Along those coastlines, swelling seas are driving water tables higher and creating worries in places where septic systems abound, but where officials often lack reliable data about their location or how many might already be compromised.

“These are ticking time bombs under the ground that, when they fail, will pollute,” said Andrew Wunderley, executive director of the nonprofit Charleston Waterkeeper, which monitors water quality in the Lowcountry of South Carolina.

The lawsuit came from two South Carolina environmental groups — the Charleston Waterkeeper and the Coastal Conservation League. Amid a building boom, the groups claim, state regulators have failed to adequately consider the effects of sea-level rise and stronger storms when approving growing numbers of septic tank permits across eight coastal counties.

“[Their] failures place the public’s health at risk and expose our state’s waterways, marshes, beaches, and fisheries to significant, documented harms that can be traced to untreated sewage from malfunctioning, ill-maintained, and/or ill-placed septic systems,” one filing read. The lawsuit, filed in late 2022, cites existing or planned housing developments in vulnerable areas, including several near the town of Awendaw, about 25 miles northeast of Charleston. There, the suit states, hundreds of homes built close together would rely on septic tanks, all near the Cape Romain National Wildlife Refuge.

“A large percentage of the developed land is actually comprised of septic drain fields,” one filing argues, saying that such a situation risks “significant, irreparable harms” to public health and that of nearby marshes, beaches and fisheries. That includes the possibility of diseases such as hepatitis A and salmonellosis, which can be transmitted through fecal matter, and degradation of water quality, the filing said.

The groups want state officials to give special consideration to the cumulative impact of such clusters of septic permits. Already, they argue, South Carolina has no comprehensive inventory of existing septic tanks and little way for the public to challenge permits before they are approved.

“Sea level rise is happening. It will keep happening,” said Leslie Lenhardt, a senior managing attorney for the South Carolina Environmental Law Project, which is working on the case. “It’s steady, and it’s coming for these developments.”