Source Water Characteristics and Water Quality
What source water chemistry actually tells you
Surface supplies (rivers, lakes, reservoirs) swing seasonally: spring runoff spikes turbidity and TOC, summer algae blooms raise pH and create taste-and-odor compounds, and fall turnover re-suspends iron, manganese, and organics. Groundwater is more stable in temperature and turbidity but is often anoxic, carrying dissolved iron, manganese, sulfide, arsenic, and radionuclides, and it may be classified as groundwater under the direct influence of surface water (GWUDI), which changes treatment and monitoring obligations.
Know what each parameter drives: turbidity (particle and pathogen load, filter performance), alkalinity and pH (coagulant effectiveness, corrosion control, CT credit), hardness (softening need), TOC and UV254 (disinfection byproduct precursor load), nitrate, and temperature (reaction rates, viscosity, disinfectant decay).
Common traps: warmer water is less viscous, so filters run faster and media may fluidize; low-alkalinity, low-pH water can be pushed below the coagulation optimum by alum; algae can blind filters and pass through sedimentation; and groundwater is not automatically safe - GWUDI sources can carry pathogens. Expect a question that only filtered surface-water systems carry the strict turbidity limit.
Rule of thumb
Alum consumes roughly 0.45 mg/L of alkalinity as CaCO3 per mg/L dosed. A source with 40 mg/L alkalinity dosed at 30 mg/L alum loses about 13.5 mg/L, leaving ~26 mg/L - usually enough to hold pH. A 60 mg/L dose would strip ~27 mg/L and drive pH down unless lime or caustic is added. Alkalinity is the first number to check when a jar test suddenly demands far more coagulant than usual.