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Soil and Stream-Water Impacts of Sewage Effluent Irrigation onto Steeply Sloping Land

ESR Ltd, P.O. Box 50348, Porirua, New Zealand;

H. A. Kettles and K. W. Vincent

Landcare Research NZ Ltd, Hamilton and Havelock North, respectively, New Zealand;

D. J. Campbell

ERANZ, Upper Hutt, New Zealand.

^* Corresponding author (tom.speir{at}esr.cri.nz).

ABSTRACT

In a pilot study, we investigated how irrigation of secondary sewage effluent onto steeply sloping land affected soil physical, chemical, and biochemical properties, the composition of soil- and surface-waters and the vegetation on the site. The 3.36-ha site received up to 44 mm effluent/wk (over a 7–11 h period), for 65 wk. Irrigation significantly improved total- and Olsen-P status of the soils and greatly enhanced nitrification potential. Respiration increased with increasing soil water content, but microbial biomass was not greatly affected by irrigation. Soil phosphatase activity decreased with increasing P fertility. Soil physical properties were not affected by effluent and hydraulic conductivities were sufficient to conduct water into and through the soil profiles. Soil- and surface-water NO^–₃-N concentrations increased markedly, especially in the second half of the trial when soil nitrification rates were also high. However, the streamwater NO₃-N concentrations remained well below the drinking water limit concentration of 11.3 g m^–3. In contrast, streamwater NH⁺₄-N and PO^3–₄-P concentrations remained low and results indicated that concentrations of PO^3–₄-P in river water, resulting from a full-scale irrigation scheme, would not exceed the target limit level of 0.0056 g m^–3. Irrigation accelerated natural successional changes in the vegetation, with a decline in undesirable fire-prone and shrnbby species and an increase in native trees and tree ferns. These results demonstrated that, in the short term at least, a carefully designed and implemented irrigation scheme on steepland could renovate secondary sewage effluent, without adversely affecting soil properties and surface water quality.

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