Aquifer Recharge Pits: What They Are and Why We Build Them

Here’s a small thing that turns a nice pool into a piece of water infrastructure.

Rain falls on your roof. It runs off your deck. It hits your driveway. On most Bali properties, all of that goes into a drain, then a river, then the sea — within about an hour.

A recharge pit catches it and puts it back into the ground instead.

That’s the whole idea. Everything below is detail.

Why Bali specifically

Two problems, running at the same time.

The aquifer is emptying. Fresh water aquifers have been drawn down to less than 20% of capacity, and in some areas the water table has fallen more than 50 metres in about a decade (IDEP Foundation).

The ground has been sealed. Every roof, driveway and pool deck replaces soil that absorbed rain with a surface that sheds it. Kuta Utara alone converted 202.11 hectares of paddy in 2024 (BPS Kabupaten Badung).

These are the same problem seen from two angles. Water that used to soak in now runs off — which is why the island floods in September and the wells run salty by October. Our Sponge Island article makes the long-form version of this argument.

A recharge pit is the household-scale answer. It won’t fix Bali. Ten thousand of them would make a real difference.

How one actually works

Simpler than it sounds.

You dig a shaft — typically around a metre across and two to three metres deep, though it depends entirely on the site. You line the sides so they don’t collapse, fill it with graded gravel, and pipe your roof and deck runoff into the top through a silt trap.

Water enters. Gravel slows it and holds it. It soaks outward and downward through the soil, which filters it on the way, and eventually reaches the water table.

The silt trap is the part people skip and shouldn’t. Leaves and grit will clog a pit within a couple of wet seasons if nothing catches them first. A simple settling chamber you can clear out once a year is the difference between a pit that works for twenty years and one that stops working in two.

The main types, and which suits what

“Recharge pit” is the one people know, but it’s a family of techniques. Managed aquifer recharge splits broadly into surface methods, which let water soak in from above, and subsurface methods, which put it further down (ITRC).

Infiltration well — sumur resapan

The standard, and what we usually mean by “recharge pit.” A vertical gravel-filled shaft, roughly 1m across, 1.5–3m deep.

Suits: most volcanic ground in Bali, where the water table is well below the base and the soil drains. Doesn’t suit: shallow water tables, rock near surface, karst.

Biopore holes — lubang resapan biopori

Much smaller: 10cm auger holes about a metre deep, often filled with organic matter so they compost as well as infiltrate. Cheap, quick, and you can put in dozens.

Badung Regency has required these since Peraturan Bupati No. 24/2010, on a formula of one per 20m² of building footprint plus one per additional 7m² (Badung JDIH).

Suits: almost anywhere as a supplement. Good in gardens and lawns. Doesn’t suit: doing the whole job on their own — the volume is small.

Infiltration trench

A long gravel-filled trench rather than a shaft. Spreads the same volume over more soil contact area.

Suits: sites with a shallower permeable layer over something tighter, or where you can’t go deep. Doesn’t suit: small plots — it takes up length.

Retention pond or basin

A shallow depression that holds water and lets it soak away. On a natural pool project this can be the regeneration zone itself, or a planted low area beside it.

Suits: larger properties with room to spare. Doubles as habitat. Doesn’t suit: tight sites, or anywhere you don’t want standing water.

Permeable paving and rain gardens

Not pits at all, but the same job. Porous surfacing instead of sealed concrete, and planted depressions that catch runoff from a roof or path.

Suits: everywhere, as part of the picture. The cheapest infiltration is the sealed surface you simply don’t build.

Deep injection wells

Pressurised injection into a deeper aquifer. Serious infrastructure, properly regulated, and not something that belongs on a villa project.

Working out what suits your site

Five questions, in order.

1. How permeable is the ground? The Indonesian standard asks for at least 2 cm/hour (SNI 03-2453-2002). You find out with a percolation test — dig a hole, fill it with water, time how fast it drops. It costs almost nothing and it’s the single most useful thing you can do before designing drainage.

2. How deep is the water table? You need clear separation between the base of the pit and the standing water level, so the water actually filters through soil on the way down. Permen PU No. 11/PRT/M/2014 asks for groundwater deeper than 1.5m in the wet season, or 3m for deeper systems. Measure in the wet season, not the dry — the dry-season level will flatter you.

3. What’s the ground made of? Volcanic tuff and sand: good. Heavy clay: slow, may not be worth it. Limestone karst: a different conversation entirely (below).

4. How steep is it, and what’s downhill? Concentrating infiltration near the top of a slope is a recognised way to destabilise it. Near a gorge or cliff crest, a pit goes on the far side of the property or not at all.

5. Is there anything you don’t want to send down there? Septic fields, fuel storage, or a history of chemical use nearby all change the answer. You’re creating a fast route to the aquifer — that’s the point, and it’s also the risk.

Where we don’t build them

Being straight about this matters more than the sales pitch.

Karst. In fractured limestone, water doesn’t percolate slowly through filtering soil — it runs through conduits. Documented contaminant transport in karst exceeds 300 metres per day, and pathogens travel “because of the absence of filtration” (The Groundwater Project).

Indonesian law has already made this call: Permen LH No. 12/2009, Pasal 3(3) explicitly exempts karst areas from the obligation to build infiltration wells and biopori (Ministry of Environment). It removes the requirement rather than imposing a ban — but the thinking has been done.

That rules out most of the Bukit Peninsula, which Perda Provinsi Bali No. 2/2023 formally designates as karst.

Cliff and gorge edges. A geotechnical study in south Badung found factor of safety dropping from 1.502 dry to 0.800 saturated, and recommended drainage to minimise infiltration at the cliff top (Universitas Pendidikan Nasional) — the exact opposite of a recharge pit.

Shallow water tables. If there’s no room to filter, you’re not recharging, you’re injecting.

Tight clay. Below about 2cm/hour the pit fills and stays full. Better to spend the money on permeable surfacing.

On sites like these we go the other way: capture and store the rainwater for irrigation and top-up, rather than trying to force it into ground that doesn’t want it.

Why we put them in pool projects

Because a natural pool is already most of the way there.

A conventional pool is a sealed tank of treated water that gets drained periodically and discharges chlorinated water into the ground. A natural pool holds its water, never drains, and discharges nothing. Add a recharge pit and the property stops taking from the water table and starts giving something back.

The other half is that chemical-free water is the precondition. You can put clean rainwater and clean overflow into the ground responsibly. You cannot do that with chlorinated water, and you cannot do it with salt. That’s the argument in one sentence, and it’s why this only really works alongside a chlorine-free pool.

There’s a regulatory tailwind too. Pergub Bali No. 24/2020 requires “Zero Run Off” compliance for PBG and SLF building permits (Bali provincial government). A pool designed with retention and recharge helps you meet that rather than fight it.

What it actually achieves

Honest scale, because overstating this would be easy.

Groundwater. IDEP’s Bali Water Protection programme, run with Bali State Polytechnic, has demonstrated a well-designed recharge well returning up to 41,000 litres per hour during rainfall.

Flooding. Modelling of infiltration wells in the Badung watershed found they could cut 50-year peak flood discharge by 50.7% and runoff volume by 74.1% (Institut Teknologi Sepuluh Nopember). That’s catchment-scale, not one property — which is exactly the point. It only works at volume.

Saltwater intrusion. In coastal areas, keeping the freshwater lens topped up is one of the few things that slows the sea moving inland. In Canggu that front is advancing 35–56 metres a year.

Your own bore. If you draw groundwater, you have a direct interest in the level not dropping.

One pit will not save Bali. That was never the claim. The claim is that Bali will be rebuilt over the next twenty years regardless, and we get to decide whether the thousands of properties built during that rebuild are extraction points or recharge points.

What to ask your builder

• Have you done a percolation test on this site, and what was the result?
• Where is the water table in the wet season, not the dry?
• Where exactly is the pit going, and why there?
• What’s the silt trap arrangement, and how do I clean it?
• What happens in a 380mm day?

If a builder tells you a recharge pit works everywhere, they haven’t looked at your ground. If they don’t mention it at all, they haven’t thought about your water.

We fit them wherever the site allows — as standard, not as an upsell — and we tell you plainly when the site doesn’t allow it. Book a free design consultation and we’ll walk your ground and give you a straight answer.