Where an aquifer crops out beneath the sea, ocean water may enter it under certain conditions. Under nonartesian conditions, sea water will be at such a depth that the overlying column of fresh groundwater will exactly balance a column of heavier sea
water, according to the Ghyben-Herzberg principle. Hence, under static conditions, if the fresh water has a specific gravity of 1 and the sea water has a specific gravity of 1.025, the interface between the heavier sea water and the overlying fresh
water in the area is pushed 40 feet below sea level for every foot that the water table stands above sea level. This is a very important point because it means that if the height of the water table above sea level is known, it is possible to
calculate the depth to which fresh water is present. (While a major concern in coastal regions, many upland regions also contain salt water or brackish water at some depth.)
For example, if the top (water table) of a fresh groundwater, body is 2 feet above sea level and, if we have a static condition, the depth to the salt water at this point is 80 feet below sea level or, at the well site, the total depth that would have
to be drilled would equal the sum of the thickness of the depth to the water table, the height of the water table above sea level, and 40 times the height of the water table above sea level.
Example: What is the depth to salt water where the thickness of the zone of aeration is 10 ft. and the water table is 3 ft. above sea level?
- The zone of aeration is 10 ft. (3 m).
- The height of the water table above sea level is 3 ft. (l m).
- 40 times the height of the water table above sea level equals 40 ´ 3 or 120 ft. (40 m).
Solution: The depth from the land surface to the salt water body is: (1) + (2) + (3) or l0 ft.+ 3 ft. + 120 ft. or 133 ft.
If sea level was constant and recharge from rainfall was uniform, the salt water/fresh water interface would remain motionless. But this interface does fluctuate because neither of these elements is constant.
V.T. Stringfield (1966, Artesian water in tertiary limestone in the southeastern states, USGS Professional Paper 517) discusses the effect of a pumping well on the salt water/fresh water boundary. "If a well is pumped and a cone of depression in the water
table is developed under such conditions, saltwater will rise below the well and will form an upright cone, theoretically having a height about 40 times the depth of the cone of depression in the water table. If pumping is continued at a constant
rate without change in the cone of depression, the contact theoretically remains stationary and saltwater remains motionless while freshwater moves from all directions toward the well. If, however, the rate of pumping is increased or because of depletion
of the supply the water table is further depressed, the apex of the saltwater cone may reach the bottom of the well, saltwater may be drawn into the well and movement in the saltwater zone may begin."
The relation between salt water and fresh water under the confined aquifer situation can be compared to a U-shaped tube, one side of which is filled with salt water and the other with fresh water. Assume that the leg of the tube on the right is the ocean
and the tube on the left is the aquifer, while the walls of the tube represent confining beds which are impermeable. In other words, we have a closed system with salt water balanced by fresh water. Stringfield writes of this situation as follows:
"If the confining bed is completely impervious and the head of water in the aquifer is not large enough to push the saltwater back to the submarine outcrop of the aquifer, the condition is one of equilibrium between two bodies of water of different densities. On the other hand, if the head of water is sufficiently great, a hydraulic gradient will be established in the aquifer, the saltwater will be pushed back to the submarine outcrop, and freshwater will escape into the sea. Under the first condition,
there is no discharge of freshwater into the sea. Hence, there is no hydraulic gradient, the head of the water in the aquifer is the same at all points, and the piezometric surface becomes an even surface at some height above sea level… If
the head is relatively high or the submarine out-crop is relatively near sea level, active conditions may prevail. Under these conditions, there is a hydraulic gradient; water is moving through the aquifer, and the soluble salts have generally been
removed if there has been sufficient time and if the artesian water is relatively free of mineral matter. Conversely, if the head is low or the submarine outcrop is at great depth, static conditions may prevail. The aquifer is plugged by pressure
of salt-water, and there is no leaking out of the soluble salts, except as artesian water may escape through the overlying confining beds or through nearby submarine outcrops where the aquifer is near enough to sea level so that the freshwater head
in the aquifer at the outcrop exceeds the back pressure from the saltwater column in the ocean."
During periods of heavy recharge in shallow coastal aquifers, the salt water/fresh water boundary moves toward the sea. This has been observed in the Miami, Florida, area where the gradient of the water table is low, but the aquifer is very permeable
and fresh water is constantly discharged into Biscayne Bay.
Dark water in the photo is fresh water, while the lighter shade is salt water. Photo courtesy NASA.
Saline groundwater problems
Away from the seacoast, salt water intrusion takes on different characteristics. Very deep groundwater is normally very saline, much of it is connate water, sea water deposited with the sediments forming the sedimentary rock containing it. Localized
salt intrusion may come from salt domes contributing chloride to the surrounding groundwater.
The soil through which water percolates can also be a prominent source of salts in aquifers. Soils in arid climates may be highly mineralized compared to soils in humid zones as little of it is leached away by natural rainfall. Where irrigation
water is also the primary source of its own recharge, large amounts of salt may be carried into the aquifer.
The groundwater system in many of the irrigated areas of the Colorado River Basin (Southwestern United States) is derived almost entirely from deep percolation of irrigation water, and seepage from irrigation conveyance and tail water collection systems. Salt pickup rates from irrigated soils in the basin vary in the different areas. Among the high salt pickup areas is the Grand Valley in western Colorado, estimated at 8 tons/acre/year. Water entering the groundwater supply from irrigation practices
in the valley amounts to about 145,000 acre-feet/year and contributes about 690,000 tons/year to the salt load of the Colorado River. Samples of base flow water from the weathered Mancos Shale Aquifer in the valley vary in salinity from about 1,500
mg/L to about 9,000 mg/L with a mean of 4,100 mg/L, while water samples from alluvial aquifers range from 305 mg/L to 124,000 mg/L with a mean of about 11,500 mg/L. Base flow returning to the river in the drains and washes has concentrations that
average about 4,200 mg/L.
Water losses and quantities of irrigation return flows can be reduced by improving farm irrigation efficiencies and by partial or complete lining of canals, laterals, and ditches. Increasing on-farm irrigation efficiency through system improvements and
irrigation scheduling is the most cost-effective measure. However, achievement of higher efficiencies will require changes in water laws to encourage conservation and revised water-pricing policies that discourage waste.
Where water supplies are pumped from very deep aquifers close to an interface with saline connate water below, salt water intrusion may be induced as the fresh water aquifer is dewatered. This will also occur because of any other significant lowering
of the water table. Small oceanic islands, such as those in the Caribbean or Hawaii, have very delicate fresh water/salt water balances as the volume of fresh groundwater is so low.
The above information is excerpted in large part from Chapter 31 of the 1999 NGWA Press publication, Ground Water Hydrology for Water Well Contractors.
For further information: Watch the video, The Interactive Roles of Surface Water & Ground Water, below.