If your home or project in Costa Rica has no sewer, the water leaving your septic tank or treatment plant almost always ends up in the ground, through a drain field. Whether that drain field works for decades or overflows in its first rainy season depends on one number: how fast your soil can absorb water. That number comes from the percolation test, called prueba de infiltración in Spanish.
The test is regulated by Decree 42075-S-MINAE, the regulation on disposing of treated domestic wastewater into the ground, published in 2020 and amended since. Its Annex 3 explains exactly how to do it. For how the whole septic system works, see our guide to septic systems in Costa Rica. Ingeniería de las Aguas uses the test results to design drain fields that meet the rule.
What the test measures
The test measures the infiltration rate: how many minutes the water takes to drop one centimeter in a test hole once the soil is saturated. It is expressed in minutes per centimeter (min/cm).
- A low number means fast soil, such as sand.
- A high number means slow soil, such as clay.
Infiltration trenches are only allowed when the rate falls between 2 and 24 min/cm. Below 2, water reaches the subsoil too fast to be cleaned on the way. Above 24, the soil cannot keep up and the system overflows. Outside that range, no drain field is accepted at that spot. Seepage pits and soakaways are banned outright for any wastewater, treated or not.
How the test is done
Annex 3 of the decree is precise. In summary:
- Number of tests. The responsible professional decides how many and where, with a minimum of two at the drain field site. Condos and developments with several systems must characterize each infiltration area.
- Large hole. Dig a hole 0.30 to 0.60 m deep and 0.80 to 1.0 m wide.
- Small hole. At its bottom, dig a second hole at least 0.30 m deep and 0.10 to 0.30 m across, so its bottom matches the planned depth of the trench.
- Preparation. Scrape the walls to remove smeared soil and add 5 cm of coarse sand or fine gravel at the bottom.
- Soaking. Fill the small hole with clean water and keep it full for at least 24 hours, preferably overnight. This reproduces rainy-season soil.
- Measuring. With the water at least 15 cm above the gravel, record the level, wait 30 minutes and record it again. Repeat for four hours, until there are eight intervals.
The rate is calculated from the last interval: 30 minutes divided by the centimeters the water dropped. If the water drops 15 cm in less than 30 minutes, or less than 1.2 cm in 30 minutes, the soil at that point is not suitable for a drain field.
Why a dry-season shortcut misleads
Guanacaste and much of the Pacific coast are bone dry from December to March. A test done in that season without the full 24-hour soak, or a quick check by pouring a bucket of water into a hole, can show dry, cracked soil swallowing water quickly. In September the same soil may be saturated and barely absorb anything. Drain fields "sized by eye", or from a shortcut test, are the most common reason septic systems overflow in the rains. Our guide on rainy season septic problems in Guanacaste covers the consequences.
What the report must include
The results report must state the date, the georeferenced location of each test, the measuring depth, the depth of the water table (or that none was found), photographs, and the name and signature of the responsible professional, who must be registered with their professional association.
Reading the result
With the infiltration rate, Table 2 of Annex 3 gives the maximum application rate, in liters per square meter per day. Some values from the table:
| Infiltration rate | Liters per m² per day |
|---|---|
| 2 min/cm | 90.33 |
| 5 min/cm | 57.13 |
| 10 min/cm | 40.40 |
| 15 min/cm | 32.98 |
| 20 min/cm | 28.57 |
| 24 min/cm | 26.08 |
The daily wastewater flow is then estimated from standard water allowances with an 80 % return factor. For coastal areas, the decree's table uses 300 liters per person per day of wastewater. The infiltration area is the flow divided by the application rate, multiplied by a rainfall factor of at least 2.5, and converted into a total length of trench. The practical lesson: on slow soil, the same house can need about three times as much trench as on fast soil. Our guide on septic system sizing walks through the full calculation.
The test is not the only filter
Even with a good rate, Decree 42075 also requires:
- the water table at least 1.5 m below the bottom of the trench;
- slopes no steeper than 30 %;
- the drain field in a private green area, never under parking, paths, decks or slabs;
- setbacks of 1 m from property lines and from the nearest building, 1.5 m from drinking water pipes, 3 m from water storage tanks and 2 m from slopes steeper than 25 %, plus the protection areas that the Water Law and Forestry Law set for rivers, wells and springs.
When you need one
- Homes and small projects, up to 190 m³ of drinking water a month: the septic tank, the drain field calculation and the test report are submitted with the construction plans. See wastewater permits for building a house.
- Larger projects: the treatment system must also be approved under Decree 39887-S-MINAE, the drain field needs a sampling box and the owner files operational reports.
- Changes to an existing system: changes to the tank, plant or drain field need Ministry of Health authorization before building, and a higher flow means meeting the requirements again.
- Buying land: a test before you buy tells you whether the lot can take a drain field at all. See buying land: water and wastewater checks.
If treated water will be reused for irrigation, Decree 39887 calls for a different test, the surface absorption test, which gives the basic infiltration in millimeters per hour for designing the irrigation.
Book a test
Send us a message on WhatsApp with the property location, the size of the lot, what you plan to build and, if you have it, the site plan. We will tell you how many tests your project needs. You can also use our contact page. For the full system, see our improved septic systems service.