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Clayton Group Service Page2 Standard Operating Procedures SOP -8 - Acquisition and Interpretation of Water -Level Data in the well below the reference point. As a check, a second measurement commonly is taken, and these should agree within 0.01 or 0.02 feet. Following these measurements, the tape should be decontaminated. Electrical Measuring Tape - More commonly, an electrical measuring tape, that is graduated in 0.01 feet, is used to measure depth to groundwater and/or product. These devises have a weighted sensing tip which registers as a noise and light on the surface equipment when water and/or product is detected. The tape is lowered into the casing and the depth is recorded at the reference point when water or product is detected. Again, a second reading should be taken, but it is not necessary to remove the pate prior to this second reading. Following measurement, the tape and sensor should be decontaminated. Measuring Tape and Sounding Weight - A weighted tape is lowered into the casing, and an audible splash and noticeable decrease in the downward force on the tape is noted. The tape reading at the reference point is then recorded. This method may not be sufficiently accurate, and generally is not recommended. Again, the equipment must be decontaminated after measurements are taken. 2. Determination of Stabilized Liquid Level - Groundwater will flow into and out of the well, eventually reaching a stabilized level representing the water level in the subsurface. ASTM recognizes two procedures for determining the groundwater level: • Conduct a series of water level measurements until the water level remains constant with time. This is the most common technique used. Take at least three liquid -level measurements at approximately equal time intervals while the liquid level approaches equilibrium. The approximate position of the stabilized liquid level in the well is calculated using the following equation: z ha = yl /<Y1 -v2) where: ho=distance the liquid level must change to reach the stabilized liquid level; yl=distance the liquid level changed during the time interval between the first two liquid -level readings; and, y2=dist5ance the liquid level changed during the time interval between the second and the third level readings. This process is repeated until the measurements are consistent with the desired accuracy. S:AEnnr\ProjASOPS\SOP-8.doc