D.G.S.W. Pitakumbura Manager (Groundwater Studies) Groundwater Section NWS&DB. Groundwater recharge or deep drainage or

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1 Groundwater Recharge and Extraction Techniques D.G.S.W. Pitakumbura Manager (Groundwater Studies) Groundwater Section NWS&DB What is Groundwater Recharge Groundwater recharge or deep drainage or deep percolation is a hydrologic process where water moves downward from surface water to groundwater. Recharge is the primary method that water enters an aquifer. This process usually occurs in the vadose zone below plant roots and is often expressed as a flux to the water table surface. 1

2 Groundwater Recharge Infiltration Percolation rate controls the infiltration rate Groundwater Recharge The vadose zone, also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone, the position at which the groundwater (the water in the soil's pores) is at atmospheric pressure ("vadose" is from the Latin for "shallow"). Hence, the vadose zone extends from the top of the ground surface to the water table. 2

3 Low Infiltration High Infiltration Low infiltration Objectives of Groundwater Recharge To maintain or augment natural groundwater as an economic resource To conserve excess surface water underground To combat progressive depletion of groundwater levels To combat unfavorable salt balance and saline water intrusion 3

4 Groundwater Recharge Natural Groundwater Recharge Artificial Groundwater Recharge Natural Groundwater Recharge Rainfall is the principal source for replenishment of moisture in the soil water system and recharge to ground water. Moisture movement in the unsaturated zone is controlled by suction pressure, moisture content and hydraulic conductivity relationships. The amount of moisture that will eventually reach the water table is defined as natural ground water recharge. The amount of this recharge depends upon the rate and duration of rainfall, the subsequent conditions at the upper boundary, the antecedent soil moisture conditions, the water table depth and the soil type. 4

5 Artificial Groundwater Recharge Artificial groundwater recharge is a process by which the groundwater reservoir is augmented at a rate exceeding the augmentation rate under natural conditions of replenishment. Artificial recharge of groundwater is achieved by putting surface water in basins. furrows, ditches, or other facilities where it infiltrates into the soil and moves downward to recharge aquifers. Design and Selection of Artificial Groundwater Recharge Site Selection of site: Recharge structures should be planned out after conducting proper hydro-geological investigations. Based on the analysis of this data (already existing i or those collected during investigation) it should be possible to: Define the sub-surface geology. Determine the presence or absence of impermeable layers or lenses that can impede percolation Define depths to water table and groundwater flow directions Establish the maximum rate of recharge that could be achieved at the site. 5

6 Design and Selection of Artificial Groundwater Recharge Site Designing the optimum capacity of the structure, the following parameters need to be considered: Size of the catchment Intensity of rainfall Rate of recharge, which depends on the geology of the site the quantity and quality of water available for recharging, have to be assessed. Methods of Artificial Groundwater Recharge Artificial recharge methods can be classified into two broad groups (i) Direct methods, and (ii) Indirect methods. 6

7 Direct Methods (a) Surface Spreading Techniques Flooding Ditches and Furrows Recharge Basins Run-off Conservation Structures (Gully plugs, bench terracing, Contour barriers) Stream-channel Modification Surface Irrigation Flooding Ditches Furrows Recharge Basin Contour Barriers 7

8 Direct Methods Sub-Surface Techniques Injection Wells Gravity-Head Recharge Wells Connector Wells Recharge pits Recharge Shafts Injection Well Connector Well Gravity - Head Recharge Well Recharge Shaft 8

9 Indirect Methods (a) Induced Recharge (b) Aquifer Modification (c) Groundwater Conservation Structures Induced Recharge Pumping Wells Collector Wells Infiltration Gallery Indirect Methods 9

10 Collector Well Collector Well Pumping Well Infiltration Gallery Aquifer Modification Bore Blasting Hydro-Fracturing Indirect Methods 10

11 Hydro fracturing (Blasting use dynamite or similar blasting materials Indirect Methods Groundwater Conservation Structures Groundwater Dams / Underground Barriers Fracture-Sealing Cementation Technique 11

12 Advantages & Disadvantages of Groundwater Recharge Advantages May provide a drought-proof water supply (from groundwater) Technology is easy to understand and operate Groundwater recharge collects water during wet season for use in dry season, when demand is highest In many river basins, surface water runoff is controlled due to aquifer recharge, resulting in less sedimentation problems Recharge with less-saline surface waters or treated effluents improve the quality of saline aquifers, facilitating the use of the water for agriculture and livestock Disadvantages Discharge of nutrients and micro-pollutants may negatively affect the receiving soil and aquifer Introduction of pollutants may have longterm impacts Potential of groundwater contamination from injected surface water runoff, especially from agricultural fields and road surfaces Recharge can degrade the aquifer unless quality control of the injected water is adequate Unless significant volumes can be injected into an aquifer, groundwater recharge may not be economically feasible During the construction of water traps, disturbances of soil and vegetation cover may cause environmental damage to the project area Groundwater Extraction The groundwater extraction process is an important part of everyday life, but many people do not fully understand how it works. Most people turn on the tap and expect the water to be there, but they never give any thought as to how it gets to your home. Water extraction, water abstraction, or groundwater extraction is the process of taking water from any source, either temporarily or permanently. This water can be used for irrigation, industry, recreation, flood control or treatment to produce drinking water. 12

13 GROUND WATER ABSTRACTING STRUCTURES The ground water abstracting structures are man made which collect & hold water for exploitation. The common ground water abstracting structures are Dug wells, Dug cum borehole wells, borehole wells, collector well & Infiltration gallery. Wells are vertical shafts excavated or holes drilled in earth to the level of water table for extraction. The wells have to be designed to get the optimum quantity of water economically from a given geological formation. 13

14 Collector Well Infiltration Gallery 14

15 WELL DESIGN- CRITERIA The choice of open well or bore well & the method of well design depends upon topography, geological conditions of the underlying strata, depth to water table, rainfall, climate and the quantity of water required. A water well design involves selection of proper dimensions like the diameter of the well & casing, length & location of the screen including slot size, shape & percentage open area. 15