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

Abstract

There is increasing anecdotal evidence from some land treatment sites in New 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 years, with sodium adsorption ratios (SAR) of the wastewater varying between 17 and 51 (mmol/L)^0.5 . Increases in exchangeable sodium percentage (ESP) were 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 sites, leaching distilled water through repacked columns of the soil pretreated with various SAR solutions caused saturated hydraulic conductivity (K _sat) to decrease below 100% at SAR greater than 3.5 and 8.5 for the Waitoa silt loam and Te Puninga silt loam, respectively. The decreases in K _sat coincided with an increase in dissolved organic carbon (DOC) in collected leachate samples, and no dispersed clay was observed in the leachate. The laboratory studies would predict that effects of past irrigation of industrial wastewater at the study site would be measurable in the field due to the large ESPs that were recorded.

Saturated and unsaturated hydraulic conductivity measurements carried out at irrigated sites in the field showed no evidence of reduced conductivity in the surface soil until a pressure head of –120 mm was applied, the decrease being greater for the Te Puninga soil than the Waitoa. These results, along with the laboratory studies, suggest that whereas there may have been some structural deterioration in the soil matrix as a result of irrigation with the wastewater, macropore flow at higher moisture contents in the field was sufficient to overcome any adverse effects. It is suggested that laboratory studies using repacked soil may have limited use in predicting effects of Na-contaminated wastewater on soil hydraulic properties in structured soils. The results also further support suggestions that organic matter dissolution in Na-affected soils may affect soil physical properties.