Irrigation of olive trees with treated wastewaters in arid and semi-arid regions is becoming a necessary
alternative to addressing issues of water shortages. The irrigation requires a careful monitoring of soil
and plants for a range of parameters including salts, nutrients, micro-elements, heavy metals, toxic pollutants,
and pathogens. A 3-years monitoring of the above parameters in soil and leaves of olive trees in
Crete, Greece was conducted using trees that were irrigated with three types of water: secondary treated
wastewater (STW), tertiary treated wastewater (TTW), and tap water (TW). Plant growth was similar for
all irrigation sources as indicated by measured trunk diameter, plant height, and photosynthetic activity.
In addition, no significant differences in mineral leaf content were observed. Accumulation of nutrients
(mainly P and K) and salts (Na) in soils 2 years after the application of both types of treated wastewater
were observed in comparison to tap water. Magnesium, calcium, and boron concentrations in treated
wastewaters were higher than TW. However no effects on soil concentrations were reported. Concentration
of polycyclic aromatic hydrocarbons in soil ranged from 349.0 g kg−1 to 395.4 g kg−1 values
not significantly different between the examined water sources. Finally, E. coli mean content in STW was
3580 ± 800 MPN 100 mL−1; however, the absence of microbial contamination was detected in leaves and
fruits of olive trees. The results of this study suggest that STW and TTW can be applied successfully for
the safe irrigation of the olive groves.