With the country’s population increasing at an alarming rate, fresh water supply is being continually depleting, increasing the importance of ground-water monitoring; ground water being the principal source for domestic, industrial, and agricultural use.
In addition, ground water supports freshwater ponds, wetlands, streams, and estuary environments, all of which represent specific and important habitats for native flora and fauna. Coastal resources, such as streams and estuaries depend on aquifer discharge to maintain yearly and seasonal flow rates and to regulate water chemistry. The particular ecological significance of the discharge depends on the rate at which freshwater mixes with saltwater. Slow mixing zones such as streams and estuaries depend on the natural influx of freshwater for sustained health of the ecosystems.
Water cycle: Water cycle is the process of collecting, purifying and distributing earth’s water. The sun evaporates water from oceans, rivers, lakes, soil and plants into the atmosphere. This water vapour is carried by winds and when cooled, condenses into clouds and falls as precipitation (rain, sleet, hail or snow). Fresh water runs off into lakes, streams and rivers, which return water back to the ocean, completing the cycle. Water also seeps deep into the ground and is stored as ground-water in aquifers (spaces between rock formations). Seawater intrusion: One of the major concerns most commonly found in coastal aquifers, is the induced flow of salt water into fresh water aquifers caused by ground-water development, known as salt-water intrusion. Seawater intrusion occurs when fresh water is withdrawn faster than it can be recharged near a coastline. Seawater generally intrudes upward and into an aquifer and around a well, though it can occur passively with any general lowering of the water table near a coastline. The transition zone (the interface where freshwater naturally mixes with sea water as it’s discharged to the sea) naturally descends towards land as a wedge within aquifers along the coastline.
Freshwater, being lighter will float on top of seawater. Mixing of the two does not take place rapidly because of their different densities and as a result, a gradational, interface boundary normally forms between them. Movement of ground water caused by pumping, changes in recharge, or tidal action can cause a larger interface layer. Where there is little movement of ground water, the interface layer may be only several feet in thickness. When substantial aquifer activity is present, the freshwater/seawater interface may be as much as several hundred feet thick.
Sea-water interface layer position is determined by the difference between the hydraulic heads of the seawater and freshwater and the volume of freshwater under-flow. If the freshwater gradient near the coastline (i.e., freshwater height above sea level) is high and the aquifer water balance is adequate to maintain flow to the seawater, the interface layers can be close to vertical and located off the shoreline. As the freshwater head decreases, the interface layer becomes less vertical and moves inland.
In this condition, seawater, being heavier, can flow under the freshwater and pushes it up slightly forming a Ghyben-Herzberg lens (i.e., the lens is the underground freshwater that floats on top of the seawater). If the freshwater head is maintained, the interface layer will remain relatively static. Under Ghyben-Herzberg conditions, a one-foot drop in freshwater head may result in a 40-foot movement in the freshwater/seawater interface layer. Consequently, relatively small decreases in a fresh water aquifer may have relatively large impacts on the intrusion of salt water into that aquifer.
Impact: An aquifer is a geological formation that has the capacity for storing and transferring water under the ground. Aquifers are replenished through recharge. Any surface activity which serves to reduce aquifer recharge has the potential to effect seawater intrusion. When the water level (head) in the aquifer is lowered, the flow of freshwater to seawater is reduced. If the flow is reduced sufficiently, the seawater-freshwater interface will move inland. Limiting pumping and assuring that proper recharge is maintained can control seawater intrusion.
Sea intrusion: Deltaic region marks Sindh’s 300km long coast with Arabian Sea. The region is dominated by subsidence and high sediment accumulation rates. The bulk of the sediments are transported by the Indus River along its 2000 km long route from its point of origin in the Himalayas. The Sindh shelf resembles a passive margin with a characteristic broad (about 100 km) shelf and relatively stables slopes. The Indus canyon has an average width of 8 km and depth (relief) of approximately 800 m. The canyon is 170 km long.
Indus River is the 23rd largest river in the world. It is the only major river on the coast of Pakistan. At present, it discharges in the Arabian Sea at two points, Turshian and Khobar. The discharge and nutrient-rich sediments load of the river have great influence on marine life (not only in the estuary but also in the near shore areas of the delta). Construction of channels, barrages, embankments, dikes and dams for hydroelectric power, irrigation and flood control has reduced water discharge into the delta region to about one fifth, while at present no water is allowed down the Kotri barrage for more than ten months of the year.
Intrusion of seawater from the Arabian Sea into the Indus River has been attributed to the diversion of river’s flow. The quantity of outflow to sea has been progressively reducing, particularly after the construction of barrages, dams and link canals after the signing of the Indus Water Treaty, 1960. The actual outflow to sea at the time of independence was about 80 MAF, which has now reduced to 36 MAF.
The duration of the flow has also reduced to less than 2 months and that also only in high flood years. Studies have shown that Indus River used to throw 400 million tons of silt in the sea every year, which has now been reduced to just about 100 million tons a year. This gradual depletion in flow of sweet river water and rich silt into sea has led to sea water intrusion leading to hyper-saline condition in the coastal area that literally kills life in all forms, livestock and vegetation on land and fish and other edible marine varieties and the rich mangroves forest in the sea. The present salinity level of 40 to 50 ppt (particles per thousand) compared to 15 to 20 ppt half a century ago has already crossed the dead level mark.
Thatta, a predominantly agricultural district in Sindh, and situated where the Indus river flows into the Arabian Sea, has been worse affected. Out of total geographic area of 4.29 million acres, and the sea has invaded about 1.2 million acres of fertile land of Thatta and adjoining areas. The effects are not just restricted to land inundation but mangrove forests, an essential breeding ground for shrimps and freshwater fish species are disappearing fast.
There has also been severe damage to livestock. Saltwater intrusion caused a dramatic decline in livestock numbers in all the three subdivisions of Thatta District, bringing about depletion and erosion of ranges, shortages of fodder, pasture and potable water. This has resulted in mass migration of inhabitants. Affected population comprises mostly of rural areas, with 90 per cent of the total population concentrated in rural localities. The area remains enslaved to poverty and illiteracy. High birth rates, low household incomes and small housing units all keep coastal community in a web of deprivation. Poverty in the areas touches 70 per cent. It is a mainly agrarian society where most of working force is involved in occupations in the primary sector (agriculture, forestry, fishing and animal husbandry} . Thousands of people, mainly farmers and fishermen, have left their ancestral places in search of livelihood after the Arabian Sea water has flooded them. Under the provisions of the Water Accord, 1991 a quantity of 10 MAF has been provisionally earmarked for outflow downstream Kotri pending further studies to be undertaken to establish the needs of minimum escapages down stream Kotri. Unfortunately the study has not been initiated even after 11 years of signing the Water Accord due to trifling objections raised on the terms of the study.
Economically, environmentally and socially sustainable water development solutions for Pakistan are vital as it is a key resource for its agricultural and industrial progress. Ironically, the issue of fresh water release below Kotri barrage for saving the eco-system has always been tackled politically or administratively, rather than environmentally.
Officials are trying to give the impression that the flow of freshwater into the sea is wastage of water, knowing well that it is a mandatory for keeping the coastal environment alive. The people who regard floodwater escapage to sea as waste and wish to store it will have to understand the nature’s cycle before reaching on a conclusion.
A major proportion of floodwater flows towards sea during the month of August, a period when sea produces extremely high tides, thus chances of backlash of seawater are very prominent. Floodwater during August acts as a natural barrier for preventing the backlash of seawater. Therefore all attempts of blocking the floodwater during high flood season will be practically the same as to altering nature’s practice thus inviting seawater to intrude the coastal areas.
The alternatives to current water crises are efficient water delivery systems for on-farm conservation and usage of ground-water. The development of strategy to shift funding and water development planning toward more sustainable projects will provide long lasting social, environmental and economic benefits for progress and prosperity. Sound alternatives need to be evolved to meet critical energy and agricultural needs. We must move towards sustainable and indigenous renewable development and restoration of the rich integrity of the natural resources of Pakistan.
The government has been putting great stress on water development projects and for that matter brought various mega projects costing billions of rupees which would ultimately reduce further the fresh water flow past Kotri Barrage, degrading agricultural productivity, destroying mangroves and riverine forests and fisheries. The fundamental thing that should be duly recognized— before embarking upon decisions of such environmental significance that we should try to know and understand the risks and economic and social costs involved.
An injection of fresh water into the Indus could restore life and property. Several studies, including one major study carried out by the IUCN, have suggested that a discharge of 35 million-acre feet of water (MFA) is required at the delta to prevent further damage to deltaic area. But inter-provincial political differences and Islamabad’s failure to assert its regulatory role in ensuring a judicious distribution of water among the provinces are seen as underlying factors obstructing a solution.
Time, however, is running out. The destruction that has so far taken place, although drastic, is still reversible. If the authorities do not ensure the flow of the required quantity of water in the Indus, things would be irremediable.