For most of the U.S. and Canada, autumn is marked by falling temperatures, falling rain and falling leaves. For municipalities and sewer inspection crews, these seasonal changes bring new challenges. While stormwater systems are typically prepared for increases in the volume of water, the added leaf litter can aggravate the treatment process, overwhelm systems and exacerbate existing pipeline defects and deterioration.
The main issue with fall foliage in a stormwater sewer system is that leaf litter contributes phosphorus and nitrogen to water. In excess these "nutrients" cause what is known as eutrophication, and can also produce harmful algal blooms (HABs). Both lead to depleted oxygen content in water, which suffocates aquatic life, and HABs produce toxic cyanobacteria which threatens human and wildlife health. These water quality problems are hazardous and costly to fix, but also preventable.
Last year, Bill Selbig of the U.S. Geological Survey (USGS) published a study that reviewed the impact of leaf litter on nutrient levels in urban stormwater. Selbig observed that keeping nutrient concentrations low by physically removing leaves was especially effective before rainstorms. He found that “27 to 80% of initial phosphorus was leachable in the first 24 [hours] of soaking,” meaning preventative measures are highly advisable. Despite the fact that many sewer systems have sumps and screens to prevent debris from polluting stormwater, leaves disintegrate quickly, so these devices are insufficient unless they are equipped with robust filters.
In nature, soil and plants would absorb the extra water and nutrients that autumn brings, but in cities, stormwater systems take it all in. And, as the effluent is rarely treated before being discharged into area waterways, the nutrient content of that effluent can have a major impact on environmental health. Reharvested effluent can be used for irrigation and agricultural applications, toilet flush water, industrial functions and even as a potable water source, making it a valuable resource. Leaf litter jeopardizes the reusability of effluent to further benefit municipalities. Ongoing street and curbside upkeep during autumn months not only improves water quality, but is also has advantages for sewer pipeline systems.
Leaves cause a number of blockages, from storm drains covers to in-pipe obstructions, and the nutrients they add to sewer systems increase the rate of microbial corrosion. These damages wreak havoc on systems and if left untreated can create health hazards. Extensive pipe rehabilitation projects and falling water quality can both be avoided with a focused effort on leaf clean up. With fall in full effect, municipal leaf removal projects should be well underway, and you can do your part to make a positive impact too. Keep sidewalks, curbsides and stormwater drains clear by raking leaves and disposing them in yard waste or compost bins. Fall foliage may be a quintessential part of autumn, but prolonging its time on the ground isn’t worth risking water quality and sewer pipeline systems. Read the 2016 USGS study here:
Water Online, Blame it on the Leaves
USGS, Fallen Leaf Removal for Urban Water Quality
Waterworld, Examining Stormwater Collection and Treatment