Water Percolation in Soil

37% is a very steep slope. The water will flow through it very quickly. How long and deep is the leach field? With such a steep slope, if the leach field isn't long or deep enough, it's possible the sewage may escape the area. Likewise, if there is a layer of impermeable clay or rock just a short distance beneath the field, the sewage may also escape the area.

Hi, DVader1000,

Sorry--I was referring to the surface slope as 37%. The actual slope on the percolation field is 2%, and it is a rectangle about 10' x 26'. It is 6' below grade. (The septic system is a Canadian-built "EcoFlow".) This system is arranged on the flat ground on the top of the steep surface slope.

What I'm asking is related to the water percolating away from this 10 x 26' footprint. Some will move straight down, but the moisture plume in the ground will be also moving away from the footprint as it moves down. (Ignoring clay steaks and bedrock.) I'm wondering if the angle of the plume will be shallow enough to intercept the ground surface line some distance down the slope...?

There is no ground water in the area and the percolation rate was measured at an average of 3 minutes per inch.

Jon.

Just taking a wild swag and with your disclaimers (bedrock, clay streaks) I would think you were fine.

I actually had to use a vertical "leach well" on my last property because of the high clay in the topsoil. But since yours is already purchased and on-hand....

But I do software so I'll shut up now

There are building codes that apply to septic systems: in this part of the country you cannot build one without having an inspector come and approve the site.. the inspector and or the codes will give you a better idea whether this is a feasible setup.

The bed at 10 X 26 is smaller than the usual system for an ordinary sized home as built in our area. Reedbed systems have a smaller footprint than conventional systems and are recommended for space constrained situations. (popular in Europe I believe). Along that line of thinking and design, there may be something you could do with appropriate vegetation on the steep slope, to offset the risk of flow escaping.

Correct. The reason for this is because the reeds provide oxygen that aids in the breakdown of the wastes. Perhaps it might be possible for the OP to dig the leachfield deeper and then grow reeds in the sand to aid in the breakdown of the wastes. A 2% slope will cause water to flow faster than the usual 1 in 100 (1%) slope obviously, but the resistance caused by the plant roots will help slow it down, while the plants themselves provide required oxygen to aid the waste breakdown, as well as help dry the soil through natural evapotranspiration.

Please tell me the powers-that-be didn't make your friend put the septic system uphill from the well (or that you have municipal water).

From the description, I predict smelly soggy spots on the slope before very long.

Country wisdom (as opposed to sanitary engineering) would indicate that the leach field should be downhill from the house, and absolutely be downslope from the water source. Country folk would also route the gray water to a separate leach (dry well?) to minimize flow to the black water system.

If I were building a leach on a hill like that, I would use a single trench running horizontally the width of the property. Install the horizontal perforated pipe absolutely dead level so as not to "bias" the flow towards any one spot. The connection to the septic tank should be in the center of the run also. If the property is wide enough, I might split the flow to two or more equally spaced points on the level horizontal leach. The more you spread the flow, the less likely a wet spot.

Unfortunately you are up against the so-called "experts". I wish your friend the best of luck.

This looks like a classic case for dosing a leach field to avoid breakout of the effluent. Check this link for data on the Flout:

http://www.inspect-ny.com/septic/altgravitydose.htm

The Flout should work very well even if you have just a small drop from the tank outlet to the leach field. By dosing, the whole leach field will be flooded, then allowed to rest. In regular (un-dosed) septic systems a constant trickle of water is flowing from the tank to just the lowest area of the trench piping, until the soil can absorb no more. Then the flow backs up the the next lowest area until that area fails too. The original area stays saturated as the water is pooled in the piping, or worse breaks out to the surface. By flooding and resting the whole leach field, effluent will not have a chance to saturate one area until it fails.

Jon, do you have any option as to what direction to lay the pipes of the leach field? You being in the Kootnays in BC; I suspect you are being forced to use the same 'standard' septic system the province is insisting everybody has to use. I have seen my friends forced to use these in several places with varying results.

If the tile bed of the leach filed is oriented across the sloping face, it would slow down the flow compared to having the pipes running away from the tank but directly down slope.

I am facing the same situation outside Prince George. We live on top of a gravel bed but immediately behind our lot, the land drops away about 15 - 20 feet. If or when the 'authorities' force me to build a new septic system, I am going to have the identical problem.

Elnav, and all you other guys who were kind enough to post a reply,

This particular leach field does not use percolation pipes. It is set up to have a permeable, layered sand/gravel bed that is approximately 12' x 24'. The long direction of this bed is arranged parallel with the edge of the bank, and as far away from the bank as possible.

It sounds like you may have the same questions as I...will we eventually see the water "break out" of the bank? You are lucky that you have gravel where you live--I lived in the PG area 20 years ago and became aware of the surface ponds in the backyards in Pineview just in the nick of time before I bought out there. I wondered why everyone had a pond out back with 17 foot high weeds around it!! I ended up buying out towards Ness Lake. I can't imagine living on a 200 foot thick layer of clay! Although I think I may have been on some pretty fine soil out there, too--the guy I bought from told me he had to pump his septic tank, but didn't know where it was--so he searched (and found) it by pushing a rod into the ground until he hit something solid! Try that in an area where there is more than one rock per square mile!!

Regarding some of the other suggestions: Yes, this system is a "dosing system". About 40 litres per hour. It is the required distances away from wells and bodies of water. As a matter of fact, the system is at the top of the hill because there is a lake at the bottom end of the property. I don't really want to get into a discussion about all the rules and details that the authorities want--it doesn't answer the question at hand.

What I am looking for is someone who knows something about how water moves in soil. I understand that there is something called "soil conductivity" or a similiar term that relates to how readily water moves in a soil. (Nothing to do with electricity) Further, my understand is if the conductivity is high, the water moves more vertically than horizontally. If the conductivity is lower, it will tend to spread out more. I'd like some help to use that information to make a prediction if the water will break out of the bank.

Thanks,

Jon.

What do you mean by depth of 'soil' ? Is it the soil that has to be added on top of the bed and usually above grade as per provincial standards or the depth of ground lying on top of either bedrock or a clay layer if there is such a layer.

Depth here refers to the depth of permeable soil before it reaches a non-permeable layer or the water table. If the top of the water table is less than 10 feet from the bottom of the permeable layer, you shouldn't use it as a leachfield at all.

Hi DVader1000,

Hope all is well with you?

Very good point made about the specific depth!!!!!

I have to say this does seem to be a little 'hit or miss' on something you cannot afford to have that attitude? Least ways it sounds like that to me.

bb

Hi BB,

I'm cool, hope things are fine with you too. Correct, without knowing the actual depth of the leachfield, it will be almost impossible to calculate the percolation and retention time, let alone the OP's biggest query, will it seep out of the field.

Hello DVader1000,

Thanks for the reply affirmation, and take care..............

bb

It looks like you lack some information such as what the soil conductivity is. Looking at DVaders reply this is what you must enter as arguments. Fine gravel is a somewhat subjective description. It does sound like the water flow will be mostly vertical as long as the fine gravel extends deeper than the bottom of the hill. If you start with the leach field buried 6 feet down at what point is the surface of the sloped hill level with the leach field?

If vertical movement is as much or more than horizontal movement ( thanks to gravity) lets assume the movement follows a 45 degree slope angle. but the surface is only 37 degrees. In other words the spread will never reach the surface. This assumes the soil remains consistently fine gravel to a depth lower than the bottom of the slope you mentioned.

Where we live, once the frost is out of the ground , any amount of water quickly disappears. Even after a heavy rain the ground is almost dry within an hour. It does become a problem for the gardening.

Elnav,

This is getting somewhere...

You pose some interesting "horse sense" ideas. Actually, the slope is 37 percent, not degrees, so that the slope angle is more like 20 degrees. The horizontal distance to the bank from the edge of the leach field is about 30 feet. The average slope, from top to bottom (lake edge) is about 50%.

To get breakout, the angle of the percolation plume would have to be more than 45 degrees--more like 65 degrees to achieve breakout at the lowest point on the property. This sounds to me like the probability of breakout is pretty low.

Happy....

Thanks,

Jon.

Hello sawmilleng,

This sounds very 'hit and miss'. The place you should be looking and dealing with before anything is started is a Soil Mechanics Site, or local Expert.

There is certain things you should not do, and most are common sense criteria.

You should not allow this kind of construction where it could possibly run into any kind of 'natural water course'. What water feed is there to the house? Is this uphill from the house/s?

I could go on but to make it short, this system may work fine for several years. But, if and when you get a rain storm, the sh-t will be washed out and has to escape somehow. Hopefully, it will do so away from your friend's place and any other dwelling. If it breaks out near or floods another dwelling, you could be in for a seriously hefty bill in sorting it out!

The company who designed it should have installed it. They could then be liable for any 'damage' done in the coming years.

The run off area hardly seems big enough to me.

One other point, the run off waste pee and poo, if uphill from the dwelling, may well end up flooding the dwelling itself. Maybe not for a few years but as you say..sh-t has to go somewhere!!!

I would seriously have 'second thoughts' on this. It sounds to me like the designed plans and or manufactured bits are from a company who may not have even seen the site. My advice is get a soil engineer and water engineer to have a look at what, if anything, you have done, and at the plans you have in mind, plus the plans of the dwelling. Extreme...........................it could end up pushing the dwelling down the slope!

Often these apparently 'cheap' deals end up costing a whole lot of trouble and expense further down the line.

Finally, I have never heard of a 'self drain' sewage system which is uphill from any dwellings. Cesspits yes, but they are usually maintained so as they do not over-flow.

Take care and several tons of luck are enclosed.

bb

I had a similar problem for a home that I constructed in the mountainous areas out west. I now live in Florida!

I resolved the problem by installing a vertical system. "That is" I had a deep hole dug and as the drain pipe was installed it was coiled and gravel was placed at the same time, spacing and supporting the pipe as it migrated upward. A civil engineer that specialized in systems did the math and determined that such a system could be constructed just as long as the lowest part of the drain field remained 2" above what had been determined as the high water level, underground for that area. I am not aware of any problems that ever existed with that installation. It does require a bit more gravel than a horizontal drain field bed.

As you stated that it is to be constructed uphill from the building or septic tank, I presume that you are planning to use a "pump & grinder, within the septic tank to elevate the sewage to the drain field.