thumbnail

Urban Stormwater Treatment Using Biofiltration—Variable Performance Across Solids, Nutrients, Major Ions, and Metals

Scientific Investigations Report 2026-5143
Prepared in cooperation with the Milwaukee Metropolitan Sewerage District
By:  and 
https://doi.org/10.3133/sir20265143

Links

Abstract

Urban runoff from streets and parking lots carries pollutants that degrade receiving waters. Green infrastructure, such as biofilters, is increasingly used to treat this runoff by mimicking natural hydrologic processes. The U.S. Geological Survey, in cooperation with the Milwaukee Metropolitan Sewerage District, evaluated a biofilter receiving roadway runoff from an industrial area in Milwaukee, Wisconsin, over a 3-year period (2022–24). Paired inlet and outlet samples were analyzed for changes in runoff volume, peak discharge, and concentrations of solids, nutrients, major ions, and metals. The biofilter reduced runoff volume by 86 percent and peak discharge by 92 percent, with substantial reductions in total suspended solids (99 percent), total phosphorus (86 percent), and particulate metals (greater than 80 percent for most analytes). However, dissolved constituents showed variable performance; dissolved phosphorus and several metals exhibited net export, likely influenced by media composition, redox conditions, and winter road salt inputs. Sodium export, despite stable chloride loads, suggests cation exchange and seasonal release dynamics. These findings highlight limitations of conventional biofilter designs for dissolved pollutants and underscore the need for improved media, vegetation management, and consideration of winter deicing practices.

Plain Language Summary

Urban stormwater runoff can carry sediment, nutrients, salts, and metals into nearby rivers and lakes, contributing to flooding and water-quality problems. To reduce these impacts, communities are increasingly using shallow, planted systems called biofilters to capture and soak up runoff. This study evaluates how well a biofilter in Milwaukee, Wisconsin, performed over three years and what its results mean for managing stormwater in urban areas. 

The biofilter was highly effective at managing stormwater volume and flow. On average, it reduced the amount of runoff leaving the site by 86 percent and reduced peak flow rates by 92 percent. These reductions help lower the risk of flooding downstream, especially during heavy rain.

The biofilter also worked very well at removing pollutants attached to soil and debris. Nearly all suspended sediment was removed, and total phosphorus was reduced by more than 80 percent. Most metals attached to sediment, such as lead and copper, were also greatly reduced. These results show that biofilters are reliable tools for controlling particulate forms of pollutants from roads, even when sediment loads are high.

However, the biofilter was less effective at treating dissolved phase pollutants. For example, dissolved phosphorus and several dissolved metals, including iron and manganese, were often higher in water leaving the biofilter than in water entering it. Sodium, a major component of road salt, was also released from the system at times. Export of dissolved phase pollutants from the biofilter likely reflects interactions between runoff, organic material in the soil, and winter deicing practices. Improving soil mixtures, managing vegetation, and reducing salt inputs may help biofilters better protect urban water quality in the future.

Suggested Citation

Selbig, W.R., and Romano, J., 2026, Urban stormwater treatment using biofiltration—Variable performance across solids, nutrients, major ions, and metals: U.S. Geological Survey Scientific Investigations Report 2026–5143, 27 p., https://doi.org/10.3133/sir20265143.

ISSN: 2328-0328 (online)

Table of Contents

  • Acknowledgments
  • Plain Language Summary
  • Abstract
  • Introduction
  • Materials and Methods
  • Results and Discussion
  • Implications for Urban Stormwater Management
  • Summary
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Urban stormwater treatment using biofiltration—Variable performance across solids, nutrients, major ions, and metals
Series title Scientific Investigations Report
Series number 2026-5143
DOI 10.3133/sir20265143
Publication Date March 18, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Upper Midwest Water Science Center
Description Report: vii, 27 p.; Data Release
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Additional publication details