How to preserve our water resources? And why not by promoting the recharge of groundwater tables?

French groundwater is used more and more for drinking water, agriculture, industry and leisure, to the point that the situation becomes critical in some departments in summer and that prefects resort to restrictions on use. of water (73 departments concerned in August 2020).

Will we run out of water tomorrow?

Painful experiences around the world show how quickly rural regions or large capitals can come close to the depletion of their water resources: Cape Town came close to disaster in 2018 when its reservoirs, after three years of drought, had fallen to 11% of their capacity. The return of the rains and rigorous management of consumption have enabled the South African capital to overcome the crisis in 2020.

How to avoid being at the mercy of the rain?

Solutions exist to maintain the fragile balance between our needs and the hazard of the natural supply in the context of climate change. We speak of "integrated water resource management", which aims to preserve the level of groundwater tables, the flow rates of rivers and to fight against flooding and the salinization of water in coastal areas.

Among these solutions, we find the controlled recharge of aquifers, which we are going to focus on.

Groundwater, unevenly distributed

Groundwater is an invisible resource contained in "aquifers", a term designating the rock formations or sediments that contain these waters ; they are renewed more or less quickly.

Some deep aquifers contain water from rains that fell when humanity was still cutting flint! Others, close to the surface, contain water which passes through in a few years. In our latitudes, it is in winter, when vegetation takes up less water, that precipitation recharges aquifers. These underground water tables are very vulnerable to pollution and variations in precipitation, sometimes leading to a recharge deficit.

The water cycle and the formation of aquifers (canal9valais / Youtube, 2021).

Almost half of this groundwater is saline, unsuitable for consumption and for its most “greedy” use, irrigation. Exploitable groundwater is thus scarce and unevenly distributed on the planet.

Very variable pressures

The pressures on the resource are also very unequal depending on the climatic zones and the degree of development. The pressure is exerted above all on resources in metropolises, megalopolises and their expanding areas of influence.

Coastal areas whose population density increases periodically with tourism and which concentrate industrial and commercial activities require water resources often beyond their capacities. Added to this is the permanent threat of saline intrusion into coastal aquifers, exacerbated by global change.

However, the overall results seem positive: on a planetary scale, the exploitation of groundwater would only represent 8% of recharge.

It is therefore regionally and locally that the balance between groundwater recharge, abstraction and flow is very fragile.

Growing needs

Unesco's sustainable development goals aim, by 2030, to “significantly increase the rational use of water resources in all sectors and ensure the sustainability of withdrawals and freshwater supplies. "

But current trends are not going in the right direction. Human action causes variations in precipitation which, rarer or more intense, favor runoff over infiltration. In addition, our needs are increasing by around 1% per year worldwide; and this movement is accelerating.

In France, this translates into a drop in the level of 0,5 to 10 meters depending on the aquifers and the climate scenarios considered.

Knowing that with the sea level which rises under the influence of global warming, the littoral tables will be more threatened by saline intrusions.

Solutions to implement

To the question, "Will we run out of water tomorrow?" ", The answer is therefore:" We already lack it, locally and more and more often ".

The solutions already exist, in the world and in France, for many years. But it is a matter of implementing them and integrating them into coherent groundwater management strategies:

- characterize, monitor and forecast, on the basis of reliable models, the evolution of resources and needs;

- practice sobriety;

- reduce the pressure on water quality by reducing the quantity of persistent and mobile chemicals;

- improve wastewater treatment;

- use and reuse unconventional water after treatment;

- retain water in the territories by slowing down flows and storing water in natural environments.

Controlled recharge of aquifers

The concept of controlled recharge of aquifers consists of temporarily storing, in an aquifer, excess water from various sources for deferred use.

The term encompasses methods for maintaining, improving and protecting groundwater under quantitative and qualitative pressure.

In quality, by slowing or repelling a salt water intrusion or by using the purifying capacity of the soil, the banks of the rivers and the aquifer itself for a natural purification of the water. In quantity, because these practices help to rebalance overexploited water tables and maintain wetlands.

Traditional well recharge system in Delhi (India). Wolfram Kloppmann / BRGM

Some of these systems have been in use for millennia, but most have developed over the past 60 years, with increasing success.

A 2018 inventory has already listed 1 examples of MAR systems in 200 countries. Behind this acronym hides a multitude of technical solutions as well as a wide range of water resources to implement them.

Wastewater, brackish and saline water treatment

Technically, the controlled recharge of aquifers involves infiltration basins, injection boreholes, baffles or retention basins on watercourses, etc. Conventional (surface water, flood water, storm water) or unconventional (wastewater, brackish water, salt water) resources can be used.

The latter, available all year round, can constitute a contribution independent of variations in precipitation. The water resources used are, according to their quality, purified beforehand. Depending on the use of the recharged aquifer, the regulations and the water quality, this pretreatment can achieve drinking water quality.

In the case of saline water, prior desalination is required.

Above all, aquifer recharge systems can improve the quality of groundwater. This “Soil Aquifer Treatment” (SAT) of contaminated surface water or wastewater is an important option of nature-based solutions, in addition to other treatments.

On the left, the dune aquifer recharge system with treated wastewater in Koksijde (Belgium). Right, the coastal aquifer recharge system with treated wastewater in Shafdan (Israel). Geraldine Picot-Colbeaux / BRGM

A complex and essential follow-up

In addition to their storage capacity, aquifers therefore often have a role of natural purification. Both aspects imply that the waters spend an extended time in underground reservoirs. Rigorous monitoring of water quality and risk analysis are therefore essential in order to preserve or even restore the environmental quality of the environments.

The systems in question are complex: flow through the unsaturated zone of the aquifer, changes in chemical conditions in the water table, role of microorganisms, behavior of possible pollutants… all characteristics that are difficult to understand and anticipate.

In the operational phase of controlled recharging systems, information and communication technology tools can control and optimize operations by combining monitoring data (online or not) and modeling results.

These evolving practices must comply with the regulations in force and the there is a great disparity between the countries on this plane.

In France, the legal framework is given by the law on water, but there is no national technical methodological guide. Controlled recharge systems for aquifers require an authorization request endorsed by prefectural decree.

Across our territory, groundwater is for some communities the main resource for drinking water supply. Reconciling the preservation of the quantity and quality of this resource is a major issue for them. All water resources and technologies must be taken into account in the integrated management of water resources. Controlled recharge of aquifers is a valuable tool in this regard.The Conversation

Geraldine Picot-Colbeaux, Hydrogeologist, BRGM; Marie pettenati, Hydrogeologist, BRGM et Wolfram kloppmann, Researcher in isotopic geochemistry, project manager, scientific expert, BRGM

This article is republished from The Conversation under a Creative Commons license.

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