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These sites were at: Palmerston North, Pahiatua, Woodville and Dannevirke. At each site, two to three piezometers were installed to allow sampling of shallow ...
VARIABILITY IN HYDROGEOCHEMICAL CONDITIONS IN SHALLOW GROUNDWATER IN THE MANAWATU RIVER CATCHMENT AND IMPLICATIONS FOR DENITRIFICATION POTENTIAL Rivas, A.,1 Singh, R.,1 Horne, D.,1 Roygard, J.,2 Matthews, A.2, Hedley, M.1 1 Massey University 2 Horizons Regional Council Aims

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Favourable hydrogeochemical conditions in soil-water systems can attenuate nitrate (NO3 ) leached from agricultural lands before it has impact on receiving surface waters. Denitrification has been identified as an important NO3 attenuation process in groundwater systems. However, the denitrification characteristics of groundwater systems vary in space and time depending on the properties of the surface and subsurface environments. The magnitude of, and variability in, attenuation capacity has implications for both the policy related to, and management of, NO3 losses from farm systems. Therefore it is important to investigate denitrification. However, in-situ measurements of denitrification are costly and time consuming. On the other hand, some water quality parameters that are commonly measured in groundwater monitoring programmes, such as redox conditions, may be used as indicators of denitrification capacity. The aims of this study were (a) to monitor hydrological and redox related water quality parameters at four selected sites in the Manawatu River catchment, (b) to determine the spatial and temporal variability in redox processes in the shallow groundwater, and (c) to identify the implications of hydrological and redox processes for denitrification at selected sites.

Method Detailed monitoring of shallow groundwater was conducted at four study sites in the Manawatu River catchment. These sites were at: Palmerston North, Pahiatua, Woodville and Dannevirke. At each site, two to three piezometers were installed to allow sampling of shallow groundwater at depths ranging from 4.4 m to 7.7 m below ground level. Shallow groundwater samples have been collected monthly since October 2014 and analysed for a range of water quality parameters. In addition to water quality parameters measured on site (pH, dissolved oxygen, electrical conductivity, temperature, and oxidation-reduction potential or ORP), samples were taken for laboratory determinations of nitrate and ammonium-nitrogen (analysed by flow injection analysis), sulphate (by hydriodic acid reagent reduction), dissolved organic carbon (by potassium dichromate wet oxidation and titration), bicarbonate (based on alkalinity measurements and on site pH measurements), and the major cations namely, iron, manganese, calcium, magnesium, potassium, and sodium (by microwave plasma - atomic emission spectrometry). The ambient redox conditions in groundwater were determined in accordance with the biochemical succession of electron-accepting processes and sequential production of final products (McMahon and Chapelle, 2008). Redox processes were inferred by comparing concentrations of relevant -1 parameters with threshold values as follows: dissolved oxygen (1.0 mg L ), nitrate-nitrogen (0.5 -1 -1 -1 -1 mg L ), manganese (0.05 mg L ), iron (0.1 mg L ) and sulphate (0.5 mg L ).

Results As expected, water tables decreased (by 23 cm to 156 cm) from October 2014 to March/April 2015, the drier period of the year, at all sites. Thereafter, water tables rose and reached the highest levels at all sites in July 2015. Dissolved oxygen (DO) levels varied significantly across the sites with two sites (Palmerston North and Woodville) showing reduced conditions (DO < 0.70 -1 mg L ) (Figure 1a). Significantly higher DO levels were observed at the Pahiatua site and in the shallower piezometers at the Dannevirke site. In general, the DO concentrations seemed to correspond with changes in groundwater level, with DO levels decreasing as groundwater levels fell (from October to March) and increasing as the groundwater levels rose in the wet months (from May to August). This indicates that conditions for denitrification are more favorable during the summer period. Other studies have made similar observations (e.g., Anderson et al., 2014). This trend appears to be reflected in the decreasing nitrate-N concentrations at the Dannevirke site (Figure 1b). However, this was not observed at the Pahiatua site where the nitrate-N concentrations were found to increase during the drier months. This may indicate low denitrification potential at this site and the observed increase in nitrate-N concentrations could be attributed to the reduced dilution during the drier months. In reduced conditions such as in

Palmerston North and Woodville sites as well as in the deepest piezometer at the Dannevirke site, very low nitrate-N concentrations were found. The significant amount of iron, which could act -1 as electron donor in the denitrification process, in these groundwaters (0.5-4.2 mg L ) is a key indicator of the denitrification potential at these locations. Initial results from measurements of redox processes support these findings with oxygen reduction identified as the dominant process at the Pahiatua site, whereas anoxic conditions and iron reduction are the dominant processes at the Palmerston North, Woodville and Dannevirke (deepest piezometer) sites. The initial results presented here clearly indicate the variability of denitrification characteristics among sites as well as within sites (particularly at the Dannevirke site), and the temporal dynamics of hydrogeochemical properties of shallow groundwater. Given that the groundwater quality parameters used in this study are included in the monitoring programmes of regional councils, conducting such analysis with existing databases would help the assessment of spatial and temporal dynamics of denitrification characteristics in groundwater systems at a larger scale.

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Figure 1: The variation in (a) dissolved oxygen and (b) nitrate-nitrogen concentrations at four sites in the Manawatu River catchment. Notes: PNR – Palmerston North site; PAH – Pahiatua site; WDV – Woodville site; and DAN – Dannevirke site. Acknowledgment

This research is part of a wider collaborative research programme by Massey University's Institute of Agriculture and Environment and Horizons Regional Council to investigate the transport and fate of nitrogen as it moves from farms to waterways in the catchment. References Anderson, T., Groffman, P., Kaushal, S., & Todd Walter, M. (2014) Shallow groundwater denitrification in riparian zones of a headwater agricultural landscape. Journal of Environmental Quality, 43, 732-744. McMahon, P. & Chapelle, F. (2008) Redox processes and water quality of selected principal aquifer systems. Ground Water, 46, 259-271.