Abstract
It is well known that soils where olive mill wastes (OMWs) are disposed of eventually become overloaded with organic matter of unknown and insufficient quality as well as inorganic nutrients and polyphenols. In European Mediterranean countries, the disposal of OMW is considered to be a major environmental issue. An extensive study on the risks of soil quality caused by the disposal of OMW, in the framework of the LIFE PROSODOL Project (http://www.prosodol.gr), resulted in the definition of eight soil indicators. The indicators specifically for soils that contain OMW are pH, organic matter, electrical conductivity, total nitrogen, polyphenols, exchangeable potassium, available phosphorus, and available iron. To confirm the validity of this set of indicators, nine OMW disposal areas were randomly selected and studied in Rethymno, Crete, without any knowledge of their history, details of OMW production and disposal, or other activities in those areas. Soil samples were collected and analyzed for particle size distribution, pH, electrical conductivity, organic matter, carbonates, total N, available P, exchangeable cations (K, Ca, and Mg), polyphenols, boron, and available Mn, Fe, Cu, and Zn. The results indicated that all soil parameters were affected but in different directions and by different magnitudes. Changes were evaluated considering the number of ponds for which (a) changes in soil parameters were observed; (b) a parameter value was measured above the excessive threshold; and (c) the change in the parameter values was >100%. It was revealed that organic matter, nitrogen, polyphenols, potassium, phosphorus, and iron are the constituents for which all three evaluation factors yielded the highest values. Moreover, zinc could be also considered as a potential indicator although it is not included in the evaluated set of indicators. With regard to pH and electrical conductivity, although no substantial changes were observed in this study, they should always be included in a set of indicators as they are valuable in evaluating soil buffering capacity and salinization threat, respectively.
Highlights
Sustainable management of olive mill waste (OMW) in Mediterranean countries is a challenging issue [1]
This is in agreement with the results by Regni et al [20], who reported that soils with a high percentage of clay appear to be the best choice for fertigation with olive mill wastewater, while the carbonate content is important for providing efficient buffering power and avoiding extreme changes in the pH value
This is in agreement with the results of previous work [37], which reported that the low pH of OMW results in the long-term loss of carbonate from the topsoil
Summary
Sustainable management of olive mill waste (OMW) in Mediterranean countries is a challenging issue [1]. Olive oil production is considered one of the oldest agricultural industries in the Mediterranean countries. The production of olive oil from different types of olive mills (three-phase, two-phase, and traditional) is a seasonal process, which results in very large quantities of waste that must be managed in a short time. An oil production unit has its own waste management system (e.g., chemical or biological treatment) [9]. Mainly due to the small capacity of oil mills and the high cost of various waste treatment technologies, oil mills discharge untreated OMW into the environment. The main recipients are soils, surface water, and groundwater in addition to seas
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