The most wide spread environmental impacts from former MGPs involve groundwater which becomes contaminated by contact with coal tar, tar contaminated soil or purifier waste. The contaminated groundwater can them move away from the site into other areas.
Typically, the most significant environmental impacts from MGP sites are related to organisms living on the bottom of nearby surface water bodies such as streams, rivers and lakes. Significant accumulations of tar-impacted sediments have been found near former MGPs. MGPs may have discharged mixtures of tar and water directly into these water bodies when they were operating. The tars settled into the sediments at the bottom of these bodies. In addition, some MGP sites may contain tar seeps, where tar emerges from subsurface soil into water bodies.
In addition to visible tar, lower levels of PAH contamination are often found in sediments near MGPs. Contaminated sediments may be carried by stream flow or tidal action to other areas. Wildlife which lives or feeds in the impacted sediments may be exposed to these MGP contaminants.
PAHs from MGP sites may harm fish and wildlife directly exposed to the contaminants. Based on similarities between coal tar and petroleum, large releases of coal tar directly into surface water may taint nearby fish, making them aesthetically unfit to eat. However, small releases are unlikely to lead to substantial contamination of fish because fish and wildlife generally metabolize PAHs, so that PAHs do not build up in edible tissues. PAHs are not passed along in the food chain, and do not accumulate in predatory fish and birds.
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