Source Water and Water Quality
Know the source before you design the treatment
Every downstream decision - coagulant dose, disinfectant demand, corrosion control - begins with source water quality. Surface supplies (reservoirs, rivers, the State Water Project and Colorado River aqueducts) shift seasonally in turbidity, temperature, algae, and TOC. Groundwater is more stable but often carries hardness, iron, manganese, sulfide, nitrate, arsenic, and low dissolved oxygen. Learn which contaminants are primary (health-based MCLs such as nitrate at 45 mg/L as NO3 and arsenic at 10 µg/L) and which are secondary (aesthetic, such as iron at 0.3 mg/L and manganese at 0.05 mg/L).
Pathogen risk is not limited to rivers. A well under the direct influence of surface water can pass Giardia and Cryptosporidium, and Crypto is notably chlorine-resistant, so its control depends on filtration, UV, or ozone. Watershed sanitary surveys, wellhead protection, and source water assessments are preventive tools the exam expects you to recognize.
Classic traps
- Confusing an MCL with a secondary standard, or a single sample result with a running annual average.
- Assuming cold water is easier to treat: low temperature slows coagulation kinetics and raises viscosity, so floc settles poorly.
- Forgetting that algae raise pH and TOC, creating both DBP precursors and extra coagulant demand.
Worked example
Two sources blend: 1.0 MGD at 4 NTU and 3.0 MGD at 12 NTU. Flow-weighted turbidity = (1×4 + 3×12) ÷ 4 = 40 ÷ 4 = 10 NTU. Blending is mass-weighted - never a plain average.
Rule of thumb: a raw-water change of roughly 10–20% or more in turbidity, pH, alkalinity, or temperature should trigger a jar test before you change dose.