Does your blending water change with the seasons? Monsoon, turnover, and the spec that has to hold in July

Key takeaways

• Source water is seasonal; a spirit is not. Raw water turbidity at Delhi’s Sonia Vihar plant runs “25 – 8,000 NTU, especially in the monsoon season” (SUEZ) — a 300-fold swing in the feed to a treatment train that has to deliver one answer.
• Groundwater moves too, just more slowly. India’s 2024 national monitoring compared the same trend stations either side of the monsoon and found nitrate exceedances rising “from 30.77% of samples pre-monsoon to 32.66% post-monsoon,” the “dual effect of rainfall, which dilutes nitrates in some areas but leads to a higher leaching of contaminants” (Mongabay India).
• The faults that travel are the ones you can smell. Geosmin and MIB have “low odour threshold concentrations of 0.00001 mg/L (10 ng/L)” (Australian Drinking Water Guidelines) and are “not removed from water using routine treatment processes” (Hunter Water).
• Even the purification plant has a summer and a winter. On an RO membrane “the permeate flow increases 3%” per °C and “salt passage increases 6% for every 1° C increase in feedwater temperature,” so systems see “higher flows and worse rejection in the summer, and lower flow and better rejection in the winter” (WaterWorld / Dow).
• The answer is a twelve-month proof, not a good certificate. WHO’s water guidance requires validation “covering at least one year of operation over different seasons” and routine monitoring of the source “to account for environmental, seasonal or supply changes” (WHO TRS 1033, Annex 3).

Yes — if it comes from a river, a reservoir, a shallow well or a municipal main, it changes with the calendar, and the changes are large enough to see without instruments. What a distillery needs from blending water is the opposite: the same water in January and in July, because the spirit it reduces is blended to one strength and one flavour profile all year. Seasonality is therefore not a curiosity about hydrology. It is the single biggest reason a proofing specification has to be written as a guaranteed maximum backed by a year of data, rather than as a typical analysis taken on a good day in February.

Why did distilleries once stop making whisky in summer?

Because the water ran out — and the industry built a tradition around it. “Traditionally, distilleries always used to shut down in the summer, because there was a lack of water,” George Grant of Glenfarclas told Scotchwhisky.com; in the drought summer of 2018 the distillery stopped distilling for ten weeks, beginning two weeks earlier than its usual shutdown. “We didn’t really have a winter, so there was no groundwater, and the rain didn’t really come” (Scotchwhisky.com).

The same feature records a subtler seasonal effect, on the spirit rather than the schedule. “When it’s hot, it’s harder to get the liquid to condense on the copper,” says Brendan McCarron of Glenmorangie — warm cooling water reduces reflux, so distillers run stills slower and less often to avoid sulphury character.

But the silent season was about how much water there was, and that is the quantity risk covered in our guide to abstraction licences and drought. This article is about what is dissolved in it — a question that does not go quiet when the burn refills, and which reaches the bottle far more directly, because the water added at reduction is never boiled, never fermented and never filtered again.

What actually changes in a surface water source between seasons?

Four things, and all four end up as work for a treatment plant.

Turbidity. The monsoon loads a river with everything the catchment can spare. Delhi’s 635 MLD Sonia Vihar plant is designed around raw water whose turbidity ranges “25 – 8,000 NTU, especially in the monsoon season” (SUEZ). Solids themselves are easy to remove; what rides in with them — organic matter, metals, microbiology — is the actual payload.

Disinfection by-products. Chlorine plus natural organic matter makes trihalomethanes, and warmth accelerates it. A study across 58 sampling locations in Karachi found median total THMs of 76.60 µg/L in summer against 62.16 µg/L in winter, with chloroform contributing 93.07% of the summer total (Siddique et al., Global NEST Journal). The same chemistry that raises THMs raises the chance of the chlorinated phenols behind chlorophenol taint, which are perceptible at parts per trillion.

Algal taste-and-odour compounds. Warm, still, nutrient-rich water grows cyanobacteria, and cyanobacteria make geosmin and 2-methylisoborneol. Their “similar low odour threshold concentrations of 0.00001 mg/L (10 ng/L)” put them among the most detectable substances in drinking water (ADWG). The seasonal pattern is explicit in utility guidance: “Seasonal increases in naturally occurring algae or bacteria in water sources can cause a rise in Geosmin and/or MIB above the taste and odour threshold” — and, critically, they “are not removed from water using routine treatment processes” (Hunter Water). Both are “virtually resistant” to ordinary oxidation; ozone manages roughly 50% removal, and activated carbon is the usual defence.

Reservoir chemistry. A stratified reservoir changes its own water without any help from the catchment. “The establishment of anoxic conditions (DO concentrations <0.5 mg/L) in sediments during thermal stratification favors soluble forms of Fe and Mn,” which then diffuse into the water column; elevated iron and manganese “cause staining, odor, and undesirable taste, which require treatment prior to potable use” (Munger et al., Lake and Reservoir Management). When the reservoir turns over, that bottom water is mixed through the whole column — and iron and manganese are precisely the trace metals a blending water specification exists to remove.

Doesn’t groundwater stay the same all year?

It is steadier, not static — and India has unusually good evidence on the point, because the Central Ground Water Board samples the same trend stations before and after the monsoon.

Across roughly 15,259 samples, the 2024 national assessment found 9.04% exceeding the fluoride limit and 7.25% exceeding on electrical conductivity. The seasonal comparison is the interesting part: nitrate exceedances rose “from 30.77% of samples pre-monsoon to 32.66% post-monsoon,” which the report attributes to “the dual effect of rainfall, which dilutes nitrates in some areas but leads to a higher leaching of contaminants from the surface to the groundwater” (Mongabay India). Monsoon recharge improved fluoride in several states; it made nitrate worse overall. Recharge is not a rinse cycle.

Pumping adds a second, man-made season. USGS work on public-supply wells found that in Modesto, California, “supply wells are more likely to produce younger groundwater with higher nitrate and uranium concentrations during the summer (high) pumping season,” while in Albuquerque they “are more likely to produce older groundwater with higher arsenic concentrations during the winter pumping season” (USGS). The same borehole, the same aquifer, a different answer depending on how hard and how often it is drawn.

Depth is the main defence. A deep, confined aquifer is slow to communicate with the surface, which is why we chose one — the basalt-filtered aquifer below Vadodara, described in why Vadodara — rather than a shallow source with a prettier story. But “deep and confined” is a claim to be demonstrated with a year of data, not assumed from a borehole log.

Does the treatment plant simply cancel the seasons out?

Mostly, and that is the point of buying treated water rather than raw water. But the plant has seasons of its own, and a buyer should know where they show up.

A reverse-osmosis membrane is temperature-sensitive in two directions at once. “For every 1° C increase in temperature, the permeate flow increases 3%,” while “salt passage increases 6% for every 1° C increase in feedwater temperature” — twice as sensitive as flux. The practical consequence, stated plainly: systems with large annual swings in feed temperature “will experience higher flows and worse rejection in the summer, and lower flow and better rejection in the winter” (WaterWorld / Dow). A plant that meets its conductivity target comfortably in January is not thereby proven in May, which is why normalised performance data matters more than a single spot reading.

Microbiology follows the thermometer too. “Higher water temperatures increase microbial growth and diversity in pipes,” and “generally, water temperatures at or greater than 15 °C favour microbial growth” (Walkerton Clean Water Centre). Purified water has no disinfectant residual to defend itself with, so summer is when a storage and distribution loop is tested hardest — the mechanism behind why pure is not sterile and why storage and pipework cause purity drift.

And some seasonal species slip past the headline barrier entirely. Geosmin and MIB are small, neutral and odorous — the class of contaminant an RO membrane is least effective against, as set out in five things an RO membrane leaves behind. Carbon handles them; carbon that was sized for winter organic loading may not.

Why does a seasonal wobble matter more in a proofing tank than in a glass of tap water?

Because of where it sits in the process. A municipal customer drinking water in August is drinking a product that will be replaced tomorrow. A distillery that proofs a batch in August has put that water into a spirit permanently, as 30–60% of the finished liquid, after every corrective step in the production chain has already happened — there is no carbon filter, no fermentation and no cask between the proofing tank and the bottle. Three consequences follow, all commercial rather than theoretical:

1. An off-note is unrecoverable. A musty geosmin character or a chlorophenolic taint is at parts-per-trillion concentration in the water and parts-per-trillion in the bottle. The options after filling are described in what happens when blending water fails its spec — and they are not good ones.
2. Consistency is the brand. A bottling that smells different in the monsoon batch than in the winter batch is a quality failure even when every number is inside a legal limit, because the drinker is comparing it to the last bottle, not to IS 10500.
3. The certificate may not catch it. A conformance test against the FSSAI and IS 10500 floor looks for health-based limits. Geosmin, MIB and low-level organics are not on that list, which is why a sensory check on the water itself belongs alongside the certificate of analysis — especially in bloom season.

How do you specify against a source that moves?

Borrow the discipline from the industry that solved this first. WHO’s guidance on water for pharmaceutical use requires that “the quality of drinking-water should be monitored routinely to account for environmental, seasonal or supply changes which may have an impact on the source water quality,” and lists among the design inputs for a treatment system “the quality of the available feedwater and the variation over time (seasonal changes)” (WHO TRS 1033, Annex 3).

Its validation requirement is the part worth copying verbatim. A three-phase approach should be used “covering at least one year of operation over different seasons,” with Phase 3 extending the programme so that all phases “cover at least 12 months” and the data “demonstrate the reliable performance of the system over this period of time covering the different seasons.” Tests on the source water are explicitly inside that programme, not alongside it.

Translated into a procurement checklist for a distillery buying blending water:

• Ask for twelve months of results, not a certificate. One analysis proves one tanker. Twelve months of trended data proves a system, and shows you the worst month rather than the best one.
• Read the spec as a maximum, never a typical. “Typical TDS < 10 mg/L” is a marketing sentence. ”≤ 10 mg/L, every batch, with a CoA per consignment” is a specification — the distinction we draw in how blending water is tested.
• Ask what happens upstream in the monsoon. Does the supplier’s feed turbidity spike? Is pre-treatment sized for it? Is carbon changed on a schedule or on breakthrough?
• Put organics and sensory on the plan seasonally. Conductivity will not see geosmin. A quarterly nose on the water, weighted towards bloom season, costs nothing.
• Check the aquifer’s own calendar. Depth, confinement and pumping regime decide how much of the surface year reaches the well — and a supplier should be able to show pre- and post-monsoon numbers from their own source.

This is also the clearest argument in the make-or-buy decision. A distillery running its own plant inherits every season its catchment has. A distillery buying to a specification transfers that variability to a supplier whose entire business is absorbing it — provided the specification is written as a guarantee that holds in July.

Glossary

• Pre-monsoon / post-monsoon — the standard Indian sampling pair, taken before and after the rains at the same location, used to separate seasonal change from long-term trend.
• NTU (nephelometric turbidity unit) — the measure of cloudiness in water from suspended particles; the headline number that moves most violently in the monsoon.
• Thermal stratification — the summer separation of a lake or reservoir into warm surface and cold bottom layers that do not mix.
• Hypolimnion — the cold, dense bottom layer of a stratified waterbody, where oxygen is consumed and iron and manganese become soluble.
• Turnover — the seasonal overturn that mixes a stratified reservoir top to bottom, redistributing whatever accumulated in the hypolimnion.
• Geosmin / 2-MIB — earthy and musty odour compounds produced by cyanobacteria and actinobacteria, detectable near 10 ng/L and not removed by routine treatment.
• THMs (trihalomethanes) — chlorination by-products formed when free chlorine meets natural organic matter; higher in warm water.
• Salt passage — the fraction of dissolved salts that crosses an RO membrane rather than being rejected; rises about 6% per °C of feed temperature.
• Trend station — a monitoring point sampled repeatedly over time so that seasonal and multi-year changes can be compared at the same place.

Frequently asked questions

Does blending water really change between the monsoon and the dry season? Raw water does, dramatically. Turbidity at one Delhi plant spans “25 – 8,000 NTU, especially in the monsoon season” (SUEZ), and India’s national groundwater monitoring recorded nitrate exceedances rising from 30.77% pre-monsoon to 32.66% post-monsoon (Mongabay India). Purified blending water bought to a specification should not change — but that is a claim the supplier has to prove with a year of data, not an inherent property of the word “demineralised”.

Why does tap water taste different in summer? Usually one of two seasonal effects. Warm water forms more chlorination by-products — one Karachi study found median THMs of 76.60 µg/L in summer against 62.16 µg/L in winter (Global NEST Journal) — and warm reservoirs grow the cyanobacteria that make geosmin and MIB, which are detectable around 10 ng/L (ADWG). Both are reasons not to dilute a good whisky with tap water, as we argue in tap vs filtered vs mineral vs demineralised.

Can a reverse-osmosis plant produce different water in summer and winter? Yes, measurably. Permeate flow rises about 3% per °C and salt passage about 6% per °C, so a system sees “higher flows and worse rejection in the summer, and lower flow and better rejection in the winter” (WaterWorld / Dow). Well-run plants normalise their data to a reference temperature precisely so that a seasonal shift is not mistaken for membrane failure, or vice versa.

How long should a water system be monitored before I trust its specification? A year. WHO requires validation “covering at least one year of operation over different seasons,” with the three phases together covering at least 12 months and the data demonstrating reliable performance “covering the different seasons” (WHO TRS 1033, Annex 3). It is the most transferable idea in pharmaceutical water practice, and it costs a spirits producer nothing to ask for.

Does deep groundwater avoid the problem entirely? It reduces it rather than removing it. Depth and confinement slow the exchange with the surface, but pumping creates its own seasons — USGS found supply wells drawing “younger groundwater with higher nitrate and uranium concentrations during the summer (high) pumping season” in one city and higher arsenic in winter in another (USGS). The honest position is that no source is seasonless; the specification, the treatment train and the monitoring are what make the delivered water behave as if it were.

Blenders International · Water Science series