Tile drain and soil water sample assessment : relationships between soil denitrification potential and redox processes

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Beyer, M.; Rissmann, C. 2016 Tile drain and soil water sample assessment : relationships between soil denitrification potential and redox processes. Lower Hutt, N.Z.: GNS Science. GNS Science report 2016/60 ii, 16 p.; doi: 10.21420/G2PP4Q

Abstract: Tile or mole pipe drains are recognised as a major pathway of contaminant transport to waterways across Southland. GNS Science was commissioned by Environment Southland (ES) to investigate if relationships between soil water redox signatures in the tile drains can to some extent be estimated by the reduction potential of soil. Chemical analysis of redox-sensitive species (dissolved oxygen, nitrate-nitrogen, dissolved iron and manganese, sulphate) of 61 tile drainage water samples collected from 47 sites located mostly in central and southern Southland agricultural areas were compared against the ES’s soil Combined Redox Potential (CRP) map (Rissmann et al., 2016). Multivariate methods were also used to assess the main processes controlling variance in redox signatures of tile drainage waters and covariance with predefined soil denitrification potential class (SDP). The effect of bypass flow on tile drain redox signatures was considered through inclusion/exclusion of samples associated with soil cracking. Although statistical analysis indicated insignificant relationships between SDP and the redox signatures in tile drains, box and whisker plots showed a clear relationship between soil CRP and tile drain sample redox signatures. The relationships between SDP category and redox sensitive species in tile drainage is consistent with earlier findings of Rissmann et al. (2016) who identified significant (95% level) and meaningful relationships between soil and geological reduction potential categories (Combined Redox Potential) and the redox signatures of surface water and shallow groundwater across Southland. The lack of statistical significance is likely due to the limited number of available data, which may also limit the resolution provided by Principal Component Analysis in understanding underlying processes. A further finding of this work suggests that the redox state and predominant redox Terminal Electron-Accepting Process (TEAP) assignments (Jurgens et. al. 2009) were not suited for estimating soil redox conditions in soil water. Rather, the best indicators of soil zone reduction appear to be endogenous Fe(II), Mn(II) and DOC. Nitrate appeared to be a less reliable, albeit still predictable, indicator of redox potentially due to a dominantly exogenous controls (i.e., land use) over its occurrence. In conclusion, although graphical analysis indicated a correlation between SDP category, meaningful statistical analysis was inhibited by a significant skewing of the results towards one of the six soil class. It is therefore recommended to undertake further sampling and analysis with a suitable spatial and temporal coverage and a higher number of samples per SDP class to test statistical significance, particularly as 73% of the Southland’s mapped soil units are associated with intensive subsurface drainage. Regionally, tile drains are recognised as a major conduit for contaminant transport to waterways and as such, perhaps the most important pathway of contaminant transport to surface waters across the region. (auth)