The U.S. EPA finalized four rules last week affecting natural gas and coal power plants. One of them marks the first time the federal government has ever forced existing coal plants to reduce their carbon emissions. If plants want to stay open past 2032, they’ll need to install carbon capture technology — which is still in development and expensive.

Another rule looks to crack down on mercury emissions from power plants that burn lignite, a low-energy, highly polluting type of coal.

And the last two rules have big implications for something we’ve followed closely at the Energy News Network: coal ash. This byproduct of burning coal contains toxins like mercury and arsenic, and it’s often stored in landfills and ponds at coal plants, putting it at risk of contaminating groundwater.