Brown Stains on Pool Bottom: Iron in Water

Introduction

Have you noticed brown, reddish, or rust-coloured stains on the bottom or walls of your pool? These stains are a common but often misdiagnosed problem. The most likely cause is the presence of metals in the water, especially iron, but it could also be manganese or copper. Correctly identifying the origin is essential for applying the right treatment.

In Margem Sul, this problem is particularly frequent in areas where well water is used, such as Moita, Montijo, Alcochete, and rural areas of Palmela and Setúbal. The iron present in these groundwater sources, when oxidised by chlorine, deposits on pool surfaces as brown or reddish stains.

In this guide, we'll explain how to identify the origin of the stains, the different treatments available, and how to prevent their recurrence.

Identifying the Origin of Stains

Before treating, it's crucial to identify whether the stains are caused by metals or other factors such as algae or organic matter.

The Vitamin C Test

This simple test helps confirm if the stains are of metallic origin:

1. Take a vitamin C tablet (ascorbic acid)
2. Rub directly on the stain for 30-60 seconds
3. If the stain lightens or disappears → it's metal (iron or manganese)
4. If there's no change → probably organic or algae

Colours and Their Origins

Factors Indicating Metals

• You use well water to fill the pool
• Stains appeared after adding chlorine
• Water became cloudy or coloured after shock treatment
• You have heating with old copper heat exchangers

Why Iron Stains Appear

Iron can be present in water in various forms:

Sources of Iron

• Well water: Main source, can contain 0.5-10+ ppm of iron
• Aged mains water: Old pipes can release iron
• Equipment corrosion: Oxidised ladders, screws, supports
• Fill water: Even mains water can have iron in older areas

The Oxidation Process

Dissolved iron in water is initially invisible (ferrous iron - Fe²⁺). When you add chlorine or another oxidiser:

1. Chlorine oxidises ferrous iron to ferric iron (Fe³⁺)
2. Ferric iron precipitates forming iron oxide (rust)
3. This oxide deposits on surfaces, creating stains

How to Remove Iron Stains

Method 1: Ascorbic Acid Treatment

The most effective and safe method for iron stains:

Materials needed:

• Ascorbic acid powder (vitamin C) – 500g per 40m³
• pH reducer
• Metal sequestrant

Procedure:

1. Preparation

• Lower pH to 7.0-7.2
• Reduce chlorine to less than 1 ppm (can wait a few days without adding)
• Backwash the filter

1. Applying ascorbic acid

• Turn off the pump
• Distribute ascorbic acid evenly throughout the pool
• For concentrated stains, apply directly and brush
• Let it work for 30-60 minutes

1. After treatment

• Turn on pump and let circulate 2-4 hours
• The dissolved iron returns to the water – don't worry
• Add metal sequestrant (shock dose)
• Let circulate 24 hours

1. Restore chemicals

• After 24-48 hours, rebalance pH
• Add chlorine gradually
• Use liquid chlorine (not granular) initially

Method 2: Metal Sequestrant

For light stains or prevention:

• Add metal sequestrant at recommended dose
• Works best for dissolved iron before it precipitates
• Can help lighten mild stains over time
• Requires regular application to maintain effectiveness

Method 3: Citric Acid (Alternative)

Similar to ascorbic acid, but may be harder to find:

• Use 500g per 40m³
• Same procedure as ascorbic acid
• May be less effective for old stains

What NOT to Do

• Don't add more chlorine – will worsen it by oxidising more iron
• Don't use shock chlorine – same reason
• Don't brush with excessive force – can damage the liner
• Don't drain the pool – stains may become permanent if they dry

Treatment for Well Water

If you use well water, treatment should be preventive:

Before Filling or Topping Up

1. Test well water for iron, manganese, and other metals
2. Add metal sequestrant to pool BEFORE adding water
3. Fill slowly if possible, adding more sequestrant proportionally
4. Avoid shocking immediately – let sequestrant work for 24-48 hours

Sequestrant Dosage for Well Water

Pre-Filtration

For very high iron concentrations (>3 ppm):

• Consider an iron filter in the filling system
• Or fill an intermediate tank and let it settle
• Use only the upper water, clean of sediments

Manganese Stains

Manganese produces black-purple stains and is harder to treat:

Characteristics

• Darker stains than iron
• Common in deep well water
• More resistant to conventional treatments

Treatment

• Ascorbic acid works, but may need larger doses
• Specific sequestrants for manganese are more effective
• Process is similar to iron, but takes longer

Copper Stains

Copper produces blue-green stains:

Origins

• Corroded copper heat exchangers
• Excess copper-based algaecides
• Copper pipes with acidic water

Treatment

• Metal sequestrant specific for copper
• Avoid low pH (<7.2) which corrodes more copper
• Check and replace damaged equipment

Preventing Metal Stains

Prevention is always preferable to treatment:

Maintenance Routine

• Use metal sequestrant weekly (maintenance dose)
• Maintain pH between 7.2-7.6 – low pH corrodes metals, high precipitates
• Avoid super-chlorination – excessive oxidation causes precipitation

When Using Well Water

• Always add sequestrant before new water
• Test iron and manganese at least annually
• Consider alternatives if levels are too high

Equipment Check

• Inspect metal ladders and supports regularly
• Replace iron screws with stainless steel
• Check heat exchangers annually

Costs and Materials

Treatment Materials

Professional Treatment

If stains are extensive or persistent:

• Assessment and treatment: €100-200
• Includes diagnosis, treatment, and prevention
• Guaranteed results

Special Cases in Margem Sul

Moita and Montijo

• Well water with high iron content common
• We recommend water testing before any filling
• Sequestrant should be a routine product

Palmela and Azeitão

• Combination of iron and limestone
• Treat iron first, then limestone
• Use compatible products

Coastal Zones

• Less iron, but watch for sea spray corrosion
• Stainless steel equipment is essential
• Check screws and supports frequently

Frequently Asked Questions

Are iron stains permanent?

Not necessarily. If treated correctly, most stains can be removed. However, old stains that have dried may be more difficult and, in extreme cases, may require professional treatment or repainting.

Can I swim with iron stains in the pool?

Yes, iron stains are not dangerous to health. Iron in water, at normal concentrations, is not toxic. However, it's a sign that water treatment needs attention.

How long does it take to remove stains?

With ascorbic acid, stains can lighten within minutes to hours. The complete process, including chemical rebalancing, takes 2-3 days.

Does iron damage pool equipment?

Iron itself doesn't, but the associated chemical imbalance can. Additionally, iron deposits can clog filters and pipes over time.

Should I drain the pool to remove stains?

In most cases, no. Ascorbic acid treatment works with the pool full. Draining can even be counterproductive, as dried stains are harder to remove.

Conclusion

Brown stains in the pool are usually caused by oxidised iron in the water, especially common when using well water. Key points for solving and preventing this problem are:

• Identify the origin with the vitamin C test
• Treat with ascorbic acid to remove existing stains
• Use metal sequestrant preventively
• Treat well water before adding to pool
• Avoid super-chlorination which oxidises and precipitates iron

If you need professional help with your pool in Margem Sul, ManutençãoPiscinas is here to help. Contact us for a no-obligation quote.