## 4 Discussion of Results
### Pipeline Scale Analysis
This area had already suffered from significant pollution-induced deforestation at the start of this study, so while bare land expansion continued, its rate was lower than in newly contaminated regions. The existing contamination limits the measurable impact of new spills on mangrove forests, as much of the affected area was already devoid of vegetation at the time of detection. Sample pipeline 3 had the highest PII rating at 0.99, marking it as an area in urgent need of intervention and reflecting the total quantity of spills distributed over this longer infrastructure segment. This pipeline's location showed little degradation at the beginning of this study, but the sudden increase in pollution during the study period led to rapid degradation of surrounding land, demonstrating the widespread impact associated with the onset of significant pollution activity, as seen in Figure 13. \begin{table}
\begin{tabular}{c c} \hline
**Segment Label** & **PII** \\ \hline Pipeline 1 & 0.18 \\ Pipeline 2 & 0.78 \\ Pipeline 3 & 0.99 \\ Pipeline 4 & 0.21 \\ Zone A & 0.06 \\ Zone B & 0.84 \\ Zone C & 0.54 \\ Zone D & 0.86 \\ \hline \end{tabular}
\end{table} 
Table 3: PII results for all sample pipelines segments labelled in Figure 11.  
Figure 11: The top panel shows the initial implementation of the Pipeline Impact Indicator (PII). Four sample pipelines are numbered in this plot, which are expanded upon in Figure 12 and represent the range of values in this PII implementation at the total pipeline extent scale. The centre panel shows the PII assessment at a more granular scale of 1 km of pipeline per segment, highlighting regions of significant impact for targeted intervention. In the centre panel, four regions of comparison are outlined to illustrate the range of PII values and are further expanded upon in Figure 13. The bottom panel shows the estimated spill quantity data from (NOSDRA, 2006) to within 2.5 km of the nearest pipeline, with Bille and Port Harcourt labelled for reference. This figure collectively provides a comprehensive view of the environmental impact associated with specific segments of petrochemical pipeline infrastructure, highlighting those sections of the pipeline with accelerated and significant impact on adjacent mangrove forest decline. Given the extensive pipeline network in Rivers State, which spans more than 600 km, it is essential to assess the environmental impact at a more granular scale. Our approach identifies pipeline infrastructure hotspots and associated locations where contamination damage is most significant and can enable targeted maintenance and improved safeguards against sabotage and oil bunkering. For instance, Figure 11 (centre panel) presents an analysis of the pipelines in 1 km increments, highlighting several areas of interest for further inspection. The infrastructure segment east of Bille (Zone B) shows a high PII rating of 0.84, significantly surpassing the rating for Zone C (0.54), which overlaps with the town of Bille itself. Despite the extensive area of degraded land surrounding Bille, observed in Figure 7, the PII reveals a higher spread rate in Zone B, suggesting recent spills contributing to the increased spread rate or severe long-term impact spreading from the epicentre of the original affected area (Figure 13). The urgency of intervention in Zone B is evident, particularly for areas such as the Touma Mangrove Forest, which is now at critical risk of degradation. This area  
Figure 12: Linear regression fit of the bare land increase for sample pipelines labelled in the top panel of Figure 11. This figure emphasises the discrepancy between pipelines exerting significant stress on adjacent mangrove forest health, with pipelines 2 and 3 showing a widespread deterioration effect. Conversely, pipelines 1 and 4 do not show the same impact in terms of the overall extent of deterioration.