Geological Classification of Aquifers

Aquifers are one of the most important parts of the groundwater system. They store and transmit groundwater through underground layers of permeable rock, sand, gravel, and other geological materials. People depend on groundwater stored in aquifers for drinking water, irrigation, industry, and many other purposes.

Unlike rivers and lakes, aquifers are hidden beneath the Earth's surface. However, they can contain large quantities of water and may extend across extensive areas. The ability of an aquifer to store and transmit water depends mainly on the properties of the rocks and sediments that form it.

Aquifers also differ greatly in their geological age, composition, depth, permeability, water quality, and relationship with surrounding layers. In Bangladesh, these differences are particularly important because much of the country's water supply comes from groundwater.

What Is an Aquifer?

An aquifer is a geological formation, group of formations, or part of a formation that contains enough permeable material to store and transmit groundwater in quantities that can be used or naturally discharged.

In simple terms, an aquifer is an underground geological layer that can store groundwater and allow that water to move through it.

Aquifers commonly consist of materials such as:

• Sand
• Gravel
• Sandstone
• Limestone
• Fractured rock
• Other permeable geological materials

The amount of groundwater an aquifer can store depends largely on its porosity, while the ease with which water moves through it depends mainly on its permeability and hydraulic conductivity.

It is important to understand that not every underground layer containing water is considered a productive aquifer. A clay layer may contain water within its pores, but because water moves through clay very slowly, it may not supply enough water to wells. Such layers are generally considered aquitards rather than aquifers.

Classification of Aquifers:

Geologically, they can be classified based on the age and composition of the sediments or rocks that host them. The four main types are:

1. Holocene Aquifers
2. Late Pleistocene-Holocene Aquifers (Unconsolidated to semi-consolidated):
3. Plio-Pleistocene Aquifers (Semi-consolidated)
4. Miocene Aquifers (Consolidated)

Holocene Aquifers:

(a) Upper Holocene Aquifers:

• The Upper Holocene Aquifers are developed all over the deltaic and flood plain areas. This does not occur in the Chandina Formation areas (Tippera Surface).
• The lower part is composed of silt and clay at the bottom, and fine sand at the top. The upper part is composed of silt and clay, and is commonly found to be inter-bedded or mixed with medium sand.
• In BWDB-UNDP (1982) classification this aquifer is mentioned as Upper Composite Aquifer, in DPHE-BGS (2001) report it is considered as Upper Shallow Aquifers and Aggarwal et al. mentions it as the First Aquifer.
• Aggarwal et al. (2000) dated water from this aquifer as about 100 years old. Water of this Upper Aquifer is also affected by arsenic contamination. Holocene aquifers contain a number of sand layers/lenses that are stacked and interconnected, which makes them of leaky type.

(b) Middle Holocene Aquifers:

• Above the Late Pleistocene-Early Holocene Aquifer lies the fine sand which becomes coarser in the upper part. This sandy sequence varies greatly both vertically and horizontally. The upper part also contains silt and peaty organic matters.
• These Mid- Holocene Aquifers may be considered as in a similar position in the geological section as the Main Aquifer (BWDB-UNDP 1982), the Second Aquifer (Aggarwal et. al. 2000) or the Lower Shallow Aquifer (DPHE-BGS 2001) in the floodplain and deltaic areas of Bangladesh. Aggarwal et al. (2000) dated water from this aquifer as about 3000 years old. Most of the ground water in Bangladesh is withdrawn from this aquifer and the water is severely affected by arsenic contamination.
• The sediment from the surface samples in the Chandina Formation areas (Tippera Surface) dates around 6,000 ka. In this area the Middle Holocene aquifer is encountered nearest to the surface, but in most of the river basin and the delta plain areas this is at different depths.

2) Late Pleistocene-Holocene Aquifers (Unconsolidated to semi-consolidated):

• The Late Pleistocene-Early Holocene Aquifers are not continuous all over the country. This to some extent corresponds to the Deep Aquifer of BWDB-UNDP study (1982), lower part of the Deep Aquifer of the DPHE-BGS study (2001) and the Third Aquifer of Aggarwal et al. (2000). Aggarwal et al. has dated water from this aquifer as about 20,000 years old.
• The sediments of this aquifer to some extent correspond to the Late Pleistocene-Early Holocene Unit of the sediment section.
• Water within this aquifer is found to be arsenic safe but heavy withdrawal from this aquifer needs further study.

3) Plio-Pleistocene Aquifers (Semi-consolidated):

• The Plio-Pleistocene Aquifers of the Dupi Tila Formation lies beneath the Pleistocene Madhupur Clay Formation.
• This aquifer is composed of light gray to yellowish brown, medium to coarse sand with pebble beds.
• All of the water for Dhaka City is withdrawn from this aquifer but the water is as yet arsenic safe. This aquifer is confined to semi-confined.

4) Miocene Aquifers (Consolidated):

• Tipam Sandstone Formation of Chittagong Fold Belt
• Upper Bokabil Sandstones: Local aquifer with low transmissivity

How Do Aquifers Work?

Aquifers function as part of the natural groundwater cycle. Water from rainfall, rivers, lakes, wetlands, and other sources can enter the ground and move downward through permeable materials.

The water may eventually reach a saturated zone where the spaces between sediment grains or within fractures are filled with groundwater.

Once groundwater enters an aquifer, it can move slowly through the interconnected pores and fractures. The direction and speed of groundwater movement depend on hydraulic gradients and the physical properties of the geological materials.

Groundwater Storage in Aquifers

Aquifers store groundwater in the pore spaces between sediment grains or within fractures and cavities in rocks.

For example, a sandy aquifer can contain groundwater between individual sand grains. A fractured-rock aquifer can store and transmit water through cracks and fractures.

The amount of water stored depends on factors such as:

• Porosity
• Thickness of the aquifer
• Area covered by the aquifer
• Degree of saturation
• Geological composition

Groundwater Movement Through Aquifers

Groundwater generally moves from areas of higher hydraulic head toward areas of lower hydraulic head.

Although groundwater can move considerably faster in highly permeable materials, its movement is usually much slower than the flow of water in rivers.

Sand and gravel can allow relatively easy groundwater movement, whereas clay restricts movement because its pores are very small and poorly connected.

Recharge and Discharge of Aquifers

Aquifer recharge occurs when water enters an aquifer and increases its groundwater storage. Rainfall is an important source of natural recharge. Water can infiltrate through soil and sediment and eventually reach an aquifer.

Aquifers can also discharge groundwater through:

• Springs
• Rivers
• Wetlands
• Lakes
• Coastal areas
• Groundwater wells

Therefore, aquifers are not isolated underground reservoirs. They are dynamic parts of the hydrological system.

Role of Porosity and Permeability

Two important properties control aquifer behavior: porosity and permeability.

Porosity describes the amount of empty space within a geological material. Permeability describes how easily fluids can move through connected pores or fractures.

A material can have high porosity but relatively low permeability if its pores are poorly connected. Clay is a common example.

In contrast, well-sorted sand and gravel can have both useful porosity and high permeability, making them excellent aquifer materials.

Characteristics of Aquifers

The ability of an aquifer to store and transmit groundwater depends on several physical and hydraulic properties.

Porosity

Porosity is the percentage or fraction of a geological material's total volume that consists of pore spaces. For example, sand contains spaces between individual grains where groundwater can accumulate.

High porosity generally provides greater potential for groundwater storage, although high porosity alone does not guarantee high groundwater flow.

Permeability

Permeability describes the ability of a geological material to allow groundwater to move through interconnected pores or fractures. Well-connected pores generally produce higher permeability. Sand and gravel often have relatively high permeability, while clay usually has much lower permeability.

Hydraulic Conductivity

Hydraulic conductivity measures how easily groundwater can move through a particular geological material under a hydraulic gradient. It depends on both the properties of the geological material and the properties of the fluid.

High hydraulic conductivity generally means that groundwater can move more easily through the material.

Transmissivity

Transmissivity describes the ability of the entire saturated thickness of an aquifer to transmit groundwater. It is influenced by the aquifer's hydraulic conductivity and saturated thickness. A thick aquifer with high hydraulic conductivity can have high transmissivity and may be capable of supplying substantial quantities of water to wells.

Storage Capacity

An aquifer's storage capacity refers to the amount of groundwater that can be stored and released from the aquifer under changing hydraulic conditions.

The storage behavior differs between confined and unconfined aquifers because the mechanisms controlling water release are different.

Hydraulic Head

Hydraulic head represents the energy level of groundwater and helps determine the direction of groundwater flow. Groundwater generally moves from areas of higher hydraulic head toward areas of lower hydraulic head.

Hydraulic head is therefore an important concept for understanding groundwater movement and well behavior.

Importance and Uses of Aquifers

Aquifers provide water for many human and environmental needs.

Drinking Water Supply

Many communities depend on groundwater from aquifers for drinking and domestic purposes. Groundwater can often be available throughout the year, even when surface-water levels change seasonally.

However, groundwater must be tested and treated when necessary because aquifer water can contain naturally occurring contaminants or pollutants.

Irrigation

Agriculture is one of the largest users of groundwater in many regions. Groundwater from aquifers can provide a reliable source of irrigation water, especially during dry seasons when rainfall and surface-water availability are limited.

Industrial Water Supply

Industries may use groundwater for processing, cooling, cleaning, and other operations. Because industrial water demand can be high, uncontrolled extraction may place considerable pressure on local aquifers.

Groundwater Resources in Bangladesh

Groundwater is especially important in Bangladesh because the country contains extensive unconsolidated sediments capable of storing and transmitting groundwater. Shallow and deep aquifers provide water for domestic use, agriculture, and urban supply.

However, groundwater quality and availability vary considerably from one geological setting to another. Therefore, understanding the characteristics and distribution of individual aquifers is essential for responsible groundwater management.

Aquifer Problems and Groundwater Quality

Although aquifers are valuable water resources, they can face serious environmental and management challenges.

Arsenic Contamination

Arsenic contamination is one of the most important groundwater-quality problems associated with many shallow aquifers in Bangladesh. Naturally occurring arsenic can be released into groundwater through geochemical processes involving sediments and groundwater conditions.

The problem is particularly important because groundwater may appear clear and clean even when it contains elevated concentrations of arsenic.

Therefore, groundwater quality testing is essential before assuming that an aquifer is safe for drinking.

Aquifer Depletion

Aquifer depletion occurs when groundwater is removed faster than it is naturally or artificially replenished over a sustained period. Long-term depletion can cause declining groundwater levels and reduce the productivity of wells.

Salinity and Saltwater Intrusion

Coastal aquifers may be vulnerable to salinity and saltwater intrusion. When freshwater levels decline, particularly because of excessive groundwater pumping, saline water can move farther inland or upward into freshwater-bearing zones.

This can reduce the suitability of groundwater for drinking and irrigation.

Groundwater Pollution

Aquifers can also become contaminated by human activities.

Potential sources include:

• Industrial waste
• Agricultural chemicals
• Sewage
• Landfills
• Leaking storage systems
• Contaminated surface water

Once contaminants enter an aquifer, they can be difficult to remove because groundwater moves slowly and contamination may spread over long periods.

Aquifer Protection

Protecting aquifers requires careful management of both groundwater extraction and land use.

Important measures include:

• Monitoring groundwater levels
• Testing groundwater quality
• Controlling excessive pumping
• Protecting recharge areas
• Managing agricultural and industrial pollution
• Improving wastewater treatment
• Monitoring coastal groundwater
• Using groundwater according to sustainable withdrawal limits

Conclusion

Aquifers are essential underground geological systems that store and transmit groundwater. Their ability to provide water depends on properties such as porosity, permeability, hydraulic conductivity, transmissivity, thickness, and groundwater pressure.

Aquifers can occur in several forms, including unconfined, confined, semi-confined, and perched aquifers. They can also be classified according to the age and geological composition of their host sediments and rocks.

In Bangladesh, important geological aquifer groups include Holocene, Late Pleistocene–Holocene, Plio-Pleistocene, and Miocene aquifers. These systems differ in their sediment characteristics, distribution, depth, groundwater age, hydraulic properties, and water quality.

The Upper and Middle Holocene Aquifers are important shallow groundwater systems, while deeper Late Pleistocene–Holocene Aquifers may provide groundwater with different quality characteristics. The Plio-Pleistocene Dupi Tila Aquifer is particularly important for Dhaka's groundwater supply, while Miocene aquifers occur in older geological formations such as the Tipam Sandstone and Upper Bokabil Sandstones.

At the same time, aquifers face challenges such as arsenic contamination, groundwater depletion, salinity, saltwater intrusion, and pollution. Sustainable groundwater management therefore requires continuous monitoring, responsible pumping, protection of recharge areas, and regular assessment of groundwater quality.