Very fast-draining soil near a well - northern Portugal

I live in northern Portugal where the ground is mostly granite and water drains incredibly fast. In the area I’m considering putting the mulch basin my perk test result is 1cm in 30 seconds.

The location is 20-30 meters from a well and 2-3 meters uphill (on a terrace) from it.

I’m wondering if there is any risk of contamination to the well considering these parameters? Should I not even attempt? Or is there something I can do to mitigate any risk?

Of course I don’t want to take any change to contaminate the water.

Welcome to the forum.

I would suggest that the well would be 50 to 75 feet from grey water.

That also depends on laws where you live.

Anne Miller wrote:Welcome to the forum.

I would suggest that the well would be 50 to 75 feet from grey water.

That also depends on laws where you live.

Thanks! Is the 50-75 feet a general rule? Or it applies to the fast draining soil scenario?

from https://www.housing.qld.gov.au/__data
RECOMMENDED SET BACK DISTANCES FOR GTEY WATER  1 march 2026 revision
Recent feedback from stakeholders indicated that the setback distances contained in Table 2.2 of the QPW
Code were insufficient and would make the containment of greywater within premises more difficult to achieve.
To address this concern amendments to the QPW Code provide for increased minimum setback distances from buildings, boundaries, swimming pools etc.
Set back distance from bores intended for human consumption 30.0

Stephen Horvath wrote:

Anne Miller wrote:Welcome to the forum.

I would suggest that the well would be 50 to 75 feet from grey water.

That also depends on laws where you live.

Thanks! Is the 50-75 feet a general rule? Or it applies to the fast draining soil scenario?

I have never seen any rules that refer to fast draining soil scenarios.

If I was worried about that then I might go with 100 feet.  I have fast draining soil due to rocky conditions and my septic is over 100 ft.

With drainage that fast, the 20–30 meter distance by itself probably isn't enough to call it safe. The missing pieces are the well depth and construction, where groundwater actually moves, and whatever setback rules apply in your part of Portugal. Since the proposed basin is uphill from drinking water, more distance seems like cheap insurance. Before digging, check the local requirement for greywater near a private well and find out whether it changes with soil permeability or well type. If there is any doubt, moving the basin farther away makes more sense than trying to engineer around the well later.

Thank you all for your feedback. It's looking like I may not have a graywater disposal option on my land. Portugal doesn't make a distinction between graywater and blackwater for disposal purposes and the setbacks are the same (50m) for both. My land is only 1500m2 and there is no spot that is 50m away from a water source.

So now I'm looking into other treatment and reuse options. I want to learn more about irrigation approaches. The line between disposal and irrigation is a little gray (no pun intended!) to me. The disposal methods I learned about involve a mulch basin with trees and plants growing around it and this seems very much like irrigation, though apparently the guidelines for irrigation and water source setbacks are different somehow? I'm not really understanding this.

I also want to know about in-house reuse, like toilet flushing. It seems like gray water must run through filtration and treatment before it can be used for flushing toilets. Are these simple systems that people are building DIY or must I shell out the money for these 8000€ systems that I see advertised? We are a family of 3.

Ideally I would be recycling water from sinks, showers, washing machines, and dishwater, but I would guess that treatment requirements vary widely for these categories. I need something I can start with right away since my current septic tanks are failing, but something I can upgrade later for more advanced and thorough solutions.

Looking forward to hearing what things you have all done and experiences you've had.