Civil-Environmental Engineering

Frequently Asked Questions*

Most counties will come out and perform a soil survey on your site.  In fact, many require it be performed by their own inspectors.  However they often request that several large holes be dug by a backhoe.  There are some simple things you can do with nothing more than a shovel and a bottle of water that can give you a good idea if this is going to be an easy site, or a difficult one. 

​There are four basic things that can cause you concern when it comes to installing a gravity leachfield:

A) Soil that is too 'loose' (i.e. it has a high sand or gravel content).  The effluent flows through it too quickly to be treated by soil bacteria before it reaches the groundwater.  Does it feel gritty, especially when wet?  Won't squeeze into a ribbon when a wet ball is pressed between thumb and forefinger, but just breaks off instead?  Then it's possibly too sandy.  An easy test is to put some in a jar and cover it with water (take out any gravel or rocks first).  Shake it vigorously, then let it sit.  If about half or more of it settles back out within a few seconds, it's problematic.  You should get a professional percolation test before you go any further with your plans for the site.

 B) The soil is too 'tight' (i.e. it has a high clay content).  The effluent flows through it so slowly that the field size required becomes excessive.  Is it very hard when dry, and sticky when wet?  Will it easily form a ribbon about 2" long when a wet ball is pressed between thumb and forefinger?  Then it likely has a high clay content.  Try putting some in a jar and covering it with water (take out any gravel or rocks first, even if you have to break up the clods with a hammer).  Shake it vigorously, then let it sit.  If about a third or more has not settled out after a couple of minutes, it has a high clay content.You should get a professional percolation test before you go any further with your plans for the site.​

C) The groundwater is too high.  There is not enough depth between the surface of the ground and the highest level the groundwater reaches to allow a standard system to be constructed, and still leave sufficient depth of soil needed to treat the effluent before it gets to the groundwater (see individual county requirements linked on the the Installers Page).​  How can you tell how high the groundwater can get?  The best way is to monitor it through one or more rainy seasons, but this obviously takes more time than most people want to spend.  But when the water table is up,  the ground is saturated and therefore lacks air.  This causes some visible changes such as: any iron particles rust, turning orange; manganese particles turn black; the soil itself often turns grey, or even blue, either in spots or entire layers; the root zone stops, since the roots can't get any oxygen.  These details are best evaluated by a trained soil specialist if there is any possibility that high groundwater might sometimes be present.​

​D) There is a restrictive layer that prevents downward flow.   It may be hardpan, clay, or rock but either way your effective soil depth is insufficient (see individual coun​ty requirements linked on the the Installers Page) and the effluent has no place to go.  ​You will definitely need an engineered system, but sometimes expensive advanced treatment can still be avoided.​

There are other site conditions that preclude using a standard gravity system, such as being in a 100 year flood plain or the site being too steep.  See FAQ #3.

*The following page is a summary for quick reference only and may not represent the latest standards.  For complete and up-to-date information, see each counties official website.