Sumba Island is situated at about 1, 500km ESE of Jakarta and is mainly composed of Neogene Tertiary and Quaternary marine sedimentary rocks. These consist of limestone, tuff and sandstone. Volcanic and intrusive rocks of the pre-Quaternary are also distributed. Most formations of marine sedimentary rocks are related to limestone and the major facies of limestone are coral reef origin.
The topographical characteristics of marine sedimentary rocks areas on the island are coastal terraces and karst. Several steps of coastal terraces and cliffs developed along the seashore. Karst features such as dolines, sinkholes and caves occur in the limestone areas. Many springs exist in these areas, especially in the Kaliangga formation area, the Waikabubak formation area and the Kananggar formation area. The total number of small spring (5>
Q≥1
l/s) +moderate spring (100>
Q≥5
l/s) +big spring (
Q≥100
l/s) on the island is at least 124. Out of 124, 43 are moderate springs and 9 are big springs. One of the famous big springs is Waikelosawa where the quantity of flowing water is more than 1000
l/s even during the dry season. Eighty percent (99 springs) of 124 springs exist below 500 m altitude.
Most groundwater is spring water, but underground river also occur, for instance the Pabinjal-ai in thePraipaha Basin. Confined groundwater, however, occurs in the Anakalang Basin. In this area, aquifers are composed of limestone rich facies and aquicluds of tuff rich facies. These facies are of the Waikabubak formation. Columner sections of the existing tube wells show that the aquifers are of limestone, marl, sand and gravel. But, judging from the field data, the greatest proportion of groundwater consists of cavern and fissure water, most of them being unconfined karst spring.
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