What are the environmental concerns for a liquid hydrogen release?

Because hydrogen is labeled as a hazardous substance, sometimes people are concerned about the environmental impact of releasing liquid hydrogen. There are currently no regulations that require reporting of hydrogen releases for environmental reasons.

However, there are safety implications. For example, the US DOT requires reporting of hazardous materials releases under certain circumstances due to the potential safety impact and to understand the effectiveness of existing regulations.
Due to LH2’s cryogenic temperatures and flammability, there are safety concerns if released into a sewer, storm drain, ditch, drainage canal, lake, river, or tidal waterway, or upon the ground, sidewalk, street ,or highway. Of particular concern would be the potential for the hydrogen cloud to collect in confined areas (such as a sewer) or ignite. Liquid hydrogen will only pool for very large releases and even then, the area of pooling is relatively small. Refer to the spill test completed by AD Little of 6000 gal for NASA for an example test spill. The LH2 vaporized in seconds.