Springshed Management Training Curriculum 2016 The Springs Initiative

Transcription

1 Springshed Management Training Curriculum 2016 The Springs Initiative

2 SESSION TITLE: Aquifers SECTION: Understanding the Resource MODULE: II. Applied Hydrogeology AUTHORS: Dr Himanshu Kulkarni CONTRIBUTING ORGANIZATIONS: ACWADAM PURPOSE: Overview of general aquifer characteristics IMPACT: Participant will be familiar with aquifer types TIME REQUIRED: 60 minutes MATERIALS: This ppt ADDITIONAL RESOURCES:

3 Outline: Defining aquifers and why they are important Baseflows How and why springs and wells produce water Confined versus unconfined Parameters- storativity, transmissivity, specific yield

4 AQUIFERS & THEIR CHARACTERISTICS

5 Base flows can contribute significantly to stream and river flows

6 Base flow The contribution of groundwater to the ground surface through springs and seepages, which then flows as surface flow is called BASE FLOW. Base flow contributes significantly to the surface runoff in a stream or river. Streams, in many parts of India, continue to flow during the dry season due to base flow contributions. Surface runoff, however, may also include other components such as pure runoff generated by rain, snow melt, releases from reservoirs upstream etc.

7 An exposed aquifer

8 An exposed aquifer Why does a spring yield water?

9 Why does a well produce water? The permeability or hydraulic conductivity of the aquifer allows water to flow towards the well. The storage of groundwater in the aquifer due to the porosity of the rocks/rock material provides storage of water to feed the well through permeable zones.

10 Why does a well produce water? The permeability or hydraulic conductivity of the aquifer allows water to flow towards the well. The storage of groundwater in the aquifer due to the porosity of the rocks/rock material provides storage of water to feed the well through permeable zones.

11 Aquifers AQUIFER (In Greek) = AQUA (water) + FERRE (to bear) In simple terms an aquifer is simply a water bearing rock formation. Underground storage reservoirs is another way of referring to aquifers. An aquifer must possess enough porosity to get saturated and must also be able to release this stored water to wells and springs which can then serve as good sources of water supply.

12 Types of Aquifer Unconfined aquifer Where groundwater is in direct contact with the atmosphere through the open pore spaces of the overlying soil or rock, then the aquifer is said to be unconfined. The upper groundwater surface in an unconfined aquifer is called the water table. Confined aquifer A confined aquifer is an aquifer bounded both at the bottom and at the top by an impermeable stratum and fully filled with water which is usually under pressure. When perforating a hole into the ground until reaching the confined aquifer, one will see the water level in the hole rise to above the top of the aquifer and perhaps even above the land surface.

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14 Unconfined and confined aquifers

15 The same layer of rock can act as unconfined and confined aquifers for wells in different locations

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17 Surface water catchment

18 Surface water catchment

19 Unconfined & confined aquifers

20 Recharge to Unconfined & Confined aquifers

21 Aquifer storage The volume of aquifer storage can be simply estimated using the relation: St= A x D x? St= Aquifer storage A= Surface area on which the aquifer is exposed, i.e. aquifer surface area D= Saturated thickness of the aquifer? But we do need another factor to account for the fact that water is stored in rocks / rock material The storage capacity of the aquifer depends on the porosity of the rock formation

22 Aquifer storage The volume of aquifer storage can be simply estimated using the relation: St= A x D x? St= Aquifer storage A= Surface area on which the aquifer is exposed, i.e. aquifer surface area D= Saturated thickness of the aquifer? But we do need another factor to account for the fact that water is stored in rocks / rock material The storage capacity of the aquifer depends on the porosity of the rock formation

23 Empty tank Empty tank

24 Tank full with water Empty tank

25 Empty tank Tank full with water

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30 Measurement of storativity When we pump out the water from an unconfined aquifer, the water level declines. A specific quantity of water is released from a unit volume of aquifer under the influence of gravity. If V R is volume of aquifer releasing V w volume of water under gravity then V W V R V W = S x V R S = V W / V R where S is Storativity

31 Specific yield unconfined aquifer Generally specific yield for an unconfined aquifer ranges between 1 to 30% or 0.01 to 0.3 Specific yield values are useful in estimating the aquifer storage and the changes therein (±Δs) for an unconfined aquifer.

32 An aquifer performs the function of TRANSMISSION (flow) of groundwater along with that of storage. TRANSMISSIVITY Groundwater moves through the porous or fracture network of an aquifer under the influence of hydraulic gradient. This movement is called transmission. If the aquifer is spread out regionally underground, then the movement of groundwater also takes place on a regional scale.

33 Low transmissivity

34 High transmissivity

35 An example m2/day

36 Transmissivity and storage coefficient - definitions The capacity of the aquifer to allow flow of groundwater across a unit cross sectional area of the aquifer is called Transmissivity (T). The capacity of the aquifer to allow a certain volume of groundwater storage within the limits of the aquifer is its storage coefficient or Storativity (S). T and S are properties of the aquifer, i.e. the rock / rock material that holds groundwater

37 How does aquifer trasmissivity affect a spring High T implies a highdischarge spring A low T implies that spring discharge is usually small

38 How does aquifer storativity affect a spring A larger storativity of the aquifer implies that the aquifer can discharge to the spring over a long period of the year perennial spring Limited storativity of the aquifer implies that the aquifer can discharge to the spring only for a part of the year seasonal spring

39 What can you say about the aquifers feeding these springs?

40 ???Confused??? Advanced Center for Water Resources Development and Management (ACWADAM) Plot 4, Lenyadri society, Sus road, Pashan, Pune Website: