Hydrology of the Greater Tongonan geothermal system, Philippines, as deduced from geochemical and isotopic data

Fluids in the Greater Tongonan geothermal system exhibit a large positive 18 O shift from the Leyte meteoric water line. However, there is also a significant shift in 2H. The δ2H−δ18O plot shows that the geothermal fluids may be derived by the mixing of meteoric water (δ18O = −7‰ and δ2H = −41‰) with local magmatic water (δ18O = 10 ± 2‰ and δ2H = −20 ± 10‰). The most enriched water in the Greater Tongonan system, in terms of δ18O, δ2H and Cl, is comprised of approximately 40% magmatic water. Baseline isotope results support a hydrogeochemical model in which there is increasing meteoric water dilution to the southeast, from Mahiao to Sambaloran and towards Malitbog. The Cl−δ18O plot confirms that the geothermal fluid in Mahanagdong, further southeast, is distinct from that of the Mahiao-Sambaloran-Malitbog system.