JAM Achiever series - Hydrogeology Part – 4

 Hydrogeology Part – 4

SPRINGS

A spring is a concentrated discharge of groundwater appearing at the ground surface as a current of flowing water. Springs occur in many forms and have been classified as to cause, rock structure, discharge, temperature, and variability. 

Bryan divided all springs into 

    those resulting from nongravitational forces and

    those resulting from gravitational forces. 

Under the former category are included volcanic springs, associated with volcanic rocks, and fissure springs, resulting from fractures extending to great depths in the earth's crust. Such springs are usually thermal (fig above). 

Gravity springs result from water flowing under hydrostatic pressure.

the following general types are recognized:

1. Depression Springs-Formed where the ground surface intersects the water table.

2. Contact Springs-Created by a permeable water-bearing formation overlying a less permeable formation that intersects the ground surface.

3. Artesian Springs-Resulting from releases of water under pressure from confined aquifers either at an outcrop of the aquifer or through an opening in the confining bed (fault).

4. Impervious Rock Springs-Occurring in tubular channels or fractures of impervious rock. Tubular or Fracture Springs-Issuing from rounded channels, such as lava tubes or solution channels, or fractures in impermeable rock connecting with groundwater.

Quality of Groundwater

The groundwater in natural systems generally contains less than 1,000 mg/l dissolved solids. unless groundwater has 

(1) encountered a highly soluble mineral, such as gypsum, 

(2) been concentrated by evapotranspiration, or 

(3) been geothermally heated

Natural groundwater generally acquires dissolved constituents by dissolution of aquifer gases, minerals, and salts. Consequently, soil zone and aquifer gases and the most soluble minerals and salts in an aquifer generally determine the chemical composition of groundwater in an aquifer

Saltwater intrusion

    Saline water is the most common pollutant in fresh groundwater. Intrusion of saline water occurs where saline water displaces or mixes with freshwater in an aquifer. The phenomenon can occur in

1. deep aquifers with the upward advance of saline waters of geologic origin, 

2. in shallow aquifers from surface waste discharges, and 

3. in coastal aquifers from an invasion of seawater. 

The interrelations of two miscible fluids in porous media have been studied extensively both theoretically and under field conditions. Management techniques that enable development of fresh water and at the same time control of saline intrusion is the prime interest of the Ghyben-Herzberg relation.

GHYBEN-HERZBERG RELATION BETWEEN FRESH AND SALINE WATERS

More than 100 years ago two groups of investigators, working independently along the European coast, found that salt water occurred underground, not at sea level but at a depth below sea level of about 40 times the height of the fresh water above sea level. This distribution was attributed to a hydrostatic equilibrium existing between the two fluids of different densities. The equation derived to explain the phenomenon is generally referred to as the Ghyben-Herzberg relation, after its originators.

The hydrostatic balance between fresh and saline water can be illustrated by the U-tube, Pressures on each side of the tube must be equal; therefore,

psgz = pfg (Z + hf)

where ps is the density of the saline water, pf is the density of the fresh water, g is the acceleration of gravity, and z and hf are the heights as shown.

 Solving for z yields

z = "pf" /"ps−pf"  hf

which is the Ghyben-Herzberg relation. For typical seawater conditions, let "ps"= 1.025 g / cm3 and "pf" = 1 g / cm3, so that

z = 40 hf

Translating the U-tube to a coastal situation, as shown in Figure hf becomes the elevation of the water table above sea level and z is the depth to the fresh-saline interface below sea level. This is a hydrodynamic rather than a hydrostatic balance because fresh water is flowing toward the sea. 

The Ghyben-Herzberg relation gives satisfactory results. Only near the shoreline, where vertical flow components become pronounced do significant errors in the position of the interface occur. 

For confined aquifers, the above derivation can also be applied by replacing the water table by the piezometric surface.

    It is important to note from the Ghyben-Herzberg relation that fresh-salt water equilibrium requires that the water table, or piezometric surface (1) lie above sea level and (2) slope downward toward the ocean. Without these conditions, seawater will advance directly inland.

Q.11 Hardness of groundwater is determined by

(A) Mohs’ scale of hardness 

(B) concentrations of calcium and magnesium

(C) Bernoulli equation 

(D) Darcy’s law (2020)

Q.20 Elevation contours of ground surface (values in parenthesis) and groundwater table (values in normal font) are given in figure below.

What do the points P, Q and R represent? 

(A) P - recharge area, Q - spring R - discharge area

(B) P - discharge area, Q - spring R - recharge area

(C) P - spring area, Q - recharge R - discharge area

(D) P - discharge area, Q - recharge R - spring area

(2014)

Reference: Introduction to Groundwater Science and Engineering  – Neven Kresic

    Applied Hydrogeology 4th edition –  C W Fetter

Groundwater Hydrology – D. K. Todd & L. W. Mays