By 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
Salinity in wells can be due to several reasons, not necessarily due to shortage of fresh water. The commonest of all reasons is that the bottom of the well is in the transition zone. Another reason is that the farmers irrigate large areas from a single well by excessive pumping and consequently a cone of saline water rises up into the well. In Chunnakam, the transition zone was at a depth of 29 meters (conductivity 2000 micro Siemens/cm-SMEC). The salinity of drinking water is considered to be less than 500 ppm of chloride or in conductivity units, less than 1000 micro-Siemens per cm; seawater being 35,000 ppm of chloride or conductivity of 50,000 micro-Siemens per cm. Sometimes the depths of wells are such that the transition layer may rise into the well making its water saline only in the dry season. Therefore, wells must be dug such that their depth is above the transition zone in late August. It is imperative to determine the cause of the salinity rather than guessing that it is caused by freshwater shortage
Suggested remedial measures
It is important to immediately quantify the amount of available freshwater, if it has not been done already. Only then will it be known whether remedial measures are needed to get additional freshwater.
Quantifying the freshwater is possible by having a network of boreholes in a grid in the peninsula and measuring the salinity of groundwater periodically at different depths. Alternatively, the same data can be obtained electronically without actually drilling the boreholes. This will require special equipment which SMEC has already identified. This investigation will involve some expenditure but much less than transporting freshwater from an external source. Reverse-osmosis desalination on the other hand is too expensive for Sri Lanka, both for installation and later with recurring expenditure. It may be even better to opt for water conservation by adopting drip system of irrigation in the peninsula. Drip irrigation, done correctly according to specifications, not only saves water but also eliminates pollution. Remedial measures should be affordable, cost effective, environmentally safe and practically feasible without depriving other areas of freshwater.
Several wells in the Jaffna town-ship have been contaminated with nitrates and e-coli greater than WHO’s safe limits. The pollution is due to ill-designed septic tanks or their proximity to wells. Pollution can be overcome by constructing the long overdue sewerage system with a sewage treatment plant for Jaffna.
In agricultural areas, excessive use of fertilisers and over-irrigation tends to leach the nitrates into groundwater. This type of pollution can be easily overcome by regulated application of fertilisers and by good irrigation practices. The above two pollution problems are the result of human behaviour and can be easily overcome by strict management practices.
- Arumugam S (1971) Development of groundwater and its exploitation in Jaffna Peninsula.- Transactions of Institute of Engineers. Ceylon 1. 31-62.
- Balendra V. S. (1969) Salt water-Fresh water interface studies in Jaffna Peninsula-Sri Lanka Water Resources Board report.
- Joshua W. D. et. al. (2013) Seasonal variation of water table and groundwater quality of the karst aquifer of the Jaffna Peninsula, Sri Lanka – J. Natn. Sci. Foundation Sri Lanka 41(1) 3-12
- Wijesinghe M.W.P. (1973) Groundwater Hydrology – Annual Sessions of the Ceylon Association for the Advancement of Science, Colombo.
- SMEC (2006) Snowy Mountain Engineering Corporation, Australia-Jaffna Peninsula Water Supply and Sanitation Feasibility studies ,