Long-term application of olive-mill wastewater affects soil chemical and microbial properties

Abstract

Disposal of untreated olive-mill wastewater (OMW) is a major environmental problem in many Mediterranean countries. This study assessed the impact of OMW application on soil microbiological properties and explored the relationship to soil chemical properties during a 9-month, periodical soil-sampling campaign in a pilot study area in Crete, South Greece. Cases studied involved: direct application of OMW on soil; OMW disposal in active evaporation ponds; sites hosting evaporation ponds that have been inactive for the past 9 years; sites downstream of active evaporation ponds; and control soils, upstream of the waste-disposal ponds. Long-term OMW disposal on land affected the main soil chemical properties. Applicability of the results from the systematic monitoring was confirmed by results obtained in other OMW disposal sites around the pilot area. Soil microbial properties (microbial activity, microbial biomass carbon, and metabolic quotient) were considerably affected by OMW disposal. Moreover, seasonal changes of soil properties revealed short-and long-term residual effects due to OMW disposal. Significant correlations were observed among soil microbial characteristics and soil chemical properties, clearly indicating a close relationship between chemical properties and the transformation of microbial communities in soil after OMW land spreading. The determination of a key set of chemical and microbiological parameters that can be used as indicators for monitoring soil quality at olive-mill waste-disposal areas will verify the efficiency of the techniques used for the land disposal of OMW and will consequently promote their sustainable management.

Evaluation of the influence of olive mill waste on soils: the case study of disposal areas in Crete, Greece

“…The increase in potassium concentration in soils that accept OMW has been reported by many researchers [10,11,13,20]. The very high exchangeable K content of the pond soils (Figure 7), especially in the upper soil layer, confirms also the observations about the main pilot area of the PROSODOL project.…”

“…These authors decided to consider as guideline values the concentrations of phenolic substances in the control soils of their study, which varied from 14 to 25 mg/kg. Other values have been proposed as thresholds such as the value of 10.0 mg/kg as a threshold concentration of phenols in paddy soil or the value of 40 mg/kg, which has been adopted by the Netherlands [10,11,21,23]. In the present study, a high percentage of the control samples (45%) had polyphenol values between 41 and 108 mg/kg, which are considered high, although the reason for that is unknown.…”

“…Untreated OMW can cause severe environmental degradation due to its very high organic and inorganic load, foul odour, and intense dark black/brown color [6,7,[10][11][12]. Furthermore, landspreading and disposal into evaporation ponds without taking protective measures are practices that are observed in many olive producing areas and mainly small mills due to the low cost of these practices and also lenient control measures by competent authorities.…”

“…Experiments with controlled addition of OMW to soils for usually a few years (1-3 years) have shown that a severe impact on soil electrical conductivity (EC), polyphenol content and pH is not expected although a significant increase is observed in potassium, phosphorus, nitrogen, and organic matter (OM) concentrations in soil [13,20]. Even though in most of these cases and after a sufficient time period, usually 4-6 months, or after harvesting of cultivated plants, the soil was restored in terms of increased value of its constituents, this is not the case for soils where untreated OMW is disposed of in an uncontrolled manner and for many years [10,11,21]. Another important difference between these two cases is soil quality.…”

“…Therefore, it is obvious that soil behaviour in these two cases does not have much in common, especially in terms of persistence, resilience, and the degradation level of pollutants. Studies carried out in the framework of the European LIFE PROSODOL Project "Strategies to protect and improve soil quality from the disposal of olive mill waste in the Mediterranean region" have shown that in areas where untreated waste is disposed of, there are changes in soil properties, which are not reverted to normal conditions even after a 10-year cessation of waste disposal [10,11,[21][22][23]. Although most of the soil properties were found to be affected to a greater or lesser degree, it was also found that some soil properties could be used as indicators for the assessment of soil quality at OMW disposal areas.…”