Drought and borehole water quality: how to protect your installation

Why does borehole water quality deteriorate in summer?

When rainfall decreases and temperatures rise, groundwater tables drop. This is known as low water. This phenomenon has direct consequences on the quality of water extracted by private boreholes and wells.

In winter, the water table is high: water flows through the upper soil layers, naturally filtered by sand and gravel. In summer, the table drops and the borehole draws from deeper layers, richer in dissolved minerals and concentrated pollutants.

The main problems observed during drought periods are:

• Iron and manganese — concentrations multiplied by 3 to 5, causing coloured water, metallic taste and stains on laundry and fixtures.
• Nitrates — the 50 mg/L threshold set by European Directive 2020/2184 is frequently exceeded in agricultural areas.
• Pesticides and VOCs — the reduced water volume concentrates phytosanitary residues.
• Bacteria and coliforms — heat promotes microbiological proliferation, especially in shallow wells.
• Turbidity — suspended particles increase, making the water visually cloudy.

Which areas are most affected?

Groundwater vulnerability to drought varies considerably by region. In France, the most exposed basins are the South-West (Adour-Garonne), the Centre (Loire-Bretagne) and the Mediterranean coast. In Belgium, Wallonia is more affected than Flanders, particularly the provinces of Hainaut and Belgian Luxembourg.

According to BRGM (Bureau of Geological and Mining Research), over 40% of French aquifers were below normal levels in August 2025. The VMM (Flemish Environment Agency) also recorded historically low levels in several Walloon piezometers.

The visible impact on borehole water

The difference between borehole water in normal conditions and during low water is often dramatic. Water can go from perfect clarity to brownish colouration within a few weeks.

This degradation is not merely cosmetic. Water loaded with iron and manganese damages pipes, clogs household appliances and makes water unfit for consumption without proper treatment. Nitrates, odourless and colourless, represent a silent health risk, particularly for infants and pregnant women.

The DIMM multi-barrier treatment chain

To guarantee consistent water quality regardless of season, DIMM recommends a multi-barrier approach: several successive treatment stages, each targeting a specific type of contaminant. This strategy aligns with recommendations from the WHO and EU Directive 2020/2184.

The 7 stages of the recommended treatment chain are:

1. Pumping (borehole) — extraction of raw water from the aquifer.
2. Sediment pre-filtration — a filter housing removes coarse particles (sand, rust, silt) protecting subsequent stages.
3. BIRM iron removal — a catalytic filter oxidises and retains dissolved iron and manganese, eliminating colouration and metallic taste.
4. Activated carbon — adsorbs pesticides, VOCs, residual chlorine and improves taste and odour.
5. UV sterilisation — destroys 99.99% of bacteria, viruses and parasites without chemical additives.
6. Reverse osmosis — removes nitrates, PFAS, heavy metals and dissolved salts at over 95%.
7. Tap distribution — safe drinking water, compliant with European standards.

A real installation in a private home

Here is an example of a complete installation in a utility room. All filtration elements are mounted in series on the main water line, between the borehole and the domestic network.

The visible components from left to right: three filter housings (sediment pre-filtration + activated carbon), followed by the UV steriliser. The assembly is connected in copper with isolation valves for easy maintenance. This standard configuration can be supplemented with a BIRM module and reverse osmosis unit depending on the raw water analysis results.

Who is this solution for?

The DIMM multi-barrier treatment chain is designed for several profiles:

• Homeowners with private boreholes or wells — rural dwellings using borehole water as a primary or supplementary source.
• Agricultural operations — livestock and crop farms requiring consistent water quality for watering and irrigation.
• Campsites and outdoor hospitality — open-air accommodations supplied by borehole, subject to reinforced health checks during summer season.
• Rural communities — small municipalities whose network partly relies on underground water sources.
• Installers and resellers — water treatment professionals seeking proven solutions to offer their clients.

DIMM offers pre-configured kits adapted to the flow rate and raw water quality, as well as technical support for custom sizing.