I just finished installing a new Grey Water Pit (4’deep, 4’wide & 32’long). 4” perforated PVC runs down the middle of pit to 8” off the bottom & tees out in both directions 16’. Then filled pit up with 15 yards of gravel to 10” from top. Then covered gravel with ( 2’x 8’x 1/8”) corrugated black plastic panels & a layer of 8 mill plastic to keep dirt from seeping down into gravel. Then covered to grade with dirt.
My neighbor says I should not have used plastic & just covered gravel with dirt so water would evaporate out the top. The pit ditch sides are pretty hard solid clay dug down to a sand layer at the very bottom of the pit, so probably not going to get water to soak into walls of pit, just the bottom at sand layer.
I live in the middle of Oklahoma panhandle & only get average of 17” of rain per year. The new G/W pit only gets the cloths washer & kitchen sink water to releave stress on my 20 year old septic tank.


Putting plastic on top when you cover with dirt will extend the life of the drain. This can be verified by looking at how to design a French drain--if you prevent the dirt from settling in the rock the drain will last much longer. Your gray water drain is a French drain.

  • Ed Beal, thanks for your reply. I feel better now regarding your description of my Gray Water pit as a French drain. I was planning on drilling ( 12” ) holes with my tractor auger down through the top of the 10”clay layer, corregated black plastic & 8 mill poly liner, & then backfill the holes with gravel to grade level, hoping to create a wicking action to help keep pit dried up. I think neighbor had a good idea about omitting the plastic seal & letting moisture escape out the top, but I like your reasoning better, thanks Oilytex – Oily Tex Nov 29 '19 at 19:21

I would have uesd geofabric/geotextile (a porous cloth-like plastic material) over the gravel to keep the dirt out.

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    Hi. Would you spend a few seconds to fix the spelling/grammatical errors? Thanks. – Daniel Griscom Nov 22 '19 at 13:18
  • You might justify your suggestion with some reasoning. We can infer the benefits, but they should be there, in your words. – isherwood Nov 22 '19 at 21:28
  • Jasen, yes that would of been a good idea also. The geo-fabric would of kept the dirt from washing down into the gravel & still let the moisture wick up & migrate through the soil, thank you very much for your reply, oilytex. – Oily Tex Nov 29 '19 at 19:51

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