Excerpts from "The Salinity of Rivers"

by Arthur F. Pillsbury

Scientific American, July 1981, 55-65.


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"...Before man began harnessing the rivers, the seasonal floods were highly effective in carrying salts to the ocean and keeping the river basin in reasonably good salt balance. Today, with river flows being regulated by storage systems, and with high consumptive use of the released water, there is not enough waste flow to achieve anything approaching balance. The salt is being stored, in one way or another, within the river basin."

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"Not only are salts getting bogged down somewhere in the system, but also various measures are being taken that deliberately impede the flow of salts to the sea. In the U.S. it is the law of the land, reflecting the demands of both environmentalists and water users, that rivers remain, if not forever, "wild and scenic," at least fresh for their entire length. The measures being planned and effectuated to accomplish this ideal are dangerous for the future. The general concept is to divert saline flows, where they can be found, into evaporation basins. There water will evaporate from the surface, leaving behind layers on layers of crystalline salts. It is proposed that the evaporation basins be situated either where the underlying ground is already saline or where the soil is relatively nonporous. Where neither is the case, the ponds are to be lined with a relatively impervious material. Such schemes, designed to store the salts in the river basins themselves, may work for a few years or decades, but are bound to be disastrous in the long run.

Why? The schemes will fail for any of several reasons. Although the ground waters under the evaporation basin may well be brackish or saline, every groundwater basin with a flow gradient must have an outlet somewhere near its lower end. The saline water in the evaporation basin will serve to increase the "head," or hydraulic pressure, on the saline waters below, and will thereby increase the rate of discharge at the natural outlet, wreaking havoc in downstream ground waters and downstream lands. If the evaporation basin is situated above soil known to be impermeable to fresh water, it will be found that the soil will gradually become more permeable when the waters are saline. This fact is well established. Many types of materials have been proposed for making evaporation basins impervious: rubber and plastic sheeting, asphaltic mixtures, and special types of concrete. Conceivably some linings will be effective for as long as 50 years, but ultimately one must expect them to fail. In all probability their lifetime when they are exposed to saline water will be much shorter than their lifetime when they are exposed to fresh water, for which they are normally intended."

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"Before the advent of intensive irrigation, the ground waters of the western [U.S.] valleys and basins were almost uniformly of high quality. The underground aquifers were largely recharged at the upper end of the valleys, where the rivers disgorged onto the valley lands. The ground waters subsequently discharged into the basin lands and for the most part into the rivers themselves in the form of diffused flow. When farmers began pumping irrigation waters from the aquifers, the net effect was to lower the water table downstream from the aquifer, thereby reducing the discharge flow back into the rivers. Eventually the water table would fall so low that there was no discharge at all [i.e., zero baseflow] and the aquifers became closed basins. Salts would then no longer be passed along downstream and simply accumulated."

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"The only effective way to keep ground water basins fresh is to pump from wells near the lower end of each basin, where the salinity is highest, and to hurry [discharge] the effluent on its way to the ocean or some other sink. At the same time it will probably be essential to augment the recharge near the upper end of each basin. Unless these steps are taken one can foresee the day when the aquifers will be destroyed by salinity."

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"The custom traditionally followed in the U.S. in developing water resources has been to expect the river itself to carry supplies of fresh water to points of diversion almost down to the river's mouth. Such a design is generally the cheapest and has the advantage of capturing floodwaters that upstream storage would miss. This ignores the basic principle, essential for the long term, of going upstream for supply and allowing the lower [reaches of] rivers to become brackish."

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"...it is the only concept that will enable the lower reaches of western rivers to achieve the salt balance necessary for the long-term health of western agriculture, on which the entire U.S., and indeed the world, has much dependence. Unless the lower rivers are allowed to reassert their natural function as exporters of salt to the ocean, today's productive lands will eventually become salt-encrusted and barren."

Lessons to be learned:
  1. Exorheic drainage systems effectively deliver salts to the oceans.

  2. A certain amount of waste runoff is needed to carry salts to the ocean.

  3. The objective of waste runoff is to allow the basin to reach a good salt balance.

  4. Unlike exorheic systems, endorheic drainage systems accumulate salts without limit.

  5. Engineered endorheic drainage systems (i.e., evaporation ponds) are unsustainable, because they cause the number and size of wastelands to grow without limit.

  6. Ground water is likely to become more saline in the presence of endorheic drainage systems.

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