Author: Customer
Company: ARO

Abstract

Due to the shortage of fresh water (FW), use of treated wastewater (TWW) effluents became a common practice in thr field of irrigation. Since TWW differs from FW in their organic and inorganic matter contents, irrigation with TWW may harm the environment, for example, by influencing soil structure and ground water quality.
Specifically, irrigation by TWW containing different groups of organic matter may
have an impact on water-soluble soil organic matter (dissolved organic matter, DOM).
DOM is known to be of high importance for multiple chemical, physical and
biological processes occurring in soil environment. The hypothesis of the work was that soil DOM composition is affected by quality of irrigation water and it may affect soil structure and stability. Therefore, the goal of this research was (1) to examine the effect of TWW on the composition of water-extractable soil DOM and (2) to find a possible connection between soil DOM characteristics and soil physical parameters – soil stability (measured as soil stability parameter) and hydraulic conductivity.
In this work, four different soil types, representing main arable Israeli soils, were
studied: loamy sand from the Coastal Plain, a sandy loam from the Northeastern
Negev, dark brown sandy clay from the Pleshet Plains and clay from the Western
Galilee, Israel. Each soil was sampled from two adjacent fields: one irrigated with FW and one that had been irrigated, for at least five years, with TWW. Soil samples were taken from two depths: 0-20 and 20-40 cm. Hence, 16 different combinations of soil type, irrigation water quality and soil depth were considered. For each combination, three samples were taken from different locations in the field; thus, altogether 48 soil samples were used in the study. All the soil samples were characterized for particle size distribution, organic carbon (OC) content, cation exchange capacity (CEC), exchangeable sodium percentage (ESP), and calcium carbonate content. In addition, each soil sample was tested for soil stability using high energy moisture characteristic (HEMC) and hydraulic conductivity of soil columns washed with distilled water.
Soil DOM was obtained from shaking ground and sieved soil (<2 mm) with deionized water ( at the 1 to 2 ratio) for two hours, filtered through 0.45 μm and
characterized by measuring dissolved organic carbon (DOC) concentration, optical density (OD254) at 254 nm and excitation emission matrices (EEM) of fluorescence.
For each soil sample, three different solutions were prepared for further analysis: (a) a solution containing the original water-soluble DOM, (b) an acidic solution containing the DOM fraction that did not precipitate at low pH (acid-soluble fraction) and (c) a solution containing the DOM fraction that precipitated at low pH and was redissolved in a basic solution (further, acid-precipitated fraction). All together, the 48 soil samples studied (4 soil types X 2 irrigation water qualities X 2 sampling depths X 3 replicates), each yielding 3 DOM solutions, resulted in total of 144 DOM solutions that were analyzed.
PARAFAC (Parallel Factor analysis) was used in order to characterize DOM's
fluorescence and quantify its composition. Using PARAFAC analysis enables
obtaining concentration scores (proportional to absolute concentrations) of fluorescent DOM components and comparing them to soil physical parameters studied. In watersoluble soil extracts from different soil samples, three major fluorescent DOM components in were obtained using PARAFAC analysis. These components included two typical humic components (I and II) differing in wavelengths of excitation (λex) and emission (λem) maxima (I: λex<250 and 320 nm, λem 432; II: λex 270 and 365 nm, λem 480 nm). Third fluorescent component (III) was assigned to a proteinous matter characterized with tryptophan-like fluorescence at excitation/emission wavelengths of 285/354 nm.
The quality of irrigation water (FW vs, TWW) was found to affect the concentration scores of all three fluorescent components, and therefore, the composition of watersoluble soil DOM. Irrigation with TWW as compared with FW involved increases of the concentration scores of all three fluorescent components in soils containing low clay content (less than 20%) and resulted in decreases of concentration scores in soils containing high clay content (more than 40%). Strong linear relations were found between optical densities at 254 nm of DOM solutions and concentration scores of fluorescent components: the coefficients of correlations between OD254 and concentrations scores were found to be - 0.98, 0.95 and 0.85 for components I, II, III, respectively. Opposite ranking of soils (except loess) in terms of soil stability was obtained by using two different approaches: with (a) normalized soil stability indexes and (b) final hydraulic conductivity values (where both the indexes and final hydraulic conductivities obtained on the TWW-irrigated soil were normalized against
FW-irrigated soil). Only in the low clay content soil (hamra), effluent irrigation was
associated with an increase in the normalized soil stability parameter. In soils with relatively low clay contents, i.e. hamra and leoss, a link may be seen between the changes in the DOM composition (the changes in concentration scores of fluorescent components) and the normalized soil stability parameters.
The obtained results demonstrate that soil DOM composition is influenced by
irrigation water quality, and it seems that the effect of the TWW irrigation on soil
stability is in correlation with the change in soil DOM composition due to the
irrigation water quality thus supporting the hypothesis raised in the research work.
This relation between changes in soil stability and DOM composition may depend on the type of soil.