Subsurface dams for groundwater storage in semi-arid regions of Northeast Brazil

Water is a scarce resource in the semi-Arid region of Northeast Brazil. As stated by Shubo et al. (2020), in the Brazilian countryside, especially in the semiarid region, there was a lack of rainfall during the time period of 1981–2019. Climate models show a trend towards increased frequency and intensity of droughts and length of dry periods in the northeast, as already has occurred in some Brazilian regions (Denys et al., 2016, in Shubo et al., op.cit.). From 2012 to 2017 another major drought affected the semiarid region and 2015 was considered the most critical year of that period (Shubo et al., op.cit.).

In Cirilo, Costa (1999)

The drought of 1990 decade has led to the development of subsurface (or underground) dams, with the aim of storing and providing water to supply small villages or communities and to agricultural activities while being able to fix people in these arid regions and avoiding their migration into the main cities. Such dams have the advantages of being able to store larger volumes of water than the natural alluvium aquifers underlain by crystalline basement rocks, and of being less susceptible to evaporative losses as the water is stored underground.

Detailed explanation

General description

This description is adapted from Shubo et al. (2020), considering also inputs from Costa (2004).

An underground dam allows rainwater to be stored under riverbeds during the rainy season, making it available for the dry season. Although simple, its construction must comply with some technical requirements regarding its location: the alluvium must be predominantly sandy; the slope must be as flat as possible; the depth of the impermeable layer must be greater than 1.5 m below the riverbed; the construction site must be at the narrowest part of the riverbed; the river basin head, where there is less water, should be avoided; and the recharge area upstream the dam should be of at least 1 km of alluvium material. Regarding water quality, low salinity rates are essential to make its implementation feasible and this should be addressed before building the dam.

The core idea of its operation is to restrict the flow of the alluvial aquifer by building an impermeable transverse septum, thus raising the level of the upstream water table. In Brazil, there are two types of underground dams suitable to local features: the submerged type, also known as the Costa & Melo type and the submersible type. Both of them make use of a buried impermeable septum to restrict the underground flow and are equipped with an Amazon type well to allow the use of the accumulated water in the saturated zone. To build the septum, a trench is dug down to the impervious layer. Then, a plastic blanket is placed over the septum and covered with the excavated material, to block the groundwater flow. Other kind of impermeabilizing options exist, as described in Costa (2004).

The first type, known as a Costa & Melo-type underground dam (submerged dam), is suitable for the bed river of temporary creeks where the thickness of the sedimentary layer is greater than 1.5 m. This style of construction uses an impermeable septum that is totally buried, retaining only the groundwater flow, making the water table in the alluvium rise upstream of the barrier. There is no physical constraint to the runoff.

In the Brazilian semiarid region, mainly in the states of Pernambuco, Ceará, and Rio Grande do Norte, this is one of the most applied techniques to deal with water shortages. Fig. 4 shows a schematic of a Costa & Melo type underground dam.

In the second type of underground dam, the submersible underground dam, apart from the buried septum, there is another one made of rocks, bricks, or clay over the riverbed. This barrier makes the superficial flow spread over the land, creating a water pond that lasts up to two to three months after the end of the wet season. The process of lake formation generates a gradual accumulation of sediments, increasing the thickness of the soil upstream of the dam, thus providing an increase of the storage capacity over time, as happens in sand dams. This technique is suitable for small rivers and water pathways. This dam over the riverbed is equipped with a spillway made of concrete to spill over excess water and preserve the barrage above the ground, limiting the water level. Upstream, close to the dam, a large diameter well is built to recover water for irrigation and for other uses, such as livestock water supply when the water level falls below to the ground level. Fig. 5 shows a schematic of the submersible underground dam.

Next a more detailed, yet synthesised, description of the building steps of a Costa & Melo type underground dam is presented.

More detailed description of a Costa & Melo subsurface dam.

This text describes the basic concepts for the construction of the Costa & Melo dam, based on the work of Costa (2004).

This dam model was developed by researchers of the UFPE, Waldir D. Costa and Pedro G. de Melo, in the beginning of the 1980s, and further adapted to local conditions by the first researcher.

It consists of: (1) excavation of a straight trench normal to the stream direction; (2) location of an impermeable septum inside the trench; (3) building one or more large diameter well, one of them located besides the impermeable septum and containing drains along the trench; (4) surface rock filling, on surface, in the impermeable septum downgradient side; (5) implementing one or more piezometers upgradient the alluvium area influenced by the dam.

The main advantages are: (1) the execution is fast (1 to 2 days if machinery is used); (2) low construction costs; (3) may be built using local manpower; (4) allows monitoring the salinity and the water level; (5) may be used for multiple water usages.

The main disadvantage is that it requires local physical conditions already stated in the previous section.

One of the main issues to implement a subsurface dam is the social aspects and the water demand. It is crucial that the opportunities provided by constructing the dam are acknowledged by the local community and that its owner in committed to preserve and exploit it at the maximum of its water availability, mainly by growing the adequate crops.

Prior to the construction of the dam, and to find sites with the adequate conditions, some activities must be developed:

(1) Data survey: search needy local communities and, if this is the case, the conditions to grow irrigated cultures; climate data; hydrogeological reports; topographical and geological maps, areal-photo maps; (2) analysis of the data survey searching for the adequate physical conditions; (3) field survey, including water quality; (4) geological with or without geophysics profiling, in order to define the geometry of the alluvium bed and its granulometry; (5) if possible and justifiable, well drilling and pumping tests to define the hydraulic properties of the alluvium; and collecting water for chemical analysis; (6) project of the subsurface dam, considering: a) length of the dam, b) average depth of the trench, c) trench width, d) recommended septum type, e) type of large diameter well, f) number of needed wells, depending on the alluvium extension, g) number and location of the piezometers; (7) final conclusive report.

Costa (2004) goes through a thorough description of all the construction details of the underground dam. To get further information the reader is encouraged to read this reference (in Portuguese).

Just a remark, related to the pumping large diameter well, to be located in the deepest part of the alluvium upstream the dam: besides allowing a complete use of the groundwater, it gives conditions to deplete the aquifer during the period of the first annual rainfalls. This action renews stored water, thus avoiding salinization processes due to progressive evaporation.

Evidence of benefits from implementation

Advantages of Underground Dams

Some of the major advantages of these dams are, from a technical point of view [3]:

• The technology used behind these dams is very simple. In fact, aside community wells, these dams are the simplest and the most effective system for serving water needs of the poorer communities of Brazil.
• These dams are highly economical as they are built using locally available resources and technology.
• These dams are also easy to install, requiring no external expertise for operation and maintenance.
• In comparison to surface water reservoirs, these dams have very low evaporation rates.
• Being based on simple technology, they can easily be implemented along with other technologies like soil and water conservation techniques, and dug wells upstream.

From a social and agricultural point of view (Silva et al., 2021):

• Contribution to family’s food sovereignty and nutritional security;
• Increased access to and multiple uses of water;
• Diversification and integration, providing greater resilience and sustainability to family-based agroecosystems;
• Strengthening social inclusion and the productive organization of women and youth;
• Surplus is sold at local free markets;
• Creates a solidary space that is self-managed and farmer-led; and
• Is aligned with five of the 17 Sustainable Development Goals (SDGs) in the UN 2030 Agenda: 1, 2, 5, 6 and 13.

Disadvantages of Underground Dams

At times, installed dams leak or fail to provide good quality water, which proves that many vital issues are to be addressed before construction of these dams are carried out within a given alluvial valley on a large scale.

This may be overcome by the operating and adequate monitoring practices of the exploitation of the alluviums and their crops.

A general disadvantage of the construction of the subsurface dams may be baseflow reduction, thus affecting some groundwater dependent ecosystems.