Many areas in Java, Indonesia, underlain by sub-recent to recent volcaniclastic rocks support a dense population (more than 1,000 persons per sq km) and contain large reserves of groundwater. Three areas are typical: Yogyakarta in central Java, Bandung in west Java, and Nganjuk-Kertosono in east Java. The volcanic material is mostly of andesitic composition.

The Yogyakarta area is underlain by about 100 m of ash, sand, gravel, and coarser aggregate from the continuously active volcano Merapi. In the Bandung area, lavas and breccias from the ancient Sunda volcano are overlain by about 120 m of lahar from the volcano

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Tangkubanparahu. The Nganjuk-Kertosono area is an intermontane basin filled with volcaniclastic rocks that have been washed away from the volcanoes Wilis, Anjasmoro, Pandan, and Kelut.

Artesian conditions are not distinct at Yogyakarta, but potential artesian basins underlie both the Bandung and the Nganjuk-Kertosono areas. In the latter a clay layer of lacustrine-paludal origin acts as a confining bed. Specific capacities at Yogyakarta average between 0.5 and 5 1/second/m of drawdown. Water-bearing parts in the lower breccias and lavas at Bandung have a specific capacity between 0.5 and 1 1/second/m of drawdown, whereas the specific capacity of the aquifers in the overlying lahars range between 1 and 5 1. A piezometric surface about 10 m above the ground is measured at wells drilled to the upper zone. Specific capacities in the Nganjuk-Kertosono area vary considerably depending upon the material which was derived from several sources.

Groundwater is expected to be used increasingly in the three areas for domestic purposes, for irrigation, and by industry.

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