We assess the plot and install anchored septic tanks with a raised field, biological wastewater treatment plants (BIO plants) or holding tanks in Riga and the Riga region. A septic tank including the tank and installation costs from €2,200; additional work depends on the water level.
How to tell whether the groundwater is high
- Dig a test pit in spring, once the snow has melted, because that is when the water level is highest. A level measured in autumn is misleading.
- Look into your well: if the water surface is close to the top in spring, the groundwater is high in the surrounding area too.
- Ask your neighbours whether there is water in their basements and septic tanks in spring.
- For a new build, the groundwater level is already stated in the geotechnical survey report.
Why high groundwater is a problem for a septic tank
The first problem is the tank. Groundwater presses on the tank from below, and an empty or half-empty plastic tank gets pushed upwards, most often right after pumping. In a concrete tank that is not watertight, groundwater flows in through the joints and overfills it.
The second problem is infiltration. A drain field treats water while it seeps through an unsaturated soil layer. If the groundwater is at drain level, this layer does not exist: wastewater reaches the groundwater almost untreated, and in spring the field floods and does not take any water.
Cabinet Regulation No. 384, point 4.4: wastewater from a septic tank enters the environment only through a purpose-built infiltration system. LBN 223-15, point 151: up to 5 m³/d, septic tanks with filtration fields, filtering drains or sand-gravel filters are used (151.1), or factory-made compact plants discharging into water bodies, drainage ditches or the ground, depending on the degree of treatment (151.2). Sources: likumi.lv, checked on 30 October 2026.
Solutions for high groundwater
| Solution | What it solves | What to consider |
|---|---|---|
| Anchored tank | The tank does not float up, even after pumping | Always needed if the water reaches the tank depth; concrete anchor slab and straps |
| Septic tank with a raised field and a pump | The drains sit above the groundwater, in a built-up sand layer | Pump chamber and electricity; the field mound rises above ground level |
| Biological treatment plant | Treats water to a level at which it can be discharged into a ditch or shallowly into the ground | Electricity, regular maintenance; the outlet point is agreed in the building documents |
| Anchored holding tank | Nothing enters the ground | Frequent pumping for the whole life of the system |
We explain how to choose a biological treatment plant on the page biological treatment plants, and how a field is built on the page drain field. A holding tank is the last option, because pumping it over its lifetime costs more than any other system.
What changes in the installation work
- Timing We plan the work for when the water level is lower: in the second half of summer or in autumn, not during the spring floods.
- Dewatering Water flows into the pit all the time, so it is pumped out throughout the installation. In sandy soil, the pit walls have to be shored up so that they do not cave in.
- Anchor slab A concrete anchor slab is laid at the bottom of the pit, and the tank is strapped to it before backfilling.
- Filling The tank is immediately filled with water and backfilled with sand. An empty tank must not be left in a wet pit even overnight.
Operation: pumping when the groundwater is high
Do not pump out an unanchored plastic tank completely during the spring floods. Pump it out partly, or fill it with water straight afterwards. Even an anchored system is better pumped in a drier period. If the tank fills faster in spring than in summer, groundwater is getting in, and the tank needs checking: see septic tank repair.
How much a septic tank for high groundwater costs
The base price (a septic tank including the tank and installation from €2,200) applies to standard conditions. High groundwater increases it: an anchor slab, dewatering during the work, imported sand for the field and a pump chamber. If you need a sewage pump station, it costs from €1,650; for a smaller pump, we set the price in the estimate. A biological treatment plant for 4–6 people costs from €1,750, installation work from €800, maintenance from €160.