The Mak-Ban (Bulalo) geothermal field has generated more than 50 TWh of electrical power since 1979. A model to explain the fluid chemical changes with time was developed as part of a broader effort of conceptual and numerical modeling of the resource. End-member compositions of distinct recharge fluids were derived by evaluating initial-state and historical reservoir chloride trends. Fluid types, barriers to flow and processes governing changes in reservoir chloride (Cl) content were determined using trends in geo-thermometers, non-condensible gases (NCG), tritium, stable isotopes, magnesium, sulfate and enthalpy (excess steam) with time. Results indicate that four types of recharge fluid sustain production: upflow, marginal recharge (MR), injectate, and meteoric recharge. Confirmation of the marginal recharge composition comes from an early exploration well located three km north of the commercial reservoir. Bulalo is being sustained by relatively hot geothermal recharge from broad zones of sub-commercial permeability at its margins. We define this marginal recharge (MR) as well-mixed fluid composed of outflow from the Bulalo geothermal field and old meteoric water (> 50 years). The initial source temperature for the shallow western recharge is 216 degreesC, the northern recharge is 249 degreesC and the deep southern recharge is 293 degreesC. The proportion of recharge has increased in response to pressure drawdown, especially since about 1996. The calculations indicate that in 1999 marginal recharge production was 30-40% of field-wide production whereas injectate was 5-10% and meteoric recharge was insignificant. Cooling rates within the reservoir were calculated from geothermometry and assigned to the dominant recharge process in each well. Overall, marginal recharge has the highest estimated cooling rate of 2.2 degreesC/yr. (C) 2003 CNR. Published by Elsevier Ltd. All rights reserved.