Guard the Ground

       As water moves across the ground, particles are picked up and transported to new places. Soil is naturally moved by rain or snow melting in a process called erosion. The Appalachian Mountains used to be the tallest mountains in the world, but over time the mountains eroded and became much smaller. Today, they are only a fraction of their original size. 

Erosion can help transport nutrients to new areas and make new habitats for animals. However, human activities have dramatically increased erosion beyond normal levels. For example, land is often stripped of trees and grass to make room for farms, roads, and buildings in a process called ‘land stripping.’ Without the protection of trees and grass, the soil is exposed to rain which makes it vulnerable to erosion. Land stripping can increase the rate of erosion over 1,000 times! The soil, and anything on the soil, is washed away and pollutes streams, lakes, rivers, and oceans. 

Soil pollution may seem like a harmless problem -- soil is natural, after all. But, soil pollution clouds the water and sunlight can no longer reach plants under the water. The plants no longer grow and can even die, and the fish that eat the plants go hungry. Then, the fish that eat those fish go hungry, especially since the cloudiness of the water makes it harder to hunt. The water can also become too cold since sunlight does not shine through cloudy water. Furthermore, the eroded soil falls to the bottom of the lake or river on top of plants that die from being covered in soil. As the plants die off, the lake or river is no longer able to hold the same amount of water and it floods. If too much soil is eroded from the land, builders have to add new soil which is costly and results in more soil pollution. 

         Construction or build sites are big sources of erosion pollution because they must remove the vegetation from the soil to make way for the road, skyscraper, tunnel, airport, house, apartment building, or whatever man-made structure will be there. Soil is left exposed to the elements during construction. The pictures below show erosion pollution from a large development in Chatham Country, North Carolina. The pictures show muddy water full of soil and other sediment going straight into the Haw River. The sediment will flow from the Haw River into Jordan Lake, which supplies drinking water to several large towns. 

Consequences of Construction Site Erosion, Chatham Park, North Carolina (River Issues 2020)

The Environmental Protection Agency (EPA) is a government agency that creates rules to protect the environment from human activities including erosion. Protecting water from soil pollution is important for animals and humans because the particles in the water can be toxic. The EPA has some rules about soil pollution from build sites, but the guidelines can be hard to meet because the methods used to keep erosion at bay are not well studied. 

Many times, companies make new, flashy products for builders to buy, but the products do not usually work as advertised and the builder gets fined anyway. This kept happening to the Alabama Department of Transportation (ALDOT) on their road and bridge projects. Therefore, ALDOT commissioned an experiment at Auburn University to test methods that decrease erosion at construction sites. ALDOT gave money to our research team to build a large “rainfall simulator.” 

The simulator consists of 10 sprinklers that stand 15 feet tall and release water onto a long, 40-foot slope. We built the slope using the type of soil ALDOT encounters most often. Our team at Auburn University tested different ways to minimize the amount of soil that eroded from the slope for ALDOT. In total, we tested four different methods of erosion control using various products purchased from the commercial market: wheat straw, blankets, hydro mulch, and chemicals called “amendments”. We applied each product to the soil, turned on the sprinklers, and measured changes in soil erosion and pollution.


Sprinkler Setup

The pictures below depict the four types of erosion control methods tested. 



Hydro mulch


The first method used wheat straw was simply bundles of wheat straw purchased from a local agricultural shop. Straw was applied to the soil three different ways: 1) loosely distributed over the slope, 2) distributed then pressed over the slope with a large crimping device, and 3) distributed over the slope then sprayed with a binding chemical to stop the straw from moving. 

The next method used three types of erosion control blankets of varying thicknesses. An erosion control blanket is a bundle of material that can be unrolled across the ground and staked into. Each blanket was made by a different company and purchased from local suppliers then rolled across the slope. 

The third method sprayed hydro mulch on the slope. Hydro mulch is an organic product like wood shavings or recycled paper that is treated with chemicals, dried, and then placed in a large water tank that is sprayed onto the soil. The mixture hardens on the top of the soil and remains in place. In total, we tested four hydro mulch products from two different manufacturers. 

Lastly, we tested two amendments: one called gypsum and another called polyacrylamide (PAM). Agricultural farmers already use gypsum as a fertilizer and to prevent erosion. PAM is a new commercial product that absorbs water and spreads out when wet, forming a sticky protective cover over the soil. In this experiment, we covered the amendments with a thin, waffle-like mat because it is a generally recommended practice.

         Before testing began, we checked the sprinklers to make sure they put out enough water to simulate erosion from rainfall. The sprinklers needed to mimic both the amount of rainfall that happens in a severe storm and the size of the raindrops. The size of the raindrops is important because large raindrops are heavier and can wash away more soil. We also replaced and compacted the soil before testing each method to mimic the soil at build sites. The sprinklers were run for one hour after applying each method. We collected all of the water that flowed off the slope in a basin at the end of the slope. The pictures below show the rainfall simulator and how water was collected at the bottom. 

Then, we measured the amount of soil suspended in each milliliter of water in the basin. This measurement is called the Total Suspended Solids (TSS). We also measured the cloudiness of the water which is called water turbidity. Then, the water was left alone so that the sediment would settle at the bottom. After settling, the water was drained, and researchers weighed the soil at the bottom. Water collected from the best erosion control products had low TSS, low turbidity (cloudiness), and only a small amount of soil in the basin. Poor erosion control products had high TSS and turbidity in the runoff and a lot of soil in the basin.  

         Two products prevented erosion very well: the thickest erosion blanket and the gypsum soil amendment. Straw performed best when used with the binding chemical to help it stick together. All four hydro mulches performed poorly. 

Indeed, slopes with hydro mulch lost up to three times the amount of soil as the straw and five times as much as the blankets. PAM was equally ineffective as hydro-mulch. In fact, the PAM and hydro mulches created just as much erosion as not putting anything on the slope at all! Hydro mulches performed poorly because the mulch detached from the soil when it got wet and washed away, which left the soil unprotected. PAM was washed off of the soil altogether. Unfortunately, yet another complication arose with these two products. 

The chemicals used in the hydro mulch binder and PAM were detected in the water that ran off of the slope. The water runoff was foamy, soapy, and frothy, as shown in the pictures below. We tested the water runoff with specialized lab equipment and found that the water had high concentrations of polymer-like substances in the water after the slope was treated with PAM or any hydro mulches. These polymer-like substances are chemicals that would pollute the environment if soil was treated with PAM or hydro mulch at build sites.

View of slope at the base (prepared with hydro mulch)

Froth forms in runoff from hydro mulch

There are three major benefits of this experiment. First, the experiment was run by Auburn University, an independent entity, instead of companies that sell products. Therefore, the results are objective and unbiased because the University is not interested in profiting off of new products unlike companies. Second, the sprinkler system was large enough to simulate actual rainfall that would occur at real construction sites. Other studies usually place soil in a small box surrounded by a small number of sprinklers. Due to the physics of water movement, this does not accurately simulate erosion. The speed of the water is very important for erosion, but in a small box the water does not gain the same amount of speed that it would on a real hillside or worksite. Therefore, the erosion measured in these small experiments is inaccurate. Finally, this experiment tested many products all at once using the same simulator. This allowed us to easily compare the different products since they were all tested under the same conditions. Therefore, all of these tests produced more reliable results than ever before. 

The experiment discovered that builders can prevent pollution and avoid EPA fines by using straw treatments, blankets, or gypsum. The experiment also discovered that builders should stay away from hydro mulches and PAM because they work poorly and leach chemicals into the very waters they were meant to protect. We shared the results with ALDOT. For legal reasons, the specific names of the manufacturers, brands, and distributors are not published in the research, however they were shared with the sponsor, ALDOT. Erosion due to man-made processes is an often-ignored source of pollution that damages lakes, rivers and streams, including sources of drinking water. This study discovered the best ways to prevent it. 

Written By: Christin Manning

Academic Editor: Ecologist

Non-Academic Editor: General Practice Lawyer

Original Paper

• Title: Comparison of Erosion Control Products Using an ASTM D6459 Rainfall Simulator: Insights and Suggestions

• Journal: Journal of Irrigation and Drainage Engineering

• Date Published: 22 May 2023

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