I live on an island dealing with water shortage issues in our sole source aquifer. The community has enacted a number of irrigation restricting laws, and there is significant discussion about various other water-saving practices such as turning off the water when you do the dishes, shorter showers, etc.

To clarify, the sole source aquifer under our island is like a lense of fresh water surrounded by salt water. If too much water is pumped out without recharge from rain, the saline water moves inland and can affect wells. So there is an obvious need for water conservation. But is water from an extended shower or letting the tap run for a long time "lost" as it would be in a municipal system drawing from a reservoir? My thinking is that such water is returned directly to the source, and would therefore not be lost.

I understand that irrigation is a completely different situation. I also understand that in some areas huge amounts of pumping could supercede the ability of water from the septic system to return to the aquifer AND that it might exceed the ability of the system to contain the water.

But in general, drawing water from the aquifer for use in a shower, and then returning it underground would have a negligible effect on the net amount of water in the aquifer. Is this thinking flawed.

  • @Paparazzi - could you elaborate on that statement?
    – That Idiot
    Commented Sep 26, 2017 at 12:21

3 Answers 3


It seems you are correct: As long as your greywater lands in a septic system that ultimately infiltrates the greywater into the aquifer, you are not really wasting water.

Also, as you indicate, irrigation would be an other issue due to evapotranspiration.

What I don't know is how the freshwater treatment on your island works. If, as you say, there's a problem with nitrates they may well have some pretreatment for denitrification, thus expending energy for every liter that leaves your tap. Maybe ask your municipality/water works?

What might also help the water situation of your island is some greywater treatment - horizontal flow constructed wetlands are generally considerd to be good at denitrification and could be built at low cost, if land is not scarce. I'm sure there's other approaches.

  • Each home has its own well and its own septic system (with a few minor exceptions). There is no municipal water works for the vast majority of the inhabitants. There IS a push now to implement low nitrogen septic systems that actively manage water to effect biological N reduction, and those systems do, indeed, use electricity. But currently it is all small passive systems.
    – That Idiot
    Commented Sep 5, 2017 at 11:27

Gray water has grease in it. Soap etc. For this reason you need a grease pit above the holding tank. Septic tank. The water can then filter from there back into the ground. Both need cleaned regular. No overflow. I live on a island also. Our water is charcoal filtered for the tap. Some what like America used in the 60s. Our well water I do not drink. Chemicals or minerals. In the valley of ore's. We have it all. iron lead gold silver zinc in the ground. So you do not know a good well from a bad well unless tested. We are more volcanic rock area. You do not want to plug the pours in said rock up. So grease removal is important there.

  • This could be a good answer if somebody could polish the English up a bit...
    – RedSonja
    Commented Oct 6, 2017 at 12:46

The main problem with recharging an aquifer with grey water is contamination, or further contamination of the water in the aquifer.

If the aquifer is the island's sole source of drinking water it is best not to contaminate the aquifer by putting grey water into it. Very soon the water from the aquifer will become undrinkable & the island's population will have an even bigger problems than it currently has.

  • Well, all septic systems on the island are also "in the aquifer." The alternative would be to "discard" the gray water through irrigation or some other disposal - and that definitely would negatively affect water quantity.
    – That Idiot
    Commented Aug 30, 2017 at 12:56
  • @ThatIdiot: when you wrote "water is returned directly to the source", in the 2nd paragraph, I read that as putting the grey water directly into the aquifer. If you're putting the grey water into septic tank & there is no interaction between the seepage from the septic tank & the aquifer you should be OK. But, if the septic tank is within the recharge zone for the aquifer then contamination problems could occur. Check how the aquifer normally get recharged.
    – Fred
    Commented Aug 30, 2017 at 16:06
  • "If you're putting the grey water into septic tank & there is no interaction between the seepage from the septic tank & the aquifer you should be OK. But, if the septic tank is within the recharge zone for the aquifer then contamination problems could occur." - This is exactly what does happen in every house on the island - there is nothing but sand/clay between the well and septic. There are Health Department regulations specifying separations which ensure that there is no bacteriological contamination, but nitrates are a large problem.
    – That Idiot
    Commented Aug 30, 2017 at 18:00
  • So my concern is with water quantity - not quality - since there is really nothing else to do with the gray water. What would you do, truck it away? It must go into the ground.
    – That Idiot
    Commented Aug 30, 2017 at 18:02

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