Report No DWI0196
SAMPLING OF GROUNDWATER WITH SPECIAL REFERENCE TO SALINE/FRESHWATER CONDITIONS
Phase 3 Final Report October 1988 - October 1989
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
3.2 Future Cell Performance
- The precision of the cell string measurements is estimated at
ñ 10%; the borehole logging and depth sampling data have errors
of about ñ 20%.
- Given these error ranges, only one cell appears to have suffered a detectable performance impairment since installation.
- Recalibration of the cells confirms that the cells have in general performed as intended.
3.3 Guidelines for Installation in Backfilled Boreholes
- This is difficult to assess in the absence of any clear time trends in cell performance.
- 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)).
- 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.
3.4 Guidelines for Location of Cells in Borehole Profiles
- 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.
3.5 Guidelines for Interpretation of Cell conductivity Data
- 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.
- 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.
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.
- Changes in measured conductivity must be greater than the
method precision before conclusions can be drawn as to long term
- 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
Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.