Water occupies a dual role in our industrial society. It is the indispensable raw material, essential to life itself. It is equally an industrial nuisance, interfering with the production of industrial minerals, making construction difficult, flooding valuable land, and generally producing problems because it is in the wrong place at the critical time.
Gravel or Water
The valley of the Ohio River is underlain by deposits of gravel, sand, silt and clay. Generally, coarse, clean, sands and gravels underlie the silts and clays. The river itself is controlled by a series of navigation dams and the channel is dredged occasionally as the needs of navigation are recognized and met.
In many places, along the bank of the river, well fields have been constructed. The water derived from such well fields is usually water that originated in the Ohio River and passed through the underlying sands and gravels until it reached the well, in the process, the water has been filtered and naturally purified/ and is bacteriologically safe without further treatment. The result is a low-cost water supply of excellent quality and computable quantity.
The well fields so established do not operate at uniform offtake rates the year around. From time to time, during periods of low flow, silt settles to the bottom of the river, reducing the permeability of the natural filter bed and, consequently, well yields. Periodically, as floods occur, the bottom is scoured, the silt goes into suspension, and well yields increase. Effectively, therefore, a greater or lesser area of river bed is utilized by the well field as a filter, depending upon the permeability of the river bottom.
The mining process consists first of dredging up the bottom materials. Then the sand and gravel is separated from the silts and clays by washing and the washed material is loaded into barges. From the standpoint of ground-water development the most significant phase of the mining process is the dumping of the fine material back into the river. It should be noted that in many reaches of the Ohio River the original materials have been removed to within a foot or two of bedrock. The continuous trench, dredged parallel to the river bank for its sand and gravel, then fills with silt and fine materials which tend to settle and become compacted and impermeable. These fine materials then act as a cut-off wall, inhibiting the movement of ground water.
Water or Limestone
Turning to another example (and this could occur in Nebraska, Iowa, Illinois or Wisconsin) in any area overlain by thick deposits of clay, there is always a great need for good aggregate for construction purposes. Similarly, there is usually a need for ground water. In one instance we find a valley in which a stream had deposited a layer of sand and gravel some ten feet thick on a shattered shale bedrock. This, in turn is underlain at an additional depth of some 30 feet by a fractured limestone. The sand and gravel is saturated with water and the farms in the area operate shallow wells to provide for domestic needs.
In this instance the road program and nearby industrial expansion created a need for aggregate. Whereupon a group of men organized a company and bought several hundred acres of land in the valley to be used as a limestone quarry. The first problem was, of course, to dewater the proposed site. Pumps were installed in holes drilled for the purpose. As pumping progressed, farm wells in the vicinity, even those ending in limestone, began to go dry.
Minerals or Water
In the development of our water resources, particularly in the West, it becomes necessary to store vast quantities of water in order to increase the low flows for irrigation purposes and to mitigate floods. The reservoirs are often large and expensive and would be uneconomic except for the production of hydroelectric power and flood control benefits. Even with such benefits, the cost of many of these reservoirs and the associated facilities closely approaches potential, benefits from all sources including irrigation, and these are questionable even when computed over a 40-year, 60-year or even a 100-year period. Moreover many of these reservoirs have been built in arid, geologically young areas, where drainage basins are subject to erosion. As the reservoirs usually “carry-over” water from one year to the next, they have a tendency to trap most of the sediments that have been eroded in the drainage basin that are being transported by the river.
These reflections on the inter-relationship of natural resources and the problems involved in their most efficient development, bring forward the inevitable question that confronts every engineer: “What do we do now?” Before discussing general principles, there are specific solutions to be proposed for the problems posed by my examples.
The tentative solutions proposed are based upon achieving the minimum waste and maximum, long-term utilization of our natural resources. To take them in order:
- For the dredging of.sand and gravel as opposed to the utilization of these sands and gravels as natural filter plants in this instance the value or utility of a limited quantity of raw material must be compared to the endless utility of the same material as a natural filter plant.
- In the controversy between pumping to waste to enable the mining of limestone as opposed to the pumping of water for drinking purposes:
In the Western states, where the doctrine of appropriation is the applicable water law, it is entirely possible that the economics of the situation would govern.
In states not operating under appropriation doctrine the situation is not at all clear. In some of these states the right to pump water for any purpose is limited to a short period of years (after which it must be re-applied for without assurance of renewal) and cannot be transferred or sold.
Inasmuch as the present mining laws would seem to encourage the waste (by disuse) of our mineral resources it is not surprising that, as a by-product, our topographic resources are being developed on a false basis, economically speaking. The present procedure of arbitrarily submerging our potential mineral resources by an accumulation of water and sediment, without first trying to survey and evaluate them, is not really satisfactory.