variations. To improve the resolution of subsurface monitoring, we have developed a method for accessing the side‐wall of a vertical or angled borehole at many discrete intervals along the entire length of the borehole. The method uses an access device that embeds sensors or sediment samplers laterally through the borehole sidewall into the undisturbed formation to distances slightly less than the diameter of the borehole. The access device can also obtain a core sample up to 15 cm long and 4 cm in diameter, and then insert a permeable sleeve for extracting fluid samples (water, gas, nonaqueous phase liquids). The system has been used under field conditions in the United States and Denmark to place electrodes capable of measuring water content (using time domain reflectometry [TDR] waveguides), Eh (using platinum electrodes), or electrical resistivity (using a four‐conductor electrode). At one site, as many as 22 water samplers and 19 resistivity electrodes were installed in a single borehole at vertical spacings as close as 7 cm. This approach was used to install horizontally oriented TDR waveguides at depths greater than 10 m, thereby extending the TDR technique to the study of deep vadose zones. Other applications include measurement of in situ Eh at a site where strong chemical oxidants were injected to remediate sediments contaminated by organic chemicals.