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.