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8.7 Freshwater/Saltwater Interaction

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.

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8.7 Freshwater/Saltwater Interaction

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  1. 8.7 Freshwater/Saltwater Interaction in coastal aquifers

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

  3. Island in an ocean • Freshwater lens • Moving interface

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

  5. Locating the Interface Ghyben-Herzberg relation

  6. 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 =

  7. 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

  8. 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

  9. Position of the interface x

  10. 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

  11. Example 8.4, contd. L

  12. 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)/1000m = 0.25 m3/d/m • Interface = = 0.5 z2 • Length of protrusion = = 1250 m

  13. 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

  14. 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

  15. Case study: saltwater intrusion control in Biscayne Aquifer, Florida

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

  17. 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

  18. HW 8.4 8.5

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