Borewell water TDS in India: why it is higher, why it climbs in summer, and what to test for.

Why borewell water reads higher than the mains

A municipal supply in most Indian cities is river or reservoir water: rain that fell recently, ran over the surface, and was settled, filtered and chlorinated before it reached you. It has had little time to dissolve anything. Borewell water is rain that fell years or decades ago, soaked down through soil and rock, and has been sitting in the gaps in that rock ever since. Water dissolves rock slowly but steadily, and the longer it sits, the more it carries. That is the whole reason: time in contact with mineral.

Two things make it worse. Groundwater is not diluted by fresh rain the way a river is, so nothing resets the reading. And a borewell draws from a particular depth in a particular rock, so what it delivers depends on the local geology in a way a treated municipal supply does not. Two wells a street apart can read differently. The scale that puts these readings in context, with IS 10500 limits and what each band calls for, is on the TDS reference page.

The seasonal rise

A borewell's TDS is not one number. It is lowest a few weeks after the monsoon, when fresh rain has soaked down and diluted the water near the top of the aquifer, and it is highest at the end of the dry season, when the water table has dropped and the pump is drawing from deeper, older, more mineralised water that the recharge never reached. In between it climbs. A well that reads 450 ppm in January can be well past 600 by May, and the difference is not the meter.

The practical rule follows from this. Test twice a year, once after the monsoon and once at the height of summer, and size the purifier for the summer figure. A membrane chosen on the January number can be out of its depth by May. The measuring guide explains how to take a reading you can compare across seasons.

Over years rather than seasons there is a second trend in many areas: when wells draw faster than the aquifer refills, the water table falls a little every year and the summer reading creeps up with it. If your log shows each May higher than the last, that is what you are seeing.

What southern India typically sees, and what we cannot tell you

Most of peninsular India sits on hard rock: granite, gneiss and the basalt of the Deccan. Water moves through the cracks in such rock rather than through pores, sits in them for a long time, and picks up mineral as it does. That is the geological reason borewells across Karnataka, Tamil Nadu, Andhra Pradesh and Telangana so often read above the 500 ppm acceptable limit and not rarely above 1000. Along the coast a second mechanism adds to it: where wells draw down a freshwater aquifer, seawater moves in underneath, and readings past 2000 follow. Parts of the peninsula are also known for fluoride in groundwater, which comes from the rock itself and has nothing to do with contamination.

What this page will not do is give you a typical TDS for your city or district. Groundwater varies by depth, by rock, by distance from the coast and by how heavily the local aquifer is pumped, and a figure that is true of one well is routinely wrong for the next. Any site that gives you "the TDS in Bangalore" is quoting one reading. Measure your own tap, in summer, and use that. If you want a sense of the range, the high TDS guide covers what is dissolved in water at each level and what it does.

The two lab tests every borewell owner should get once

The TDS meter tells you how much mineral the water carries. It cannot tell you about the two contaminants that most often make borewell water genuinely unsafe rather than merely salty, and both are common in groundwater in India.

Fluoride. Comes from the rock. Tasteless, invisible, and harmful to bones and teeth over years above the IS 10500:2012 permissible limit of 1.5 mg/L (acceptable 1.0). Reverse osmosis removes it; a UV purifier does not.

Nitrate. Comes from sewage and fertiliser soaking down. The standard sets 45 mg/L with no relaxation because the risk to infants is direct. RO removes it; UV, carbon and boiling do not.

Add a bacteriological test after the first monsoon you own the well, because a flooded bore can draw in surface water. Get all three done once at a laboratory, keep the report, and your purifier decision is made on facts. The guide on what TDS cannot see goes through the full list.

Silt, and what it does to cartridges

Borewell water carries more suspended solids than treated mains water — sand, rust from the casing, fine silt after the monsoon — and none of it shows on a TDS meter, because it is not dissolved. It shows on the sediment cartridge instead. A cartridge rated to twelve months on municipal water commonly needs changing at six to eight on a silty bore, and a clogged one drops the pressure the membrane needs, which raises the output TDS and shortens the membrane's life. Change it on time; it is the cheapest part in the system.

Choosing the purifier for a borewell

Size for the summer reading, and read the lab report before choosing the membrane. Below 2000 ppm a standard 1812 membrane in any of the three LUXGEN purifiers handles the water; the Aura is listed for municipal or borewell supply and has a TDS controller, which on a clean report lets you keep some of the mineral for taste. Above 2000, fit the molded high-TDS membrane, rated to 3000 ppm, and put a resin softener ahead of it if the hardness is high. For the silt, a pre-filter housing bowl on the inlet with a 5-micron wound sediment cartridge takes most of it before the purifier sees it.