Meet your Chicagoland Stormwater Storage Requirements with a rain garden!

Permanent and Seasonal Water Tables

Water TableThe idea of a rain garden is that you are directing water to a location where it is detained and then soaks down into the soil.

If that soil is already saturated you have nowhere for the water to soak down into.

The elevation at which a soil is saturated is known as the water table and can be seasonal or permanent.

I will cover these two types of water tables and describe how to identify them and how they should be handled in regards to a rain garden.

Permanent Water Table

If you have a High Permanent Water Table, water is in the upper couple of feet of the soil, or above, for most of the year.

Indicators of a High Permanent Water Table are:
The presence of a mostly permanent wetland, river, lake or stream nearby that is only slightly lower in elevation than your location.
Low flat lands
Dark black sulfur smelling soils overlaying blueish-grey soils.
Wetland trees such as Cottonwood, Willows, Cypress, or plants like Reed Canary Grass or Cattails growing in the area.
Excessive shifting of structures as seen in sinking and cracked pavements and foundations.
Constantly running sump pump even in the driest of conditions in summer.
Check with Web Soil Survey (Below)

I do not recommend putting a rain garden where you have a high permanent water table as sending more water to a location that is already fully saturated gives the water nowhere to go. That said, a rain garden that overflows directly into a body of water as part of its design can be very beneficial as it acts as a filtering strip as it removes debris and pollutants.

Seasonal Water Table

If you have a High Seasonal Water Table, water is in the upper couple of feet of the soil for a portion of the year.

Indicators of a High Seasonal Water Table are:
Clay soils, which typically have a High Seasonal Water Table due to their slow percolation rates and capillary action.
Blueish-grey streaks or spots in the soil
Land located at the bottom of a hill.
Water is slow to drain in the spring.
Holes dug a couple feet deep fill with water in the spring.
Constantly running sump pump in the spring and early summer.
Check with Web Soil Survey (Below)

You can put a rain garden in a location with a High Seasonal Water Table, but you should take into account how high and for how long your Seasonal Water Table goes when you decide on your design.

Web Soil Survey

There is an interactive online resource known as the Web Soil Survey produced by the National Resources Conservation Service that displays soil information collected by the USDA.

Although this information is not accurate down to the level we might like, it does have a lot of good information that can be used in formulating the overall picture of your permanent and seasonal water tables.

To use this website follow these steps:
1.) Click on green button labeled “START WSS”
2.) On the left of the new pop-up page you will see “Quick Navigation”, expand the tag “Address” and enter in the address you want to locate.
3.) Once your map comes up with your location, go to the task bar above the map labeled “Area of Interest Interactive Map” and click the button with a red rectangle and the letters “AOI” under it.
4.) Mouse over your map and then click and drag a window over about 1/2 the map, centered around your location in the middle of the map. It will buffer for a while as it says “Creating AOI”
5.) Now your Area of Interest has been created, and all the soil data in this area can be searched through and displayed. There is a wealth of information here, and it can be fun to go through all the different maps, but I will point out the two that I use mainly for figuring out water tables.

Water Table
1.) Select the uppermost tab labeled “Soil Data Explorer”
2.) Then select lower tab labeled “Soil Properties and Qualities”
3.) Then on left hand list select “Water Features”
4.) Under this tab select “Depth to Water Table”
5.) Leave all the settings as they are and select “View Rating”
6.) After the map comes up, Use the Number and Letter Code for your area and match it up with the table below to see the water table depth in centimeters.
7.) 1 centimeter = .39 Inch
8.) This gives you one simple number for your Seasonal Water Table for a period of at least one month.

For a bigger picture of what your water table is doing when
1.) Select the uppermost tab labeled “Soil Data Explorer”
2.) Then select lower tab labeled “Soil Reports”
3.) Then on left hand list select “Water Features”
4.) Under this tab select “Water Features”
5.) Leave all the settings as they are and select “View Rating”
6.) After the map comes up, Use the Number and Letter Code for your area and match it up with the table below to see a list of great information, most importantly the Months that there is a upper and lower water table elevation which is in feet.
7.) You can also find information great information on the Soil’s Hydrological Group, Surface Runoff, Ponding, and Flooding Characteristics.

Again, the scope of the survey can be quite large, so you will have to look at the soil map and then at your location and see how your small location might play in relation to the overall group.

Nonetheless, it is a powerful resource with a huge amount of information.

One last tip

Another method for figuring out your water table levels is to contact a deck/fence builder in your area and see what they have seen as far as water in their post holes.

Deck/Fence builders need to auger down below the frost line for their concrete support piers.

At this depth, these contractors will be familiar with the area’s water levels as these holes fill in with water shortly after they are excavated.

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