4. Relationships between environmental factors and faecal contamination
Analysis of the factors determining faecal contamination was undertaken both within individual sites and, through the use of median values, between sites across the region. The intra-site variability was assumed to be determined by those factors that vary on a temporal basis, whilst differences between sites are attributed to spatial variability. In the following analysis, flow, turbidity and seasonality were assumed to be the dominant controls upon variability within a site. Clearly though, other factors vary in a temporal manner, for example, the discharge of point sources, and stock density. Data describing these, however, was either provided in a mean annual format, or collated from one-off surveys (section 2). Consequently, in the analysis such data is necessarily treated as invariant in a temporal sense.
Bivariate relationships were used to examine the impact of each spatially varying factor upon median E. coli across the region. Pearson product-moment correlation (R) and the coefficient of determination (R2) are given in Table 2 for each relationship. Since not all the independent variables are necessarily linearly related to median E. coli (for example, the percentage of all non-pastoral vegetation, Figure 11b), the Spearman Rank correlation (Rs) was also derived (Table 2) as this measures the degree of monotonicity in a relationship, rather than the degree of linearity. The Spearman Rank correlation is also able to better account for extreme values that increase the skewness of a dataset. Correlation between the environmental factors is given in Table 3.
All median E. coli concentrations are expressed as cfu/100 mL since Log10 transformations resulted in a poorer prediction of median faecal contamination by all the environmental variables examined. Non-median concentrations used in the E. coli – flow – turbidity relationships (Figures 8, 9 and 10) have been log transformed.
Table 2. Summary of the coefficient of determination (R2), Pearson product-moment correlation (R), and Spearman Rank correlation (Rs) between median E. coli and each independent variable.
|
Independent factor |
R2 |
R |
Rs |
|
%_Pasture |
0.23 |
0.48 |
0.59 |
|
Total_stockunits/km2 |
0.29 |
0.54 |
0.58 |
|
Cattle_stockunits/km2 |
0.34 |
0.58 |
0.60 |
|
%Indigenous_forest |
0.15 |
-0.39 |
-0.38 |
|
%Non-Pastoral_Vegetation |
0.27 |
-0.52 |
-0.63 |
|
%Wetland (all watersheds) |
0.02 |
- 0.15 |
0.014 |
|
%Wetland (only watersheds with wetlands) |
0.10 |
-0.32 |
-0.47 |
|
%Urban |
0.15 |
0.39 |
0.47 |
|
%Well-drained_soil |
0.18 |
-0.42 |
-0.57 |
|
%Poor_drained_soil |
0.48 |
0.69 |
0.61 |
|
Rainfall |
0.11 |
-0.33 |
-0.28 |
|
DominantSlope |
0.17 |
-0.41 |
-0.29 |
|
%SteepSlope |
0.24 |
-0.49 |
-0.32 |
|
Catchment_area |
0.03 |
0.17 |
0.08 |
|
Median_turbidity |
0.42 |
0.65 |
0.71 |
|
%Inland_water |
0.01 |
0.10 |
0.06 |
|
Num_Dairy_effluent_discharges_to_land/km2 |
0.02 |
0.14 |
0.3 |
|
Num_Non_Dairy_effluent_discharges_to_land/km2 |
0.21 |
0.46 |
0.51 |
|
Num_Utility_effluent_discharges_to_land/km2 |
0.03 |
0.17 |
0.18 |
|
Volume_Dairy_Point_Sources/km2 |
0.10 |
0.32 |
0.51 |
|
Volume_Non_Dairy_Point_Sources/km2 |
0.15 |
0.39 |
0.34 |
|
Volume_Utility_Point_Sources/km2 |
0.05 |
0.22 |
0.36 |
|
Num_DairyPonds/km2 |
0.11 |
0.33 |
0.58 |
Table 3. Correlation (R) between the independent variables. 
Figure 8. Relationships between E. coli concentration (log10 cfu/100mL) and flow (L/s). The prefix on each axis label indicates the site number.
Figure 9. Relationships between turbidity (NTU) and flow (L/s). The prefix on each axis label indicates the site number.
Figure 10. Relationships between E. coli (log10 cfu/100mL) and turbidity (NTU). The prefix on each axis label indicates the site number.
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