Source selection – which source generally yields the least impurities? Considering typical natural sources, which source usually provides water with the least impurities for public supply (before any treatment)?

Introduction / Context:
The initial quality of source water influences the complexity and cost of treatment. Waters originating from subsurface pathways in unpolluted upland areas are commonly the clearest and least contaminated.

Given Data / Assumptions:

• Sources considered: upland springs, reservoirs, plain streams, lowland lakes.
• Generalized impurity levels prior to significant anthropogenic impacts.

Concept / Approach:
Hill springs are often fed by groundwater that has undergone natural filtration through soil and rock, removing suspended matter and many pathogens. In contrast, plain streams and lowland lakes collect runoff, sediments, and pollutants, typically exhibiting higher turbidity and variable quality. Reservoirs may stratify and experience algal growth, necessitating more treatment steps.

Step-by-Step Solution:
Identify the source with natural filtration and isolation → hillside springs.Contrast with surface impoundments and lowland systems subject to runoff and algal issues.Choose “Springs along hill slopes.”

Verification / Alternative check:
Many historic gravity-supply systems leveraged upland springs due to their clarity and low microbial counts, though modern practice still disinfects all potable water.

Why Other Options Are Wrong:

• (a) Reservoirs can have algae, manganese release, and turbidity spikes.
• (b) Plain streams carry silt and pollutants.
• (c) Lowland lakes may be eutrophic with taste and odour issues.

Common Pitfalls:
Assuming spring water needs no treatment; disinfection is still required, and protection of the recharge area is essential.

Final Answer:
Springs along hill slopes