Increased nutrient concentrations in Lake Brunner have been noted over the last 15 years (as of 2008). Data regarding nutrient inputs via surface streams have been collected by the West Coast Regional Council and NIWA but little is known about the associated groundwater pathway. Surface stream data indicates that nutrient inputs are related to major land uses within stream catchments. This research focussed on the Inchbonnie catchment on the south side of Lake Brunner and consisted of one year of monitoring data. Major ions and a selection of different forms of nitrogen and phosphorus were measured.
Groundwater data developed during this research indicates that the direction of groundwater flow in the Inchbonnie catchment is to the north toward Lake Brunner. There is likely an exchange of groundwater with associated surface streams. Some groundwater may also enter Lake Brunner directly. It appeared that concentrations of nitrate-nitrogen in groundwater increase in a downgradient direction toward the lake (from around 0.1 mg/L to as much as 0.8 mg/L) and that this is the case for most of the other variables measured except phosphorus. Groundwater concentrations of phosphorus were relatively low in all cases (i.e., below or only marginally above the detection limit).
Estimates of groundwater flow and mass flow rates of nutrients within the Inchbonnie catchment were prepared. These indicate that groundwater flow within the catchment and the mass of nitrate-nitrogen moving through the groundwater pathway exceeds the contribution of Pigeon Creek but is less than that of the Orangipuku River. In contrast, the amount of phosphorus involved in the groundwater pathway is very small compared to the surface water pathway and may be considered negligible.
Click here to download the Groundwater Nutrient Movement - Inchbonnie Catchment 2009 (PDF, 7.0MB)