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soil mottling and septic systems

soil mottling and septic systems

soil mottling and septic systems

anybody know anything about soil mottling as an indicator for high season water table concerning septic systems?

RE: soil mottling and septic systems

Soil mottling (blotchy discloration in the vertical soil profile) is an indication of oxidation, usually attributed to contact with groundwater. With regards to septic systems, I am not sure whether your asking about mottling caused by changing groundwater elevation or by saturation of the soil with septic effluent. Could you please be more specific?


RE: soil mottling and septic systems

This will be very confusing.
We have an area that has very lage wholesale plant nursery for the last seventy years. 50 sq miles +. These areas run irrigation daily. Now due to economic and westward movement these areas are becoming mid income housing community. By using test boring no water is encounter at ten feet but county keep replying that high water mottling is at 18 inches. The material is a SP material. I say it is a fault indicator. Usually in this area water table does not vary more than a couple feet any time of year.

Question: What is your defination of soil mottling?

RE: soil mottling and septic systems

Soil mottling is a blotchy discoloration of the vertical soil section, i.e. as would be seen with depth in a test pit. The discoloration is the reaction of soil components with changes in soil moisture. The most common of these is the oxidation of iron-rich soil causing a rust color.

It is possible that the significant amount of irrigation at the property could be causing the mottling. However, this mottling would then be present from ground surface on down. In this case, distinguishing the mottling from natural soil color may be difficult because of the lack of unwetted soil for comparison.

The lack of groundwater within ten feet, and the suspect mottling seen at 18 inches, do not coincide. This would suggest an 8.5-foot water fluctuation, and thus, either an unusual recharge system for the aquifer or else localized recharge events. The expected cappilary rise in SP type sediments is only 50 cm (1.6 feet) under ideal conditions. Therefore, it may be that the mottling at 18 inches is the result of the extensive irrigation activity.

Hopes this info helps.

RE: soil mottling and septic systems

The extensive irrigation activty was my argument since the property was use daily for nursery land for forty years. with no activty in the last ten. I thought the 18 inch mottling was a falses indicator. Today I installed 15 foot mointor wells to prove this point, But the county health department is going by the book and since this type of incident is not common, they are alway right.
The problem is I must have a distance of 18 inches between any mottling and my field lines which puts me above ground. So a mounds system is required. The other fifty houses are doing fine with regular systems.

RE: soil mottling and septic systems

Because you already installed monitoring wells, could you complete a years worth of water levels to show the regulators that groundwater does not come that close to the surface? This may be a fair trade off between the cost of a standard septic system and a mounded/pressure dose system.

Just a thought.

RE: soil mottling and septic systems

That's another rule-of-thumb that belongs where the sun doesn't shine. I see a few things that could be done.

First, find the root (pun intended) reference for the county's assertion that mottling indicates the presence of a seasonally high GWT. Is it a reference that is applicable in your region? If it is and the reference's author is available, you may want to consult him to see if he agrees with the interpertation by the county personnel.

Second, presuming that the current GWT is at or below 10 feet, look at the climatological data for the region (and the watershed) to see if the aquifer could fluctuate to the degree inferred by the near-surface mottling.

Third, check with the county agricultural extension service and/or USDA to see if there has been a soil survey conducted in the region. These documents will generally outline the seasonal range for the GWT. (You might be able to find these references on the web by now -- do a search on USDA SOIL SURVEY.)

Fourth, now that you have the wells installed, be sure to keep track of the weather conditions so you can get detailed level information concident with rainfall events. Correlate the changes in the GWT with the amount of rain. (If the area is remote, you can use a good quality datalogger & pressure transducer normalized to atmospheric pressure.)

If your driller can get high-quality relatively undisturbed bulk soil samples from the near-surface through the zone immediately above the current GWT, you may be able to characterize the degree of mottling with depth. If it is a sandy soil, the mottling should be present through the entire profile -- but with diminishing intensity. Here, the key may be intense mottling near-surface then diminishing with depth only to become more prominant in the vicinity of the current GWT.

In lieu of trying to get undisturbed soil samples via drilling, you may have a better shot of characterizing the profile by excavating a few test pits down to the current GWT. However, be very careful to bench the excavations so nobody gets hurt.

I would be reluctant to go back to the county until I had a substantial database and expert opinion to counter their responses. You're now probably working against an ego.

Finally, try to determine the source of irrigation water used for the nursery. If it was from a deep well, it probably had a very high content of dissolved minerals (particularly iron). Over a period of several decades there could have been a substatial amount of minerals deposited in the upper soils - particularly in the root zone (i.e. the upper 18-inches)!

Finally (this is really finally), there has to be some areas of the region that were never irrigated. Look at some old aerial photos and see if there are some locations where you can look at profiles that were heavily irrigated, lightly irrigated and never irrigated. Is the degree of mottling the same in each area? If you can do this, be sure to make sure the parent soil types are at least similar.

Good Luck,


RE: soil mottling and septic systems

to answer your last paragraph. Out of the perk test deals, I was able to purchase a lot for myself only 200 feet from these problem areas but my lot has virgin timber on it, and was never nursery property. My lot has no mottling at all to a depth of ten feet. But still the county taken lot by lot and it does not apply.

RE: soil mottling and septic systems

Here's the rub -- the county personnel won't deviate from the "book" because they have nothing to lose if they follow it and are subject to castigation if they don't and then something goes wrong. The solution is probably embodied in their "book" with wording something like "...or as approved by a registered professional engineer...". So, with your background, I'd suggest that you call a few geotechnical consultants in your area to see if they can provide the necessary arguments for a reasonable fee.

RE: soil mottling and septic systems

That the problem, I work for a local geotechnical firm as a field tech, I have five civil/geo that agree with me, even a ph,d but according to county there are never wrong.

RE: soil mottling and septic systems

Then, as a last resort, go to my last post and change "geotechnical consultants" to lawyers. But first, if you are sure that the county personnel are out of line, then I'd try going up the chain-of-command within the department and then, if you cannot get any satisfaction, talk to your County Commissioner or the Supervisor (or Executive). It would seem prudent to get the backing of local developers. Also, try to determine if there have been any problems in the region that have caused the county personnel to be inflexible. If such problems exist, then you should be prepared to explain the differences between your area and the "problem" one -- particularly if you have to go to the politicians.

To back up a bit to the mottling, it is nothing more than an indicator of some form of differential chemical activity within the soil mass. This may be due to roots, microbiologic activity, water percolating through the system, a variable near-surface water table or other factors. However, you can find many areas with near-surface variable water levels that do not have soil mottling. Therefore, to depend on the presence of mottling (or lack thereof) to make an informed decision is ridiculous and will not stand up to scientific reasoning.

You also may need to establish a proper definition for mottling. It may be a local term of art that has somehow gained one or more poorly based definitions. For example, it is normal in our area to see the term "mottled brown and gray silty clay" on soil boring logs. This form of "mottling" is, in fact, directly related to mineralization around vertical desiccation cracks that have formed in the upper 18-feet of the clay formation.

In other cases where test pits have been excavated down to a near-surface GWT in medium to fine glacial outwash sands we often find a zone of reddish partially cemented sand within a few inches above the GWT. The color is imparted by iron precipitate, as is the cementation -- but the overlying soil is not necessarily mottled!

You've got what I think is a fun problem -- to bring the county personnel up to better understanding of what they are working with. Seems like it could turn into a profitable exercise for your company if you can get some finincial backing from the local developers. The cost of a reasonable investigation shouldn't be much more than the cost of a few engineered built-up septic fields.

RE: soil mottling and septic systems

You need to speak with a soil scientist who is familar with the soils in your area and who knows the County Staff. He/she can usually smooth over a situation such as the one you are currently involved in.

Don't call the plumber to fix an electrical problem!




RE: soil mottling and septic systems

How long does it take for mottling characteristics to form in the soil?

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