When I started growing Drosera seedlings, I started thinking about how much solids we have in our tap water and if I could use it to water the carnivores. I bought TDS (Total Dissolved Solids) meter and started measuring sources of water nearby.

Total Dissolved solids refer to all salts, minerals and metals that are dissolved in water. TDS is sum of all dissolved charged ions in water. TDS of pure water is 0 PPM. Dissolved solids come from inorganic materials, such as rocks, polluted air or atmospheric gasses that can chemically react with water, forming ions. Organic source of dissolved solids is waste water or sewage, decomposing organic materials. Using chemical fertilizers, pesticides and herbicides is also major source of TDS in water.

As expected, I measured lower concentrations of dissolved solids in rainwater and standing water in a pond. Pond is filled with water plants and tadpoles and occasionally filled with tap water, if there’s not enough rain to replace water that evaporates. When it rains, TDS of pond water drops considerably.
I thought groundwater and water taken from nearby stream would be lower than TDS of tap water, but there was not as much difference as I expected. The best and cheapest way to water the carnivore plants is to store rainwater. There’s only minimal difference between distilled water and rain caught into plastic container. Rain water during wet autumn weather can fall all the way down to 2PPM and can also be higher after long period of dry weather and in case amount of rain isn’t high enough to remove the particles in the air. It’s a good Idea to wait for a while before starting to collect the rain when TDS drops, but it’s not necessary. Using any other sources of water have proven to have too high TDS values.

Nearby stream 208 PPM (almost no precipitation in about a month)
Pond 140 PPM
Tap water 248 PPM
Groundwater 238 PPM
Rain water (summer) 4PPM
Rain water (fall) 2PPM
Distilled water 2PPM

In the end, even water from the pond would be OK to use for watering, but would require watering from the top, removing water from the tray and washing down the soil every now and then with rain water to remove possible accumulated salts.