When rain falls, part of it infiltrates into the soil. Water remains in the upper part of the soil absorbs by the plants is called as soil moisture. The other part percolates into subsurface and accumulated as groundwater on a rock or impermeable soil layer. The top of the groundwater is called groundwater table and a little amount of water rises above the water table due to capillary pressure and is called as capillary water. An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a deep tube well. However, in shallow aquifers only a small dug well is enough for water supply. 

Aquifers in Sri Lanka can be classified into 6 categories (Figure 9) based on the base material (Panabokke and Perera, 2005). First, shallow karstic aquifers present in Jaffna Peninsula. These are considered as high yielding aquifers due to sink holes, caverns and stream passages present in the Miocene limestone. However, since these aquifers are close to the sea, fresh water in the aquifer can be contaminated by saltwater intrusion quickly if the groundwater extraction is high.

Second aquifer system, deep confined aquifers are found in the northwestern and northern coastal plains of Sri Lanka. They serve as the largest sedimentary aquifer system in the island. The highly karstic sedimentary limestone in the area is mainly Miocene and Quaternary limestone formations. Many faults have separated the aquifer into series of isolated blocks.

Ninety percent of the island is covered by crystalline metamorphic rocks originated in the Precambrian era. Two third lies in the dry zone of the country. These unweathered rocks are relatively impervious and show no primary porosity. Secondary porosity is present due to fracturing. Water is stored in these fractures are extracted by deep tube wells. However, due to low transmissivity and low storitivity the groundwater potential of these aquifers are limited.

Coastal sand aquifers are present along the 4/5 of the coastline. Panabokke (2006) has categorized this coastal sandy aquifer into 3 sub groups; shallow aquifers on coastal spits and bars, shallow aquifers on raised beaches, low sand dunes and moderately deep aquifers on prior beach plains. Shallow aquifers on coastal spits and bars consist of unconsolidated material with salt marsh mangroves and saline flats (Kalpitiya, Poonaryn and Mannar island). Raised beaches are usually about 3 m above sea level and can be seen all along the eastern coast line (Batticaloa, Nilaveli). In the southern coast, around Hambantota, shallow aquifers are found inside the sand dunes. Moderately deep aquifers mainly confined into the old beach plain at the west coast (Chilaw, Negombo). All these shallow coastal aquifers are directly recharged by rain fall and develop as a lens of freshwater on top of saltwater beneath or the bed rock. Since the groundwater table is very shallow (<5 m from the surface) access to groundwater is easy and cost is low. However, these are very vulnerable to any contamination due to high transmissivity of the system. High water abstraction can cause saltwater upconing and create permanent contamination of well water.

Alluvial aquifers are found where alluvial formations such as flood plains and inland valleys are located. Deep and large alluvial aquifers occur along the downstream portion of the major rivers (Mahaweli Ganga, Kelani Ganga, Deduru Oya). Since these valleys consist of alluvial beds with different types of sediments, aquifers are highly productive.

South western part of the country receives extensive precipitation throughout the year with the help of hot weather results intensive weathering of parent rock. The highly soluble minerals such as Sodium, Potassium, Calcium and Magnesium leach out of the weathering zone and insoluble minerals like iron and aluminum reside. Vesicular nature of laterite provides high porosity and permeability. Also it represents a water holding capacity. These create the laterite formation as favorable for groundwater.

Groundwater resources have been extensively used since ancient times for domestic purposes using shallow open wells and deep tube wells in almost all parts of the country. There are about 15,000 tube-wells in the country (Figure 10). The quality of the groundwater is generally fairly good and relatively constant throughout the year.

Groundwater in Sri Lanka is classified into 4 main water types (Dissanayake and Weerasooriya, 1985).

·                     Calcium type

·                     Magnesium type

·                     Sodium/Potassium type

·                     Non-dominant cation type

These have been understood by analyzing groundwater for its major and secondary constitutions. Each type then further subdivided into 4 sub groups; Chloride, Sulphate, Bicarbonate and non-dominant anion type. Furthermore, Dissanayake and Weerasooriya (1986) have produced 'The hydrogeochemical atlas of Sri Lanka' includes description on these water types and more importantly the maps showing the distribution of water quality parameters throughout the country. This gives an overall idea on the hydrogeochemistry of Sri Lanka.

Read on Water quality of the Dry Zone of Sri Lanka

Surface water

Water pollution