Source Water and Water Quality Characteristics
Why source water matters
Every treatment decision downstream is driven by what arrives at the intake. Surface supplies (rivers, reservoirs, lakes) fluctuate seasonally in turbidity, color, algae, temperature, and chlorine demand; groundwater is usually clearer and more stable but may carry iron, manganese, hardness, nitrate, radon, or sulfide, and is often disinfected without filtration.
Parameters to know cold
Turbidity (NTU, a light-scatter measurement, not a concentration), color (TCU), pH, alkalinity (buffering capacity, mg/L as CaCO3), hardness (calcium plus magnesium, mg/L as CaCO3), TDS and specific conductance, temperature, dissolved oxygen, TOC (a disinfection byproduct precursor), iron/manganese, nitrate, fluoride, and coliform/E. coli. Learn reservoir stratification and turnover: turnover can pull anoxic, iron- and manganese-rich, ammonia-laden water into the intake and double chlorine demand overnight. Sources under the direct influence of surface water (GWUDI) must be treated as surface water.
Traps examiners use
Hardness and alkalinity are not the same thing; neither is pH the same as alkalinity. Turbidity is never reported in mg/L. High TOC means more coagulant and more TTHM/HAA5 potential. Low water temperature slows settling and disinfection, so required CT goes up. Nitrate commonly points to fertilizer or septic influence; iron and manganese cause staining and taste complaints, not health violations at secondary levels. Watch for questions that compare two percentages computed on different bases.
Rule of thumb
Percent removal = (influent − effluent) ÷ influent × 100. Example: raw turbidity 12 NTU, settled/filtered target 0.10 NTU → (12 − 0.10) ÷ 12 × 100 = 99.2% removal required.