Water Sources, Quality, and Regulations
This domain covers the origins of drinking water, the physical, chemical, and biological characteristics that determine treatability, and the federal and state regulations that govern public water systems. The exam tests your ability to match source water properties to treatment strategies and to apply regulatory requirements correctly.
Source Water Types and Characteristics
- Groundwater is typically low in turbidity and microbial contamination but may have high hardness, dissolved iron and manganese, and occasionally radionuclides or arsenic. It is often anoxic and may contain hydrogen sulfide.
- Surface water (rivers, lakes, reservoirs) varies widely in turbidity, temperature, and microbial load. It requires filtration and disinfection under the Surface Water Treatment Rules (SWTR).
- Groundwater under the direct influence of surface water (GWUDI) must be determined by site-specific investigation; if classified as GWUDI, the system must meet surface water treatment requirements.
Key Water Quality Parameters
- Turbidity is a measure of light scattering by suspended particles; it is a surrogate for microbial contamination. For conventional filtration, the combined filter effluent must be ≤0.3 NTU in 95% of monthly samples and never exceed 1 NTU. For slow sand or diatomaceous earth filtration, the limit is ≤1 NTU in 95% of samples.
- Disinfection byproduct (DBP) precursors such as natural organic matter (NOM) react with chlorine to form trihalomethanes (THMs) and haloacetic acids (HAAs). TOC removal requirements depend on source water TOC and alkalinity.
- pH and alkalinity affect coagulation efficiency and corrosion control. Alkalinity is the acid-neutralizing capacity, primarily from bicarbonate, carbonate, and hydroxide.
Regulatory Framework
- The Safe Drinking Water Act (SDWA) authorizes EPA to set National Primary Drinking Water Regulations (NPDWRs), which are enforceable maximum contaminant levels (MCLs) or treatment techniques.
- National Secondary Drinking Water Regulations (NSDWRs) are non-enforceable guidelines for aesthetic parameters (e.g., chloride at 250 mg/L, sulfate at 250 mg/L, iron at 0.3 mg/L).
- Consumer Confidence Report (CCR) must be delivered to customers annually by July 1, summarizing source water, detected contaminants, and compliance.
- Public notification is required for violations: Tier 1 (immediate, within 24 hours) for acute health risks; Tier 2 (within 30 days) for other MCL violations; Tier 3 (within 1 year) for monitoring/reporting violations.
Worked Example
A surface water plant uses conventional treatment. The combined filter effluent turbidity for a month has 95th percentile of 0.28 NTU and a maximum of 0.9 NTU. Is the plant in compliance?
- Rule: ≤0.3 NTU in at least 95% of monthly samples and never >1 NTU.
- 95th percentile 0.28 ≤0.3: passes the 95% criterion.
- Maximum 0.9 ≤1.0: passes the absolute limit.
- Conclusion: compliant. If the 95th percentile were 0.35, it would fail even if the maximum were 0.5.
Common Traps
- Confusing MCLs (enforceable) with secondary standards (non-enforceable).
- Forgetting that turbidity compliance is based on the 95th percentile, not the average.
- Assuming all groundwater is automatically safe; GWUDI determination is critical.
- Mixing up Tier 1 and Tier 2 public notice deadlines.
- Overlooking that TOC removal requirements apply only to conventional filtration plants serving ≥10,000 people, with alternative compliance criteria possible.