In groundwater, the total dissolved solids will change ______ and the dissolved oxygen will change _______, with the dissolved oxygen lowest most probably around ________.

• A. Slowly, rapidly, 4 p.m.
• B. Rapidly, slowly, 4 p.m.
• C. Seasonally, daily, early morning
• D. Frequently, daily, late evening

Answer: (C)

In groundwater, different quality parameters respond on different time scales. Total dissolved solids reflect long-term processes like recharge, dilution from rainfall, evaporation, and ongoing mineral weathering, which shift with seasonal changes. That’s why TDS tends to vary seasonally. Dissolved oxygen, by contrast, is governed by shorter-term processes such as diffusion with the surrounding soil and air and microbial consumption, leading to daily fluctuations. Oxygen is typically lowest just before daybreak, after a night of continued consumption and before daytime re-aeration and mixing raise its level. So the pattern described—seasonal changes in TDS, daily changes in DO, with DO being lowest around early morning—fits groundwater behavior.