Save the treasure hidden underground

We live in a country blessed with natural resources. Together with indigenous flora and fauna, mountains, rivers, lakes, and other scenic wonders represent our wealth; but perhaps, we have forgotten that we are rich with groundwater too. It is a treasure hidden underground, and not found in abundance everywhere. The general perception of groundwater as a freely available, unlimited resource, has under-estimated the ecological and economic value of groundwater. This is ofcourse the tragedy of the commons that the famous ecologist Garrette Hardin pointed out several decades earlier. Still it is not too late to understand the uprising tragedy of groundwater, at least in the local context.

Inland fresh water resources exist as groundwater or surface water. Compared to surface water bodies such as rivers and lakes which are frequently affected by human activities, groundwater is less susceptible to direct pollution. Groundwater is usually considered as the most reliable, safe, and economical source of drinking water. Hence it is not a surprise that about 80% of Sri Lanka's rural population depend on groundwater for drinking and sanitation purposes (Water Resources Board). Groundwater serves not only the living beings, but the natural ecosystems by feeding surface waters.

Unfortunately, the stress on groundwater systems has been intensified by the rapid social and industrial developments taking place all over the country during this postwar development arena. The pollution of rivers, lakes, and other surface water systems is readily observable in a change of colour, smell, or appearance. But in contrast, groundwater pollution occurs undetected over long time periods until some unexpected incident triggers our attention. The recent disaster at Rathupaswela is one such incident which triggered our attention. Without waiting for the next incident, it is time to understand how groundwater pollution occurs and to initiate preventive and corrective measures. The purpose of this article is to make readers aware of the significance and the potential sources of groundwater pollution in Sri Lanka.

Significance of groundwater pollution

The behaviour of groundwater systems is complex. Groundwater is stored in porous media such as sand or gravel, below the topsoil. Unlike surface water, groundwater flow through porous media is relatively slow and dispersive. Groundwater residence times in aquifers can be hundreds of years. Groundwater recharge through rainwater percolation from earth and groundwater discharge into surface water systems are both important stages of the natural water cycle. Once groundwater is polluted at one location, the pollution can spread over several dimensions in long timescales. Pollutants can be detected from wells or boreholes located far away from the source, after several years. Certain solutes like nitrate, once mixed in groundwater, keep on flowing with groundwater for decades and centuries until being discharged into a surface water outlet. Thus, groundwater fed surface water bodies are also polluted by polluted groundwater. Due to its slow movement, natural cleanup of groundwater takes many years; treatment and remediation is also expensive. So it should be understood that groundwater pollution has severe and long term consequences.

Agricultural pollution

Many developed countries including the United States have identified agriculture as the most significant threat to their freshwater resources. Usually the policy makers, farmers, and even the general public are aware of the environmental damages caused by agricultural runoff carrying chemical fertilizers, pesticides, and livestock effluents into surface water bodies. But the majority is not aware that the worst damage caused by agriculture is groundwater pollution by chemical compounds such as chlorides, nitrates, potassium and arsenic leaching into groundwater systems from agricultural land.

Since nitrogen is an essential nutrient for the growth of plants, and plants can effectively uptake nitrogen in the form nitrate, fertilizer containing nitrate is extensively used in agriculture to improve crop harvest. When in access, nitrate in the soil has a greater potential to leach into underlying groundwater systems. Excess nitrate in drinking water can cause serious illness in humans. Recent research has revealed that in many agricultural areas of Sri Lanka, particularly in the North and North Western provinces, groundwater nitrate levels have risen well above 50 mg/l which is the maximum acceptable nitrate concentration recommended by the World Health Organization.

Urbanization and sewage disposal

By 2030, 70% of Sri Lanka's population will live in urban areas (Ministry of Environment). Though the population in urban and suburban areas is growing fast, neither public water supply systems nor public sewage and wastewater disposal systems are available in most of those areas. In such highly residential areas, both domestic water supply wells and soakage pits are located within small blocks of land. Soakage pits running deep below the topsoil (the topsoil facilitates absorption and biodegradation), allows seepage of hazardous compounds such as nitrates and phosphates contained in sewage. On the other hand, when many residential wells are located closely and more water is taken out, less water is left in the aquifers to dilute the incoming pollutants. A UNEP (United Nations Environmental Program) report has highlighted that in some areas of Jaffna peninsula, groundwater nitrate levels have risen above 100 mg/l due to both sewage and agricultural pollution.

The urban and regional councils do enforce regulations such as a minimum distance between water supply wells and soakage pits to ensure the safety of drinking water. But due to the dispersed and transient nature of groundwater flow, such regulations are not sufficient to protect groundwater from sewage pollution.

Impervious cover

Impervious cover is any impermeable surface such as concrete, roads, and rooftops that impedes the infiltration of rainwater into earth. With all sorts of developments including construction of highways, buildings, and parking lots, the amount of impervious cover increases. It effectively reduces groundwater recharge, the natural phenomena of water percolating through the land surface and replenishing the groundwater system underneath. When groundwater aquifers are heavily utilized without frequent replenishment, depletion is unavoidable.

Various strategies can be adopted to reduce the negative impacts of impervious cover, and most of these are preventive measures to be incorporated with development. For example, storm water management techniques can be adopted to direct storm water to landscape areas, increasing infiltration and reducing surface flows.

Landfill sites

Garbage landfill sites are now becoming a nuisance for urban residents, for passengers on urban roads, and for the entire country. In rainy seasons, rain water percolating through landfill sites can carry several pollutants into the groundwater system.

Industrial waste sumps

Reports on high pH levels, oils, and chemicals in groundwater wells surrounding manufacturing factories have been allegedly linked with industrial wastewater. When industrial wastewater is discharged into open land or dug wells, pollutant leaking can take place. While sufficient treatment is required before releasing industrial wastewater into the environment, industrial wastewater tanks should be properly designed to avoid leaking of hazardous substances.

Mining

Intensive sand mining in rivers deepen the river beds, subsequently lowering groundwater levels.

Over-abstraction

Owing to the increasing demand for fresh water from domestic, agricultural and industrial users, groundwater abstraction from aquifers has been intensified in recent years. Over-abstraction lowers groundwater levels. When the groundwater levels drop below surface water levels, polluted surface water can flow into the groundwater system. Similarly, in coastal areas, over-abstraction can cause salt water intrusion into the groundwater system.

Groundwater is a precious natural resource. Because it is hidden underground, groundwater is less susceptible to direct pollution; but for the very reason, the consequences of groundwater pollution are also overlooked. The economic development and luxuries living always come with threats to the natural resources. It is important that we recognize the potential threats as early as possible and initiate preventive measures. Towards this mission, the environmental regulators, scientists, and other stakeholders have to work together to save our groundwater resources.

(The writer is a senior lecturer in Wayamba University of Sri Lanka. He has conducted research on controlling groundwater pollution)