Summary

The geology of Central Java can be defined in terms of distance from volcanic centres located along the spine of the island. Coarse ejecta are reworked on the upper volcanic slopes to form a scree, and coarse to medium grade fluviatile sequences slope downwards. At the base, shallow marine sedimentation along a rapidly retreating coast line has created a clay-sand sequence. This variety of depositional facies has produced a lithological inhomogeneity that poses problems of aquifer delineation, and makes the cuttings from direct-flush rotary drilling difficult to interpret. Geophysical logging of the volcanic sediments also presents difficulties as gamma logs lack features because of high background counts so that resistivity logs have to be used to locate well screens.

Unconfined conditions exist on the upper and intermediate slopes of the volcanoes while beneath the lower slopes and coastal plains is an upper unconfined aquifer overlying lower, confined aquifers. Despite geological and hydrogeological inhomogeneity, approximate groundwater head distributions can be reasonably determined in the unconfined areas; multilayering makes this difficult in the confined areas.

Groundwater development for irrigation is seen as a long-term aim in the unconfined areas of the intermediate slopes of the volcanoes. Further groundwater development, by manipulating the aquifers' specific yield and its high potential recharge currently lost to flood flow, appears to be feasible. Accurate assessment of the potential development is difficult due to the complex hydrogeology and extensive existing groundwater use. Development will therefore be slow and require careful planning.

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