Miami has around 120,000 septic systems still in service, and the county has spent the better part of a decade documenting how many of them no longer work reliably. The 2018 assessment ordered under Resolution R-911-16 found roughly 105,000 parcels on septic, of which about 58,349 — some 56 percent — are periodically compromised during storms or wet years, with around 1,000 likely failing under ordinary conditions and 88 percent of those in unincorporated Miami-Dade. By 2040 the county projects more than 67,000 parcels, about 64 percent, in the periodically compromised category. The cause is not neglect. It is that a drain field needs unsaturated soil under it, Miami-Dade's water table is already within two to six feet of the surface, and mean sea level at the Virginia Key gauge has risen more than four inches since 1994 with a regional projection of six to ten inches above 1992 levels by 2030 and another ten to 26 inches by 2060. Groundwater rises with it. This is the context anyone hiring a septic contractor in Miami is working inside, and it is why the county changed its rules in 2023.
Soil Conditions
Miami-Dade's soils are thin skins of marl and gravelly loam over porous Miami Limestone, and the depth to that rock is usually the whole story. The Biscayne series is very poorly drained with bedrock at roughly 15 inches and a water table within 10 inches of the surface for four to six months of the year, receding only to about 20 inches in dry spells. The Perrine marly silt loam is also very poorly drained, with a seasonal high water table at 0 to 6 inches and moderately high to high conductivity through the marl but effectively none into the rock beneath it. Krome very gravelly marly loam, on the low rises of the Miami Rock Ridge, is the better card: moderately well drained, limestone at about seven inches, water held in the bedrock at 40 inches or more. Tamiami muck and Udorthents-urban land fill out the map. Across most of the urbanised county the hydrologic soil group is C or D — slow to very slow infiltration — with A and B only where sand or rockfill has been brought in.
The engineering problem in Miami-Dade is that the soil is too thin to do the work asked of it. Conventional septic treatment happens in the unsaturated zone, where soil microbes break down pathogens and nutrients before effluent reaches groundwater. On the Biscayne series that zone is at most 15 inches deep before hard limestone, and it is saturated for four to six months a year. On Perrine the seasonal high water table sits at 0 to 6 inches and the marl conducts water moderately well while the rock beneath conducts almost nothing at the scale of a drain field — so effluent moves laterally rather than down, and it moves fast. Krome, on the slightly higher ground of the Miami Rock Ridge, is the most workable of the group at about seven inches to rock with the water held at 40 inches or more in the limestone itself, which is why the ridge carries more of the county's viable conventional siting than the flats do. Beneath all of it is the Biscayne aquifer, unconfined and highly transmissive, which is both the region's drinking water source and the reason the county stopped approving conventional systems.
Water Table: USGS monitoring wells in the Biscayne aquifer put the water table generally two to six feet below land surface across Miami-Dade, with medians near four feet in the central county and under three feet in the south, and wet-season minimums that come within a few inches of the ground. Florida requires 24 inches of unsaturated soil between the bottom of a drain field and the seasonal high water table for a new system. Because a typical drain field bottom sits 18 inches down, the county's own vulnerability work treats a system as compromised once wet-season groundwater comes within 42 inches of the surface — a threshold much of Miami-Dade already crosses in an ordinary summer.
Local Regulations
Since 1 January 2023, under Miami-Dade Ordinance 22-83 and Section 24-42.7 of the county code, conventional septic systems are no longer approved for new installations or complete replacements anywhere in Miami-Dade. New systems must meet the county's advanced treatment standard, must be reviewed and approved in writing by DERM before installation, and must be registered with DERM; owners of pre-existing systems were required to register them by 1 January 2024. Type 3 and Type 4 systems must carry remote telemetry reporting status at least every 15 minutes, though single-family and duplex homes are exempt from that provision. In the Northwest Wellfield protection area, sewage loading for certain uses is capped at 70 gallons per day per unsubmerged acre. Statewide requirements still apply on top of all of this: Chapter 62-6, F.A.C., 24 inches of separation from the seasonal high water table for a new system, and the state's setback distances. Alongside the rules, the county runs Connect 2 Protect, a septic-to-sewer conversion programme through the Water and Sewer Department that identifies about 9,000 systems as vulnerable under current groundwater conditions, rising to roughly 13,500 by 2040, and offers homesteaded owners up to $15,000 in grants and low- or zero-interest loans toward conversion, income-tiered and first-come first-served.
Two agencies have to say yes before a septic system goes in the ground in Miami-Dade, which is unusual for Florida. The construction permit comes from the Environmental Public Health program of the Florida Department of Health in Miami-Dade County, on DEP Form 4015, under Chapter 62-6, F.A.C. — county health departments still run septic permitting through most of the state under DEP's authority, and Miami-Dade is one of them. Separately, Miami-Dade's own Ordinance 22-83 added Section 24-42.7 to the county code: since 1 January 2023 it is unlawful to install a new or whole-replacement system without prior written approval from the Department of Regulatory and Economic Resources, every new and replacement system must be registered with DERM, and pre-existing systems were required to register by 1 January 2024. The same ordinance ended conventional systems for new and complete-replacement work countywide — new installations must meet the advanced treatment standard — and requires remote telemetry reporting operating status at least every 15 minutes on Type 3 and Type 4 systems, exempting single-family and duplex homes. Plan submittals to DERM's Water and Wastewater Division include a site plan, floor plans, net-area flow calculations under Chapter 62-6, and a topographic and boundary survey showing elevations.