Report No DWI0196

SAMPLING OF GROUNDWATER WITH SPECIAL REFERENCE TO SALINE/FRESHWATER CONDITIONS

Phase 3 Final Report October 1988 - October 1989

DWI0196

Nov 1989

EXECUTIVE SUMMARY

1 Background to Project and Aims of Phase 3

This investigation has consisted of three Phases. Phase 1 (October 1983 - October 1985) assessed the then current saline groundwater monitoring methods, developed a string of electrical conductivity cells designed to give accurate values over long-term maintenance-free installation in boreholes, installed the cell strings at six sites throughout England, monitored the performance of the cells, and designed a data logging system. Phase 2 (October 1985 - October 1988) continued monitoring at five of the six original sites in order to assess long term performance of the cells. Phase 3 (October 1988 - October 1989) concludes the investigation. Its aims have been: (i) to conclude as to the performance of the cells over the four to five year installation period; (ii) to assess the probable future performance of the cells; (iii) to provide guidelines for installation of the cell strings in backfilled boreholes; (iv) to provide guidelines for location of cells in the borehole profiles; (v) to provide guidelines for the interpretation of cell data.

2 Work Carried Out During Phase 3

Monitoring using the cell strings, borehole logging, and water sampling was continued. A detailed error study was carried out for each method of monitoring. Cell strings were recovered from the sites where the cells had not been installed in backfilled boreholes, and recalibrated twice, once before cleaning and once after cleaning the cells. An electron microscope study was undertaken in order to investigate cell material degradation. Precipitates on the cells were examined, and bacterial enumeration was carried out in each borehole. Laboratory and theoretical studies were used to examine the design of backfilled borehole installations. A newly developed borehole test procedure was applied and developed in order to improve both the efficiency of the location of the cells in the borehole profile and the interpretation of the monitoring data.

3 Results of the Phase 3 Study

3.1 Cell Performance

    1. The precision of the cell string measurements is estimated at 10%; the borehole logging and depth sampling data have errors of about 20%.
    2. Given these error ranges, only one cell appears to have suffered a detectable performance impairment since installation.
    3. Recalibration of the cells confirms that the cells have in general performed as intended.
3.2 Future Cell Performance

    1. This is difficult to assess in the absence of any clear time trends in cell performance.
    2. Visual and electron microscope inspection suggest that most cells have suffered little material degradation, though a small amount of chemical precipitate is present on all cells. The precipitate appears to have affected the performance of only one cell (3 .1 (ii)).
    3. It would appear that cell performance is likely to be adequate for installation periods of the order of ten years, except where chemical evidence suggests very heavy precipitation is likely.
3.3 Guidelines for Installation in Backfilled Boreholes

    1. Care needs to be taken in installation in backfilled boreholes. Recommendations have been made with regard to sealants and particle migration. Cell orientation is unimportant.
3.4 Guidelines for Location of Cells in Borehole Profiles

    1. Vertical flows in uncased boreholes can distort salinity profiles to such an extent that they bear little relationship to the salinity profile in the aquifer.
    2. A newly developed borehole test procedure (reversed flow test (RFT)) has been adapted for use in locating cells, and has been successfully applied at two of the field sites.
3.5 Guidelines for Interpretation of Cell conductivity Data

    1. Changes in measured conductivity must be greater than the method precision before conclusions can be drawn as to long term salinity changes.
    2. The effect of local groundwater flow on the cell responses can be estimated by using the RFT results and a crude model of the borehole hydrogeology.
4 Conclusion

Phase 3 has demonstrated that the conductivity cell strings are performing as originally designed in Phase 1 and that this method of monitoring is at least as good as more traditional methods and certainly less labour-intensive. The large errors associated with monitoring have been evaluated, and a new method for evaluation of borehole hydrogeology developed. There remains, however, significantly more work to be done in further developing understanding about flows in boreholes and how they relate to aquifer flows, and in understanding the chemical aspects of precipitation within the borehole water column.

Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.