The water you showered with this morning might have been delivered three weeks ago. The water you’ll brush your teeth with tonight could have been sitting in your cistern since before your last grocery run. You’d never eat three-week-old food from the counter. But most people in Los Cabos use three-week-old water without a second thought.
Water doesn’t spoil the way food does — it doesn’t smell, change color, or develop visible mold (usually). But it does change. Significantly. Every day it sits in your cistern, it becomes a little less protected and a little more hospitable to things you don’t want in your water. Understanding water age is the key to understanding why cistern maintenance matters, why tank sizing isn’t just about capacity, and why the “set it and forget it” approach to water storage is quietly undermining your water quality.
How Long Can Water Sit in a Cistern?
There’s no hard expiration date, but the practical answer for Cabo conditions: water stored in a well-maintained cistern with residual chlorine is reasonably protected for 3 to 5 days. After that, free chlorine typically reaches zero in Cabo’s warm ground temperatures, and the water enters an unprotected phase where bacterial populations can grow without constraint. Water stored for 2 to 3 weeks — common in households with large cisterns and moderate consumption — has lost all chemical protection and has been incubating bacteria at 25–30°C for days. It’s not necessarily dangerous, but it’s not what was delivered.
The concept that matters is turnover time: how many days it takes your household to use one full cistern volume. A 10,000L cistern in a household using 500L per day has a 20-day turnover. That means on average, water sits for 10 days before use. If consumption is lower — a vacation rental that sits empty between guests, for example — the water could sit for weeks or months.
Why Water Age Hits Harder in Los Cabos
Water age is a concern everywhere water is stored. But three features of Los Cabos make it significantly worse:
Temperature. The ground temperature in Cabo, even underground where cisterns sit, stays between 22 and 28°C year-round. In the tinaco on the roof, water temperature can reach 35–40°C on a summer afternoon. Bacterial growth rate roughly doubles for every 10°C increase. A cistern in Minnesota at 10°C buys you a week or more of chlorine protection. In Cabo at 25°C, you might get 3–5 days.
Delivery-based supply. In cities with continuous municipal water, there’s constant flow through the system. Fresh water enters as water is consumed — turnover is measured in hours, not weeks. In Cabo’s pipa-based system, your cistern receives a large volume all at once (8,000–12,000 liters) and then sits until the next delivery. There’s no continuous fresh input. The batch delivery model means high water age is structural, not accidental.
Vacation properties. A substantial portion of Los Cabos housing is vacation rentals and second homes that sit empty for days, weeks, or months. During vacancy, water consumption drops to near zero. The cistern sits full. The water ages. When guests arrive, they shower, cook, and brush their teeth with water that may have been sitting for a month or more. This is one of the hidden quality problems in Cabo’s rental market that almost nobody talks about.
What Happens Day by Day
Here’s the timeline of what happens inside a typical Cabo cistern after a pipa delivery, assuming the cistern was reasonably clean and the delivered water had some residual chlorine:
Hours 0–6: The turbulence phase. The pipa fill port is usually at the top of the cistern. Ten thousand liters of water pouring in from a hose creates significant turbulence. This stirs up any sediment sitting on the bottom, resuspending it throughout the water column. Turbidity — the cloudiness of the water — spikes. If you turn on your tap shortly after a delivery, you might notice slightly cloudy or discolored water. This is normal but unpleasant, and it’s one reason inlet filtration is so valuable — it catches incoming sediment before it reaches the cistern floor.
Days 1–3: The protected window. If the delivered water had adequate residual chlorine (0.2–0.5 mg/L free chlorine), the water is microbiologically protected during this period. Chlorine is actively killing bacteria. The water is as good as it’s going to be. This is the ideal time to be using it. Resuspended sediment has mostly settled back to the bottom.
Days 3–7: The decay zone. Chlorine is depleting. In Cabo’s ground temperatures, free chlorine concentration is falling by roughly 0.05–0.15 mg/L per day (faster at higher temperatures, faster in cisterns with more organic matter or biofilm). Somewhere in this window — often around day 4–5 — free chlorine hits zero. The water is now unprotected. Bacteria that survived chlorine treatment, or that were sheltered inside sediment and biofilm, begin to proliferate without constraint.
Days 7–14: The growth phase. Bacterial populations are increasing exponentially. The water still looks fine — clear, no obvious odor. But a microbial test would show rising coliform counts. Biofilm on the cistern walls is actively contributing bacteria to the water column. If the cistern hasn’t been cleaned recently, the biofilm community is mature and continuously seeding the stored water with organisms.
Days 14–21+: The stagnation phase. Water at the bottom of the cistern, near the sediment layer, has the highest bacterial concentrations. Water at the top may be somewhat better, but stratification means there’s little mixing. Dead zones — corners and areas far from the pump intake — have the worst quality. The water is now significantly different from what was delivered. Not necessarily dangerous to touch or bathe in, but not something you’d choose to drink if you could see the test results.
This timeline resets partially with each new delivery — the turbulence of filling remixes the water and any chlorine in the new delivery provides temporary protection. But partial refills (topping off a half-full cistern) dilute the protection. And the sediment layer and biofilm are never reset by new water — they only accumulate.
Stratification: Not All Water in Your Cistern Is Equal
Water in your cistern isn’t a uniform, well-mixed pool. It stratifies — layers develop based on temperature and density differences.
The warm layer (top). Solar heat conducted through the cistern lid or ground surface warms the top layer. Warmer water is less dense and stays on top. This layer has the highest temperature and the fastest chlorine decay rate.
The middle zone. The bulk of the water. Temperature is relatively stable. This is where the pump intake usually draws from.
The cool zone (bottom). Slightly cooler, denser water settles here. This is also where sediment accumulates, creating a zone of high turbidity and high microbial activity near the floor.
Dead zones. Corners, areas behind structural walls or baffles, and zones far from the pump intake or fill port see very little water movement. Water in dead zones can sit for weeks even as the rest of the cistern turns over. These pockets develop the worst water quality in the entire system.
The practical consequence: even if your turnover time is reasonable (say, 10 days on average), parts of your cistern contain water that’s much older than the average. The first-in-first-out model doesn’t apply because the water doesn’t flow through in a linear path — it pools, layers, and stagnates.
The Bigger Tank Myth — Reframed
This is where water age connects to cistern sizing in a way most people never consider.
The conventional wisdom: “Get the biggest cistern possible. More capacity means fewer deliveries, which means lower cost per liter and less risk of running dry.”
The water age reality: a bigger cistern does reduce delivery frequency and cost. But it also increases the average water age proportionally. A family using 500 liters per day:
- In a 5,000L cistern: 10-day turnover → water is on average 5 days old
- In a 10,000L cistern: 20-day turnover → water is on average 10 days old
- In a 20,000L cistern: 40-day turnover → water is on average 20 days old
At 20 days average, your water has been unprotected by chlorine for roughly two weeks. Bacterial populations have been growing for two weeks. Sediment resuspension from each delivery stirs up two weeks’ worth of settled material. And cleaning that larger cistern is more expensive and more disruptive.
The optimal size balances delivery economics (larger is cheaper per liter) against water freshness (smaller is fresher). For most Cabo households, the sweet spot is a cistern that turns over in 7–14 days — large enough to provide comfortable buffer against delivery delays, small enough that water doesn’t sit for weeks. The cistern calculator can help you find your number.
What You Can Do About Water Age
The free fix: Calculate your turnover time right now. Take your cistern capacity (ask your landlord or check property records — common sizes are 5,000L, 10,000L, 15,000L, 20,000L) and divide by your estimated daily consumption (a rough guide: 200–400 liters per person per day in Cabo, higher if you have a pool or garden irrigation). The result is your turnover in days. If it’s over 14 days, water age is a factor in your water quality.
The cheap fix ($50–100 MXN): If you have a pipa delivery schedule, consider more frequent, smaller deliveries rather than filling to capacity. Two half-tank deliveries create more turbulence and mixing, introduce fresh chlorine more frequently, and keep average water age lower. The per-liter cost may be slightly higher, but the quality benefit is real. Also: if your property will be vacant for more than two weeks, consider draining to a lower level and scheduling a delivery shortly before arrival.
The right fix: Optimize your cistern size for your actual consumption (if building or renovating), schedule deliveries based on turnover time rather than just tank level, and maintain a regular cleaning schedule calibrated to your water age — higher water age means more frequent cleaning. For vacation rentals, implement a pre-guest protocol: flush the system, run all taps for several minutes, and schedule a fresh delivery before each occupancy period.
The Hidden Cost of Old Water
Water age doesn’t generate an invoice. But its downstream effects do.
Older water with no residual chlorine develops higher bacterial counts. Higher bacterial counts contribute to biofilm growth, which shortens the effective life of your cistern cleaning — meaning you need to clean more frequently. More biofilm means more organic matter reaching your filters, meaning faster filter clogging and more frequent replacement. Older, more stagnant water also correlates with more sediment reaching your pump, accelerating impeller wear.
For vacation rentals, there’s a reputational cost too. Guests who notice off-tasting water, cloudy tap flow, or musty odors from the taps write reviews. They don’t say “the cistern had high water age.” They say “the water was gross.” In a market where a single negative review about water quality can cost thousands in lost bookings, managing water age is asset protection.
Frequently Asked Questions
Can I add chlorine to my cistern to extend the protection window? ¿Puedo agregar cloro a mi cisterna para extender la protección? Yes, household bleach (sodium hypochlorite, 5.25% — available as “blanqueador” at any grocery store) can be used to re-chlorinate cistern water. The target concentration is 0.5–1.0 mg/L of free chlorine. For a 10,000L cistern, this is approximately 100–200 mL of standard bleach. However, this is a temporary measure — the chlorine will decay again. It’s more practical to manage water age through delivery scheduling and tank sizing than ongoing chemical treatment.
Does boiling old cistern water make it safe? ¿Hervir el agua vieja de la cisterna la hace segura? Boiling kills bacteria and most pathogens, making the water safe for drinking. However, boiling doesn’t remove sediment, dissolved minerals, or chemical contaminants. It also doesn’t address the aesthetic issues (taste, odor) associated with aged water. For drinking water, point-of-use filtration is a more practical solution than boiling.
My vacation rental sits empty for months. What should I do? Mi propiedad vacacional está vacía por meses. ¿Qué debo hacer? Before guests arrive: run all taps for 5–10 minutes to flush stagnant water from the lines and tinaco. Schedule a pipa delivery 1–2 days before arrival to introduce fresh water. If the property has been vacant for over a month, consider a cistern cleaning before the season starts. For properties with regular seasonal patterns, schedule cleaning at the end of vacancy rather than the beginning of occupancy.
Is water age a problem in condos too? ¿La edad del agua también es problema en condominios? Yes, but the dynamics are different. A condo’s shared cistern often has higher consumption across all units, which means faster turnover and lower water age. However, individual unit tinacos can still have stagnation problems, especially in units that are occupied part-time.
Keep Reading
How does tank size connect to water age? The math might change how you think about “bigger is better.” Cistern Sizing: The Turnover Time Method.
What’s growing in old water? The biofilm in your cistern is a living community that gets stronger the longer water sits. Biofilm: The Invisible Colony in Your Water Tank.
Calculate your turnover time. Enter your cistern size and household consumption in the Cistern Size Calculator and see how your water age stacks up.
How Old Is Your Water?
The Water Health Diagnostic factors in your cistern size, consumption pattern, and delivery schedule to estimate your water age and assess its impact on quality. Takes 3 minutes.