Investigation of rssouhcs of a limestone aquifer using a digital technique

Abstract

Modeling of a limestone aquifer in the north west of Sri Lanka is attempted. The Vanathavillu basin situated 18km north of Puttalum, covering an area of approximately 80sq.km has been studied, using a mathematical model based on an implicit finite difference scheme. The study area situated in the dry zone of Sri Lanka, receives a seasonal rainfall of about 900 mm/year, the most of which falls during the months of October to December. In the Vanathavillu base in there are essentially two water bearing formations: - the miocene sedimentary strata, - the quarternary deposits which overlie the miocene strata. (referred as Moongil Aru formation) Moongil Aru formation consists of a series of clays and silts which partly confines the miocene formation. The piezo-metric levels in the miocene aquifer is lower than the phreatic surface by up to 30 m in the central parts of the region. In the north the piezometric levels are slightly higher than the phreatic surface. The two water bearing formations are interdependent as leakage takes place in and out of the Miocene formation. The model was developed only for the miocene formation and the water table elevation in the Moongil Aru formation assumed constant. The miocene formation is bounded in the east by basement rock outcrops which are relatively impermeable. To the west a fault exists which runs along the coast line. This fault restricts the flow in westerly direction. It is believed that two minor faults exists along two drainage paths of Kala Oya and Moongil Aru. 'The piezometric levels .in the norths suggest that the aquifer discharges into Kala Oya which could be treated as a constant head boundary. In the south the flow directions is entirely towards north. The area has been studied by the Irrigation Dept. and the Water Resources Board of Sri Lanka. On the basis of these investigations aquifer parameters, recharge and abstraction from the limestone aquifer have been estimated. Development plans have been prepared on the basis of these estimates. The purpose of the present model investigation was to access reliability of this estimation and also to provide a tool for planning future development and management of this valuable water resource. A number of model runs with different sets of data representing aquifer parameters, boundaries and flow were made. The results were compared with an available two year record of piezometric levels in the limestone aquifer. Computations were made with one layer model as well as with a simple two layer model. A single layer model with constant inflows or a simple two layer model with water table elevation treated as a constant adequately describe the behavior of the aquifer under the presence level of abstraction. But the behavior of the aquifer with highly increased abstraction can only be model adequately by a two layer model representing both unconfined and semiconfined aquifers together.