## 4 Conclusions  
From the results obtained above, the following conclusions can be made. 1. GPR: The GPR is effective in detecting leaks in sand/soil with a moisture content of 2-5%. The detection of leaks using the GPR was effective when the moisture content of the sand surrounding the pipes was 2%. Furthermore, although the GPR was able to detect leaks when the moisture content of the sand was 5%, the visual inspection of the GPR scans required special attention to details as the sand was already moisty. Contrary to this, the GPR was not able to effectively detect leaks when the moisture content was approximately 10%. In addition, the GPR was able to detect leaks in all pipe materials. 2. Spectrometer: The spectrometer used in this study was able to detect leaks in all three types of pipes (PE, PPR and PVC) at 2% moisture content. However, the results did not follow a constant trend and identifying the leak was tricky. As the moisture content approached 5%, the spectrometer was only able to identify the leak in two cases, which were the PVC pipe with a crack and PPR pipe with a hole. Hence, it can be concluded that this spectrometer (spectral range of 0.20-0.85 \(\upmu\)m) with its limited infrared spectral range cannot be deemed as an effective NDT to detect leaks in pipes when the moisture content is 5% or higher. Based on the results obtained from the IR camera, it is suggested that spectrometers with wider infrared ranges would be successful in detecting such leaks. 3. IR camera: The IR camera (spectral range: 7.5-13.0 \(\upmu\)m) used in this study was effective in detecting leaks when the moisture content of the soil surrounding the pipes was approximately 2-5%. Moreover, the IR camera can detect leaks in all three pipe materials (PE, PPR and PVC). 4. Effect of soil moisture content: The ability of the three chosen NDTs to detect water leaks decreases with the increase in the moisture content of the soil. Therefore, the NDTs are only successful in capturing the significant increase in the soil moisture content due to the leaks. 5. Pipe material: There was no significant effect on the ability of the NDTs to detect water leaks when the material of the pipe was PE, PPR, or PVC. Hence, if there is leak in any of the three types of pipes used in this study, it can be detected by means of the tested NDTs. 6. Types of leaks: The tested NDTs were able to successfully detect the leaks due to a crack or hole in a pipe. However, the GPR was inefficient in detecting leaks from the faulty joint due to the surface ponding. In summary, out of the three instruments used in the present research study, the IR camera was the most effective in identifying the leaks. The spectrometer, however, given its limited spectral range, was the least reliable NDT to detect leaks. In addition to this, the NDTs were more effective when the moisture content of the sand surrounding the pipes was low (2-5%). This research will be a significant step towards building a comprehensive knowledge related to the use of NDTs for water leak detection, as comprehensive studies on water leak detection are lacking in literature. Existing knowledge is often limited to the applicability of the individual technologies. The effect of different factors on the use of these NDTs will be a major contribution for this paper as it will ensure the versatility of these technologies. However, the outcomes of this study may not always represent the real-site conditions. The experimental setup used in this study was controlled and scaled. It reduced the potential interferences from other weather and soil environments. However, in real situations, the lack of boundary walls in the vicinity may reduce the efficiency of the NDTs slightly. The extent of that impact was outside the scope of this study. Future studies should aim to focus on the applicability of these technologies in the real-site conditions under varying operational, atmospheric, and weather conditions. This is particularly essential for all the manufacturers of NDTs.