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The average electrical conductivity of the return water is increasing to c. 300 m but is thenalmost constant during the remaining drilling period. The most probable explanation is that theshallow groundwater inflow dominates completely in KFM06C.Content of dissolved oxygen in flushing waterIn Figure 5-15, the level of dissolved oxygen is plotted versus time. The content of dissolvedoxygen has generally been kept between 3–4.5 mg/L. Only during one week in late June thedissolved oxygen content raised above the approved upper limit of 5 mg/L, probably becauseof lack of nitrogen.Flushing water qualityThe results from chemical analyses of flushing water from the supply well HFM05 are compiledin /6/. The flushing water was sampled during drilling, for the following reasons:• Initially, to check if the quality was satisfactory. The main concern is the content of organicconstituents, which should be low, preferably below 5 mg/L. The reason is that introductionof hydrocarbons may affect the microbiological flora in the borehole, which would obstructa reliable characterization of the in situ microbiological conditions.• To monitor the groundwater chemical composition during drilling. The chemical compositionof the flushing water is important when estimating the effect, or correcting for the effect,of remaining flushing water in water samples collected from borehole KFM06C for chemicalanalyses.The microbe content in the flushing water was not determined during drilling of KFM06C. Themicrobe results from drilling of the preceding boreholes KFM05A and KFM06A /7, 8/ showedconvincingly that the cleaning procedure works well. It was therefore concluded that checkof microbes at all drilling occasions was no longer necessary. Analysis of the microbe contentreveals the abundance of microorganisms included in the flushing water pumped into the coredrilled borehole. The microorganisms originate partly from the flushing water well and partlyfrom the flushing water supply system between the well and the core drilled borehole.7O2_In [mg/L]KFM06C654321004-27 05-02 7 12 17 22 27 06-01 6 11 16 21 26 month-daySTART :05/04/26 06:00:00 INTERVAL: >= 1 Hour STOP :05/06/30 17:59:59Figure 5-15. Dissolved oxygen content in the flushing water versus time when drilling KFM06C.41