GALLATIN, Tenn. — The hulking Gallatin Fossil Plant sits on a scenic bend of the Cumberland River about 30 miles upstream from Nashville. In addition to generating electricity, the plant, built in the early 1950s by the Tennessee Valley Authority, produces more than 200,000 tons of coal residue a year. That coal ash, mixed with water and sluiced into pits and ponds on the plant property, has been making its way into groundwater and the river, potentially threatening drinking water supplies, according to two current lawsuits.

Coal ash, the hazardous byproduct of burning coal to produce power, is a particularly insidious legacy of the nation’s dependence on coal. Unlike the visible and heavily regulated airborne emissions from power plant smokestacks, coal ash is largely unseen unless there is a major spill and, until recently, far less effectively regulated.

More than 100 million tons of coal ash is produced every year, one of the nation’s largest and most vexing streams of toxic waste. The hazardous dust and sludge — containing arsenic, mercury, lead and other heavy metals — fill more than a thousand landfills and bodies of water in nearly every state, threatening air, land, water and human health.

The Gallatin power plant is facing citizens’ complaints and two major lawsuits over its handling of coal ash. One suit, filed in 2015 by an environmental advocacy group in federal court, says the utility violated the Clean Water Act by allowing toxic leaks from its coal ash disposal ponds. A second, also filed in 2015, by the state’s attorney general and its environmental enforcement agency, asserts that the Tennessee Valley Authority broke state pollution laws and endangered public health.