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Hydrogeology Of Springs In A Tropical Humid Terrain In Sri Lanka -- A Study On Water Supply Alternative

Congress: 2015
Author(s): P.G.A.N. Mahindawansha, K. M. Prematilake , H.A.H. Jayasena
Institute of Applied Geosciences, Technical Universitat Darmstadt, Germany1, National Water Supply and Drainage Board2, University of Peradeniya3

Keyword(s): Sub-theme 1: Water supply and demand,
AbstractIntroduction: Majority of Sri Lankan rural population depends on groundwater for their domestic requirements. The dug wells are the major source, however, springs play a crucial role in certain regions. Springs in hard rock terrains are formed at contacts of quartzite, marble and highly fractured granites with the adjacent rock. A study was carried out in Matale to evaluate the hydrogeological characteristics of springs with geology, structure, topography and weathered overburden. In hard rock terrains deeper groundwater occurs in the structural discontinuities such as joints, faults, fractures and shear zones. Favorable inter-connection of these joints, fractures, faults and shear zones could make fairly good fractured aquifers, which could provide high yielding wells. The types and orientation of such discontinuities could also indicate potential pollutant migration paths. Therefore, socioeconomic impacts within the geohydrologic framework imposed significance of spring water in the total environment. Study area: The area is confined to the uplands within the central Sri Lanka around Matale. It lies within 80° 35´-80° 40´ easting and 7° 20´-7° 40´ northing and consists of series of north-south strike ridges and valleys. The selected catchment is bounded by elongated ridges with steep slopes on west and up to 35°slopes in the east. The catchment is about 25 km long while the width vary from 8.4 km in the south to 1.5 km in the north. It covers an approximate area of about 110km2. The elevation varies from 240 to 1200 m above sea level. Materials and Methods: Field mapping was carried out concurrent with yield measurements from 27 springs during rainy and dry seasons in 2010. Spring discharges were compared with geology, structure, morphology and drainage before analyzing to establish relationships. Rose diagrams and equal area plots were constructed to demarcate lineament distribution and maximum stress directions. DEM, TIN and slope analysis were prepared using ArcGIS. Borehole records were examined and using inverse distance weighted method, a 3D model was developed for the subsurface. Monthly rainfall and pan evaporation data were compared with moving averages in order to examine the performance of springs with seasonal hydro-meteorological conditions. Discharge was measured using V notch and area velocity methods, however, when the flow was a trickle time to fill a known volume was measured. Results and Discussion: Geological, Structural and Hydrogeological framework: The basement comprises with high grade metamorphic rocks such as marble, garnet biotite gneiss, garnet sillimanite biotite gneiss ± graphite (khondalite), quartzo feldspathic gneiss, charnockitic gneiss and quartzite. Quartzite is present as minor bands within khondalite, however marble is well exposed as continuous bands. General strike of the rocks are in north south direction with few degrees towards NNW or NNE. However, dips vary from 10° to vertical showing wide variations. Major lineaments are running in E-W and N-S directions. Rose diagrams clearly show E-W and N-S system with minor NE-SW and NW-SE joint systems. Maximum stress is nearly in the north-south direction, which resulted in E-W joint set and lineaments. The area is covered by a thin mantle of weathered products either derived from the parent rock (residual) or transported material. Colluvium and residual weathered products consist of a wide range of poorly sorted material with varying sizes from boulders to clay. Alluvial materials tend to be sorted along the flood plain whereas the level of sorting depends on the provenance. Reddish brown lateritic soil is developed on the crystalline limestone and show varying thicknesses up to 50 m. Brown sandy clay on khondalite however, contains a hardpan developed by the subsurface soil water movement. Groundwater table exposed at springs in the central valley and extended to 39 meters below ground level in the elevated rocky hinterlands. Springs: Springs are located along with marble and discharge their waters through karst formations and along solution paths of joints, fractures and coarse grains. Rainfall is the main recharging factor for these springs which shows a bimodal distribution with a clear cyclicity. A considerable seasonal yield variations as observed at certain springs clearly reflect the rainfall distribution. The yield of rainy season varies from 20-1500 l/min while in the dry season it varies from 10-876 l/min with a general trend of increasing yields towards north. Based on borehole records and field observations, groundwater level and the bed rock surface of the study area varies from the surface up to 39 and 50m depths respectively. High yields can be envisaged in areas where springs emitting out from marble, especially when shallow water levels, significant fracture intensities and solution cavities are present. Three marble bands are prominent in the study area. There is no any systematic distribution of springs among them, however, clusters are concentrated in the middle band and in association with folding of rock layers. Four different genetic types of springs can be identified viz: 1. associated with well-developed karstic formations, 2 along with primary joints and fractures, 3. through secondary porosity and permeability, 4. locations where coarse grain marble present. Among them Matale springs are dominated by fractures and joints. Many are formed in association with E-W and NE-SW trending fractures. However, meso scale folds are responsible for few springs. Springs are generally confined to the areas where slope angle is up to 10° within the regional catchment. Many springs are located within 50 meters from the stream indicating significant groundwater supply from the effluent system, so that the recharge is most likely originated from the rainfall in the elevated highlands. Based on cumulative discharge of 7560 l/min and assuming per capita daily requirement of 110 l, over 100,000 persons can be served by the spring water. Conclusion: This study concluded that in the heart of Sri Lankan highlands, spring water could play a major role in domestic circles. It could also effectively support small scale commercial and industrial establishments and irrigation sectors. The springs emitting through karstic formations have high yields compared with those emitting from fractures and solution cavities. Since the springs are mainly recharged by ample rainfall, they provide good yields throughout the year. The forgoing discussion reveals that springs in marble formation indicate a good potential for developing as a community water supply scheme where it could serve a one fourth of the community living in the area. Arumugam, S., Rnathunga, P.U., 1974. Springs of Sri Lanka-Survey of the Island’s Water Resources: Water Resources Board, Colombo, Sri Lanka. Jayasena, H.A.H. and Dhanapala, T.R.W.S., 2007. Prediction of Ungaged basins (PUB) in the Victoria - Randenigala - Rantambe (VRR) sanctuary of Sri Lanka – Approach based on Synthetic Unit Hydrographs. Journal of the Geological Society of Sri Lanka 12: 07-20 Jayasena, H.A.H., 1993. Geological and structural significance in variation of groundwater quality in hard crystalline rocks of Sri Lanka. In Hydrogeology of Fractured rocks (Eds. Sheila and David Banks), Memoirs of XXIVth IAH Congress Oslo, Norway. Section C8: 450-471. Jayasena, H.A.H., 1995. An analysis of fluid flow through fractured rocks. In K. Dahanayake (Editor), Handbook on Geology and Mineral Resources of Sri Lanka, Second South Asia Geological Congress Souvenir Publication, Colombo, Sri Lanka. pp 87-90. Jayasena, H.A.H., 1998. Hydrologic assessment of the Deduru Oya Basin in Sri Lanka. Multi-Disciplinary International Conference on the Occasion of 50th Anniversary of Independence of Sri Lanka. University of Peradeniya, Sri Lanka. Section – G, Science and Technology 13 pp. Jayasena, H.A.H., Singh, B.K. and Dissanayake, C.B., 1986. Groundwater occurrences in the hard rock terrain of Sri Lanka - A Case Study. AQUA., 4: 214-219. Johansson, D., 2005. Influence of lineaments on the productivity of water supply boreholes in a hard rock terrain: A systematic approach in Pinga Oya Basin, Kandy, Sri Lanka, Department of Civil and Environmental Engineering, Chalmers University of Technology, Denmark, Unpublished Master Thesis 2005:32, Goteborg, pp. 21-43. Karanth, K.R., (1989) Hydrogeology: Tata Mcgraw-Hill Publishing Company Limited. Mahindawansha, P.G.A.N., 2010. Hydrogeology of Springs in Matale Marble Formation – A Case Study. Unpublished B.Sc. Thesis pp 74. Singh, B.K. and Jayasena, H.A.H., 1984. Hydrogeology, exploratory drilling and groundwater resources potential in the Kurunegala District. Water Resource Board Report, Colombo, Sri Lanka., 293 pp.
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