Desalinised Water For Jaffna Peninsula – Is It Necessary?
Willie D. Joshua
Expensive, unnecessary and environmentally damaging desalination projects for Jaffna can be totally avoided by a simple study to measure groundwater availability and optimised, evidence based water use management methods.
There have been many articles in the media about the necessity for a desalinisation project to supply potable water for Jaffna. The reason often quoted is that there is freshwater shortage due to over-usage of groundwater resulting in salinity and pollution. This reasoning will be correct only if it could be proven quantitatively by actual measurements, that the fresh water is insufficient for agriculture and domestic use. So far, no one seems to have made any systematic studies in estimating the actual quantity of available fresh water for human use throughout the dry season. This should be given the utmost priority and done immediately instead paying only lip-service. It can be only be proved by measuring salinity of water at different depths because fresh water is underlain by saline water. In addition, this information can be used to predict to what depth the wells should be dug to keep the water free of salinity always.
Computer modelling to quantify fresh water is not likely to be applicable to such limestone aquifers. Due to randomness of dissolution channels and pores, modelling inputs such as transmissibility and specific yield are widely variable at different locations, and it will not be possible to get reasonable results.
Illusion or fact
It has been proven beyond any doubt that the average rainfall during October to January recharges the aquifer with fresh water to much more than its maximum holding capacity ( Arumugam 1971, Wijesinghe 1973, Joshua et.al.2013). Only in occasional drought-years is maximum recharge not attained. By April-May, almost sixty percent of this overall storage of groundwater seeps to the sea/lagoon by subsurface outflow. This rapid depletion is a consequence of the increased outward pressure caused by the high watertable at the end of the rainy season. Normally, the stored water remaining after May is similar every year and has been sufficient for intensive agriculture and domestic use even for a population of 700,000 in the 1970s. So, is this water shortage and salinity, an illusion or a fact?
Groundwater formation and properties
Percolated rainwater into soil forms a fresh water layer, in the shape of a convex lens over the denser seawater in the aquifer. The lens will be thick at the centre and thins out towards the coasts. In all probability, Vadamaradchi, Thenmaradhi, Valikamam and the Islands have separate lenses of fresh water. In reality however, the lenses have fresh water only from the watertable down to certain depths, while the lower part of the lenses consist of a transition zone of increasing salinity. The transition zone is at variable depths at different location and probably results from the diffusion of salts from the underlying seawater into the lens. The existence of the transition zone has been proved by actual measurements (Arumugam-1971, Balendra 1969 and SMEC 2006). The thickness of the lens keeps on decreasing during the dry season with human usage and slow seepage till it is replenished by rainfall. For every unit drop in the watertable, the lens-seawater interface at the bottom of lens rises by about forty units due to density differences.
Depth of wells and salinity
