Effect of long-term application of oil refinery wastewater on soil health with special reference to microbiological characteristics

The ground water (GW), treated wastewater (TW) and the soil at the agricultural farm in the premises of the Mathura Oil Refinery have been monitored for various physico-chemical characteristics, over the last 12 years (1987-1998). It was noted that the heavy metal content in the soil, repeatedly irrigated with TW, was increasing because the wastewater itself was a better source of these metals than GW. However, the level of these metals, both in the wastewater and the soil, was very much below the permissible limits. The quantitative analysis of various functional groups of microorganism and their interaction with metals revealed that the viable count of aerobic heterotrophs, asymbiotic nitrogen fixers, actinomycetes and fungi were in the ranges of 2.6 x 10(6)-7.2 x 10(7), 5.0 x 10(4)-6.8 x 10(5), 2.0 x 10(4)-2.8 x 10(5) and 2.5 x 10(4)-7.5 x 10(5), respectively. The presence of Rhizobium spp. in the soil was shown by the nodulation in the leguminous crops cultivated in the experimental field. The significant viable counts of above functional groups of bacteria on each heavy metal supplemented plate indicated that these populations could tolerate high levels (greater than or equal to 200 microgram/ml) of metal. Among them, nitrogen fixers and aerobic heterotrophs (largely, Gram-negative) had a higher degree of resistance to heavy metals than actinomycetes. Similarly, the strains of Rhizobium, isolated from the experimental field, showed a higher degree of resistance under in vitro conditions and the minimum inhibitory concentration (MIC) was in the range of 50-800 microgram/ml, determined by a plate dilution method. It may, therefore, be deduced that 12 years repeated application of treated wastewater had not significantly changed the microbial dynamics of the soil and possibly the bacteria had adapted to the changed soil environment by developing various levels of metal resistance.