River and stream water quality and ecology - Greater Wellington ...

River and stream water quality and ecology in the Wellington region: State and trends
(b) TOC
Meaningful increasing trends in TOC concentration were exhibited in the raw
data sets for six RSoE sites (Table 4.10), with rates of change ranging from
0.10 mg/L/year to 0.41 mg/L/year. Two sites (Mangaroa River at Te Marua
and Waiorongomai River at Forest Park) also exhibited these trends after flowadjustment.
Both Otaki River sites also exhibited increasing trends in TOC
concentration after flow-adjustment while a meaningful decreasing trend was
reported for Mangaone Stream at Sims Road Bridge.
The reason for the observed trends is not clear. Given that increasing trends
were found at several forested reference sites (eg, Otaki River at Pukehinau,
Beef Creek at Headwaters), the trends may be natural (eg, climate-driven).
Table 4.10: Summary of RSoE sites that recorded meaningful trends (ie, p1% per year) in TOC concentrations (mg/L) over July 2006
to June 2011. MASS=median annual Sen slope (mg/L/year) calculated using the
Seasonal Kendall test. The arrow indicates the direction of the trend
Site Site name Median n MASS
Raw data Flow-adjusted data
Rate of
change
(%/yr)
Trend n MASS
Rate of
change
(%/yr)
RS05 Otaki R at Pukehinau 1.10 No trend 60 0.083 7.5 ↑
RS06 Otaki R at Mouth 1.04 No trend 60 0.085 8.2 ↑
RS07 Mangaone S at Sims Rd Br 4.90 No trend 60 -0.249 -5.1 ↓
RS10 Waikanae R at Greenaway Rd 1.20 60 0.100 8.3 ↑ Not assessed (no flow data)
RS11 Whareroa S at Waterfall Rd 3.70 60 0.250 6.8 ↑ No trend
RS24 Mangaroa R at Te Marua 4.50 60 0.300 6.7 ↑ 59 0.256 5.7 ↑
RS33 Ruamahanga R at Gladstone 2.55 60 0.412 16.2 ↑ No trend
RS49 Beef Ck at Headwaters 1.70 60 0.100 5.9 ↑ Not assessed (no flow data)
RS56 Waiorongomai R at Forest Pk 2.45 60 0.298 12.2 ↑ 54 0.281 11.5 ↑
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Trend
(c) Nitrogen
Meaningful trends were recorded in either raw and/or flow-adjusted data sets
for 18, 2 and 15 RSoE sites for concentrations of nitrite-nitrate nitrogen
(NNN), ammoniacal nitrogen (Amm N) and total nitrogen (TN), respectively
(Table 4.11). All of these trends represented decreasing concentrations during
the 2006 to 2011 reporting period and are indicative of potential
‘improvements’ in water quality. Seven sites exhibited decreasing trends in
both NNN and TN concentrations, one of which (Ngarara Stream at Field
Way), also recorded declining concentrations of Amm N.
The rates of change in nitrogen concentrations were highly variable across
sites. Typically sites that had low median concentrations (eg, reference sites
and other sites dominated by indigenous forest in their upstream catchment)
had relatively small rates of change when compared to sites located in more
developed or impacted catchments. For example, at Otaki River at Pukehinau
(a reference site), concentrations of NNN decreased at a rate of 0.006
mg/L/year whereas at Whangaehu River 250 m from confluence (a site located
in a pastoral catchment) concentrations of NNN decreased at a rate of 0.214
mg/L/year.