Three Quick and Easy Methods to Test for Soil Saturation

Installers don’t always have time for long-term monitoring to find wet conditions at a build site. If you’re in a hurry, follow these alternative methods to identify suitable drainfield locations.

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We used two recent columns to define, identify and interpret redoximorphic features in soils to estimate occurrence of seasonally saturated conditions. We also acknowledged some instances where even though the soil is saturated, redoximorphic features do not form and identifying whether the features reflect the current or previous conditions can also be a challenge.

When redoximorphic features are not prominent or present, the occurrence of saturation and reduction must be determined by directly observing the conditions or by using long-term monitoring. Since most decisions we make about where to site and install systems do not afford the luxury of monitoring soil conditions for longer than a few days or weeks, there are some quicker methods used to determine whether a soil is saturated and will be a problem for the proposed soil treatment unit.

THREE IDEAS

We will briefly describe some other methods soil and wetland scientists employ in the field to determine soil saturation and reducing conditions. With a little training and background information, these techniques can be used by site evaluators, installers and local regulatory authorities. The three field methods are use of a dye (alpha-alpha-dipyridyl) that changes color under reduced soil conditions, indicator of reduction in soil (IRIS) tubes, and monitoring wells or piezometers.

Use of alpha-alpha-dipyridyl involves preparing a 0.2% solution of the dye with ammonium acetate and water. When the solution is sprayed on a field-moist sample and a pink color occurs, it means that horizon or location has reduced iron and is subject to saturation.

There are some cautions for use. The dye should not be applied to soil that has been in contact with iron (soil augers, backhoe buckets, knives), has been exposed to direct sunlight after the dye was applied, or was tested for carbonates with hydrochloric acid. The dye can be harmful if inhaled or absorbed through the skin. It can irritate eyes, the respiratory system and skin and can be toxic if swallowed. Therefore, it should be used with caution.

IRIS tubes are sections of 1/2-inch Schedule 40 PVC pipe painted with an iron oxide paint. The tubes are inserted into the soil during a period where the soils are expected to be saturated. In saturated conditions, some soil microbes deplete dissolved oxygen as they break down organic matter. Once the oxygen is depleted — which may take as little as a couple of days to a few weeks — the microbes use the iron oxide paint for their oxygen, which makes the iron soluble in water. It is removed from the tube, exposing the white color of the PVC pipe instead of the reddish paint. If more than 5% of the paint is removed, the soil is under reduced conditions.

Installing monitoring wells or piezometers may be the easiest method to obtain and install, but it’s perhaps more time consuming depending on site conditions. Installation of wells and piezometers are very similar. Basically, a hole is bored in the soil to the desired depth, a pipe is inserted in the unlined borehole, the space around the pipe is packed with a granular material (sand) and the hole is sealed at the top using bentonite or similar material to prevent surface water from entering the hole along the pipe.

DEPTH OF SATURATION

The pipe in a monitoring well will have perforations (holes) the length of the pipe from just below the soil surface to the bottom of the hole, where the pipe is seated on a sand base at least 1 inch thick. If there is a saturated zone anywhere along the length of the pipe, water will flow into the pipe and fill the pipe to the level of the saturated zone — even if the soil at the bottom of the pipe is unsaturated. If monitoring wells are used, the best approach is to install a series of them at different depths in the soil so the depth of saturation can be determined more accurately.

Piezometers have holes only at the bottom of the pipe, which is sealed above that point with bentonite before backfilling the rest of the way with sand and capped at the top with bentonite again. A piezometer will gauge whether saturation is occurring at the depth where the pipe is installed. Water will rise in the pipe to the top level of saturation in the soil profile, giving an accurate picture of the depth of saturation. Of course, multiple piezometers can also be installed if desired.

In both cases, it is best if they are installed during dry periods, although they can be installed when the soil is saturated. Although if you bore a hole and water runs in, your question about saturation should be answered and a monitoring period is probably not necessary. Otherwise they should be monitored during wet periods or seasons to determine seasonal saturated conditions.

One condition to be very aware of is when soils are slowly permeable. When a boring is conducted to determine if the soil is saturated, the hole should be left open a minimum of 24 hours and then checked to see if water is present. It takes that long for water to move out of the soil into the open borehole.

We hope this gives you some ideas to work out soil saturation questions with your local regulators. Of course, bringing in a soil professional may answer the question; or they might suggest using one of these methods if they are unsure.



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