For people travelling along national highways, scrapped slices of road piled along the countryside are a much common sight. In fact, they have become so much a part of the scenery that most people pass by without even noticing while some consider it as trash marring their view and only a few actually foresee the consequences for subsoil and groundwater contamination.

These days a vast area on the lower periphery of Rawal Dam, Islamabad, is cluttered with road debris removed from old pavements to build a new six-lane freeway linking the airport and the city. Here, the soil remains saturated for at least part of the year due to the seepage from the lake and hence concerns arise about the leachate from the saturated soil matrix containing polycyclic aromatic hydrocarbons (PAHs), which will ultimately appear in the groundwater. While the use of coal tar in road construction presents unacceptable hazards to environment, dumping the debris containing coal tar is simply unimaginable. Realising the overall environmental consequences, it pains to see authorities responsible for the safety of the environment so indifferent to these issues of today. Let us analyze what can go wrong.

Historically, coal tar has been extensively used for coating road stone to improve adhesion between stones. In the last few decades, coal tar was replaced with bitumen-based products to reduce environmental hazards. Use of asphalt is recommended now over coal tar and bitumen for environmental reasons. This is because coal tar is a condensation by-product obtained by burning coal in the absence of air. More than 400 compounds have been identified in coal tar including complex combinations of hydrocarbons, phenols and heterocyclic oxygen, sulphur and nitrogen.

Bitumen, on the other hand, is derived from the distillation of crude petroleum, and is also a complex mixture containing a large number of chemical components but of relatively high molecular weight.

Both coal tar and bitumen contain PAHs but the later has significantly lesser concentration. PAHs are carcinogenic while the chronic effects of some of the constituents in coal tar include damage to the liver and harmful effects on the kidney, heart, lungs and the nervous system. PAH mixtures can possibly be phototoxic as well.

Since the concentration of PAHs found in coal tar products is higher than in bitumens, the carcinogenic potential of coal tar was well recognized and widely accepted since 1985 by the International Agency for Research on Cancer. Coal tar is completely soluble in a wide range of organic solvents but it is also slightly soluble in water and may enter in the groundwater by leaching. The water soluble compounds will tend to move to groundwater and/or subsurface soils (where degradation rates are typically slower) when the soil gets saturated especially during the monsoons. The leachability of coal tar products is especially high due to the fact that these compounds are not adsorbed by soil particles. New concepts of solute movement through porous media suggest that the leaching substance can actually by-pass much of the matrix and can appear in groundwater quickly.

When open to atmosphere, coal tar breaks down very fast into benzene, toluene, phenol, naphthalene, anthracene, and other volatile compounds in hot, windy, and sunny climates. The product compounds are expected to disappear quickly by evaporation and degradation into the environment in our climatic conditions. The fact that coal tar contains compounds of substantially lower molecular weight, many of which are liquid at ambient temperatures, is a matter of great concern.

Therefore, spreading coal tar debris should be prohibited to safeguard the environment - at least in the capital. The continued disposal of coal tar products flies in the face of global best practice.

The Pakistan Environmental Agency, Ministry of Environment, the body trusted with the responsibility to safeguard our environment, probably never raised the issue at any forum neither did it talk about the realization and capacity to measure the parameters.