What the Drain Water Knows

The Baglihar project on the Chenab is not an old dam. It was commissioned in 2009. Yet scientists from the Central Soil and Materials Research Station, the Ministry of Jal Shakti's own materials laboratory, recorded that the trouble began almost immediately: heavy seepage in the inspection galleries, and leachate deposited on the roof and walls, starting soon after the reservoir was first filled.

They tested the water. Across the samples the pH ran alkaline, between 7.65 and 9.17, but the number that matters was the difference between the measured pH and the saturated pH, and it was negative in every sample. Four of the seven fell on the aggressive side of the Langelier Index, the saturation index that the International Commission on Large Dams recommends in its Bulletin 71 for exactly this purpose. A negative index means the water is not chemically full. It has room for more calcium, and it will take it from wherever it can. The CSMRS paper puts the regional problem in one sentence.

The less travelled Himalayan Rivers generally have soft water i.e. water deficient of dissolved salts. To maintain the chemical equilibrium, it tends to leach out the salts, mainly calcium and magnesium salts from the concrete.

Vyas, Pathak and Vidyarthi, Central Soil and Materials Research Station, "Impact of Soft Water Attack on Dam Concrete of Hydro Power Structures in the Himalayan Region: Case Studies," CBIP, Rishikesh, 2022.

This is the part that surprises people. Pure water is the dangerous water. Hardened cement paste holds calcium hydroxide, which is soluble, and soft water, water low in dissolved salts, is calcium-hungry in a way that hard water is not. It is not a small effect. In a controlled study published in 2024, mortar exposed to soft water deteriorated 53 percent deeper than the same mortar in hard water over 300 days. Snowmelt and young mountain rivers are about as low in dissolved salts as natural water gets, which is why a Himalayan dam sits in more chemically aggressive water than a dam on a mineral-rich plains river.

And the white deposits are the receipt. The dissolved lime travels out with the seepage, meets the air in the gallery, reacts with carbon dioxide, and precipitates as calcium carbonate on the walls and roof. Those crusts and stalactites are not something the water brought in from outside. They are the dam, relocated. At Trinity Dam in the United States, the Bureau of Reclamation measured the calcite in one chamber averaging half an inch to an inch thick, and in places six inches.

Which is the useful part, because it means the diagnosis is already running out of your drains. The water leaving the gallery has passed through the concrete and carries what it took. CSMRS monitors seepage water quality and leached material at Himalayan projects for exactly this reason, and the Langelier Index costs a water sample and a laboratory afternoon. It tells you whether the water arriving at your dam is chemically hungry. Comparing the calcium going in against the calcium coming out tells you whether it is being fed.

Now the contrast, because the other mechanism engineers worry about behaves differently here. Sulfate attack is the reaction of sulfates with hydrated cement, and the standard describes the two products plainly.

The formation of ettringite can result in an increase in solid volume, leading to expansion and cracking. The formation of gypsum can lead to softening and loss of concrete strength.

ACI 201.2R-01, Guide to Durable Concrete, Section 2.2.2, on the mechanisms of sulfate attack.

The best-documented Indian sulfate case is Pandoh, the 76-metre earth-cum-rockfill dam on the Beas commissioned in 1977. A reddish-brown material was seeping from drainage holes in three galleries. Chemical analysis found sulfate radicals, with a concentration of 861.50 parts per million in the seepage. Petrography and X-ray analysis identified chalcopyrite and pyrrhotite, and the conversion of monosulphate phases into ettringite, as the cause of the expansion and seepage. Note where the sulfate came from. Not the reservoir and not the soil, but sulfide minerals in the bedrock, oxidising and delivering sulfate to the concrete from underneath. That is the part worth carrying away. Finding sulfate in a dam does not tell you where the sulfate came from, and the answer changes what you do about it.

That distinction matters because the reflex answer to sulfate is to specify sulfate-resisting cement, and IS 12330 defines it by limiting the tricalcium aluminate to 5 percent, since C3A is what ettringite feeds on. It is the right answer for ordinary sulfate attack and the wrong answer twice over. Thaumasite sulfate attack does not involve aluminium at all: it attacks the calcium silicate hydrate, the binder itself, and low-C3A cement does not protect against it. And IS 456 notes that where chloride is encountered along with sulphates in soil or ground water, ordinary Portland cement with C3A between 5 and 8 percent is desirable instead of sulphate-resisting cement. The code gives the requirement without the reason; the reason is that very low C3A leaves less aluminate available to bind chloride. The cement that saves you from one attack can expose you to another.

The lesson: Age is not the enemy; unmanaged deterioration is, and dams that are looked after last for generations. But leaching is slow, cumulative and quiet, which is exactly why it goes unmeasured: it does not announce itself in a crack. Read the water. Test what enters the reservoir and what leaves the drains, run the Langelier Index against ICOLD Bulletin 71, and identify the leachate rather than sweeping the calcite off the gallery floor as housekeeping. If sulfate does appear, find out where it came from before reaching for sulfate-resisting cement, because at Pandoh the answer was the rock. The first Comprehensive Dam Safety Evaluation under Section 38 of the Dam Safety Act is due by 30 December 2026, and while the section does not name water chemistry, it requires evaluation of any other condition that constitutes a hazard to the integrity of the structure. A dam quietly giving up its lime to the reservoir is such a condition, and the evidence is already in a drain.

Read more: Concrete Deterioration in Indian Dams: Warning Signs Every Dam Owner Should Recognise →