Gosford City Council Historical Water Quality Review & Analysis

RESULTS OF WATER QUALITY DATA ANALYSIS 3-10
3.1.2.3 Total Nitrogen
Annual median Total Nitrogen (TN) concentrations in the Gosford waterways are presented in Table
3.16, while quarterly average concentrations are shown in Table 3.17. Total nitrogen tends to be a
parameter that can be highly variable, particularly in semi-enclosed estuaries like Brisbane Water,
where the level is constantly being affected by oceanic inputs and catchment runoff. In general, the
TN concentrations in Brisbane Water tend to be lower than in the coastal lagoons, due to the greater
and permanent tidal flushing of the semi-enclosed estuary, compared to the intermittently closed
coastal lagoons.
The quarterly results highlight the highly variable nature of TN concentrations, particularly at the
creek sites and in the coastal lagoons. The creek sites generally exhibited higher TN concentrations
following rainfall. However, the coastal lagoons showed greater variability during dry weather times,
possibly suggesting alternative nitrogen-based processes within the lagoons. Haines (in prep.) has
found that many coastal lagoons, which are mostly closed to the ocean, exhibit highly variable
nitrogen concentrations that are independent of catchment runoff events, and postulates sediment
interactions as a significant nitrogen input to such lagoons.
Table 3.16 Annual Medians for Total Nitrogen
Total Nitrogen (mg/L)
Site No. and Location 2001/ 2002 2000/ 2001 1999/ 2000
n=11 n=12 n=9 Trend
Site : 77 - Booker Bay 1.10 0.70 0.90
Site : 78 - Narara Creek Entrance 1.10 1.05 1.50
Site : 79 - Erina Creek Entrance 1.00 1.15 1.20
Site : 80 - Kincumber Creek Entrance 1.30 1.00 0.40
Site : 81 - Cockle Creek 0.90 1.05 0.80
Site : 82 - Woy Woy Creek 0.85 1.63 0.90
Site : 83 - Wamberal Lagoon 1.80 2.05 1.80
Site : 84 - Terrigal Lagoon 1.10 2.65 1.40
Site : 85 - Avoca Lagoon 1.70 1.55 1.10
Site : 86 - Cockrone Lagoon 0.70 1.15 1.40
Average Rainfall at Gosford 1248mm 1038mm 1228mm
Table 3.17 Quarterly Means for Total Nitrogen
Total Nitrogen (mg/L)
Site No. and Location
Apr - Jun Jan - Mar Oct - Dec Jul - Sep Apr - Jun
2002 2002 2001 2001 2001
n=4 n=1 n=3 n=3 n=3 Trend
Site : 77 - Booker Bay 1.00 0.40 0.90 1.47 0.70 Highly var
Site : 78 - Narara Creek Entrance 4.15 0.40 0.80 1.27 4.43 Highly var
Site : 79 - Erina Creek Entrance 2.35 1.00 0.80 1.00 18.00* Highly var
Site : 80 - Kincumber Creek Entrance 2.33 0.70 1.00 3.10 2.70 Highly var
Site : 81 - Cockle Creek 0.83 0.50 1.03 1.50 1.00 Highly var
Site : 82 - Woy Woy Creek 1.20 0.60 0.73 2.63 1.47 Highly var
Site : 83 - Wamberal Lagoon 2.68 0.80 2.03 4.30 2.20 Highly var
Site : 84 - Terrigal Lagoon 1.20 0.50 1.63 2.57 5.80 Highly var
Site : 85 - Avoca Lagoon 1.60 0.30 0.97 3.80 0.97 Highly var
Site : 86 - Cockrone Lagoon 1.73 0.80 0.37 1.53 4.30 Highly var
Average Rainfall at Gosford 150mm 735mm 189mm 174mm 445mm
* value distorted due to very high concentration (52.4mg/L) measured in May 2001.
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