Environmental Implications of Treated Wastewater Irrigation Effects On Soil Physical Properties - Results From a Case Study In Israel.

Irrigation with treated wastewater (TWW) is gaining in importance in times of declining water availability in dry regions. In addition to other potential adverse effects on soil quality, TWW irrigation can cause hydrophobic effects on soil surfaces, reducing initial sorptivity and promoting the formation of preferential flow paths. This could lead to increased runoff, soil erosion, irregular subsoil wetting behavior and finally increased leaching of pollutants to the groundwater. Soil structure deterioration is another matter of concern, resulting in lower soil aggregate stability and reduced infiltration capacity.

In spring and summer 2010, in-situ infiltration measurements using Mini Disk Tension Infiltrometer (Decagon Devices Inc.) were performed in various agricultural plantations in Israel to assess the impact of different irrigation water qualities on selected soil properties. In most plantations long term test sites were accessed, comparing adjacent plots irrigated either with TWW or fresh water (FW). Topsoil samples were collected and analyzed regarding selected physical and chemical characteristics. Soil aggregate stability (SAS) of soil aggregates between 1 and 2 mm diameter from clayey sites was determined by wet sieving.

EC_e values were consistently higher on TWW sites (+20.9 to 92.9 %). Soil SAR was strongly elevated in 4 out of 5 TWW sites (+214.7 to +639 %). Soil water repellency index R was elevated on 4 out of 5 TWW sites from +25.1 up to +135.7 %. Hydraulic conductivity k was lower on 4 out of 5 TWW sites (-25.4 to -61.24 %). At the clayey sites, SAS was consistently lower on the TWW plots (-11.3 to -32.4 %).

The results show that irrigation water quality clearly influences physical and chemical properties of the top soil. Thus, reusing TWW for irrigation imposes side effects which have to be taken into account, in particular regarding water use efficiency and groundwater protection.