Annual report 2006-2007 - Taranaki Regional Council

23
respect of special condition 1 of consent 0108 was not achieved at the time of this
survey. This is the third consecutive year that this non-compliance has occurred. The
BOD5 in the discharge (180 g/m 3) during this low flow survey was relatively high
compared to previous years, and coupled with a high chemical oxygen demand
suggested that the aerobic pond was not performing well. Considering it appears the
Kahouri Stream had recorded lower flows prior to this water quality survey, it is
likely that greater increases in the BOD5 may have occurred in the Kahouri Stream
downstream of the abattoir discharge during March and early April 2007. Further, at
BOD5 concentrations greater than 2 g/m 3, there may be some potential for
undesirable heterotrophic growths to bloom in the stream. No such growths were
detected in the biological monitoring conducted on 17 April 2007 (refer to biological
section below).
The un-ionised ammonia concentration 50 m downstream of the tributary was
0.01155 g/m 3, compared to the consent limit of 0.025 g/m 3. Compliance in respect of
special condition 2 of consent 0108 was therefore achieved at this time. Un-ionised
ammonia is the most toxic form of ammonia, and at these concentrations, is unlikely
to cause significant adverse effects on the biological communities of the Kahouri
Stream.
While this concentration of unionised ammonia may not be expected to cause
significant adverse effects it still reflects an increase of more than 100 times the
background figure recorded upstream. Furthermore, the ammoniacal nitrogen (NH4)
increased significantly to 1.87 g/m 3, being 155 times higher downstream of the
tributary that receives the abattoir wastewater discharge, compared to levels
upstream. Compared to the ANZECC Water Quality guidelines (ANZECC 2000)
which provide limits to prevent nuisance growths of algae, this is extremely high
(when compared with the limit for slightly disturbed systems in upland rivers). The
recommended limit for ammoniacal nitrogen is 0.01 g/m 3. In the Kahouri Stream at
Flint Road, this concentration had dropped somewhat, but remained high (0.226
g/m 3), thus indicating ammonia concentrations appear to fall significantly by this
point 3 km downstream (TRC site code KHI000400).
Dissolved reactive phosphorus was raised from 0.094 to 0.263 g/m 3 below the
discharge. However, this had decreased slightly to 0.134 g/m 3 at the site in the
Kahouri Stream at Flint Road. The ANZECC guideline for DRP in slightly disturbed
systems in upland rivers is 0.009 g/m 3 (ANZECC, 2000). The previous survey had
seen ammonia and phosphorus concentrations between these two sites drop to much
lower concentrations, indicating that the higher concentrations seen in the discharge
during this survey somewhat overloaded the assimilative ability of the Kahouri
Stream.
The discharge was not observed to cause any conspicuous effects upon the water of
the Kahouri Stream at the downstream sampling point. The turbidity and suspended
solids results did not change markedly at the Kahouri site downstream of the
confluence with the tributary that carries the abattoir effluent.
During the low flow survey, an additional sample was taken from the tributary into
which the abattoir discharges but approximately 30 metres upstream from the
abattoir’s discharge point. This provides an indicator of the background
concentrations that are in the tributary prior to the discharge entering it.