In times of drought, a farmer using a borehole or well often has one primary concern: monitoring the quantity of water available. Is the level dropping? Is the flow rate still sufficient? Will the borehole be able to meet the farm's needs until the next rains arrive?

But another question deserves to be asked: does the remaining water retain the same quality?

An underground aquifer is not a simple reservoir whose composition remains identical until it is empty. When its level drops, flow rates can decrease, water circulation can change, and physicochemical conditions can be altered. In some situations, this can lead to an increase in the concentration of certain contaminants or alter the composition of the abstracted water.

For a farm that uses this resource for livestock watering, cleaning, or certain agricultural uses, monitoring only the available volume may therefore be insufficient.

When a water table drops, what happens underground?

To understand the phenomenon, we must first recall how an underground aquifer functions.

A portion of precipitation infiltrates the soil and then joins the aquifers. When rainfall is sufficient and vegetation consumes less water, generally during the autumn and winter periods, the aquifers can recharge.

Conversely, from spring to autumn, effective precipitation generally becomes lower while the needs of vegetation and abstraction levels increase. The aquifers then gradually enter their depletion period.

However, they do not all react in the same way. The BRGM (French Geological Survey) distinguishes between "reactive" aquifers, which are capable of reacting quickly to rainfall and rainfall deficits, and more "inertial" aquifers, whose evolution is slower.

In periods of prolonged drought, the level of certain aquifers can therefore decrease rapidly. This situation obviously has a quantitative consequence: a well may produce less water, a spring may dry up, and certain extractions may become more difficult.

But the BRGM also highlights a less visible consequence: dropping levels can have an impact on groundwater quality.

Drought: does less water necessarily mean more pollutants?

Not systematically. This is an essential nuance.

It is sometimes said that in times of drought, "there is less water, so pollutants are necessarily more concentrated." This explanation may apply to certain situations, but the actual functioning of an aquifer is more complex.

The BRGM nevertheless clearly indicates that when flow rates become low, contaminants can be more concentrated. A very low level can also lead to dewatering, i.e., the lowering of the water table surface, which may change geochemical conditions and, consequently, water quality.

The composition of the abstracted water may also change because the borehole no longer captures exactly the same proportions of water from the different parts of the aquifer.

Extractions also play a role. When water requirements increase during a dry period, particularly for irrigation or livestock watering, pumping can become more intense. This change in groundwater flows can, depending on the configuration of the aquifer, mobilize water with different characteristics.

Therefore, one should avoid a rule that is too simplistic, such as:

"borehole level divided by two = pollutant concentration multiplied by two."

This relationship does not generally exist. The evolution depends on the type of aquifer, the geology, the depth of the intake, existing pollution, precipitation, extraction rates, and exchanges with surface water.

On the other hand, a significant or unusual drop in the level is a good reason to take an interest in water quality again.

Which parameters can change in agricultural borehole water?

There is no list of contaminants that would systematically increase during every drought. The parameters to monitor depend on the environment of the borehole and the activities present around the resource.

Nitrates

Nitrates are one of the important parameters in many agricultural areas. They can reach groundwater after infiltrating the soil and persist there for long periods.

Their concentration in a borehole therefore does not depend solely on the agricultural practices of the current year. Groundwater can reflect much older inputs.

Depending on the circulation of water in the aquifer, changes in the level, and pumping conditions, the measured concentration can evolve. An analysis carried out several years earlier does not necessarily reflect the current situation.

Pesticides and their metabolites

Pesticides are also one of the identified causes of chemical degradation of certain French groundwater bodies.

As with nitrates, their behavior depends on many characteristics: the molecule in question, the nature of the soil, mobility, persistence, depth of the water table, and hydrogeological conditions.

A drop in an aquifer therefore does not automatically mean an increase in all pesticides. However, a change in water circulation or pumping can alter the concentrations encountered at an intake point.

Iron, manganese, and mineralization

A variation in an aquifer's level can also modify the physicochemical conditions within it.

The BRGM particularly emphasizes that dewatering can lead to a change in geochemical conditions. Depending on the nature of the subsoil, this can have consequences for certain elements naturally present in the water.

On the farm, these variations can sometimes manifest as a change in the color of the water, the appearance of deposits, or a change in taste or odor.

But here again, the absence of visible change does not guarantee that the composition has remained identical.

Microbiological contamination

Wells and some shallow intakes can also be vulnerable to microbiological contamination, especially when their protection or maintenance is insufficient.

Bacteria, however, do not follow the exact same logic as chemical substances. Their presence can be influenced by the design of the intake, surface infiltration, runoff, or the proximity of contamination sources.

A dry period therefore does not automatically mean an increase in bacteria. Significant rainfall episodes after a drought can also quickly modify transfers from the surface.

Salinity in coastal areas

For farms located near the coast, another phenomenon deserves special attention: saline intrusion.

When the level of a coastal aquifer drops sharply, particularly under the combined effect of drought and extraction, the balance between fresh groundwater and salt water can be altered.

Salt water can then advance into the aquifer. The BRGM highlights this risk in particular for certain French coastal aquifers.

In these territories, an increase in salinity can become a significant issue for agricultural uses.

Why is water quality particularly important for livestock watering?

When a borehole supplies a livestock farm, the potential consequences do not only concern equipment.

Water is an essential part of animal feed. Water whose taste, odor, or composition changes can influence intake. Available scientific knowledge shows that water availability and quality must be taken into account when trying to explain certain variations in consumption or performance in cattle.

A scientific review devoted to the quality of water for cattle thus highlights the value of combining regular water analysis with feed analysis, particularly to monitor concentrations of dissolved solids, nitrates, or sulfates.

In dairy cows, this issue takes on a particular dimension since water needs increase with milk production and heat. A lactating cow can consume several dozen liters of water per day, and high-producing animals can approach or exceed 100 liters under certain conditions.

We detail this link in our article Does the water my cows drink affect their milk production?.

It would, however, be incorrect to state that a change in water quality automatically leads to a drop in production. Health, ration, temperature, stage of lactation, and herd management all intervene simultaneously.

Water should instead be considered as an additional parameter to monitor, particularly when the resource changes significantly during drought periods.

What signs should prompt you to have your water analyzed?

Ideally, one should not wait for a visible problem to know the composition of borehole water.

Having a reference analysis allows for results to be compared when conditions change. Several situations may warrant a new analysis:

• a significant or unusual drop in the level of the borehole or well;
• a decrease in available flow rate;
• a long or particularly intense drought;
• a significant increase in the duration or intensity of pumping;
• a change in the color, odor, or taste of the water;
• the appearance of deposits in pipes or troughs;
• an unexplained change in the animals' water consumption;
• major work or changes near the intake.

Another interesting time is the return of rain after a long dry period. The first significant rainfall can modify transfers from the soil to certain resources, particularly intakes vulnerable to surface influences.

It is therefore more relevant to reason in terms of monitoring over time than a single analysis performed when the borehole is created.

What should be analyzed in agricultural borehole water?

The analysis program must be adapted to the use of the water and the context of the farm.

For livestock watering, it may be relevant to look at microbiological parameters, mineralization, and, depending on the context, nitrates, sulfates, iron, manganese, or other contaminants likely to be present in the resource.

The proximity of crops, industrial activities, effluent storage, or other potential sources of pollution can guide additional analyses.

A laboratory analysis makes it possible above all to move away from reasoning based on appearance.

Clear water is not necessarily water whose composition is suitable for all agricultural uses. Nitrates, certain pesticides, or certain microbiological contaminants are not necessarily detectable with the naked eye.

Conversely, water colored by iron is not necessarily problematic for the same reasons as water showing microbiological contamination.

To learn more about the issues related to the various uses of water on a farm, also consult our article Why is quality water important on a farm?.

What if the borehole water quality degrades?

The first step is not to choose a filter. It is to identify the cause.

High iron content, microbiological contamination, the presence of pesticides, or excessive salinity do not require the same solutions.

Depending on the diagnosis, several levers can be studied: maintenance or repair of the intake, modification of pumping conditions, reinforced protection of the structure, use of or mixing with another resource when possible, or the implementation of an adapted treatment.

When filtration is considered, it must therefore be sized based on water analysis, required flow rate, and farm usage.

This is the approach used with the FiltraLife Agricultural Solution. Before it is installed on well or borehole water, an analysis of the resource is performed in order to determine its characteristics and verify the adequacy of the system.

The solution can treat up to 5,000 liters of water per hour and combines several filtration stages, including stainless steel filters, activated carbon, and a technology combining 0.1 µm hollow fibers and electro-adsorption.

However, it is not intended to make monitoring the resource unnecessary. Groundwater can evolve, particularly when hydrological conditions change. Analysis therefore remains an essential element of water management on the farm.

Drought: do not monitor only the quantity of water

Drought is often approached as a volume problem: how many cubic meters remain available and how long will the borehole be able to continue supplying the farm?

But a drop in an aquifer can also have qualitative consequences.

Lower flow rates can favor a higher concentration of certain contaminants. The lowering of the water table can alter geochemical conditions. Pumping can influence groundwater circulation. And in coastal aquifers, a significant drop in level can promote saltwater intrusion.

None of these phenomena mean that borehole water automatically becomes "bad" as soon as its level drops.

They do, however, show why an analysis carried out under normal conditions should not be considered a snapshot valid indefinitely.

In times of drought, monitoring the resource means looking at two indicators: how much water is available, but also what water is actually being extracted.

Frequently Asked Questions

Does drought increase the concentration of pollutants in a borehole?
It can lead to a higher concentration of certain contaminants when flow rates become low, but this phenomenon is neither automatic nor identical for all pollutants. The evolution depends on the aquifer, the contaminant, the extraction, and the hydrogeological conditions.

Can clear borehole water be contaminated?
Yes. Some chemical or microbiological parameters do not necessarily change the color, odor, or taste of the water. An analysis is necessary to know its composition precisely.

When should agricultural borehole water be analyzed?
It is useful to have a reference analysis and to perform new checks when the resource conditions change significantly: unusual drop in level, prolonged drought, change in flow rate, modification of pumping, or the appearance of a visible anomaly.

What contaminants can be found in agricultural groundwater?
Depending on the territory and the intake, different parameters may be involved: nitrates, pesticides and their metabolites, minerals, iron, manganese, sulfates, or even microbiological contamination. The profile must be determined by analysis.

Should borehole water always be filtered?
No. Treatment must respond to an identified problem. A preliminary analysis makes it possible to determine if filtration is necessary and which technology is adapted.

Sources

• BRGM – Groundwater, drought and flooding: frequently asked questions
  Explanations on the consequences of low water table levels, contaminant concentration, geochemical changes, and the risk of saline intrusion in coastal aquifers.
• BRGM – Situation of groundwater tables in France
  Data and analysis on recharge, aquifer depletion, their reactivity, and their sensitivity to drought periods.
• BRGM / SIGES – Chemical quality of groundwater
  Information on the chemical state of groundwater bodies and the role of nitrates, pesticides, and metabolites in their degradation.
• Journal of Dairy Science – Invited review: Freedom from thirst – Do dairy cows and calves have sufficient access to drinking water?
  Scientific review devoted to the water needs of dairy cattle, access to watering, and the influence of water quality on consumption.
• Applied Sciences – Nitrate and Bacterial Loads in Dairy Cattle Drinking Water and Potential Treatment Options for Pollutants, 2025
  Scientific review on the presence of nitrates and bacteria in groundwater used for dairy cattle watering and issues related to their treatment.
• Animal Frontiers / Applied Animal Science – Water consumption, drinking behavior of beef cattle and effects of water quality
  Scientific summary on drinking water quality, dissolved solids, nitrates, sulfates, and the value of regular water monitoring.