Which of the following is a possible use of automatic flow-proportional samplers?

Automatic flow-proportional samplers are designed to collect water samples in proportions that correlate with the flow rate of the water being sampled. This method ensures that the samples represent the variability of the water quality over time and across different flow conditions. By collecting samples in direct proportion to the flow, these devices help capture an accurate depiction of the water's characteristics, which can be vital for environmental monitoring, regulatory compliance, and assessing pollution levels.

The other options do not utilize the specific capabilities of flow-proportional samplers. For instance, mapping water source locations pertains to geographical and hydrological studies, which is unrelated to the sampling of water quality. Monitoring air quality involves different equipment altogether, tailored specifically for analyzing airborne pollutants rather than water sampling. Testing soil contaminants focuses on solid materials rather than liquid samples that flow through systems, which is outside the application of flow-proportional sampling technology.