A commonly used trilinear diagram in hydrogeology is used to present data on natural water. What does it show?

• A. The pH and alkalinity distribution of natural water
• B. The percentage composition of major cations and anions in natural water
• C. The mineralogical composition of rocks
• D. The temperature profile of groundwater

Answer: (B)

Trilinear diagrams in hydrogeology are used to show the relative proportions of the major ions in natural waters. On the diagram, one triangle represents the cations (calcium, magnesium, and sodium plus potassium) as a percent of total cations, while the other triangle represents the anions (bicarbonate plus carbonate, sulfate, and chloride) as a percent of total anions. Each water sample is plotted in both triangles, and the resulting central diamond helps identify the overall water type or hydrogeochemical facies. This visualization makes it easy to see patterns such as calcium–bicarbonate waters in fresh groundwater or sodium–chloride waters in saline conditions, and to infer geochemical processes like mineral weathering, ion exchange, or mixing of waters. The diagram is not about pH and alkalinity, rock mineralogy, or temperature profiles, which are not depicted by this plotting method.