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| Seawater Intrusion | Chloride |
| Pajaro Valley | Water Concerns Around the World |
| Sources of Groundwater Salinization | Sodium Adsorption Ratio (SAR) |
Elevated salt content (salinity) of soils and fresh water supplies is a major problem on both a local and global scales. Problems of increasing salinity are most pronounced in arid regions with high rates of evapotranspiration and irrigated agriculture. This includes most of the US southwest, as well as large areas in Africa, Australia, Spain, Chile, the Middle East, and Asia (Gleick, 1993).
The primary source of salt contamination in the United States is from irrigation in semi-arid regions. Approximately 10% of the US agricultural land is irrigated (Postel, 1990). Of those 19 x 106 hectares or 47 x 106 acres of irrigated cropland, around 5.2 x 106 ha or 14 x 106 acres are adversely impacted by elevated salt concentrations. Another 1 x 106 ha or 2.5 x 106 acres have been adversely affected by saline seeps. Additionally, salt water intrusion is now threatening many coastal aquifers in the US. Therefore, it is estimated that 3 to 5 % of the agricultural land in the US is now being damaged by excess salinity, and that percentage is expected to increase.
Gleick, P.H. (Editor). 1993. Water in crisis. Oxford University Press, New York, NY, 473 pp.
National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. National Academy Press, Washington, D.C. 516 pp.
Postel, S. 1990. Saving water for agriculture. In: State of the World (L.R. Brown, Project Director), Norton, New York, NY, pp. 39-58.
Seawater intrusion is the migration of salt water into a fresh water aquifer. It occurs when there is a reduction in the freshwater head and flow at the sea water interface. This commonly occurs when there is overpumping or insufficient groundwater recharge of an aquifer in the coastal zone. It is a serious problem on both a local (Santa Cruz and Monterey) and global scale (Gleick, 1993). It is being exacerbated by (1) increasing freshwater demands that are depleting coastal aquifers and (2) the projected sea level rise associated with global warming.
Simple Equilibrium Model
Studies of seawater intrusion were initiated by studies in Amsterdam (Ghyben, 1988) and the North Sea (Herzberg, 1901). The analyses were based on an equilibrium model, which assumed simple hydrostatic conditions in a homogeneous, unconfined coastal aquifer. Under those hydrostatic conditions, the weight of fresh water is balanced by the weight of sea water at their interface under the aquifer:
where = density of saltwater (s) and freshwater (f), g = gravitational acceleration, and z = height of sea water (s) and fresh water (s + w) at the interface.
In those initial studies, the salinity of the salt water was 25 o/oo (parts per thousand or practical salinity units, psu). The densities are then expressed as
, and the preceding equation is simplified to:
which is termed the Ghyben-Herzberg relationship.
That relationship indicates that there is a 40:1 change in the water table elevations under equilibrium conditions:
For example, if the water table in an unconfined aquifer is lowered by 1 m, there will be a 40 m rise in the saltwater interface. Consequently, relatively small decreases in a fresh water aquifer may have relatively large impacts on the intrusion of salt water into that aquifer.
Steady State Models
However, aquifers are more appropriately modeled under steady state conditions. These involve balanced freshwater flows into and out of the aquifer. These are illustrated in numerous ways:
Since the assumptions of the Ghyben-Herzberg relation do not hold under steady state conditions, a more realistic steady state model was developed by Hubbert (1940).
Diffusion Models
Finally, salt water and fresh water do not form an inert interface. There is diffusive mixing of salt ions across the interface, which diminishes the interface and accelerates salt water intrusion. The extent of mixing is a function of both fresh water flow rates and geologic characteristics of the aquifer. Consequently, the most accurate models of salt water intrusion incorporate diffusion components based on sediment composition and flow patterns (Cooper et al., 1964).
Additional Reading and Illustrations
The preceding notes are based on pages 375-378 in the classic text, Groundwater, by R. Allen Freeze and John A. Cherry (1979). It includes illustrations and citations of the references. More recent references may be found in the case study reports on salt water intrusion in Pajaro Valley.
References
Cooper, H.H., Jr., F.A. Kohout, H.R. Henry and R.E. Glover. 1964. Sea water in coastal aquifers. U.S. Geological Survey Water-Supply Paper 1613-C, 84 pp.
Freeze, R.A. and J.A. Cherry. 1979. Groundwater. Prentice-Hall, Inc., Englewood Cliffs, NJ, 604 pp.
Ghyben, W.B. 1988. Nota in verband met de voorgenomen putboring nabij Amsterdam. Tijdschrift van Let Koninklijk Inst. Van Ing. (as cited in Freeze and Cherry, 1979).
Herzberg, A. 1901. Die Wasserverorgung einiger Nordseebader. J. Gaasbeleucht. Wassersorg., 44: 815-189 (as cited in Freeze and Cherry, 1979).
Hubbert, M.K. 1940. The theory of groundwater motion. Journal of Geology 48: 785-944.
Natural Saline Groundwater
regionally occurring saline groundwater that underlies freshwater aquifers there are multiple origins: residual (connate) water from the time of deposition in a saline environment solution of mineral matter in unsaturated and saturated zones concentration by evaporation intrusion of seawater a mixture of any or all of the above
Halite Solution
many areas in the US have thick layers of rock salt (halite)
depositional beds and salt diapers or salt domes
Seawater Intrusion
while groundwater flow in unconfined and shallow-confined aquifers is usually toward the coast
due to topography - overpumping may reverse the flow
Oil-Field Brine
"produced water" separated in the production of oil and gas
Agricultural Sources
associated with irrigation, animal wastes, commercial chemicals
fertilizers contain nitrate, phosphate, and potassium
Saline Seep
"recently developed saline soils in nonirrigated areas that are wet some or all of the time, often with white salt crusts, and where crop or grass production is reduced or eliminated"
primarily caused by evaporation from a shallow water table, in contrast to leaching in irrigation-return flows
Road Salt
application of salt to de-ice roads
Additional Sources and Causes of Salinity
Reference
Richter, B.C. and C.W. Kreiter. 1993. Geochemical Techniques for Identifying Sources of Ground-Water salinization. CRC Press, Boca Raton, FL pp 258.
Ion Concentrations in WaterIn natural waters chloride (Cl-) is very soluble and extremely mobile
little reactivity with other ions
little complex formation
little adsorption on mineral surfaces
not active in biogeochemical cyclesUsed as a reference ion to demonstrate the effects of evaporation.
log: log plot with [Cl-]:[X] = 45° for evaporation
deviation fn (nonconservative processes)
Hardie-Eugester model for the evaporation of natural watersRadioactive chlorine (36Cl) is used to date old groundwater
t1/2 = 3.01 x 105 years
dating to 1 million years (10 t1/2 = 3 x 106 years)There is an inverse proportionality between [Cl-] and runoff.
evidence of point source inputs from weathering and pollution
Main sources of chloride in surface waters
(global scale)
cyclic sea salt
18%
rain and dry deposition
dissolution of halites
57%
bedded evaporites and dispersed shales
thermal and mineral springs in volcanic areas
8%
hydrothermal alteration of volcanics
redissolution of saline crusts in desert basins
local
not a primary source
pollution
30%
sewage, oil brines, mining, road salt
ex. St. Lawrence and lower Rhine
------------
> 100%
Chloride in rivers (m/L)
total
natural
%contaminant
Africa
4.1
3.4
17
Asia
10.0
7.6
24
S. America
4.1
4.1
N. America
9.2
7.0
24
Europe
20.0
4.7
76
Oceania
6.8
5.9
13
World
8.3
5.8
30
Berner, K.A. and R.A. Berner. 1987. The Global Water Cycle: Geochemistry and Environment. Prentice-Hall, Inc. New Jersey, pp. 397.
The two primary effects of salt on soil productivity are governed by the total electrolyte content (EC) and the sodium content, relative to the other major cations. The latter is referred to as the sodium adsorption ration (SAR). It is defined as:
SAR = [Na+] / ( ½ ([Ca2+] + [Mg2+]) )-2 where the ion concentrations, [X], are expressed in millimoles per liter.
Problems in drainage occur when there is an excess of sodium, relative to calcium and magnesium ions. The latter divalent cations are most tightly bound to clays than monovalent sodium cations, and are they are preferentially adsorbed onto most clays, which generally have negative surface charges.
However, an excess of sodium ions will result in the displacement of the other adsorbed cations due to the law of mass action. This swelling will disperse the clays, and decrease the pore spaces between them. The resultant decreased pore size will decrease the soil permeability, and increase the potential for water logging. Consequently, the combination of a low EC and a high SAR creates the poorest soil conditions for agriculture.
This is, at least to me, counterintuitive. My original perception was that a little salt is bad and more salt is worse. The confounding factors of EC and SAR are illustrated by the case study of the contrasting suitabilities of water in Biloxi, MI and San Diego, CA for agricultural and domestic water uses (Tchobanoglous and Schroeder, 1987).
References
National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. National Academy Press, Washington, D.C. 516 pp.
Tchobanoglous, G. and E.D. Schroeder. 1987. Water QualityAddison-Wesley Publishing Company, Reading, MA, 768 pp.
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