Leaf Pollution Update

continued from page 7 homeowners and by lawn care vendors to remove these leaves from lawns. Despite this effort an enormous amount of dead leaves accumulate in streets, street gutters and catch basins of the storm drain/culverts.

An even greater amount of leaves are gathered up and bagged for removal by entities providing trash services. Each home that bags leaves likely gathers upwards of ten or more bags of leaves. This is likely a gross underestimation. If one assumes that each bag of leaves weight on average 10 pounds, the following amounts of nutrients are prevented from entering the storm drains that feed directly into Lake Sherwood:

• 4.7 pounds of carbon

• 0.1 pounds of nitrogen

• 0.01 pounds of phosphorus

• 0.01 pounds of potassium

• 0.16 pounds of calcium

• 0.02 pounds of magnesium

• 0.01 pounds of sulfur

• And iron, zinc and other micronutrients.

We know that one pound of phosphorus produces 500 pounds of aquatic vegetation including algae. If some type of removal of the leaves does not occur, ALL of these nutrients enter the lake. Multiply the number of residences by ten bags of leaves and that number by the amount of nutrients in each bag and the amount of available nutrients for algae production is staggering.

Assuming that 5,000 homes are contained in the drainage basin with each removing ten bags of leaves each weighing ten pounds, this amounts to 500 pounds of phosphorus kept from entering the lake. That equates to 250,000 pounds of algae NOT being produced in the lake. Unfortunately many leaves do enter the lake through the drainage systems and are blown in by wind. The question is what impact does this have on the lake?

When leaves break down they release phosphorus. Depending on the volume of leaves this may be one of the largest sources of phosphorus pollution in urban waterways. When leaves are not removed from the street gutters they will enter the drop basins of storm drains/ culverts. There the leaves sit until they are washed into the lake. Before that happens, the leaves will be soaked with any precipitation. The phosphorus contained within leaves is readily leached out creating essentially a “phosphorus tea.” This dissolved phosphorus runs directly into the lake. When the leaves themselves wash into the lake two things – both adverse – occur. First, more phosphorus and nitrogen are made available in the lake water due to decomposition. Second, the decomposed leaves add to the organic muck portion of the lake’s sediment load.

In an effort to quantitate the amount of phosphorus and nitrogen loading that occurs from urban leaf drops, the United States Geological Survey service conducted a study in Madison, Wisconsin, in the 2010s. Results were reported in 2019. Two residential areas in Madison were selected for study based on a high degree of similarity to each other. One of these served as a control and the other as the study area. Very aggressive management of fallen leaves in the study area was undertaken. The study conclusions were impressive: fall leaf litter is one of the primary sources of nutrient pollution in stormwater, especially for phosphorus!

There are two types of leaf degradation that occur. The first is the phosphorus tea and the second is particulate matter that remains that still has significant amounts of nutrients to release. By aggressive, active, thorough leaf removal and street cleaning to remove residual particulate leaf matter, total and dissolved phosphorus loads in stormwater runoff were reduced by 84% for total phosphorus and 83% for dissolved phosphorus. For nitrogen, if the fall season when leaves are falling were studied, total and dissolved nitrogen was reduced by 74% and 71% respectively by aggressive leaf management.

In the fall the majority of nutrient pollution occurs from dissolved nutrients (from leaching) which makes keeping leaves from accumulating on streets, in gutters and in piles near the street curb very effective in reducing nutrient pollution in the lake.

While this USGS study underscores that leaf collection is one treatment option available that can significantly reduce the amount of dissolved nutrients (especially phosphorus) in stormwater runoff, other interventions are also available. Leaf pickup and removal is useful and currently employed. The use of a mulcher blade to pulverize downed leaves helps prevent these from blowing into the gutters and returns nutrients to a lawn’s soil. A win-win.

The timing of any intervention is critical. Leaves are highly leachable by nature. Therefore, removal of leaves BEFORE any precipitation event significantly reduces the loads of phosphorus and nitrogen. This prevents both the formation of “nutrient tea” and the washing into the storm drains/ culverts of fallen leaves that contribute to nutrient tea and ultimately wash into the lake creating muck.

The LQEC is currently researching further mitigation strategies and more information will be forthcoming.