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New London Hydrogen Sulfide (H₂S) Study

A calm lake with a bridge, trees, sunset, and a garden with vibrant flowers in the foreground.
Upstream pool of Mill Pond on the Middle Fork Crow River. The Mill Pond is connected to Monongalia or Mud Lake further upstream.
A water channel flowing through a tunnel with railings, surrounded by greenery.
New London Dam Before Reconstruction (Photo Credit MN DNR). The dam was over 100 years old and had many environmental and mechanical concerns. Reconstruction of the dam was conducted in early 2010 through Lunda Construction Co. and Mn DNR.
A small dam or weir with cascading water, surrounded by greenery and a distant lake.
After Reconstruction (Photo Credit MN DNR).
A tranquil river scene with green trees and sunlight filtering through, leading to a bridge in the background.
Downstream of Mill Pond Dam. The Middle Fork Crow River connects the Mill Ponds to Nest Lake, Green Lake, and Calhoun Lake.
Calm lake scene with clear sky, distant trees, and rocks along the shore.
A wooden dock extending over a calm lake with clear blue skies and trees in the background.
Mill Pond Dock.
A calm lake with a bridge, trees, sunset, and a garden with vibrant flowers in the foreground.
A water channel flowing through a tunnel with railings, surrounded by greenery.
A small dam or weir with cascading water, surrounded by greenery and a distant lake.
A tranquil river scene with green trees and sunlight filtering through, leading to a bridge in the background.
Calm lake scene with clear sky, distant trees, and rocks along the shore.
A wooden dock extending over a calm lake with clear blue skies and trees in the background.

Background

Each spring, residents and visitors to New London may notice a strong “rotten egg” odor coming from the Mill Ponds area. This smell is caused by hydrogen sulfide (H₂S)—a naturally occurring gas that can be released from the New London Dam and surrounding waters.

To better understand and address this ongoing issue, the Middle Fork Crow River Watershed District (MFCRWD), in partnership with the City of New London, is conducting a Hydrogen Sulfide Study and Community Survey. Throughout this study, MFCRWD will continuously monitor H₂S levels in downtown New London, where the odor is typically most noticeable.

About the Survey

Beginning January 5, all New London residents and business owners are encouraged to complete the New London Hydrogen Sulfide Community Survey. The survey will remain open until January 25 at 11:59 p.m.

If you live in New London, your feedback is essential. It will help us better understand how hydrogen sulfide odors impact daily life, business operations, tourism, and community well-being. The results will guide future efforts to identify and implement effective mitigation strategies.

For questions about the survey or hydrogen sulfide mitigation, please contact Erin Vruwink at erin@mfcrow.org or (320) 796-0888. 

Understanding Hydrogen Sulfide (H₂S)

Hydrogen sulfide is a colorless gas known for its distinct rotten egg smell. It forms naturally when organic materials—such as leaves, algae, and other plant matter—decompose in low-oxygen or stagnant water conditions.

In New London, the Mill Ponds and dam structure can create environments where oxygen levels drop and organic materials break down, producing H₂S gas. When this gas rises to the surface or is released over the dam, it produces the recognizable odor many notice in the spring.

Potential Health and Environmental Impacts

Hydrogen sulfide may irritate the eyes, nose, and throat, and may cause symptoms such as:

  • Watery eyes or burning sensations
  • Coughing or shortness of breath
  • Headaches, fatigue, or difficulty sleeping
  • Nausea or dizziness
  • More severe eye and respiratory irritation

(Source: OSHA Hydrogen Sulfide Fact Sheet, OSHA Hazards - Hydrogen Sulfide).

Prevention and Safety Measures

While hydrogen sulfide is naturally occurring, a few practical steps can help reduce exposure and odor buildup:

  • Ensure good ventilation around enclosed spaces near water bodies.
  • Avoid prolonged exposure in low-lying or confined areas where odors are strong.

(Source: OSHA Quick Card - Hydrogen Sulfide). 

This collaborative effort between MFCRWD and the City of New London aims to better understand the source, intensity, and impact of hydrogen sulfide emissions—and to help identify long-term solutions that benefit residents, businesses, and the local environment.