What method is often used to improve the quality of stormwater runoff before it enters a watershed?

Using constructed wetlands is an effective method for improving the quality of stormwater runoff before it enters a watershed. Constructed wetlands are artificial ecosystems designed to mimic the natural filtration processes of wetlands, where plants, soil, and microorganisms work together to filter and treat pollutants found in stormwater.

These systems can enhance water quality by removing sediments, nutrients (such as nitrogen and phosphorus), heavy metals, and pathogens through various processes including sedimentation, chemical adsorption, and biological uptake. The presence of vegetation in constructed wetlands also helps stabilize the soil and provide habitat for wildlife, further contributing to ecosystem health.

In contrast, direct disposal does not involve any treatment or filtration of stormwater, which can lead to increased pollution entering waterways. Increased urbanization typically exacerbates stormwater quality issues due to increased runoff and the introduction of more impervious surfaces that prevent natural absorption and filtration. Subsurface drainage systems often focus on managing excess water rather than specifically treating it for quality improvements. Thus, constructed wetlands stand out as the most effective choice for enhancing stormwater quality before it reaches a watershed.