The Mangahahuru Stream, which begins in Pinus radiata forestry southeast of Hikurangi, is a small tributary of the Wairua River. Other than the exotic forestry in the headwaters the catchment is predominantly agricultural land use, with an increasing number of lifestyle blocks. Riparian management in the catchment is variable. The underlying geology is hard sedimentary.
The 2005-2006 results for the Mangahahuru Stream at Apotu Road are summarised in the table below including the median, range and percentage of sampling occasions that comply with relevant guidelines for the 12 sampling occasions. Medians shown in red are outside the recommended guidelines.
|Parameter||Median||Range||% comply with guideline|
|Temperature (deg. cel.)||16.0||11.1 - 20.4|
|Dissolved oxygen (mg/L)||10.2||8.7 - 13.4||100|
|Dissolved oxygen (% Sat.)||100.9||87.8 - 124||8|
|Conductivity (mSm)||13.3||11.1 - 15.3|
|Water clarity (m)||1.20||0.2 - 2.05||82|
|Turbidity (NTU)||5.3||2 - 102||58|
|E. coli (n/100mL)||485||249 - 15531||0|
|Dissolved reactive phosphorus (mg/L)||0.038||0.012 - 0.085||0|
|Total phosphorus (mg/L)||0.077||0.054 - 0.274||0|
|Ammoniacal nitrogen (mg/L)||0.020||0.005 - 0.11||58|
|Total nitrogen (mg/L)||0.643||0.292 - 1.41||42|
|pH||7.1||6.5 - 7.4||33|
The Mangahahuru Stream is significantly impacted on, with the phosphorus and
E. coli results for 2005-2006 exceeding their respective guideline values on all sampling occasions and the median for total nitrogen also exceeding guidelines.
Non-point diffuse surface run off is the likely source of most of the contamination, as spikes in contamination are associated with elevated river flows. However some may also be contributed to poorly maintained septic tanks, stock access to the stream or point source consented discharges such as the Hikurangi Oxidation Pond or Dairy Factory discharges.
There is cause for reserved optimism though, as the long-term trends for ammoniacal nitrogen, turbidity, clarity and pH all suggest that the situation is improving in the long term.
There is however some negative trends for the Mangahahuru Stream, including increasing conductivity, increasing dissolved oxygen and decreasing flow. The latter is of the most concern in terms of demands on water resources in this area; however, caution needs to be taken interpreting this as a true result as this trend is only based on the flow rate at the time of water quality sampling. This concerning trend will be investigated further by the hydrology team using the entire flow dataset for this site.