TRACE ELEMENT ASSESSMENT IN WATER OF RIVER KASSA SYSTEM, JOS-PLATEAU NIGERIA

Detailed geochemical investigation of water from River Kassa was carried out to assess trace element distribution in the river system. The results showed significant enrichment of these elements Pb, As, Co, Cr, V, Ni and Fe. The value of index of geoaccumulation (Igeo) is approximately 2; for Zn and Pb which indicates, moderate contamination. Areas of the river system with anomalous value of trace element concentrations are those where mine tailings have been deposited close to the river channel or places where run off from adjoining farmlands and settlements enter the river. The major sources of contamination are mining and agricultural activities on the flood plain of this river. The anomalous concentration of As, V, Cr, Co and Pb need further investigation in view of their health implications on man and aquatic lives.


INTRODUCTION
River Kassa is a major river which flows through several mining settlements and inhabitants of these areas depend on it for their domestic, agricultural and mining activities. The river drains an area of more than 1000km 2 during periods of high flow with a water shed of about 100km 2 . Trace elements may enter soil and aquatic environments via mine waste, industrial waste disposal, atmospheric deposition, application of fertilizers and pesticides (Forstner, 1983). The introduction of waste products into rivers and estuaries, especially those in industrial and highly populated cities has led to a significant increase in metal contamination (Fanfani, 1997, Cravotta and Bilger 2001, Horowitz et al 1995, Wood, 1996, Rabe and Bauer 1997.
Mine tailings from mining firms in these localities are discharged directly into the river system and flood plain of this river. Tributaries of Kassa River are channeled into their concessional area for processing of tin ore. The Kassa river system which is the southeastern part of Jos-Plateau begins at the point of major hub of tin mineralized communities. It empties into the Gongola River. Land use on the path of flow is mainly agricultural and washing of tin ores.
The flood plains are cultivated yearly for dry season farming and the farmers use several pesticides, herbicides and fertilizers to control weeds and nourishment of their farmlands which on being washed by rainfall are equally discharged into this river.
This could make the water body unfit for human consumption and thus constitute health hazards for inhabitants of these communities that depend on this river for their water needs because of complete absence of potable water supply in most rural areas of Nigeria.
The objective of this paper is to assess the distribution and levels of trace elements in the water Kassa with a view to highlight some hazards that local communities will have to face in the near future. The focus is to identify important contamination sources: a dump on the flood plains made up of mine tailings and the leachates from abandoned tailings on the Jos-Plateau that drains into this river during rainy season.

STUDY AREA
Kassa river is located on Jos-Plateau with an areal coverage of about 1,200km2 (Fig. 2).The climate is typically tropical with a mean annual rainfall of about 1400mm.The annual water flow rate of the river is 5200m 3 (Hossain et al 1986).The main direction of flow is from south-west towards the northern part of the area due to structural trend The geology is dominated by rocks of the Younger Granite Complex. Four main types of granites are recognized in the area (Ekwueme, 1993). The first type is hornblende pyroxene fayalite (Hpf) granite that usually forms ring dyke while the second group is hornblende biotite granite (Hbg). The third type is biotite granite (Bg) which forms the most abundant and widespread rock type in entire area. The fourth group of granites is the riebekites granites (Rg) which are mainly found in the northern part of the area as small sheet like intrusions.

MATERIALS AND METHODS
Twenty sampling sites in the Kassa River system were selected along the path of flow with the aid of a grab samples (Fig. 2). The sampling was carried out in February 2008 during dry season when dilution effect of storm runoff can be eliminated. Water samples were collected in acid-rinsed polyethylene bottles and acidified with nitric acid (HN0 3 ) to a pH of 2 and stored in ice-chested coolers. Samples were analysed for Pb, Zn, Cu, Cd, Fe, As and Hg by inductively coupled plasma atomic emission spectrometry (ICP-AES) at Activation laboratory in Canada.
A. Momoh, Department of Geology and Mining, University of Jos, Nigeria E.G. Ameh, Department of Earth Sciences, Kogi state University, Anyigba Nigeria.

Data Evaluation
Quantitative index was used to describe the distribution pattern of trace elements and to allow easy comparison of the analysed parameters. The index is index of geo-accumulation as proposed by Mueller (1979). Index of geoaccumulation (Igeo) has been used to evaluate the degree of metal contamination in terrestrial and aquatic environments (Tijani, 2004, Sutherland 2000and Manjunatha et al 2001. It is expressed as: Where Cm is the measured concentration of metal in water and Bm is the background concentration value of metal (m) (Taylor and Mclennan,1985). 1.5 is a factor for possible variation in the background concentration due to lithologenic differences. Igeo is classified into seven classes as follows: 0 = practically uncontaminated; 0 -1 = slightly contaminated, 1 -2 moderately contaminated, 2 -3 moderately -highly contaminated, 3 -4 highly contaminated, 4-5 highly to very highly contaminated. >5 very highly contaminated and Igeo of 6 and above is said to be very highly contaminated.

RESULTS
The concentrations of trace element analyses results for water samples and WHO Standard (1999) alongside with mean composition of World Rivers (MCWR, Hem, 1985) are presented in table 1. The mean concentration values in the samples are generally less than 1mg/l except Fe whose mean value is 1.43 mg/l. The higher concentrations of iron could be as a result of weathering processes of parent rocks resulting in the production of iron oxides along side with the clay minerals (Davis and Eary, 1997;Ramstedt et al 2003 andLevy et al 1997). The concentration of Co and Fe increases from Hpf to Rbg/Bg while other trace elements shows no defined trend in concentration from one rock type to the other.
Average concentrations of V, Cr, Ni, As and Pb are about five times that of WHO Standards while average Co concentration is about twice that of WHO Standards of 1999. The order of concentration is Fe > V ≥ Cu > Cr > Ni > Co > Zn > Pb > As. (Fig. 3).
On comparing the analysed trace elements with the WHO standards, the observed range of concentrations of the following trace Elements Fe, V, Cr, Ni, Co, Pb, As are above the permissible levels of drinking water.
While Zn and Cu are below the permissible levels. These do not imply absence of contamination as the observed lower concentrations could be attributed to partitioning and accumulation within the sediment phase of Kassa River System. Possible changes in chemo-thermodynamic conditions can lead to remobilization into liquid phase (Tijani 2004).
Evaluation of trace element distribution pattern was determined by using index of geoaccumulation (Igeo) as the quantitative index.

A. MOMOH AND E.G. AMEH
The respective values of mean composition of world rivers (MCWR) were used as baseline values to give an idea about the levels of trace elements in the river system.

DISCUSSION
The results of the trace element analysis on Kassa River show that metals such as Vr, Cr, Ni, As, Pb, Co, and Fe have different concentrations on the entire flow part (Table 1). The upstream of the river and close to the historical mining and smelting sites, showed enrichment of analysed elements which decreases from upstream to downstream of the river. With the exception of Cu and Zn that fell below WHO (1999) standard, other trace elements were above WHO (1999) standard.
The index of geoaccumulation (Igeo) as presented in table 2 indicates Igeo value of 2 for Zn and Pb indicating moderate contamination. V, Cu, Cr, Ni have Igeo of above 6 which is indicative of 100 foldenrichment of the metal (Mueller, 1979). It should be noted that the order of contamination defined by the Igeo is different from that of the absolute concentration outlined earlier. This is as a result of normalization with respect to mean composition of World Rivers which gives more reliable estimates of the degree of contamination of the metals with respect to the natural geogenic inputs (Tijani et al 2004).

CONCLUSION
The chemistry of river water is controlled by weathering of different lithologies and by anthropogenic inputs. The concentration of Co and Fe increases from Hpf to Rbg/Bg while other trace elements shows no defined trend in concentration from one rock type to the other Trace element concentrations in the upper Kassa River are generally higher than the lower portion.
Heaps of several mining wastes are dumped in this portion of the river and south-shore tributaries. Elements concentrations in this part are highly governed by dilution effects due to inputs of inorganic particles from mining wastes and run-off from the tailing dump on the flood plain.
In conclusion, long term and detailed research is necessary to resolve the extent of mining contamination, such research requires considerable financing and the combined efforts of Federal and State agencies of environmental protection are necessary. Consequently, the following are some recommendations: Rain-proofing of the mine tailings that were dumped on the flood plain of this river should be embarked upon by relevant government agency and a limitation of the use of this river by mining companies on Jos-Plateau may also be suggested.