||Discharge and/or downhole sampling of production, injection and monitor wells has been conducted at Broadlands-Ohaaki to establish a precommissioning baseline; this will allow changes related to future production to be assessed. Changes related to long term discharge testing have been essentially due to two processes, (1) steam gain or loss from the formation and (2) dilution by shallow and marginal steam-heated waters. These changes are predicted to continue once f u l l scale production is started, but the interpretation may be complicated by returns of high chloride reinjection waters. t There appears to be similar concentrations of in the production liquid and the steam-heated h th H p p q waters; however, the H2S is greatly depleted in the steam-heated waters, causing a strong increase in COp/HpS ratios when dilution occurs. This signature may nelp to predict the incursion of dilution waters, and also to distinguish these fluids from trends related to reinjection returns. The planned assessment of monitoring results of production wells will greatly assist in the reservoir management of the system.