Water Sources, Hydrology, and Raw Water Quality
Where raw water comes from
Surface supplies (rivers, reservoirs, lakes) and groundwater (wells, springs) behave very differently, and most questions test whether you can predict which one changes faster and why. Surface water is exposed to runoff, so turbidity, color, bacteria, and temperature swing with storms and seasons. Groundwater moves slowly through porous media, gaining natural filtration, stable cool temperatures, and often high mineral content - iron, manganese, hardness, sulfide, and sometimes radon or nitrate from septic and agricultural sources.
Key physical ideas
- Porosity is how much water a formation holds; permeability is how easily it flows. A clay can be porous but nearly impermeable.
- Safe yield is the rate a well or reservoir can sustain without mining storage or pulling in saltwater.
- Stratification traps cold, oxygen-poor water at the bottom of a reservoir; fall turnover mixes it and can abruptly worsen raw water quality.
- Eutrophication from nutrients fuels algae, which raise pH, clog filters, cause taste and odor, and add TOC - a disinfection byproduct precursor.
Traps
Candidates confuse alkalinity (acid-neutralizing capacity, mostly bicarbonate) with hardness (calcium and magnesium). They also assume groundwater needs no disinfection; under the Groundwater Rule, undisinfected systems still monitor for fecal indicators, and wells under the direct influence of surface water are regulated as surface water.
Rule of thumb and worked example
1 cfs ≈ 2 acre-feet per day. A 60-acre-foot reservoir receiving 0.5 cfs has a detention time of about 60 ÷ (0.5 × 1.98) ≈ 61 days. Long storage settles turbidity but grows algae and raises THM precursors, so intake depth and season matter.