Embedded sidewall samplers and sensors to monitor the subsurface

Subsurface properties such as moisture content, hydraulic head, or chemical composition may vary markedly over short vertical distances in soil and gr ound water systems, but conventional samplers and sensors placed in vertica l boreholes are often unable to resolve these variations, To improve the re solution of subsurface monitoring, me 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 t hat embeds sensors or sediment samplers laterally through the borehole side wall into the undisturbed formation to distances slightly less than the dia meter of the borehole, The access device can also obtain a core sample up t o 15 cm long and 4 cm in diameter, and then insert a permeable sleeve for e xtracting fluid samples (water, gas, nonaqueous phase liquids). The system has been used under field conditions in the United States and Denmark to pl ace electrodes capable of measuring water content (using time domain reflec tometry [TDR] waveguides), Eh (using platinum electrodes), or electrical re sistivity (using a four-conductor electrode). At one site, as many as 22 wa ter samplers and 19 resistivity electrodes were installed in a single boreh ole at vertical spacings as close as 7 cm, This approach was used to instal l horizontally oriented TDR waveguides at depths greater than 10 m, thereby extending the TDR technique to the study of deep vadose zones. Other appli cations include measurement of in situ Eh at a site where strong chemical o xidants were injected to remediate sediments contaminated by organic chemic als.