Prediction of subsurface redox status for Hauraki and Coromandel Catchments

TR 2016/35
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Report: TR 2016/35

Author: Murray Close (Institute of Environmental Science and Research Ltd)

About this report

Waikato Regional Council contracted Environmental Science and Research Ltd (ESR) to apply regional models for predicting subsurface redox status in the Hauraki and Coromandel sub-catchments in support of healthy Rivers plan change 2. The objective was to estimate the extent and distribution of nitrogen attenuation in groundwater in the area.

A brief summary of the method development was provided together with figures giving the overlay of the sub-catchment boundaries on the predicted subsurface redox status for the shallow (<25 m) and medium (25 to 100 m) depth groundwater. The percentages of predicted reducing, mixed and oxidising redox status for the shallow and medium depth groundwater are given in the report, together with some comments about the likely degree of nitrate attenuation for each sub-catchment. Overall the shallow and medium depth predictions indicated similar patterns of oxic, mixed and reduced groundwater, with the overall amounts being approximately 40%, 10%, and 25% for the oxic, mixed and reduced redox state groundwater, respectively.

In general, nitrate-rich shallow groundwater is likely to encounter reducing conditions in the Hauraki catchment and will mostly encounter oxic conditions in the Coromandel area. The actual amount of attenuation due to denitrification for a particular catchment depends on the amount of reducing and mixed redox state groundwater there is and may also depend somewhat on how the reducing zones are distributed in each sub-catchment.

The results substantiate the conceptual model that most groundwater nitrogen migrating along the Hauraki Plains is attenuated before reaching the Firth of Thames or emerges into surface water. Less attenuation occurs in the Coromandel Peninsula.    

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Prediction of subsurface redox status for Hauraki and Coromandel Catchments [PDF, 2.2 MB]