Hydro GroundwaTERMS

Subsurface Flow

The movement of water below the earth’s surface within the subsurface layers or geological formations.

is a significant component of the water cycle, contributing to the overall movement and storage of water in the subsurface environment.

Four Categories

1. Aquifer

2. Aquitard

3. Aquiclude

4. Aquifuge

Aquifer

Is a saturated formation of earth material which not only stores water but yields it in sufficient quantity. An aquifer transmits water relatively easily due to its high permeability.

Aquitard

Is a formation through which only seepage is possible and thus the yield is insignificant compared to an aquifer. It is partly permeable.

Aquiclude

Is a geological formation which is essentially impermeable to the flow of water. It may be considered as closed to water movement even though it may contain large amounts of water due to its high porosity.

Aquifuge

Is a geological formation which is neither porous nor permeable. There are no interconnected openings and hence it cannot transmit water.

TYPES OF AQUIFER

✓An unconfined aquifer is one in which a free water surface

i.e. a water table exists.

✓A confined aquifer, also known as artesian aquifer, is an

aquifer which is confined between two impervious beds

such as aquicludes or aquifuges.

➢A confined aquifer is called a leaky aquifer if either or

both of its confining beds are aquitards.

FACTS ABOUT WATER TABLE

❑A water table is the free water surface in an unconfined aquifer.

❑The static level well penetrating an unconfined aquifer indicates the level of the water table

at that point.

❑The water table is constantly in motion adjusting its surface to achieve a balance between

the recharge and outflow from the subsurface storage.

❑The water table follows the topographic features of the surface.

PERCHED WATER TABLE

Sometimes, a lens or localized patch of impervious stratum can occur inside an unconfined aquifer in such a way that it retains a water table above the general water table. Such water table around the impervious material is known as perched water table.

Effluent stream

Also known as a gaining stream, receives water from the surrounding groundwater.

Influent stream

Losing stream, loses water to the groundwater system.

Porosity

The amount of pore space per unit volume of the aquifer material

Specific Yield (Sy)

The actual volume of water that can be extracted by the force of gravity from a unit volume of aquifer material

Specific Retention (Sr)

The fraction of water held back in the aquifer

Henry Darcy,

French hydraulic engineer, on the basis of his experimental findings proposed a law relating the velocity of flow in a porous medium

Total Discharge

Refers to the volumetric flow rate of water passing through a given cross-sectional area of a porous medium per unit time.

Specific Discharge

Also known as Darcy Velocity or Darcy Flux, is defined as the discharge per unit area of the porous medium.

Static Water Level

refers to the water level in the well prior to pumping

Cone of Depression

Refers to the region surrounding a pumped well where the water table has been lowered due to groundwater extraction

Drawdown

Refers to the lowering of the groundwater level around a well as a result of water being pumped out

Radius of Influence

Refers to the radial distance from a pumped well to the point where the drawdown becomes negligible or zero

Area of Influence

Refers to the area surrounding a pumping well where the groundwater levels are affected by the well’s extraction activities

Recuperation or Recovery

Refers to the process by which the water table returns to its original level after the cessation of pumping from a well

Steady State

Refers to a condition where the groundwater system has reached equilibrium, with no significant changes occurring over time. This condition implies that the rates of recharge and discharge are balanced, and there are no net changes in the groundwater storage over time.

Unsteady State

Refers to a condition where the groundwater system is undergoing changes over time

THIEM’S EQUATION

A fundamental tool used to analyze steady-state radial flow to a well in both confined and unconfined aquifers. It was derived in 1906. The equation helps determine the transmissivity and permeability of an aquifer based on measurements from a pumping test.

ASSUMPTIONS

1. The aquifer is homogeneous and isotropic.

2. The well fully penetrates the aquifer and receives water through its entire thickness.

3. The pumping has continued long enough for the system to reach a steady state, where the drawdown is stabilized.

4. The flow to the well is horizontal and radial, originating from a circular source around the well.

5. The rate of discharge from the well is constant.

explain the behavior of water level in wells in confined aquifers due to change in the atmospheric pressure.

In confined aquifers, water is under pressure from overlying layers. When atmospheric pressure increases, it pushes down on the aquifer, causing the water level in the well to drop. A decrease in pressure allows the level to rise. This change in water level reflects the effect of air pressure on the aquifer.

Describe the recovery test to estimate the transmissive of a confined aquifer.

The recovery test is done after pumping stops, and the water level rise is measured. The speed of recovery helps calculate the aquifer's transmissivity, which shows how easily water flows through it. It’s useful when full pumping tests are not possible.

Discuss the principle of recuperation test of an open well.

In a recuperation test, the well is pumped to lower the water level, then allowed to recover. The rate at which water refills the well shows its yield. The test depends on the difference between the water level in the well and the surrounding groundwater.

What are the commonly used methods to assess the recharge of groundwater in an area?

Groundwater recharge is commonly estimated by water table fluctuation, rainfall infiltration, and tracer methods. These methods measure how much water reaches the aquifer. Each has different levels of accuracy depending on site conditions.

Describe the groundwater resources and its utilization.

Groundwater is stored in aquifers and used for drinking, farming, and industry. It is especially important in dry areas. Overuse can cause depletion and pollution, so careful management is needed to keep it sustainable.