Effects of sodium-contaminated wastewater on soil permeability of two New Zealand soils

There is increasing anecdotal evidence from some land treatment sites in Ne w Zealand that irrigating sodium-contaminated wastewaters onto soils may be causing soil structural problems and reduced permeability. In this study, the effect of irrigating such waste (derived from agricultural industries) on soil physical and chemical properties was investigated in an Allophanic Soil (Te Puninga silt loam) and a Gley Soil (Waitoa silt loam). Wastewater irrigation at the sites investigated had taken place for the previous 5 yea rs, with sodium adsorption ratios (SAR) of the wastewater varying between 1 7 and 51 (mmol/L)(0.5). Increases in exchangeable sodium percentage (ESP) w ere recorded to 300 mm depth in both soils. At the soil surface (0-20 mm), ESP had increased to 31%, compared with 0.4% at control sites. In laboratory studies using soil from the 0-20 mm layer in non-irrigated si tes, leaching distilled water through repacked columns of the soil pretreat ed with various SAR solutions caused saturated hydraulic conductivity (K-sa t) to decrease below 100% at SAR greater than 3.5 and 8.5 for the Waitoa si lt loam and Te Puninga silt loam, respectively. The decreases in K-sat coin cided with an increase in dissolved organic carbon (DOC) in collected leach ate samples, and no dispersed clay was observed in the leachate. The labora tory studies would predict that effects of past irrigation of industrial wa stewater at the study site would be measurable in the field due to the larg e ESPs that were recorded. Saturated and unsaturated hydraulic conductivity measurements carried out a t irrigated sites in the field showed no evidence of reduced conductivity i n the surface soil until a pressure head of -120 mm was applied, the decrea se being greater for the Te Puninga soil than the Waitoa. These results, al ong with the laboratory studies, suggest that whereas there may have been s ome structural deterioration in the soil matrix as a result of irrigation w ith the wastewater, macropore flow at higher moisture contents in the field was sufficient to overcome any adverse effects. It is suggested that labor atory studies using repacked soil may have limited use in predicting effect s of Na-contaminated wastewater on soil hydraulic properties in structured soils. The results also further support suggestions that organic matter dis solution in Na-affected soils may affect soil physical properties.