A paper company discharges treated wastewater into a river. What would be the best control group to evaluate the treated wastewater?

• A. A sample of water downstream from the same river
• B. A sample of water upstream from the same river
• C. A sample of river sediment downstream
• D. A sample of groundwater from a distant location

Answer: (B)

The main idea is using a proper control site that shares the same river conditions but isn’t influenced by the discharge. Water upstream from the discharge point provides that baseline: it reflects the river’s natural chemistry and seasonal variations without the added treated wastewater. By comparing upstream water to water downstream, you can attribute any differences in water quality to the effect of the discharged effluent.

Using water downstream isn’t a true control because it has already been affected by the wastewater, so it wouldn’t isolate the impact of the discharge. Sediment downstream measures deposition and long-term material accumulation, not the immediate aqueous conditions you’d want to compare for the discharge’s effect. Groundwater from a distant location is a different system altogether and would introduce many unrelated variables, making it a poor basis for evaluating the wastewater’s impact on this river.