Geoelectrical Signatures of Spreading and Mixing
Yan Iskandarsyah est Professeur à l’Universitas Padjadjaran de Bandung.
Volcano could be distinguished into two groups due to its chemical origin properties. First is basaltic volcanoes from hotspots or rift-zone activity, and the second is andesitic volcanoes around subduction zone. The first type has been highly studied in the last 50 years (Peterson, 1972; Custodio, 1989), but the second one is less known (Charlier et al., 2011; Vittecoq et al., 2019).
Located on the subduction zone, Indonesia has a lot of andesitic volcanoes. One of amazing volcanic complex with a high potentcy of groundwater resouces is Gede – Pangrango – Salak. Data used in this study were collected by Universitas Padjadjaran or Unpad (2006-2017), as well as by the new investigations of Unpad and Sorbonne Université (2019-2022).
Such updated data include hydrology, geology, geophysic, hydrogeology, and hydro-isotope. Four watersheds related to Aqua – Danone production wells, i.e. Gekbrong – Cianjur (Mount Gede), Ciherang – Bogor (Mount Pangrango), Lido – Sukabumi (Mount Pangrango and Salak), and Kubang (Mount Salak), have been observed during 2006-2022.
One other watershed on the eastern flank of Mount Gede (Cibanteng – Cianjur) has been being studied since 2021 to compare the results. Even more than basaltic one, these andesitic volcanoes need a multidisciplinary and multiscale study of groundwater to realize its behaviour and its flowing within a complex configuration of aquifer layers. Water balance has been estimated; stream discharges have been periodically measured; geometry of aquifers were developed; physical and chemical properties of groundwater have been measured; groundwater facies were defined; and also hydro-isotope of the groundwater have been completely analyzed.
All of those performed studies have resulted similar groundwater conceptual models. Due to its high lithological variety and complex structure, a half of effective precipitation has been infiltrated into the sub-surface (45%-53%).
Thus, such water formed a deep or intermediate circulation of groundwater that might be recharged from the proximal or the medial zones of volcanic edivice (about 2718 m asl at Ciherang to 551 m asl at Lido). This groundwater flows through pores and fractures system of confined aquifers and may discharge in the distal zone, because of fault existences, as the artesian springs. On the other side, some flowing of groundwater occur near the surface as a local circulation. Groundwater facies then verifies the groundwater flow system.
Most of the facies are dominated by Ca – Mg and HCO3. Ca and Mg cations reflect an evident of ion exchange through rock interaction that may occur in a long circulation of groundwater, whereas HCO3 anion indicates that the interaction of penetration of CO2-rich surface water has occured.
As well as andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene, Ca–Mg HCO3 facies can indicate a typical characteristic of andesitic groundwater. However, their conceptual models are still being developed due to some uncertainties such as the configuration of aquifers around the recharge zones, the changes of recharge elevation, and the regional scale interpretation of the complex behaviour of groundwater in Gede – Pangrango – Salak.
Sorbonne Université – Campus Pierre et Marie Curie
METIS-IPSL, salle Darcy