2.3 Watershed Protection & Source Water Contaminants

2.3 Watershed Protection & Source Water Contaminants

Every dollar spent keeping contamination out of the source is a dollar not spent removing it at the plant. The multiple barrier concept starts here, upstream of the intake, and the standardized exam tests whether you can connect a watershed condition to the treatment problem it will create on your shift.

---

1. Point vs. Nonpoint Sources

Nonpoint pollution is the harder problem precisely because there is no pipe to permit. For a drinking water operator, the practical signature is that nonpoint loading is event-driven — it arrives with the storm.

---

2. Nutrients and Eutrophication

Eutrophication is nutrient enrichment of a water body, principally by nitrogen and phosphorus. In fresh water, phosphorus is usually the limiting nutrient — the one in shortest supply relative to what algae need — so a small phosphorus increase can produce a disproportionate bloom.

The chain of consequences an operator feels

1. Algal bloom — dense growth of algae and cyanobacteria in the photic zone.
2. Taste and odor — Anabaena and Actinomycetes release geosmin and 2-methylisoborneol (MIB), detectable at nanogram-per-liter concentrations and unaffected by ordinary treatment. Powdered or granular activated carbon is the usual answer.
3. Filter clogging and shortened runs — algal cells blind the media, driving headloss up and filter run length down.
4. pH swings — daytime photosynthesis strips carbon dioxide and drives pH up, upsetting coagulation.
5. Cyanotoxins — Microcystis and related cyanobacteria can release microcystins, a genuine health concern. Avoid pre-oxidation that lyses cells and releases intracellular toxin into the water.
6. Organic loading feeds DBP formation — more natural organic matter means more total organic carbon, and more TOC means more trihalomethanes and haloacetic acids after chlorination.
7. Oxygen depletion — the bloom dies, decomposes, and consumes dissolved oxygen, driving the hypolimnion anoxic.

---

3. Reservoir Stratification and Turnover

A deep reservoir in the Southeast stratifies thermally through the warm months into three layers:

Turnover

In fall, surface water cools until it approaches the density of the bottom water and wind mixes the reservoir top to bottom. A spring turnover occurs as ice-free surface water warms toward 4 °C. Turnover blends anoxic hypolimnetic water into the whole column, and the intake suddenly receives:

• A spike in dissolved iron and manganese — colored water complaints, manganese post-precipitation in the distribution system.
• Hydrogen sulfide — rotten-egg odor, chlorine demand.
• Taste-and-odor compounds and elevated TOC — higher DBP precursor loading.
• Turbidity and ammonia — ammonia converts free chlorine to chloramine, wrecking a free-residual disinfection strategy and the CT credit that depends on it.

Operator response: anticipate it. Increase raw-water monitoring in the weeks before expected turnover, be ready to switch intake depth, adjust oxidant selection and dose, pre-position PAC, and reassess your chlorine demand curve rather than chasing residual after the fact.

---

4. Intake Design and Operation

Multi-level intakes are the single most useful source-water tool a surface water plant has. Ports at several depths let the operator draw from the layer with the lowest turbidity, iron, manganese, and algae as conditions shift. Operators should also manage:

• Intake screens — trash racks and traveling screens; monitor differential headloss across them.
• Zebra and Asian clam fouling — biofouling of intake structures and piping.
• Presedimentation or off-stream raw storage — evens out storm turbidity spikes and provides response time after an upstream spill.

---

5. Source Water Assessment and Wellhead Protection

Every public water system in the state has a source water assessment, built from three steps:

1. Delineation — map the area contributing water to the intake or well. For surface water it is the watershed upstream; for a well it is the zone of contribution, based on aquifer characteristics and pumping rate.
2. Contaminant source inventory — catalog the potential sources inside that area: landfills, underground storage tanks, industrial sites, CAFOs, septic systems, transportation corridors, abandoned wells.
3. Susceptibility determination — rate how vulnerable the source is, given the delineated area, the inventory, and the natural protection the setting provides.

A wellhead protection program applies the same logic to groundwater, adding management measures such as land-use controls in the contribution zone, proper abandonment of unused wells, and spill response planning along nearby transport routes.

Exam framing: questions in this area usually give you a watershed condition and ask for the plant consequence. Learn the pairs — nutrients → algae → geosmin/MIB and DBP precursors; turnover → iron, manganese, sulfide, ammonia; storm event → turbidity and nonpoint loading; anoxic hypolimnion → dissolved metals.