Water-source protection zones in Portugal: building and septic-tank rules for a rural plot (Portarias 367–375/2026)

Like every perimeter approved under Decreto-Lei n.º 382/99, de 22 de setembro, each portaria delimits three nested zones of decreasing restriction, moving outward from the catchment. The same three-zone regime governs the Santiago do Cacém pair (Portaria n.º 367/2026/1, Portaria n.º 372/2026/1) and the Montijo trio (Portaria n.º 373/2026/1, Portaria n.º 374/2026/1, Portaria n.º 375/2026/1). The most relevant to a buyer are the intermediate and enlarged zones.

Critically, the intermediate and enlarged zones are around two orders of magnitude largerthan the immediate one — from tenths of a hectare out to tens of hectares. That is the practical headline: the restriction is not confined to the small fenced wellhead on the catchment's own ground, but reaches across neighbouring, third-party parcels — including private land a buyer may be considering. The exact boundaries are the coordinate polygons in the annexes of each portaria, which this page does not reproduce.

Zona imediata (immediate)

The plot closest to the borehole. Everyinstallation and activity is interdicted except those for the catchment's conservation, maintenance and better operation. The ground must be fenced and kept clear of any residues, products or liquids that could infiltrate. This is the zone where you effectively cannot build.

Zona intermédia (intermediate)

This is the zone buyers most often land in. The interdicts (article 3.º n.º 2) cover airports, vehicle workshops, fuel stations, hydrocarbon/radioactive/hazardous waste depots, toxic product pipelines, landfills, waste-water collectors and treatment plants, polluting industry, cemeteries, quarries and mines, scrapyards, and unlined water-retention works or research boreholes. The conditions (article 3.º n.º 3) are what matter for a rural plot:

• Building construction may be permitted provided the property is connected to the municipal sewerage network or, where that is not possible, fitted with a watertight fossa do tipo estanque.
• Fossas séticas (septic tanks) are allowed only where they meet strict estanquidade criteria; existing ones must be replaced or converted into sealed systems, and decommissioned once public sewerage becomes available.
• Agriculture, livestock and grazing are allowed only where they do not significantly affect water resources; mobile/persistent pesticides are restricted.

Zona alargada (enlarged)

The outermost zone. Interdicts (article 4.º n.º 2) cover hydrocarbon/radioactive transport, hazardous waste/recharge depots, toxic pipelines, refineries and chemical industry, landfills, and — notably — installing a fossa sética where a public wastewater sewerage system is available. Conditions (article 4.º n.º 3) allow wastewater collectors and treatment plants on strict estanquidade criteria, restrict mobile/persistent pesticides, and — again — permit a fossa sética only if it is watertight and will be decommissioned on connection to public sewerage.

For buyers of a rural plot in Santiago do Cacém or Montijo:establish, before exchanging, whether the plot falls inside a protection perimeter and, if so, which of the three zones. The boundaries are the coordinate polygons set out in the annexes of each portaria — that is the authoritative, official delimitation, not a general rule of thumb. Ask your conveyancing lawyer for a search against the published perimeters and for a site-specific confirmation that any intended build satisfies the zone's condition (municipal sewerage connection or a watertight fossa estanque).

For buyers in Montijo specifically: do not rely on the older, still-most-indexed Portaria n.º 220/2014 or Portaria n.º 58/2022. The 2026 trio (Portaria n.º 373/2026/1, Portaria n.º 374/2026/1, Portaria n.º 375/2026/1) repeals both by name and replaces the polygons and condition lists with new ones. Reading the 2014 diploma is the classic source-expiry trap — the boundaries and conditions it states no longer apply.

For owners with an existing septic tank: if the property lies in an intermédia or alargada zone, the portaria allows the fossa only where it is watertight, requires existing ones to be replaced or converted into a sealed system, and requires decommissioning and connection once public sewerage is available in the zone. If sewerage already exists, a fossa sética in the alargada zone is prohibited.

What these perimeters do NOT do: they do not impose a blanket building ban, and they are not a listing or zoning that revalues the plot. They are a groundwater-protection measure that conditions certain activities. The protection is asymmetric — town-planning uses remain governed by the municipal Plano Diretor Municipal, and the perimeter imposes an additional environmental condition, not a substitute for planning permission.