We know that our blog’s regular readers are knowledgeable, intelligent, well-informed people with a strong sense of environmental responsibility (and no, we’re not just sucking up – we really are in awe of our fantastic followers).  We also know that water infrastructure is a confounding and often intimidating structure.  Even when you take the “infrastructure” out and just talk “water,” there are so many terms: blue water, grey water, black water – what’s with all the colors?

There’s not a full water rainbow – I have yet to come across anyone using the terms “red water” or “pink water,” but there do seem to be several different classifications of water, and each of those classifications use colors as indicators.  So this week, we’ll apply a metaphorical prism to the system and refract some answers, hopefully providing some clarity in the process.  So, let’s drop the tortured metaphors and get to the colors:

Blue Water: This is water that is collected on earth – the water that sits in reservoirs, rivers, lakes, etc.  It includes naturally gathered water (lakes) and unnaturally gathered water (reservoirs.)  Blue water is a metric used to evaluate the sustainability of crops and products that must draw on blue water reserves for irrigation, processing, etc.

Green Water: Green water is another standard used when calculating the water footprint of products.  It’s not standing water, like blue water, nor is it groundwater.  Instead, it’s the water sitting on the surface level of the ground.  It’s the water that stays in the soil after a rainfall, or after irrigation, that will eventually feed plants or evaporate.

Grey Water: Grey, while not the most dazzling of colors, is perhaps the most metaphorically rich.  It’s often used to describe being between extremes – moral grey areas, shades of grey, etc.  Grey water is something in-between as well.  Specifically, it’s between being safe to drink and being black water (more on that below).  It’s water that has household uses, but it’s quite clean enough to be good for ingesting.  To learn more, check out our previous posts on grey water.


Black Water: If grey water is halfway between drinking water and black, then black water, naturally, is the opposite of drinking water.  This is water that isn’t safe to be used for any purpose.  It includes water contaminated by natural waste (like what you flush down your toilet) as well as synthetic waste (runoff from factories).  While there are some practices that exist to purify black water, they are often expensive, energy intensive, and have limited return.  In short, any discussion of water conservation must include a discussion of eliminating black water as much as possible.  One of the biggest challenges in the modern world is that most cities have only one sewer system to dispose all the water, meaning black water is mixed with otherwise useable greywater and renders the whole thing unusable.