Geologists working for the Environmental Protection Agency monitoring surface waters to assess the effects of acid rain on aquatic ecosystems would most likely track pH levels. Acid rain directly impacts the acidity of surface waters, and monitoring pH levels is crucial for understanding how these changes can affect aquatic life and ecosystems.
While other factors like gas emissions, human activity, and climate patterns might also be relevant in broader environmental studies, pH levels are the most directly associated with the effects of acid rain on water quality and aquatic organisms.