Modifications of Physical and Chemical Soil Properties by Application of Treated Petrochemical Effluent
Wastewaters generated by Pólo Petroquímico do Sul (South Petrochemical Complex, Triunfo, Brazil) and treated up to tertiary level at SITEL (the integrated effluent treatment plant of the complex) are disposed of on land since 1983 at average rates of 140 m3/ha.day. With the purpose of studying the cumulative effect on soil properties of effluent applied under distinct conditions, four sites have been selected for comparison with adjacent blank areas. Soil samples have been taken for analyses from three different depths (0–30, 30–60 and 60–120 cm) and infiltration tests have been conducted on site. Soil pH, electrical conductivity and extractable Na, Ca and S increased in treated areas, while exchangeable Al and extractable Zn decreased. Exchangeable K and Mg and extractable P, Cu, Mn and B, as well as other toxic metals, did not show significant modifications in comparison with blank areas, with the exception of cadmium. Infiltration rates showed an average fourfold decrease in soils which underwent effluent application, due to increase in pH and exchangeable Na and decrease in Al, all these factors contributing to clay dispersion. The main alterations of forest and grassland environments consisted of trees falling caused by progressive weakening of the root system, due, in turn, to the change to a predominantly anaerobic soil environment and damage to some grass and shrub species, due to the direct impact of sprayed effluent. The observed results are compatible with the average effluent characteristics: low concentrations of toxic metals and negligible contents of residual organic toxicants, together with a high Sodium Adsorption Ratio. With respect to the chemical status of the investigated soils the service life of the disposal system can be extended to decades, provided the discharges of Cd are restricted. As far as infiltration rates are concerned, the results are worrying, however. The soils can be reclaimed by fallowing of the application areas and/or addition of Ca and Mg in order to decrease the Exchangeable Sodium Percentage.