HOMS COMPONENT

PERMANENT ELECTRODE SYSTEMS FOR GROUNDWATER QUALITY MONITORING

1.   Purpose and Objectives

     Permanent on site, in-the-borehole monitoring of changes in groundwater quality.

2.   Description 

     An electrode system consists of a 26-strand cable, onto which 13 helical electrode pairs are attached. The permanent electrode array allows the electrical resistivity of the subsoil to be measured at minimum seven to maximum thirteen meters depths. Cables of the required length can be supplied and are installed as soon as the borehole has been drilled. The electrical resistivity can be measured in a few minutes, to ascertain the water quality. If no changes are recorded over time it is safe to assume 
that the groundwater quality has remained unchanged; water analysis can be dispensed with, or limited to the bare minimum. This method is also used on a large scale to monitor groundwater contamination associated with the dumping of waste, or to trace the flow of infiltrating groundwater. The main advantage of the use of permanent electrode systems can be briefly summarized as follows: - The number of observation points is not limited by the diameter of the borehole. - A series of observations takes only a few minutes. - The measurements do not affect the groundwater situation in a way the extraction of water samples does under unfavorable circumstances. 

3.   Input

     Electrical resistivity of the subsoil at various depths.

4.   Output

     When the formation resistivity is known, the Cl- content can be derived if information on the composition of the formation water is available from analysis of water samples from observation screens. At low Cl- contents (lower than 50 mg/l), chloride is not the only factor determining the resistivity of the formation water. HCO3- and SO2- have their influence too. This influence, however, diminishes with higher chloride contents. 

     The observations can be converted directly into the degree of change in the chloride content resistivity of the groundwater. This can either be done manually, using a graph, or with a computer program. The resistivity can be converted to chloride content.

     Using these graphs is time-consuming if several cables have to be measured regularly. Therefore a simple computation program has been developed which uses a formula to automatically compute the chloride content. 

5.   Operational Requirements and Restrictions 

     The exact position of the cable in the borehole is determined by geophysical well logging. The cable is taped to the deepest observation screen and built into the borehole together with that filter. 

6.   Forms of Presentation 

     Two measuring devices are available: one digital and one digital with datalogger. The advantage of the latter is that no mistakes are made when reading off the value. An Instruction Manual (in English) for installing and operating the system is available. 

7.   Operational Experience 

     The first version of the system has been launched in 1965. It has been updated and improved regularly and has been sold all over the world. 

8.   Originator and Technical Support

     TNO Institute of Applied Geoscience, P.O.Box 6012, 2600 JA DELFT, The Netherlands, Phone: (31 15) 269 69 00, fax: (31 15) 256 48 00

9.   Availability

     From TNO Institute of Applied Geoscience, The Netherlands.

10.  Conditions on Use

      For sale at commercial rates, depending on length of cable.  

Last updated: 19 JAN 98)
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