10 Groundwater measurements

10.1 Calibration

10.1.1 General

10.1.1.1 All measuring systems used shall be calibrated prior to commissioning the piezometer. This applies to both new and reused equipment. All parts of the measuring system that affect the accuracy of the measurements shall be calibrated.

10.1.1.2 The calibration results shall be documented in a report which, in addition to a description of the calibration procedure, shall include all information required to evaluate the measurements (see 12.1.8.2).

10.1.2 Open systems

Open groundwater measuring systems only need be calibrated if a pressure transducer is used in the piezometer pipe. The water level in the piezometer pipe shall be determined by measuring the difference between the level of the measuring point (i.e. upper edge of the piezometer pipe) and that of the pressure transducer relevant to the measurements (e.g. membrane).

10.1.3 Closed systems

Transducers in closed systems groundwater measuring systems shall be calibrated prior to installation of the completed measuring system in the ground as, unlike open systems, subsequent checking of the calibration is usually not possible. Transducers shall be calibrated together with the readout device to be used in the field, step by step, until the specified maximum pressure is reached. The difference between the level of the membrane in the transducers of pneumatic or electrical measuring systems and the mid-point of the filter, which is usually located below them, shall be established to enable the values measured in the field to be corrected. Pneumatic measuring systems shall be calibrated complete with all equipment and tubing to be used in the field and with the gas flow required to make the membrane lift when the measurements are performed. Electrical systems shall also be calibrated complete with all equipment and tubing to be used in the field. When calibrating transducers that measure absolute pressure, the atmospheric pressure shall be measured simultaneously.

10.2 Performance of the measurements

10.2.1 General

10.2.1.1 Measurements shall be checked if an influence from installation of the measuring system is detected or if unexplainable time lags or groundwater fluctuations occur, compared to other measurements.

NOTE Effects of installation are, e.g. effects of flushing medium (change of water density), excess pore pressure, clogging,short cuts between aquifers.

10.2.1.2 The results of the measurements shall be documented in a report which shall enable the values measured to be related to a particular stratum and interpreted unambiguously (see 12.1.8).

10.2.2 Open systems

10.2.2.1 Groundwater measurements in open piezometers can be performed at separate specified times (e.g. manually by electric contact meter) or continuously (e.g. by pressure transducers, pipes and loggers). The atmospheric pressure shall also be measured when using pressure transducers measuring absolute pressure. For manual measurements, the head of the groundwater potential is determined by measuring the distance between the identified level of the measuring point at the head of the piezometer and the water level in the piezometer pipe. When measuring the pressure in the piezometer pipe, the head of the groundwater potential is determined taking into account the distance between the level of the measuring point and the measuring level of the pressure transducer, and the atmospheric pressure if necessary. Continuous automated measurements shall be checked at least every six months by measuring the water level in the piezometer pipe manually.

10.2.2.2 The time lag of the open groundwater measuring system shall be determined.

10.2.3 Closed systems

10.2.3.1 Measurements in closed piezometers with pneumatic transducers are carried out by increasing the gas pressure in the supply tube until the specified flow rate required to lift the membrane is reached. The pressure shall be controlled gradually to allow equalisation of the back-pressure and the pore pressure acting on the other side of the membrane and thus to avoid overloading the membrane. The values measured shall be corrected by the hydrostatic pressure difference calculated from the height difference between the level of the membrane and the mid-point of the filter on the basis of the calibration.

10.2.3.2 Closed piezometers with electrical transducers should be used in particular for continuous data recording at regular, short intervals, the values being recorded by a logger. For transducers without equalisation of atmospheric pressure, the atmospheric pressure at the ground surface shall also be recorded at the same measuring times. Where necessary, the values measured shall be corrected by the atmospheric pressure on the basis of the calibration and by the difference in hydrostatic pressure from the difference in level between the membrane and the mid-point of the filter in the same way as for pneumatic systems.