In situ measurements in fractured till using sidewall sensors

Subsurface parameters, such as hydraulic head, often vary markedly with dep th in fine-grained glacial sediments, but sensors placed in vertical boreho les are poorly suited to resolve these variations. One problem is that conv entional methods only allow one, or perhaps a few, sensors to be placed in each borehole. To address such limitations we have developed a method for a ccessing the sidewall of a borehole. The method uses a device that pushes s ensors or sediment samplers laterally into the sidewall to distances slight ly less than the diameter of the borehole. The device can obtain a core sam ple 15 cm long and 4 cm in diameter, and then insert a permeable sleeve for extracting water samples. The same device has been used to insert several types of electrodes capable of measuring water content (using TDR waveguide s), Eh (using platinum electrodes), or electrical resistivity (using a mini ature Wenner-type array). At a site near Flakkebjerg, Denmark, we installed 22 water samplers and 19 resistivity electrodes in a single borehole to me asure hydraulic head gradients in detail and to monitor the vertical migrat ion of ionic tracers. This approach can be used to install horizontally ori ented TDR waveguides at virtually any depth, thereby extending the TDR tech nique to the study of deep vadose zones. At a contaminated site in the USA, TDR wave guides were installed to a depth of 12 m in glacial till. Other a pplications include measurement of Eh at a site where in situ chemical oxid ization was used, and the in situ sensors provided results that are similar to data obtained from soil cores.