Managing Runoff Contamination: Silt, Nutrients, and Algae Blooms

Heavy rain washed silty, nutrient-laden construction runoff into a two-pond system. The water turned chalky. Do you need to drain and refill? Not necessarily.

Assess the Contaminants

• Inert silt/clay: Will settle naturally or be filtered out.
• Nutrient-rich runoff: Expect an algae bloom within days.
• Chemicals (fuel, concrete washout): Test water; if toxins suspected, partial water changes are needed.

Immediate Actions

1. Restart the pump and filters. Circulation aids settling and oxygenation.
2. Add temporary fine filter media (polyester floss, sponge) at the filter intake; clean daily.
3. Use a polymer coagulant (e.g., AccuClear) to flocculate fine particles for easier removal—costly for large ponds but effective.
4. Monitor ammonia/nitrite. Decomposing organic matter can spike levels.

Long-Term Prevention

Your ditches help. Also consider a sediment forebay or vegetated swale upstream to trap runoff before it reaches the ponds.

Nature handles sediment well; your job is to accelerate the process and watch for nutrient-driven algae.