Karachi’s monsoon is short and violent. Most of the year’s rain falls between July and early September, often in a handful of storms that flood streets within an hour. For stored water this is the riskiest stretch of the year, and the reason is not the rain itself. It is what the rain does to drains, sewers, power and the ground around your tank.

This guide explains the monsoon water risk in Karachi from the point of view of the tank owner: how contamination gets in, which tanks are most exposed, and a simple before, during and after plan.

In brief

• The danger is not rain falling into a tank. It is flooded ground, overflowing sewers and low pressure in leaking mains.
• Underground tanks with a hatch at floor level are the most exposed.
• Clean in June, before the rains, and fix the lid and hatch at the same time.
• After flooding, clean first and disinfect second. Chlorine tablets alone are not enough.
• Boil drinking water for the whole household through the season.

Why the rains are a water quality problem

Sewers and storm drains overflow together

In heavy rain, sewage and stormwater end up in the same flooded streets. Water mains run through that ground. Karachi’s supply is intermittent and, according to a World Bank project document, often delivered at very low pressure, and the same document estimates that more than half the water put into the network is lost through leaks and other losses. A leaking pipe sitting in flooded, sewage-laden soil with little or no pressure inside it is an open door.

Underground tanks sit below flood level

Most Karachi houses have an underground tank with a hatch at or just above ground level. When the lane or courtyard floods, water finds its way in around the lid, through pipe entries and through cracks in old RCC walls. It does not take much floodwater to contaminate a full tank.

Power fails when it is needed

Storms bring feeder trips and long outages. Pumping stations stop, pressure in the mains drops, and household motors cannot lift water to the roof, so water sits still for longer. Then supply returns in a surge that carries disturbed sediment with it. We explain this cycle in how load shedding affects water quality.

Warm, humid weather keeps bacteria active

Monsoon days are slightly cooler than May and June but still warm enough for rapid bacterial growth in a rooftop tank, a point covered in our article on heat and bacterial growth in tanks. Add fresh organic matter from dirty supply water and the tank has both the temperature and the food.

Mosquitoes breed in standing water

Dengue cases in Karachi rise after the rains. An uncovered or loosely covered tank is a ready breeding site, and the WHO advises households to cover, empty and clean water storage containers to deny the mosquito a place to lay eggs.

What the record shows

Outbreaks of waterborne disease in Sindh are well documented. During the cholera outbreak of 2022, the WHO reported 234 laboratory-confirmed cases in Sindh between 15 January and 27 May, noted that most water samples tested in Karachi contained coliform bacteria, and named safe drinking water and sanitation as the most effective controls. Typhoid follows a similar pattern, and Karachi accounted for 69 percent of cases in the XDR typhoid outbreak recorded by WHO up to the end of 2018.

We do not have reliable public figures for how much illness rises in any single monsoon, so we will not quote a percentage. What is predictable is the mechanism: flooded ground, low pressure and leaking pipes put more silt and more sewage-borne bacteria into the supply, and all of it settles in the tank at the end of the line.

Which tanks are most exposed

Tank or situation	Main monsoon risk	What to check
Underground RCC tank in a low-lying lane	Floodwater entering around the hatch or through cracks	Hatch height above ground, lid seal, wall and floor cracks
Underground tank beside a gutter line or soak pit	Sewage seepage when the ground is saturated	Distance from sewer lines, damp patches or staining on the inside wall
Rooftop plastic or fibre tank	Rainwater, dust and insects through a loose lid; mosquito breeding	Lid fit, cracks, vent and overflow mesh
Shared tank in an apartment block	One contamination event affects every flat	Who checks it after rain, and when it was last cleaned
Tanker-fed tank	Tanker water of unknown origin when piped supply fails	Ask where the tanker filled; clean more often

Low-lying parts of the city flood first, but no area is exempt. If you live in a block of flats, our article on who is responsible for an apartment building’s tank explains who should be organising the checks.

How floodwater gets into an underground tank

Understanding the entry points tells you what to fix. In the tanks we inspect, water gets in by four routes:

1. Over the top. A hatch that is level with the courtyard floor lets surface water run straight in. A kerb of even four to six inches around the opening stops most of it.
2. Around the lid. Thin steel covers rust and warp, and concrete slabs chip at the edges. Any visible gap is too much.
3. Through pipe entries. The hole where the inlet or suction pipe passes through the wall is often left unsealed.
4. Through the structure. Old RCC develops hairline cracks and honeycombing. When the soil outside is saturated, water pushes through under pressure, and so does anything in that soil.

The first three are cheap repairs. The fourth needs a mason and sometimes waterproofing, and it is best done when the tank is empty after a cleaning.

Before the rains: June

1. Clean the tanks. If you clean once in the year, do it now. A tank that starts the season without sludge and biofilm has far less for incoming contamination to feed. The Sindh health department also advises cleaning tanks before summer each year.
2. Fix the lid and the hatch. Replace cracked covers. If the underground hatch is flush with the floor, have a raised kerb built around it.
3. Seal pipe entries and visible cracks. A mason can do this in an hour while the tank is empty after cleaning.
4. Fit mesh on vents and overflows.
5. Clear the drain nearest the tank so water runs away from the hatch, not towards it.

During the rains

• Keep both tanks closed. Do not open the underground hatch while the area around it is wet or flooded.
• If the lane is flooded, switch off the motor so it does not pull doubtful water up to the roof tank.
• Boil all drinking water for the whole household, not only for children.
• When supply returns after a long gap, let the first flow run to waste if you can.
• Do not send anyone into an underground tank during or just after flooding. Gas builds up in closed tanks and the risk is real, as we describe in why untrained tank cleaning is dangerous.

After the rains: do you need to clean again?

Not always. Use this as a guide:

• Clean immediately if floodwater reached the hatch, if the water has changed colour or smell, if there is debris floating in the tank, or if several people in the house have fallen ill.
• Clean within a few weeks if the underground tank is old, sits in a low spot, or you relied on tankers during the rains.
• Inspect and carry on if the tank was cleaned in June, the lids held, the water is clear and odourless, and the tank floor looks clean when you shine a torch in.

The symptoms that should prompt action are listed in signs your drinking water is making you sick, and children deserve particular care in this season, as covered in our guide for parents.

Can chlorine tablets replace a cleaning?

No, though they have a place. Tablets disinfect the water sitting in the tank. They do not remove silt from the floor or biofilm from the walls, and dirt in the water uses up chlorine before it can do its job. After flooding, the order should be: clean out the dirt, disinfect the surfaces, then refill. The WHO’s own guidance on cleaning and disinfecting storage tanks follows the same sequence of cleaning first and disinfecting second.

How we handle monsoon call-outs

The work follows our standard eight-step procedure: drain, ventilate and gas-test underground tanks before entry, jet wash at 3,000+ PSI, vacuum out the silt, disinfect, rinse and report. After flooding we pay particular attention to the floor corners where silt packs down, and to cracks and pipe entries, which we photograph for the service report so you can have them repaired.

Both branches answer around the clock in the rainy season: DHA, Clifton and South Karachi on 0333 0293174 and Gulshan, Jauhar and East Karachi on 0340 2717530. You can also get an instant price online and book ahead of the rains. For the regular schedule through the rest of the year, see how often to clean a tank in Karachi.

Monsoon water risk in Karachi for businesses and shared buildings

Everything above applies with more force where one tank serves many people. Apartment unions, schools, restaurants and clinics should clean before the rains as a matter of routine, inspect after every major storm, and keep the dated service report on file. Our sector guides cover the detail for schools, hotels and restaurants, hospitals and factories.

Frequently asked questions

It rained heavily. Should I empty the tank?

Only if there is a reason to think floodwater got in: water around the hatch, a change in colour or smell, or debris in the tank. Heavy rain on a sealed rooftop tank does not by itself call for a re-clean.

When is the best time of year to clean a tank in Karachi?

June, before the monsoon, and again after the rains end if the tank was exposed. Households that clean twice a year usually choose June and the winter months.

Is rainwater that gets into a rooftop tank harmful?

Rain itself is fairly clean, but it washes roof dust, bird droppings and debris in with it. The fix is a lid that seals.

How quickly can you come after flooding?

Both branches take calls 24 hours a day and aim for same-day visits. After a major storm demand is high, so call early in the day.

Should I switch off the water motor during heavy rain?

If the lane or courtyard around the underground tank is flooded, yes. It stops possibly contaminated water being lifted to the roof tank and spread through the house.

Floodwater got into my underground tank. Can I still use the roof tank?

Only if it was filled before the flooding and the motor has not run since. Use it sparingly for washing, boil anything you drink, and have both tanks cleaned before pumping again.

Does a tank need to dry out after monsoon cleaning?

No. After disinfection and a clear-water rinse it can be refilled straight away.

Sources

Figures in this article were checked in October 2026 against the following:

• World Bank, 2019: Karachi Water and Sewerage Services Improvement Project, project information document
• WHO Disease Outbreak News, 17 June 2022: Cholera, Pakistan
• WHO Disease Outbreak News, 27 December 2018: Typhoid fever, Pakistan
• WHO: Dengue fact sheet
• WHO Technical Note No. 3: Cleaning and disinfecting water storage tanks and tankers
• Geo News, 13 September 2025: Naegleria claims another life in Karachi, toll hits five in Sindh

About Khan Tank Cleaning

Khan Tank Cleaning has cleaned residential, commercial and industrial water tanks in Karachi since 2005. The company was founded by Mateen Khan, who still oversees quality control at both branches. More about us, or get in touch: 0333 0293174 for DHA and South Karachi, 0340 2717530 for Gulshan and East Karachi, both answered 24 hours.