Groundwater Recharge

Techniques

By

CH.APPARAO BEM-12-001

Introduction

Ground water is a precious and the most widely distributed resource of the earth and unlike any other mineral resource, it gets its annual replenishment from the meteoric precipitation.

At present nearly one fifth of all the water used in the world is obtained from ground water resources. Agriculture is the greatest user of water accounting for 80% of all consumption. (H.M. Raghunath , 2006).

Due to rapid urbanization, infiltration of rainwater into sub-soil has drastically decreased, and recharging of groundwater has diminished over the years.

This creates serious impact on socio-economic and environmental degradation of

the area.

It has therefore, become imperative to promote in-situ water harvesting to

augment groundwater recharge.

Out of 400 Mha.m rainwater, although 215 Mha.m percolates into the ground

(about 165 Mha.m is retained as soil moisture), but only 50 Mha.m joins the

groundwater (Michael, 1990).

Ground water recharge is the process where by the amount of water present in

or flowing through the interstices of the sub-soil increases by natural or artificial

means.

Rainfall is the principal source for replenishment of recharge of ground water.

Other sources include recharge from rivers, streams, irrigation water etc.

An unconfined aquifer is recharged directly by local rainfall, rivers, and lakes,

and the rate of recharge will be influenced by the permeability of overlying rocks

and soils.

A confined aquifer, on the other hand, is characterized by an overlying bed that

is impermeable, and local rainfall does not influence the aquifer. It is normally

recharged from lakes, rivers, and rainfall that may occur at distances ranging

from a few kilometers to thousands of kilometers.

Groundwater in Hydrologic Cycle

Fig. 1 A graphical distribution of the locations of water on Earth.

Fig.2: Ground water Resources Availability and Utilization in India

Source : Ministry of water resources, Central ground water board(2009)

Dynamic Ground Water Resources

Annual Replenishable Ground water Resource 36.50 BCM

Net Annual Ground Water Availability 32.95 BCM

Annual Ground Water Draft 14.90 BCM

Stage of Ground Water Development 45 %

http://cgwb.gov.in/gw_profiles/st_ap.htm

Groundwater resources of Andhrapradesh

Groundwater accounts for small percentage of Earth’s total water, groundwater comprises approximately thirty percent of the Earth’s freshwater.

Groundwater is the primary source of water for over 1.5 billion people worldwide.

Depletion of groundwater may be the most substantial threat to irrigated agriculture.

(Alley, et al., 2002)

Impact of Climate Change on Groundwater

It is important to consider the potential impacts of climate change on groundwater systems.

Although the most noticeable impacts of climate change could be fluctuations in surface water levels and quality.

Because groundwater aquifers are recharged mainly by precipitation or through interaction with surface water bodies, the direct influence of climate change on precipitation and surface water ultimately affects groundwater systems.

Methods and techniques for ground water recharge

Urban Areas Rural Areas• Recharge Pit

• Recharge Trench

• Tube well

• Recharge Well

• Gully Plug

• Contour Bund

• Gabion Structure

• Percolation tank

• Check Dam/ Cement Plug/ Nala Bund

• Recharge shaft

• Dugwell Recharge

• Ground Water Dams/Subsurface Dyke

1. Gully plug

•These are built using local stones, clay and bushes across small gullies and streams.

•Gully Plugs help in conservation of soil and moisture.

2. Contour bund

•To conserve soil moisture in watershed for long duration.

•These are suitable in low rain fall areas.

Fig. 3: Gully plug and Contour bund

Ground Water Recharge in Rural Areas

3. Gabion structure

•A small bund across the stream is made by putting locally available boulders in a mesh of steel wires and anchored to the stream banks.

•The excess water over flows this structure storing some water to serve as source of recharge.

Fig.4: Gabion structure

4. Percolation tank•Percolation tank is an artificially created surface water body, submerging in its reservoir a highly permeable land, so that surface runoff is made to percolate and recharge the ground water storage.

Fig. 5: Percolation tank

5. Check dams• Check dams are constructed across small streams having gentle slope. 

• The site selected should have sufficient thickness of permeable bed to facilitate recharge of stored water within short span of time.

Fig. 6: Check dams

6. Recharge shaft This is the most efficient and cost effective technique to recharge unconfined

aquifer overlain by poorly permeable strata.

By constructing recharge shaft in tanks, surplus water can be recharged to ground water.

Fig. 7: Recharge shaft

7. Dugwell recharge•Existing and abandoned dug wells may be utilized as recharge structure after cleaning and desilting the same.

•The recharge water is guided through a pipe from desilting chamber to the bottom of well or below the water level to avoid scouring of bottom and entrapment of air bubbles in the aquifer.

Fig. 8: Dugwell recharge

8. Ground Water Dams or Sub-Surface DykesIt is a subsurface barrier across stream which retards the base flow and stores water upstream below ground surface. 

Since the water is stored within the aquifer, submergence of land can be avoided and land above the reservoir can be utilized even after the construction of the dam. 

Fig. 9: Ground water dams or sub-surface dykes

1.Recharge Pit•In alluvial areas where permeable rocks are exposed on the land surface or are located at very shallow depth, rain water harvesting can be done through recharge pits.

•These are constructed for recharging the shallow aquifers.

2. Recharge Trench•Recharge trenches are suitable for buildings having roof area of 200-300 sq. m. and where permeable strata is available at shallow depths.

•Trench may be 0.5 to 1 m wide, 1 to 1.5m. deep and 10 to 20 m. long depending upon availability of water to be recharge.

Ground Water Recharge in Urban Areas

3. Tube Wells•In areas where the shallow aquifers have dried up and existing tubewells are tapping deeper aquifer, rain water harvesting through existing tubewell can be adopted to recharge the deeper aquifers.

4. Trench with Recharge Well•In areas where the surface soil is impervious and large quantities of roof water or surface runoff is available within a very short period of heavy rainfall, the use of trench/ pits is made to store the water in a filter media and subsequently recharge to ground water through specially constructed recharge wells.

•This technique is ideally suited for area where permeable horizon is within 3m below ground level.

Recharge pit Recharge trench

Tube wells Trench with recharge well

The following methods are commonly in use for estimating ground

water recharge:

1) Ground water level fluctuation method 

2) Ground water balance method 

3) Hydrochemical (chloride mass balance)

GROUND WATER LEVEL FLUCTUATION METHOD   

              

This is an indirect method of deducing the recharge from the fluctuation of the water table.

The rise in the water table during the rainy season is used to estimate the recharge, the rainfall recharge, Ri is given by,  Ri = Sy∆s + Tp RT

Where , Sy = specific yield,Tp = The abstraction during the rainy season divide by the study area,

andRT = The return flow due to any irrigation which occurs during the rainy season.

The basic limitation of the above equation is that it neglects the subsurface inflow and outflow and assumes that every inflow and outflow is uniformly distributed over the area.

GROUND WATER BALANCE METHOD

I – O = ∆W / ∆t

Where, I = inflow (m3/day) during time ∆t, O = Outflow (m3/day) during time ∆t, and W = Change in water volume (m3).

Considering the various inflow and outflow components, the ground water balance equation for a time period ∆t is given as:

Ri+Rc+RT+Rt+Si+Ig = Et+Tp+Se+Og+ ∆W

Where, Ri = Recharge from rainfall Rc = Recharge from canal seepageRT = Recharge from field irrigation Rt = Recharge from tankSi = Influent recharge from rivers Ig = Inflow from other basinsEt = Evapotranspiration Tp = Draft from ground waterSe = Effluent recharge to rivers Og = Outflow to other basins∆W = Change in ground water storage.

The above equation is the general ground water balance equation for an unconfined aquifer.

Chloride Mass Balance Recharge Estimation

To determine the mean annual recharge using the chloride method it is assumed that the only possible source of chloride ion in groundwaters of the study area is at the soil surface (either in precipitation or as dry fallout) and that there is no contribution from weathering.

Recharge as shown in the following relationship developed by Eriksson and Khunakasem (1969):

Recharge (mm) = rainfall (mm) × Cl concentration in rainfall (mg/L)/Cl concentration in groundwater (mg/L)