Connection: Cleaning Powers Of Soil

Sally Brown

Sally Brown
BioCycle April 2013, Vol. 54, No. 4, p. 52

Whether you prefer tap, Smart or Britta filtered, the fact is the water you are drinking has been filtered by the soil. This natural filtering has been done by soils for thousands of years. My own favorite self-indulgence is a bottle of the Italian version, sourced from limestone springs, which also passed through the soil on the way to those springs. This natural filtration has been so successful that today, many cities are trying to apply the lessons learned from soil filtration of drinking water to storm water management. Grey infrastructure is being replaced with green alternatives that include green roofs, rain gardens and bioretention systems. This new infrastructure can look beautiful from many different perspectives, creating greener, prettier cities both literally and figuratively. And save money (see the Portland, Oregon link below).

These green practices are a wonderful thing that should be embraced. Their acceptance and use is in the early stages though, and many of those involved are cautious about replacing something that has very well defined specs (grey infrastructure) with a living system. Let me tell you something about living things: performance isn’t constant over time. My right shoulder is a very good case in point. Luckily for these green infrastructure systems, the time frame is a lot different than for humans. Soils take thousands of years to develop and so will generally perform well in a bioretention mix for decades, longer than it takes for a pipe to start cracking.

Why the new interest in a green alternative to grey? Cities have relied on grey infrastructure for a very long time. In urban areas we have replaced soil with concrete and asphalt, which up until recently only came in the impervious form. No infiltration meant that storm water needed an alternate exit strategy. In many cities this exit strategy has been direct flow into sewer pipes. All fine and good for streets in that the water goes away fast but not so good for streams and lakes. For streams and lakes it is either feast or famine. In times of famine, absence of infiltration into soils means no slow recharge and very low stream flows in dry weather. For feasts, high rainfall means flashy streams and lakes that sometimes get specially seasoned with combined sewer overflows (CSO).

Increasing regulation of CSOs gives municipalities two alternatives: increase storm water storage capacity through grey or engineered solutions or increase the permeability of surface areas in cities to let water go into the soil rather than over the surface. Even if we make lots of green roofs, bioretention systems and the like, we will still have a lot of asphalt in cities. This means that each component of the green infrastructure will be tasked with ”treating” more than its faIr share of storm water. To do that well, we have to understand how soil does it and then how to supersize the process.

Multidimensional Cleaning Powers

The water cleaning technology found in soil is multidimensional. The first thing soil does is slow water movement down. That lets the heavier contaminants in the water (typically sand, silt or clay from eroded soil) settle out and become part of the soil. The next thing soil does is provide glue. Here glue is another term for adsorptive surfaces, officially referred to as the soil’s cation exchange capacity (CEC). Many of the contaminants in water are actually metals in solution. Copper is one example. Copper is a serious contaminant in freshwater bodies. When added in dissolved form to soil, it is a charged ion. This ion will stick to the charged clay particles or organic matter where it is repurposed into a plant nutrient.

Dissolved organics do the same thing. Here the glue is soil organic matter. Organics would much rather stick to their own than float around in water. The other trick that soil adds to fill out its filtering potential is its living component. Each gram of soil contains about 6 million bacteria. Those bacteria get hungry. The dissolved organics in water are food for those hungry millions. The more complicated or larger the organic contaminant is, the likelier it is to stick to soil (think oil particles); the more simple or smaller it is (think spilled soda or coffee) the more likely it is to turn into dinner. These soil bacteria have no shame and will even eat their own if forced to. This translates into pathogen removal when water is filtered through soil. Pathogens like cryptosporidium may be a concern when you drink water from a stream in the forest. However, water that has been filtered by soil is pathogen free.

Finally both those bacteria and the plants think of the nitrogen and phosphorus in water as nutrients rather than contaminants. In other words, soil provides a natural and highly effective filtering system for water, one that has been functioning for thousands and thousands of years. However, soil in its natural state doesn’t have to absorb all of the storm water from the neighboring highway. That is why we need to try to copy soil but do it one better to make these green infrastructure systems work.

Power Of Composts

Sand is really good at soaking up water — just picture the waves disappearing at the beach. As a result, sand is often the primary ingredient in the recipes for these green systems. However, putting sand pits all over cities is not likely to win you a lot of converts. And while sand is good at filtration, it isn’t the best for the filtration part of green infrastructure systems. For that you want something that mimics soil. But you don’t want just any soil, you want soil that can filter more than its share. Here different types of composts are ideal because they have: 1) High adsorption capacity for organics, nutrients and metals; 2) High water holding capacity so the systems won’t get droughty during dry periods; 3) High organic matter content and thus can support microbes to eat pathogens; and 4) Slow release nutrients so plants can be put into the systems. With appropriate composts, the plants in these systems will even stay green over time. This is good because green plants are nicer to look at than dead plants. And it is also good because green infrastructure without the ”green” just doesn’t seem right. Finally it is good because the neighbors who will maintain these systems won’t end up dosing them with fertilizers to keep the plants alive.

In many areas, specs for these systems are very cautious about adding too much compost or the wrong kind of compost. That is understandable. Remember that this ”green” infrastructure is replacing grey. There are concerns that compost will decompose and that it will be a source rather than a sink for contaminants. However, organic matter is universally recognized as the key to soil health. Creating green infrastructure with an appreciation of the need for and value of composts is a great way to build highly functional systems that will outlive pipes and my shoulder.

We are just learning how to best create these systems. An article in last month’s BioCycle (“Managing Nutrient Release In Compost-Amended Bioswale Soils,” March 2013) showed the potential value of water treatment residuals as part of the mix. Recipes will get more refined over time and no single recipe will likely be appropriate for all climates and situations. But either way, you’ll want to have a good amount of compost in the mix.

Sally Brown — Research Associate Professor at the University of Washington in Seattle — authors this regular column. Email Dr. Brown at

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