CHAPTER 14   SALT WATER INTRUSION

SALT WATER INTRUSION Coastal Areas - CA, NY, TX, AL, WA, LA, FL   Coastal Areas - CA, NY, TX, AL, WA, LA, FL Inland Areas - OK, KS Ghyben - Herzberg Principle A

Since sea water is heavier than freshwater, a fresh water interface formed and fresh water drains into the ocean. Hydrostatic equilibrium – Z=hs

rs=1.025 for sea water (1.02 – 1.03) rf= 1.000 for fresh water

Assumptions 1. Fresh water moves horizontally to the ocean; vertical gradients of flow neglected. 2. Fresh-salt water interface is abrupt; that is, concentration varies from fresh water to that of seawater suddenly. Sharp interface. 3. Relationship not valid near point A, as seepage face must exist for fresh water outflow into the ocean. A

Hydrodynamic Equilibrium - M. K. Hubbert (1940) Considering dynamic equilibrium of F-SW Interface showed discrepancy between actual depth and calculated depth (Ghyben - Herberg principle).

Total pressure along AB equipotential line is equal Total pressure along AB equipotential line is equal. AB points don’t lie in a vertical line as equipotential line is curved, vertical distance between MSL and interface will be more than 40 hf.

Potential Concept – where p - pressure z - elevation Eliminate p,  

Slope of interface:   Since and Use Darcys’ Law qf - specific discharge = vf (unit area normal to flow)

Thus,   90 at the aquifer outcrop.   If salt water stagnant, and  increases with qf ; slope of interface increases as outcrop approached. Thus,   90 at the aquifer outcrop.

Influences on interface: 1. Pumping in coastal and inland aquifer 2. Recharge 3. Tidal fluctuation

Interface Equation

q - F.W. flow per unit length of coastline K - Perm of aquifer x, z. coords.  

When z = 0, AB   When x = 0, AC    Thus,

Upconing of Interface   Pumping the well causes interface to rise below well. Pressure of fresh water reduced on interface. If well bottom is close to interface or well discharge relatively high, saltwater cone may reach into well. Well discharges salt water.

Assumptions 1. Steady, horizontal F.W. flow to well 2. No lateral movement of salt water 3. Sharp interface

Steady state -  

Transient Flow – Z - Rise of salt water cone at time t, ft Q - Well discharge, gpm  ft3/d L - Distance between well bottom and interface before pumping, ft. Kx - Permeability of aquifer in horizontal direction, ft/d Kz - Permeability of aquifer in vertical direction, ft/d  - Aquifer porisity t - Time since pumping began, d

For t = 0, Equilibrium height of salt water cone below well center:   Critical cone height 0.4 L and 0.6 L. When cone height > critical value, zt not  Q. Cone reached well bottom with a sudden jump, indicating instability conds. Upconing important for determining safe depth and well pumpage that prevent entry of saline water into well.

Oceanic Islands -   Most islands are permeable -- sand, lava, limestone, coral. Fresh water in islands entirely by rainfall.

Using G-H relationship (Hydrostatic relation) -- Outward flow Q at radius r: (1) Change in flow thru a cylinder of radius r and thickness dr Integrate and note that (2)

Equating eq. 1 & 2: Integrate and use BC h = o at r = R Depth to saltwater interface = f (rainfall recharge, island size, perm)

Tidal and seasonal fluctuations and pumpage may form a transition zone. Pumping wells should skim water from top of lenses. Shallow well should be located in center of island. Drawdowns of few inches to few feet will provide enough water supplies.

Control of Sea Water Intrusion Stop pumping Reduction or rearrangement of pumping ·       reduce pumping draft on a coastal aquifer ·       locate wells inland of aquifer

3. Subsurface Barrier ·       Tar, clay or bentonite, polyethylene ·       Reduce permeability of aquifer to prevent inflow of sea water

Artificial Recharge ·  Artificially recharge aquifer from spreading basins, or recharge wells. ·  Overdraft eliminated and water level and gradient of W.T. sloping toward ocean. ·  Spreading basings for unconfined aquifer recharge, wells for confined aquifer. ·   Imported high quality water, injection into aquifer and pumping back - expensive cycle. Economics should be considered.

5. Pumping Trough A line of pumping wells adjacent and parallel to the coast. Trough formed in water level. Disadvantages: ·       reduces usable fresh water storage ·       costly to install and operate ·       fresh water wasted in mixture of fresh and sea waters pumped from trough ·       generally not economically feasible. Can be used as a temporary measure until other method is operated.

6. Pressure Ridge In unconfined aquifer, use spreading basins. In confined aquifer, use recharge wells. Ridge must be sufficiently high to repel seawater.

6. Pressure Ridge cont. - Costly Method    - Not much fresh water recharge lost to ocean. Manhattan Beach, cal. - (LA County) Colorado water imported to ridge area -- Filtered, softened, chlorinated.

Asbestos pipe used for wells -- corrosion proof dia. - 12” Spacing - 500 ft - to form 4000 ft long line Q = 0.4 - 0.5 cfs Recharge line - 2000-ft inland from ocean. Major areas in U.S. - Florida, New York, Texas, California