The impact of coal ash and slag dump on the quality of surface and ground waters – A case study

This paper presents the assessment of the impact of coal ash and slag from the “Maljevac” dump on the quality of water of Paleški Creek, Montenegro. The obtained results confirm the negative influence of surface and groundwater from the dump on the water from the Paleški Creek. The results obtained by testing of the water samples, collected from Paleški Creek upstream of the dump, indicate that the water is qualified to be used for drinking, after simple physical treatment and disinfection. The results obtained for the water samples collected from the Paleški creek downstream from the dump indicate that the water could be safe for drinking only after the treatment that requires an intensive physical, chemical and biological processing, including some extended treatment.


INTRODUCTION
Although the material from ash and slag dumps may be used in the construction industry, 1 the largest amount of this material is stored in dumps, where leaching of the elements with toxic effects occurs. 2 Such dumps pose a serious threat to the surrounding soil, surface and groundwater 3 as well as to aquatic ecosystems. 4The ash produced from coal combustion is regarded globally as the waste which presents a serious health risk 5 and which must be treated in an adequate manner.
In order to provide sufficient space for the disposal of coal combustion by-products, ash and slag, generated at Pljevlja Thermal Power Plant, the earthen dam "Maljevac" was built in 1982 in the riverbed of Paleški Creek, approx-528 ĆIPRANIĆ et al.
imately 7 km from the town of Pljevlja, Montenegro.At the time it was the common practice to dispose of ash and slag waste directly onto the soil surface, without setting up any protection layers.Hence, the waste material has been stored directly onto soil in the riverbed of Paleški Creek for decades.
The aim of this paper is to determine the impact of coal ash and slag dump of a thermal power plant on the quality of ground and surface water.The impact evaluation is based on the investigations carried out in the area of the coal ash and slag dump "Maljevac" of the thermal power plant in Pljevlja (Montenegro).

RESULTS AND DISCUSSION
For the purpose of examining the effects of dumps on the surface water quality, the analysis of water in Paleški Creek was conducted as the dump directly influences the water of that stream (leachate from the dump is discharged into the stream).The water samples were collected upstream and downstream of the dump.The results of the water samples testing are presented in Table I  The results obtained by the testing of the water samples collected from Paleški Creek, upstream of the dump, indicate that the water of the stream is qualified as clear spring water.
Paleški creek downstream of the dump is characterized by a large number of the water quality parameters that are above the permissible limits (in Montenegro, determined by the directive 6 ): pH value, electrolytic conductivity, the content of sulphates, calcium, sodium, cyanide, etc.The water collected from Paleški Creek upstream of the dump is classified as "A 1 " category, while the one collected downstream of the dump is classified as "A 3 " category.According to the directive 6 , the category "A 1 " refers to the waters which can be used for drinking after simple physical treatment and disinfection.On the other hand, the category "A 3 " encompasses the water that could be safe for drinking only after the treatment requiring an intensive physical, chemical and biological processing, including extended disinfection and chlorination, coagulation, flocculation, decantation, filtration, activated carbon adsorption as well as disinfection with ozone and chlorine.
The groundwater samples were surveyed from the two boreholes drilled in the body of the dam and in its plinth.Based on the pH values presented in Table I, it is noted that the mean value ranges from neutral to mildly acidic.Even though the pH value of water near the surface is basic, the obtained mean value results from the fact that the dam was constructed of marly clay aterial considered acidic.Electrolytic conductivity is increased.The elevated concentrations of calcium, magnesium, potassium, sodium, cadmium and sulphates are registered.The elevated concentration of Ca is a result of calcium oxide dissolution from the coal ash.In addition, there are traces of arsenic within the borehole in the plinth of the dam.The core of the problem is closely related to the solubility of the ash components in water.In terms of the pollution control of ground waters, the SO 4 2-are the most significant as it does not bind tightly to the soil.Due to this feature, sulphates can be used as "tracers", i.e., it would be possible to determine the pollution level of ground water from the dump by means of tracking the ions. 7he water samples at the dump were collected from one point at the very surface of the dump.Based on the obtained results it is concluded that these samples are characterized by increased the electrolytic conductivity.Moreover, the pH value is alkaline.The pH value of the water from boreholes depends on the depth of the borehole.The minimal and the maximal concentrations from Table I are similar, which indicates that there is a constant inflow of wastewater.

CONCLUSION
The impact of the coal ash and the slag dump of the thermal power plant on the quality of surface and groundwater is exceedingly harmful.The quality parameters of the surface water samples (pH value, electrolytic conductivity, the content of sulphates, calcium, sodium, etc.) are above the permissible limits.The groundwater in the vicinity of the dump is characterized by the increased elec-trolytic conductivity, high concentrations of zinc, sulphates, calcium, magnesium, potassium, sodium, cadmium, etc.The harmful effects of the dump on the quality of surface and groundwater should be eliminated in these cases (when the dump was built directly on the ground, without necessary protection) by means of employing the adequate measures of remediation and recultivation.In addition, the material should be removed and used in making concrete and concrete blocks (used in construction industry) as well as for other industrial purposes.
. The sampling points are marked in Fig. S-1 of the Supplementary material to this paper.