Stable water isotope 18O and Deuterium (D) have been widely used in groundwater hydrology, notably to estimate recharge elevation. We analyze water stable isotopes in self-flowing (artesian) wells, prior and after borehole annulus are cemented in order to sample aquifer layers within two categories, shallow or deep. The tested wells are located on the distal plain from different active volcanoes, one in Subang, West Java and the other in Pasuruan, East Java. Both wells have drilling depth ranging from 100 to 120 meters, discharge rates are about 1,500 m3/day and 6,000 m3/day respectively after grouting. Free-flowing occurred when the drilling hit the confined aquifer at depth 65 to 70 meters below the surface. The isotope results from Subang well, value δ18O and δD before cementing is -7.5‰ and -45.6‰ and after cementing -8.0‰ and -50.4‰. In Pasuruan, prior grouting value δ18O is -5.8‰ and δD -40.0‰ and after grouting -6.3‰ and -41.0‰. Mixing computations to determine mixing rates based also on hydrochemistry confirm the depletion of the deepest layers as regards the less deep. Temporarily, we conclude two hypotheses; (i) grouted annulus borehole i.e. deeper layers of the aquifer deliver depleted isotope 18O and D while in the upper layers are enriched. Depleted isotope value mostly scattered nearby the local meteoric water line (LMWL) which means the discharge from the self-flowing well relatively recharged by local rainwater at higher altitude. (ii) in the perspective confined (deep) or unconfined (shallow) aquifer, the deeper aquifer relies more on the depleted water stable isotope and vice versa for shallow one. This approach is still limited at two tested artesian aquifers but, in Pasuruan, as isotope samplings were also performed in boreholes of various depth, comparison of isotopic results with depth will also enable to help draw conclusions that will be presented in the paper.
Keywords: stable isotopes, aquifer, enriched, depleted