[en] Seventeen exploration drill holes in the vicinity of the Jabiluka One and Jabiluka Two deposits were logged for Eh-pH and conductivity at 5 metre intervals to depths of up to 195 metres below ground surface. Forty-seven water samples from exploration drill holes, augered holes on the Magela flood plain and from two billabongs in the vicinity of the deposits were collected and analyzed. Analyses for pH and Fe were conducted in the field, and further analyses for major ions Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄^2-, Cl^-, HCO₃- and Si and minorelements Zn, Cd, Pb, Cu and U were conducted in the laboratory. The in situ Eh-pH and conductivity measurements, and analyses for major and minor elements of ground waters suggest that deep-lying chlorite-graphite schists containing the uranium mineralization are well protected from, or do not react rapidly with, ground water under present-day conditions, i.e. the schists of the Cahill Formation are a stable host for uranium mineralization at depth. In the vicinity of the Magela flood plain where the Cahill Formation and the permanent water table are close to the surface, some samples were found to contain high concentrations of sulphate, zinc, lead and iron. These same samples were characterized by low pH's in the pH range 3.0-4.0. The anomalies suggest weathering of sulphides associated with the mineralized Cahill Formation, where the schists are at shallow depths and in an oxidizing environment. The anomalies are not, however, necessarily indicative of zones of uranium enrichment in this formation. (author)