Borehole vs Tap Water: Which Is Better for Your Swimming Pool?
Many South African pool owners are weighing up whether to top up their pools with borehole water or stick with municipal tap water. With water restrictions, rising costs, and a growing number of homes already on borehole supply, it’s a practical question. The answer depends on what’s in the water.
This guide explains the key differences between borehole and tap water for swimming pool use, what each does to your chemical balance, and the steps to take if you’re using borehole water to keep your pool clean, clear and properly balanced.
Why this matters in South Africa
In Johannesburg, Pretoria and across Gauteng, boreholes have become a common solution to municipal supply issues, water restrictions, and the cost of refilling a 30,000–50,000 litre pool from the tap.
But borehole water is not the same as tap water. It hasn’t been treated, and the mineral content varies dramatically depending on the geology around your property. What this means for your pool: borehole water can disrupt chemical balance and, in some cases, cause staining or scaling if it isn’t managed properly.
How tap water behaves in your pool
Municipal tap water is treated to drinking-water standards. It generally has:
- A relatively stable pH (around 7–8)
- Some chlorine already in it
- Predictable mineral content within local supply norms
- Low risk of introducing iron, manganese or other staining metals
For most pools, tap water can be added directly with minor pH and chlorine adjustment afterwards. It’s the simplest way to top up a pool – and the most expensive.
How borehole water behaves in your pool
Borehole water is untreated groundwater. Its quality depends entirely on the geology around your property, which means:
- pH can be acidic or alkaline depending on the aquifer
- Mineral content (calcium, magnesium, iron, manganese, sulphates) is often higher
- It may carry organic matter, sediment or bacteria
- Total dissolved solids (TDS) are typically higher than tap water
Without treatment, borehole water can throw your pool’s chemistry off – sometimes badly enough to cause visible problems within days.
Common issues with borehole water
Iron and manganese staining
The most common problem. Even small amounts of iron in borehole water will cause brown, orange or rust-coloured stains on pool plaster, marbelite or tiles. Manganese causes darker, purple or black stains.
Once these stains set in, they’re hard to remove without acid washing or surface treatment.
Hardness and scaling
High calcium and magnesium content makes water “hard.” Over time, this can lead to white, chalky scaling on tiles, waterlines and inside pool equipment like heaters and salt chlorinators.
pH instability
Borehole water often has a different pH baseline than your existing pool water. Topping up with a large volume can cause sudden pH swings, which affect chlorine effectiveness and can damage pool surfaces.
Sanitiser demand
Untreated water carrying organic matter or bacteria immediately consumes chlorine. Add a large volume and your chlorine reading can drop fast – leaving the pool unsanitised until it’s shocked.
Cloudy water
High TDS, sediment or precipitating minerals can cause cloudy water that won’t clear with normal filtration.
How to use borehole water in your pool, properly
If you’ve got borehole water and want to use it for top-ups, or to fill a pool from scratch, the rules are simple.
1. Test the water first
Get a borehole water analysis from a local lab. It’s usually R200–R500 and tells you exactly what’s in your water – pH, hardness, iron, manganese, TDS. Without this, you’re guessing.
2. Pre-filter where possible
A sediment filter on the borehole pump removes coarse particles. Whole-house filters with iron-removal cartridges can dramatically reduce staining risk before water reaches the pool.
3. Use a metal sequestrant
A metal sequestrant (also called a chelating agent) binds dissolved iron, copper and manganese in the water so they can’t react with pool surfaces. Add it before topping up, or as soon as you’ve added new water.
4. Top up gradually, not all at once
Adding a few thousand litres at a time gives your existing pool chemistry time to absorb the new water without dramatic swings. Avoid filling the pool right to the top in one session.
5. Test and adjust after every top-up
After adding borehole water, test:
- pH – target 7.2 to 7.6
- Total alkalinity – target 80 to 120 ppm
- Calcium hardness – target 200 to 400 ppm for plaster pools
- Free chlorine – target 1 to 3 ppm
Adjust as needed.
6. Shock the pool after major top-ups
A chlorine shock after adding more than around 10% new borehole water destroys any organic contaminants that came with it.
Mixing borehole and tap water
If you’re on borehole supply but have tap water available, mixing the two is often the best approach. It dilutes the mineral load, smooths out pH, and reduces the risk of staining.
Some properties run a borehole-fed system but use municipal water specifically for the pool. If your borehole has high iron content, this is worth considering.
When tap water is the better choice
- Filling a brand-new pool – starting from balanced, treated water is far easier than dealing with mineral problems from day one.
- Light-coloured pool finishes – pale plaster, white marbelite or light tiling shows stains very visibly.
- You don’t want extra maintenance – using tap water means you skip the sequestrant, pre-filter and frequent testing.
When borehole water is the better choice
- Topping up an existing pool – borehole is far cheaper for ongoing top-ups.
- During water restrictions – using borehole water for the pool keeps you within municipal rules.
- You already have a treated borehole supply – if your borehole feeds the rest of the property and is filtered, the pool is no different.
In short
Tap water is easier; borehole water is cheaper. Both can keep a pool healthy – borehole just takes more attention.
If your pool is staining, scaling or losing chlorine fast and you’re using borehole water, the fix is usually one of three things: filter the water before it goes in, add a metal sequestrant, or test and adjust more often.
If you’re not sure which approach makes sense for your pool, contact Chosen Pools and we’ll help work out the right setup for your borehole and your pool