Calculating Soil Infiltration Rate

Youth Rain Action Storyboard

Post 4: Calculating Soil Infiltration Rate

When building a rain garden, one important consideration is the soil. If the garden is going to absorb water, the soil needs to be permeable. Before we designed the rain garden, Toronto and Region Conservation Authorities Sustainable Technologies team tested our soil by conducting a soil infiltration test.

staking out for hole diggingThe first step was to measure out where the test would take place. They outlined a hole 1m x 1.5m with stakes.

 

 

 

digging out test areaThen they had to dig the hole 65 cm deep. This took an incredible amount of effort as the soil was extremely hard.

 

 

 

clay sample

It became clear very quickly that the soil was mostly clay. Clay is nutrient rich, but bad at absorbing water. This means our infiltration rate will likely be very slow.

 

 

double ringsNormally, infiltration rate is measured using a set of double rings. The outside ring is filled with water to make sure the soil around the test area is saturated. Then the time it takes for the water in the centre ring to disappear is measured.

 

 

PVC tube added to infiltration pitThe soil at our site was so hard, they could not place the rings into the soil. Instead, they used a different method that didn’t require anymore digging. First they put a piece of PVC tube filled with holes attached to rebar in the centre of the infiltration pit.

 

 

 

 

filling infiltration pit with waterThen, the infiltration pit was filled with water.

 

 

 

data loggersFinally, they placed a data logger at the bottom of the pipe. The data logger measures pressure. Another data logger placed at ground level nearby records the atmospheric pressure is. This can change with weather, so it is important to measure it at the same time. By subtracting atmospheric pressure from the pressure at the bottom of the infiltration pit, the height of water can easily be calculated. The data loggers were set up to take a measurement every 5 minutes. The Sustainable Technologies team thought it might take days for the water to infiltrate the soil!

After a hard day’s work, on one of the hottest days of the year, all that’s left is to wait. In a few days someone will come back to collect the data loggers and determine infiltration rate. Since there is so much clay, we will have to bring in soil rich in sand and loam to make our rain garden effective.

cleaning up

 


Visit the Youth Rain Action project page to learn more or contact Emily Dutton at edutton@ecosource.ca.

The Youth Rain Action Project is led by Ecosource in partnership with Toronto and Region Conservation Authority, the Burnhamthorpe Sustainable Neighbourhood Retrofit Action Plan (SNAP), and Sheridan Nurseries, and is generously supported by the Government of Ontario’s Great Lakes Guardian Community Fund.