Between Dominical and Ojochal, the coastal mountains drop almost straight into the Pacific. Most new homes, villas and vacation rentals here are built on those slopes, with long views over the ocean and the Marino Ballena National Park. There is no public sewer in most of the area, so every property treats its own wastewater on site.
Two things set this stretch of coast apart: steep ground and a lot of rain. Put together, they make the drain field the hardest part of the design.
A practical note on jurisdiction: Dominical belongs to the canton of Pérez Zeledón, while Uvita and Ojochal belong to the canton of Osa. Each municipality processes its own construction permits.
Steep lots: find the drain field first
Decree 42075-S-MINAE, the national regulation for disposing of treated wastewater into the ground, sets limits that hit hillside lots hard:
- No trenches on slopes steeper than 30 %.
- Level trenches. Each trench is built horizontal, so on a slope the trenches follow contour lines, with a maximum length per trench and inspection boxes along the way.
- 2 meters from the top of any slope steeper than 25 %. Every cut you make for the driveway, the house pad or the pool creates a new slope and a new setback.
- Private green areas only. Never under the driveway, parking, decks or pavers.
The most common mistake we see is designing the house first and looking for the drain field at the end, only to find that no area is left that complies. On these lots, the order should be reversed: identify where the soil, slope and setbacks allow infiltration, then place the buildings.
When the only suitable area is below the house, or on the other side of the lot, a small pumping station can move the treated water there. It needs a backup plan for power cuts, which are not rare during storms.
Heavy rain: the real test
The National Meteorological Institute (IMN) explains that the Intertropical Convergence Zone has its strongest influence on Pacific rainfall toward the south of the country, and that the Pacific's wettest months are September and October. Locals do not need the explanation: they know what a southern Pacific October looks like.
For a septic system, that means:
- Saturated soil. A drain field that absorbs well in March can stop absorbing in October. That is why the percolation test must be done after soaking the ground for 24 hours, and why the drain field area is multiplied by a precipitation factor of at least 2.5. Our page on the percolation test explains the procedure.
- A rising water table. The regulation requires at least 1.5 meters between the trench bottom and the water table. On flatter land near rivers and the coast, the wet season is when that rule is tested.
- Runoff from the hill. Water running down the slope after a storm can flood a drain field from the surface. Interceptor drains and a proper stormwater drainage design keep it out.
- Roof water. Gutters and downspouts must never connect to the septic system.
If your system already overflows during the rains, see septic smells and overflows.
A tank that gives the drain field a chance
An improved septic tank, which adds an upflow anaerobic filter after the tank, sends cleaner water to the drain field and helps it last. It does not rescue a drain field placed in the wrong spot, but on a good site it adds margin, especially for rental villas that fill up in the holidays. To size the tank and trenches for real occupancy, see our guide on septic system sizing.
Before you buy
Many buyers fall in love with a view and discover later that the lot has almost no area where a drain field is allowed. Before closing, check the slope, look for streams and springs (they have their own protection areas), ask for the water availability letter for that lot, and if possible run a percolation test in the area where the drain field would go. Our guide on buying land: water and wastewater checks has the full list.
How we help in Dominical, Uvita and Ojochal
Ingeniería de las Aguas, based in Palmares, Alajuela, works in Puntarenas, including the Southern Zone. We:
- design and build improved septic systems and drain fields from the percolation test results;
- design pumping stations and drainage for hillside lots;
- design treatment plants for small hotels and groups of villas;
- review systems that fail in the rainy season and redesign them;
- handle permits with the Ministry of Health and the municipalities of Osa and Pérez Zeledón, and supervise construction.
Heading north along the coast? See Jacó and Herradura.
Send us the location, a few photos of the lot or the current system and, if you have them, the topographic plan and house plans, on WhatsApp, or use our contact page.