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Clayton Group Services ACQUISITION AND INTERPRETATION OF WATER -LEVEL DATA SOP -8 In hydrogeologic investigations it commonly is necessary to know the depth to groundwater or other liquid surface relative to a fixed datum (generally a locally defined datum or mean sea level). From the depth to water level, the elevation of the groundwater surface (potentiometric surface) can be determined. If at least three monitoring wells are in place, groundwater flow velocities, direction of flow, and hydraulic gradients often can be determined which can then be used to assess contaminant movement in groundwater. When depth to groundwater over time is measured, it can reflect changes in flow direction and water levels caused by seasonal, human or other factors. When used in conjunction with aquifer testing, water level measurements provide information on the hydraulic properties of the aquifer. The procedure for determining and interpreting liquid level measurements in monitoring wells shall follow ASTM standards, Designation: D 4750 87 - Standard Test Method for Determining Subsurface Liquid Levels in a Borehole or Monitoring Well (Observation Well). By reference, this standard is incorporated into these Standard Operating Procedures. 1. Prior to measuring depth to groundwater, the elevations of each groundwater monitoring well must be surveyed relative to each other and relative to a fixed local, assumed datum or a National Geodetic Vertical Datum elevation benchmark above mean sea level. Generally a reference mark is placed at a point on the top of the well casing using indelible ink or magic marker, and relative elevation is determined for that point. 2. Liquid level measurements are made relative to this marked reference point. ASTM standards recognized three procedures for measuring depth to groundwater or other subsurface liquid: Measuring_Tape - Chalk or liquid- and product -finding paste are placed along the lower few feet of a measuring tape. The weighted tape is lowered into the well casing until the liquid is penetrated. The reading on the tape at the reference mark is recorded to the nearest 0.01 foot, and the tape is withdrawn from the well. The difference in tape readings from this recorded value and the demarcation between the wetted and dry portion of the tape is the depth to groundwater or other liquid