What Is A Groundwater Recharge? Processes Of Groundwater Recharge
What Is A Groundwater Recharge?
Groundwater recharge is the process where water flows from the surface to underground aquifers, replenishing the groundwater supply. It happens both naturally, through precipitation, and artificially, like when rainwater or treated water is directed to the subsurface.
The primary way water enters an aquifer is through recharge. Estimating recharge rates can be done using various methods, such as chloride mass balance, soil physics, environmental and isotopic tracers, groundwater level fluctuation, water balance methods, and estimation of baseflow to rivers.
Processes Of Groundwater Recharge
Groundwater Recharge refers to the process where water from the earth’s surface filters down into aquifers and is collected. This process is also referred to as deep drainage or deep percolation. There are five main mechanisms for groundwater recharge, which include:
Diffuse Or Focused Mechanisms
Groundwater recharge can occur through diffuse or focused mechanisms. Diffuse recharge occurs when precipitation infiltrates the soil and reaches the water table, which is distributed over a large area. Focussed recharge takes place when water leaks from rivers, lakes, wadis, or wetlands, and it becomes more prevalent in arid regions.
Natural Recharge
Groundwater recharge can also occur naturally through processes such as rain, snow melt, and to a lesser extent, surface water.
However, human activities such as paving, development, and logging can obstruct the recharge process, leading to reduced water infiltration and an increased runoff. Overpumping groundwater for irrigation purposes can also lower the water table, so it is essential to manage groundwater in a sustainable manner.
Recharge can help shift excess salts from the root zone to deeper soil layers or the groundwater system, and tree roots can increase groundwater saturation, reducing runoff.
Wetlands
Wetlands help maintain the water table and control the hydraulic head, providing the force for groundwater recharge and discharge. The extent of recharge depends on various factors, including soil, vegetation, site, perimeter to volume ratio, and water table gradient.
Groundwater recharge occurs mainly through mineral soils near the edges of wetlands. Small wetlands, such as prairie potholes, have a high perimeter to volume ratio, which increases the surface area for water infiltration and contributes significantly to regional groundwater recharge. Some studies have found that wetlands can recharge up to 20% of their volume per season.
Artificial Recharge
Artificial groundwater recharge strategies include streambed channel modification, bank filtration, water spreading, and recharge wells. This method is becoming increasingly crucial in India, where farmers have depleted underground resources through overpumping.
In 2007, the Indian government allocated funds to support dug-well recharge projects in seven states to address the issue. However, the disposal of waste through water flux, such as dairy farms and industrial runoff, poses an environmental challenge.
Stormwater runoff collects in retention basins, where degradable contaminants can be accelerated by biodegradation. The design of detention ponds, retention ponds, and rain gardens must consider water table levels to ensure the appropriate treatment of pollutants.
Depression-Focused Recharge
Depression-focused recharge occurs when water falls uniformly over a field and exceeds the soil’s field capacity, leading to percolation and groundwater recharge. The more focused the infiltration, the larger the relative contributing runoff area.
This recharge process happens frequently in arid regions and has a profound impact on contaminant transport into groundwater, particularly in karst geological formations. In these regions, depressions intended to trap runoff water can connect underground over time and carry pollutants directly into the groundwater supply.
The quality of water collecting in infiltration basins is essential to consider in depression-focused recharge.
Factors Affecting Groundwater Recharge
Climate change and urbanization are two important factors that affect groundwater. Climate change has an indirect impact on groundwater through its effects on irrigation water demand which can result in a decline in groundwater storage.
This is due to increased groundwater usage for agriculture, especially in drylands made worse by the changes in the water cycle caused by climate change. The exact impacts of climate change on groundwater are still under investigation, but they could lead to reduced groundwater recharge and water quality deterioration.
Urbanization, on the other hand, increases the rate of groundwater recharge which can result in flash floods as the local ecosystem adjusts to the changes in the surrounding environment.
Urban areas have a higher recharge rate (up to ten times higher) compared to rural areas, due to the water supply and sewage networks and the infrastructure that prevents surface water from infiltrating into the soil.
The ecosystem has to accommodate the sudden influx of groundwater surplus which can lead to flash flooding as the road networks are less permeable and result in higher amounts of surface runoff.
