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Section No 6 Revision No 1 40 Date January 13, 1992 Page 30 of 55 6 5 2 Rotary-wash Drilling Rotary-wash drilling methods will be implemented to collect disturbed and undisturbed soil samples when ■ hollow-stem continuous-flight augers cannot be used because of heaving sands and coarse gravels, ■ solid-stem continuous-flight augers cannot—Fe—used— ecause the boreholes will not stay open below the water table because of heaving sand deposits and coarse gravels which collapse the hole, ■ cable tool drilling is excessively slow and casing must be driven as the hole advances thus precluding the use of geophysical logging, a critical tool required for proper placement of well screens to monitor discrete zones, ■ air rotary and down-the-hole hammers with casing hammers are not suited for relatively fine-grained deposits below the water table and the casing would preclude the use of geophysical logging, . ■ reverse circulation rotary requires special sampling devices, causes - erosion of - the- boring walls, and- requires large- volumes of high- - velocity=circulation_water,this=method--is=not=well—suited—for ` installing relatively small diameter monitoring wells When possible, formation water will be used as the drilling fluid Fully expanded, sodium-free bentonite will be added to the drilling fluid only in amounts to stabilize the borehole Drilling fluids will be contained in approved holding bins Specific drilling and sampling details including the borehole diameter, sampling intervals, and other pertinent information will be addressed in the Work Plan for each given task Prior to drilling/sampling, the drill rods and bit(s) will be measured and recorded by the project geologist Disturbed samples will be collected as the drill rods are advanced by sieving the drilling fluids with a fine mesh screen as the fluids are forced • from the borehole As the sample is collected, the geologist will note the 6-30