Geochemical and Biochemical Properties of Coalmine Tailings of North Eastern Collieries of Assam, India.
Hari Prasanna Deka Boruah, Regional Research Laboratory, (CSIR), Jorhat, RRl-Jorhat, NH37, Jorhat, 785006, India
The coal-mining region of Assam located in northeastern India lies between latitude 170 23 / - 270 23 / N and longitude 950 35 / - 960 00 E. Opencast mining over the past century in this area has been producing waste rock (as mine tailings) at a stripping ratio of 1:14 with respect to coal. We compared physicochemical and biochemical properties of mine tailings known as overburden (OB) from the Tirap mines in Assam to soil samples from un-mined areas as well as to soils having no record of cultivation (fallow) for 25 years. The microbial population in tailings was about one-tenth that found in un-mined and fallow soil. The particles of – 0.355 mm size in mine tailings were 0.5% of the total content of the boulders, cobble, and pebble and of stones and were highly acidic (pH 2.0-2.5); however in un-mined and fallow soils (both predominantly -0.335 mm) the pH was neutral. In OB dumps, dehydrogenase and b-glucosidase enzyme activity were not detected and rhodanase (1.83 m mol thyocyanate g –1 h -1), phosphatase (6 m mol p-nitrophenol g –1 h -1), and urease (6 m mol p-NH3 g –1 h –1) activities were very low. These results were also confirmed by the low microbial biomass (64.33 mg kg -1) in OB. Thus, OB materials have no catabolic activity and a low capacity to transform nitrogen and phosphorus into bioavailable form. Geochemically, Al, Fe, Mg, Ca, Mn and S values are 3- to-5-fold higher in OB than in un-mined soil and of fallow soil. Cd, Hg, Pb, and Co do not occur in un-mined soil. Hg and Co do not occur in fallow soil, and OB contain 0.4 ppb Hg, and 128.12, 20.11 and 27.29 ppm Pb, Cd and Co, respectively. Some of these metals, as essential and non-essential elements, are above permissible limit in OB. Thus, although they comprise only 0.5% of the tailings, the -0.355 size fraction is highly acidic, low in biological activity, and contains above-threshold values of trace and heavy metals. Keywords: Biochemical, Geochemical, Mine tailings, Overburden (OB); Metals.