Redox characteristics of shallow groundwater in the Tararua Ground Water Management Zone : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science at Massey University, Manawatū, New Zealand
Groundwater redox conditions have a major influence on transport and transformation of nutrients such as nitrate from farms to rivers and lakes. This study focused on measurement and analysis of chemical and physical characteristics of groundwater to determine the spatial distribution of redox characteristics across the Tararua Ground Water Management Zone in the Manawatu River catchment. The influence of catchment characteristics such as soil texture and drainage, and rock type have on groundwater chemistry and its redox characteristics across the Tararua GWMZ is investigated using multivariate statistical analysis.
Existing geographical information was collated and analysed to map spatial distributions of land use, soil characteristics and lithologies across the study area. This information was utilised to identify potential site locations for sampling and analysis of shallow groundwater in the Tararua GWMZ. A direct-push system capable of penetrating a range of substrates including deep, imbricated, and coarse gravels was developed. Using this system, shallow groundwater samples were recovered from contrasting hydrogeological settings, areas where water wells are rarely installed; such as along the margins of the axial ranges, and from areas considered not to have groundwater; e.g. the mudstone country on the east of the Tararua District.
Data collected with the direct-push method was combined with similar data collected from existing wells by Rivas et al. (2017) and classified according to redox status. The data was subjected to multivariate statistical assessment using Hierarchical Cluster Analysis to determine the water type, and Principal Component Analysis to determine the influence of discrete catchment characteristics on redox reactions occurring in shallow groundwater of the Tararua GWMZ.
The in-field and chemical analysis revealed significant variation of groundwater quality parameters and redox characteristics across the Tararua GWMZ. The regional trend was for reducing conditions in gravel aquifers in the north western areas of the Tararua GWMZ and oxidising in gravel aquifers of the south western; although statistically significant variations of redox characteristics is also recognised within these areas. Groundwater samples were collected from mudstone where little, if any, groundwater research has been conducted previously. Groundwater characteristics from mudstone are generally classified as anoxic and strongly reducing, with very high specific conductivity and analyte levels such as bromide, chlorine, sodium, fluorine, dissolved inorganic carbon and magnesium. Identifying the influence of discrete catchment characteristics on groundwater chemistry and redox characteristics was complex and difficult to quantify. Extrapolation of the principal component inferred to be associated with redox characteristics provides a useful means to evaluate the influence of discrete catchment characteristics on redox conditions in shallow groundwater of the Tararua GWMZ. The direct-push method provided an opportunity to compare groundwater chemistry between samples collected proximal and distal to production wells. Statistically significant differences in redox related parameters such as DOC, Eh, Fe2+, Mn2+, NH4+-N, and N02--N were detected in groundwater samples collected from existing wells compared to groundwater samples collected with the direct-push method. Factors contributing to this effect were explored but found to be difficult to isolate.