8.7 Freshwater/Saltwater Interaction in coastal aquifers

Coastal aquifers Density difference, zone of mixing (diffusion) Interface Saltwater front

Island in an ocean Freshwater lens Moving interface

Saltwater intrusion Pumping of groundwater  diverts groundwater from flowing toward sea  seawater movement toward aquifer Causes deterioration of water quality in coastal aquifers

Locating the Interface Ghyben-Herzberg relation

Ghyben-Herzberg Relation Hydrostatic conditions: Weight of column of freshwater extending from water table to interface weight of column of saltwater extending from sea level to the same depth =

Ghyben-Herzberg Relation: f = freshwater density = 1.0g/cm3 s = saltwater density = 1.025g/cm3 z = height of saltwater column hf = hydraulic head above sea level hf + z = height of freshwater column

Ghyben-Herzberg Relation: The depth to the interface between freshwater and saltwater is approximately 40 times the height of the water table above sea level

Position of the interface x

Example 8.4 f = 1.0 g/cm3, s = 1.025 Water levels in two wells far from shoreline : 0.5 m , 1.0 m Distance between two wells = 1000 m K = 10 m/d, b = 50 m Calculate: Length of saltwater wedge Position of interface

Example 8.4, contd. L

Example 8.4, solution Discharge from aquifer to the sea per unit length of shoreline: Q’ = Kb dh/dx = (10 m/d)(50 m)(1.0 m -0.5 m)/1000 m = 0.25 m3/d/m Interface = = 0.5 z2 Length of protrusion = = 1250 m

Methods for limiting seawater intrusion Controls on pumping: Reducing pumping rates Reducing number of pumped wells Artificial recharge through ponds Using imported water or reclaimed wastewater

Methods for limiting seawater intrusion Pumping trough barriers Using a series of pumped wells to remove saltwater and form a potentiometric barrier Freshwater injection To establish a seaward hydraulic gradient

Case study: saltwater intrusion control in Biscayne Aquifer, Florida

Upconing of Interface Upconing = rise of interface as a result of groundwater pumping Rise of interface (upconing) = z = Maximum permitted pumping rate =Qmax

(2x3.1416)(100m)(10 m/d)(1.025 g/cm3 – 1.0 g/cm3) Example 8.5 The distance from the base of a pumping well to the freshwater-saltwater interface is 100 m, the pumping rate is 3000 m3/day, and the hydraulic conductivity is 10 m/d. What will be the position of the interface? What’s the maximum permitted pumping rate? --------------------SOLUTION------------------ Rise = z = (3000 m3/d)(1.0 g/cm3)/ (2x3.1416)(100m)(10 m/d)(1.025 g/cm3 – 1.0 g/cm3) = 13.26 m Q max = 4.7x103 m3/day

HW 8.4 8.5