Source Water and Groundwater
This section covers Source Water and Groundwater. The exam tests source protection, well construction, aquifer vulnerability, and the link between raw-water quality and treatment choices.
Core rules
- A public water system's source water assessment identifies contamination threats and susceptibility. Wellhead protection areas and sanitary setbacks reduce risk near wells.
- Well construction matters: casing, grout, sanitary seal, screened interval, and a proper cap keep surface water and shallow contamination out. Abandoned wells must be properly sealed.
- Groundwater under the direct influence of surface water (GWUDI) is determined by site-specific evaluation; GWUDI sources face surface-water treatment and monitoring requirements.
- Common groundwater contaminants include nitrate, arsenic, radionuclides, volatile organic compounds, and pathogens. Surface water adds turbidity, algae, and seasonal organics.
- Protect the intake and source area; treatment is not a substitute for source protection.
Worked example
Well A produces 100 gpm at 12 mg/L nitrate as N; Well B produces 300 gpm at 2 mg/L. Blend concentration = (100 × 12 + 300 × 2) ÷ 400 = (1,200 + 600) ÷ 400 = 4.5 mg/L as N. The MCL is 10 mg/L as N, so the blend is compliant. Reasoning: flow-weight the concentrations; do not average the two wells equally. Also confirm nitrate is reported as N, not as NO3.
Traps
Averaging wells without flow weighting; confusing nitrate as N with nitrate as NO3; sampling before adequate purging; treating a sanitary survey as a compliance sample; missing GWUDI status; overlooking abandoned wells or unsealed annular spaces.