Journal of Applied Sciences and Environmental Management, Vol. 11, No. 4, 2007, pp. 19-24
Physicochemical properties of pit-water from ishiagu lead/ zinc (Pb/Zn) mine as an index for alkaline classification of the mine drainage.
AROH, K N1; EZE, C. L2; ABAM, T.K.S3 ; GOBO,A.E4 AND UBONG, I .U5
1,2,3,4 Department of Environmental Management, Institute of Geosciences
and Space Technology (IGST)
E.mail: email@example.com (+234 080 560 391 46)
*Corresponding author: Aroh, K N1
Code Number: ja07087
The nature and chemical composition of mine pit-water was investigated in Ishiagu lead/zinc (Pb/Zn) mine in southeastern, Nigeria. Laboratory determination of some physicochemical parameters namely, temperature, total dissolved solids (TDS), conductivity , chloride, pH, alkalinity, hardness, sulphate and dissolved metal constituents like calcium and magnesium were assessed in the three mine pits using standard methods. The results showed that Ishiagu Pb/Zn mine is slightly acidic with moderate levels of dissolved metal constituents. Seasonal variation observed in the parameters indicate higher levels for pH (6.7-8.1), alkalinity (36.4--115.3mg/l) and sulphate (6.8-84.4) in the wet season when compared with corresponding values, pH (6.1-7.3), alkalinity (31.3-98.4mg/l) and sulphate (6.1-78mg/l) in the dry season. The consistently the higher levels of pH, alkalinity, hardness, calcium, magnesium, total dissolved solids (TDS) and conductivity in more than 66% of the mine pits (mine 1and mine 3) investigated across the season is an indication of changing state of the mine pit-water to more alkaline mine drainage. This phenomenon is attributable to preponderance and buffering effect of carbonate minerals in the area.
Open-pit mining involves the excavation of large quantities of waste rock in order to extract the desired mineral ore. This was the predominant mining method employed in the extraction of lead/zinc (Pb/Zn) mineral in Ishiagu, Ebonyi State, southeastern, Nigeria.
The mine became operational in 1965 and closed 17 years after intensive extractive activity. The poorly decommissioned mine left as part of its environmental legacy to the community, stock piles of marcasite and abandoned mine pits measuring about 15 meters in depth. Eze and Uko (2003) reported that part of the waste in Ishiagu Pb/Zn mine include shales , sulphide and carbonate minerals. The oxidation of sulphide in these waste according to the authors have contributed to AMD in the mine. Similar findings were also documented by Aroh (2003). Acid mine drainage (AMD) occurs when sulphide bearing minerals in rock is exposed to air and water changing the sulphide sulphur to sulphuric acid (Carlos and James, 1997). The acid can dissolve heavy metals contained in the waste rock and tailings such as Lead, Zinc, Copper, Arsenic, Selenium, Mercury and Cadmium into ground water and surface water.
The waste rock and the exposed bed rock walls from which it is excavated are the source of most of the metal pollution caused by mining (Carlos and James, 1997).
Acid mine drainage (AMD) has been identified as one of the greatest environmental impacts of mining on the environment. Significantly, the impact on surface and ground water varies depending on the nature and chemical composition of the mine water. In Ishiagu Pb/Zn mine, over 60% of the mine drainage composition from the three mine pits are slightly acidic. The mean pH values ranged from 6.7 to 8.1 in wet season and 6.1 to 7.3 in dry season (Aroh 2003). Eze and Uko (2003) recorded a much lower mine pH of 5.6 and 5.1 for wet and dry season respectively in a separate study. Williams and Rose (1982) enumerated the factors that influence the generation of acid mine drainage to include abundant pyrites in overburden, presence of iron oxidizing bacteria and low pH. Conversely, significant presence of limestone, bicarbonate or other neutralizers and restricted access to air and unexposed pyrites lower it generation. Cravotta et al ( 1990), has also shown through series of chemical equation that significant carbonate minerals can neutralize acid mine drainage (AMD). The carbonate rock which produces significant alkalinity or bicarbonate concentration in ground water, surface water and mine drainage samples are limestone and dolomite and the principal carbonate minerals are typically calcites (calcium carbonate) and dolomite (calcium magnesium carbonate) (Brady and Hornberger, 1990). Plumlee et al (1993, while sampling metal mine drainage in Colorado agreed that drainage quality was controlled by ore deposit geology, climate and mining method. Pyrites content and acid buffering capacity were also considered to be the most important geological controls on pH and metal content of the mine water.
In Ishiagu Pb/Zn mine, the mine water composition is considered slightly acidic with moderate loads of dissolved metal constituents (Aroh, 2003, Aroh, et al 2006). Considering the significant volume of mine waste and large scale presence of carbonate minerals particularly dolomite and siderite in the area, one wonders if indeed the AMD in the area is not drifting towards neutral or alkaline state. The conditions that favour the formation of neutral or alkaline mine drainage (NAMD) in Ishiagu mine were partly documented by the preliminary works of Eze and Uko (2003), Aroh (2003) and Aroh et al (20006).The huge presence of limestone deposits in the area have exposed more carbonate minerals to chemical processes. Again the long period of inactivation of the mine may have equally masked existing pyretic compounds from sustained weathering process and have thus paved way for more weaker sedimentary deposits to predominate. The study therefore looks at the physicochemical properties of the mine pit-water in relation to conditions that favour the formation of alkaline mine drainage.
MATERIALS AND METHODS
Ishiagu Pb/Zn mine is located between latitude 60 05”N and longitude 70 35” E in the South-eastern end of Nigeria (Figure 1). The mine site is an extension of the huge Pb/Zn ore deposit from Ishiagu through Benue up to Zurak in the North (Nehikhare, 1987). The site is characterized by significant quantity of weathered and unweathered sulphide minerals including shales, siderite and dolomites (Eze and Uko,2003) . The climate is typical of the tropics. The average temperature range is between 25 and 31° c while an average rainfall of about 1500mm to 2400mm is prevalent (Akintola, 1986 and Ofomata, 1965). Following rainfall events precipitation and surface runoff results in seasonal fluctuations in volume of the mine pit water. There are three mine pits coded mine pit 1 (M1), mine pit 2 (M2) and mine pit 3 (M3) in the area.
SAMPLE COLLECTION AND ANALYSIS
Mine-pit water samples were collected at three mine pits station: M1, M2 and M3 with 2-liter polythene bottles at about 20cm depth. The samples were analyzed at I.PS laboratory of the Rivers State University of Science and Technology, Port Harcourt.
The pH of the sampling was measured using corning pH meter (model -7), Conductivity with Horiba water checker (model-v-10) and total dissolved solid (TDS) with lovibod instrument (model-cm-21). Temperature measurement was done in the field with a mercury in glass thermometer. Other parameters like alkalinity, hardness, Calcium, Magnesium and Chloride were determined titrimetrically while sulphate was determined by spectrophotometric method (Spectronic instrument model 21D) in line with APHA(1995) guidelines.
RESULTS AND DISCUSSION
The physicochemical properties of the surface water from three open-pit mines of Ishiagu Pb/Zn mine are presented on Table 1. The result is an illustration of the general trend in some physicochemical properties of the mine pit-water from Ishiagu mine. The data show that mean pH of mine 1(M1) and 2 (M2) were slightly acidic (6.1-6.8) while mine 3 (M3) was alkaline (7.3-8.1) on the pH scale .In the study it was observed that high mean pH values were recorded in the wet season (6.7-8.1) compared to the dry season’s (6.2-7.3) levels. The alkaline composition observed in mine pit 3 (M3) pit unlike the other two reflects the increasing capacity of (buffering capacity) of mine 3 to neutralize the acidic minerals (pyrites FeS2 ) to the observed pH seen. This finding is further strengthened by the high alkalinity levels (98.4-115.3 CaCO3 mg/l) observed in mine pit 3. A comparison of the mean alkalinity values (115.3 CaCO3 mg/l) (Table 1) recorded in the wet season with the concentration (160 Ca CO3 mg/l ) reported by Eze and Uko (2003) in the same season confirms the influence of carbonate minerals of dolomite and siderite earlier identified by the authors as contributory factors. In Pennsylvanian mine, a comparative data from mine water investigation shown in Table 2 indicate a similarly high pH and associated parameters in a mine generally characterized by abundance of carbonate minerals.
Details in Table 1 also reveal that temperature of the mine water samples in the wet season ranged from 30.3 to 30.7 0 and slightly higher mean values (31.7 to 33 0 C) in the dry season period. The higher temperature usually experienced during the dry season spell may be responsible for the higher dry season values. Moreso, the semi closed nature of the mine ponds which tend to concentrate the solar energy in a smaller surface area may have moderately raised the temperature values unlike in streams that tend to disperse this same energy along its flow course (Aroh 2003).The pH and concentration of alkalinity and sulphate were moderately higher in the wet season compared to other parameters investigated. The increased rainfall and associated mine drainage runoff could be attributed to this observation. The range for alkalinity was least in mine pit 1 (M1) (31.3-36.4 CaCO3, mg/l ) and highest in mine pit 3 (M3) (98.4-115.3 CaCO3 mg/l).
Although alkalinity was significantly (P<0.05) higher in mine pit 3 (M3) than mine 1(M1) and mine 2 (M2) pits, it equally exhibited similarly raised wet season values with sulphate. In Table 1 it was further observed that an exceptionally mean peak values for conductivity, total dissolved solids (TDS), hardness, calcium, magnesium and chloride were recorded in mine pit 1 (M1) and mine pit 3 (M3) compared to mine pit 2 (M2).The mean level of these parameters were observed to be slightly higher in the dry season than the wet season values. This may be due to the higher evaporation from increased temperature peculiar to the dry season period. There is no doubt that this has a concentrating effect on dissolved substances in water. Eze and Uko (2003) posited that the process of rock weathering was strongly influenced by temperature and by the amount and distribution of precipitation.
Acid mine drainage formation was found to be more severe in humid area with moderate rainfall where rapid oxidation and dissolution of exposed minerals can occur (Hem,1970).The consistency in the elevated levels of these parameters in mine 1 (M1) and mine 3 (M3) presents the existence and occurrence of closely related minerals in the mine pits. Rement (1996) and other investigators (Eze and Uko 2003) had separately noted the presence of carbonate minerals particularly dolomite as the primary minerals of the ore deposits in the area. APHA (1995) also reported that calcium results from passage of water through limestone deposits including dolomite, gypsum and gypsiferous shale. This however contributes to total hardness of water. The exceptionally high level of total dissolved solids (TDS) and conductivity in the two mine pits (M1 and M3) is also an indication of the degree dissolved metal constituents in the water (Ubong and Gobo 2001).
The study has revealed a gradual rise in pH and some physicochemical parameter notably alkalinity, Calcium, Magnesium, Chloride and Hardness due to the proportionally high deposit of carbonate minerals in two of the three mine-pits investigated. A comparison of the data in the present study with results of similar mines elsewhere in the world, shows that Ishiagu Pb/Zn mine is gradually drifting to neutral or alkaline mine drainage state. However, the period of this shift is poorly known considering a possible resurgence of AMD on exhaustion of the potentially neutralizing source in the mine.
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