Surface water is weather you can pump
A borehole changes slowly; a dam or river changes with the sky. The measure that matters is turbidity — how much suspended mud and fine matter the water carries, reported in NTU. South African rivers swing wildly: the Welbedacht Dam on the Caledon recorded raw water from 200 NTU to 90 000 NTU within a single year as storm flows came down the catchment. Even a quiet farm dam sits far above the level treatment needs — and South African small-supply guidance (the WRC treatment guide, TT 181/02) draws the line simply: raw surface water above about 50 NTU needs the full treatment train, not just a filter.
Dams do half the work for you by acting as settling basins — water drawn from a large, still dam is far clearer than the river feeding it. Design for the worst week after a storm, not the average, and the system stays ahead of the weather.
The bloom that has killed herds before
In hot, dry weather a shrinking dam becomes exactly what blue-green algae (cyanobacteria) want: shallow, still, warm, and rich in concentrated nutrients. Their blooms look like green paint or pea soup blown against one bank, and some strains release microcystin, a liver toxin. This is not a theoretical risk in South Africa — farmers around Free State and Mpumalanga pans reported stock deaths from the 1920s; the Vaal Dam bloom of 1942–43 killed stock along both banks, with the deaths following the scum as the wind moved it; and at Kareedouw in 1996 a bloom in a cement reservoir killed 290 dairy cows in milk, with 70 more slaughtered — a whole herd, from one water point.
The South African livestock guideline is blunt: no visible scum, and a count above 2 000 Microcystis cells/mL means a bloom is building and scum can form within days. If scum is visible, stock must not drink there — fencing the water point off, on its own, ended the deaths in the earliest recorded cases.
What kills the algae does not kill the poison
Three facts about the toxin decide what works:
- Wind piles the scum onto one shore. Keep livestock and pump intakes away from the downwind bank, draw water from well below the surface, and switch to an alternative supply while a bloom stands.
- Killing the bloom releases the toxin. Dosing a bloom with algicide bursts the cells and puts the poison in the water; precautionary guidance keeps stock off treated water for two to four weeks afterwards.
- Boiling does nothing. Microcystin survives boiling. Settling and plain sand filtration remove the cells but not toxin already dissolved; dissolved toxin is removed by activated carbon or destroyed by strong oxidation (properly dosed chlorine, or ozone).
The farm-scale treatment train
Taking dam or river water to drinking standard — for the house, the dairy or the poultry shed — follows the same sequence every waterworks uses, shrunk to farm size:
- Screen and settle. Coarse screening at the intake; a settling tank or the dam itself drops the heavy silt.
- Coagulate when it's muddy. A dosed coagulant makes fine mud clump and settle. The dose is found with a jar test — typically somewhere between 5 and 40 mg/L depending on how dirty the water is.
- Filter. A backwashing media filter (sand or glass, run at about 5 m/h) does the standard duty. The old slow sand filter still earns its keep on clear, stable feeds: run at 0.1 m/h it polishes biologically and needs nothing but a scrape every one to six months — but it clogs helplessly on muddy water.
- Disinfect — clear water only. Chlorine works when the water entering it is below 1 NTU: dose so a free chlorine residual of at least 0.5 mg/L remains after 30 minutes of contact, with the water below pH 8 (a dam in an algal bloom can sit at pH 9–10, where chlorine is mostly the weak hypochlorite form — correct the pH first), checked with an ordinary test kit; expect 0.2–0.5 mg/L at the furthest tap. UV is a clean alternative on well-filtered water. Where taste, odour or an algae event intrudes, powdered activated carbon dosed with the coagulant carries the toxin and the musty taste out together.
- Cover the storage. Sunlight strips chlorine within hours and grows the next crop of algae. A closed tank protects both.
Rule of thumb
Size the treatment for the dirtiest week, not the yearly average. Watch the downwind bank in hot weather — scum means fence, switch supply, and never rely on boiling. And remember the order that makes surface water safe: settle, coagulate, filter, then disinfect clear water and keep it covered. Skipping a step is how a dam stays a dam instead of becoming a supply.