Rectangular Schlumberger Resistivity Arrays For Delineating Vadose Zone Clay-Lined Fractures In Shallow Tuff

Rectangular Schlumberger arrays can be used for 2-dimensional lateral profiling of apparent<br>resistivity at a unique current electrode separation, hence single depth of penetration. Numerous<br>apparent resistivity measurements are collected moving the potential electrodes (fixed MN spacing)<br>within a rectangle of defined dimensions. The method provides a fast, cost-effective means for the<br>collection of dense resistivity data to provide high-resolution information on subsurface hydeogeologic<br>conditions.<br>Several rectangular Schlumberger resistivity arrays were employed at Los Alamos National<br>Laboratory (LANL) from 1989 through 1995 in an area adjacent to and downhill from an outfall pipe,<br>septic tank, septic drainfield, and sump. Six rectangular arrays with 2 AB spacings were used to<br>delineate lateral low resistivity anomalies that may be related to fractures that contain clay and/or<br>vadose zone water. Duplicate arrays collected over a three year time period exhibited very good data<br>repeatability.<br>The properties of tritium make it an excellent groundwater tracer. Because tritium was present<br>in discharged water from all of the anthropogenic sources in the vicinity it was used for this purpose.<br>One major low resistivity anomaly correlates with relatively high tritium concentrations in the tuff. This<br>was determined from borehole samples collected within and outside of the anomalous zone. The<br>anomaly is interpreted to be due to fractures that contain clay from the soil profile. The clay was<br>deposited in the fractures by aeolian processes and by surface water infiltration. The fractures likely<br>served as a shallow vadose zone groundwater pathway.