HYDROTHINK
Conceptual Models Count
William J. Stone, MEM-2164
wstone04@gmail.com
Playas or salt pans, the flat, periodically flooded areas in the center of arid basins or bolsons, are common features of basin-and-range country. Soluble salts (for example, borax, trona, etc.) left by evaporating water may reach thick- nesses of economic significance (think Death Valley). Sound exploration for such deposits requires a sound conceptual hydrogeologic model.
The water that ponds in playas has one of two contrasting origins: surface water or ground water. Surface water is the most common source and originates as runoff from adjacent mountain slopes and streams. The amount of salt left by evaporation is minor because runoff is ephemeral and total- dissolved- solids content of runoff is not great. Alternatively, since the basin center is often the discharge area for the hydrologic system within the adjacent mountain range, the water in a playa may also come from ground-water surfacing there. The amount of salt left by evaporation in this case is more substantial because discharge is more constant and total-dissolved-solids content of the ground water is greater than that of runoff (greater contact time). Therefore, eco- nomic salt deposits are more likely to be associated with the ground-water-discharge origin.
A west-coast company exploring for economic salt deposits hit upon a playa in the Southwest where I was employed by the state geological survey. The company apparently assumed (hoped?) the ground-water-discharge model applied. After considerable drilling yielded nothing, they abandoned the project. But their disappointment was avoidable. We had already done considerable hydrogeologic work in the basin, including some drilling. However, they were so secretive that they never spoke to anyone in the state survey, which has historically been confidentially supportive of exploration activities.
I later included the playa as a stop on a field trip for my hydrogeology class. We walked to the middle of the playa where there was about one inch of standing water. I had the students hand auger down about one foot and then asked them if this was a runoff or ground-water playa. The answer was obvious: beyond an inch or so the clayey sediment was dry. No ground-water discharge here! Maybe the salt com- pany didn’t have a hand auger. Tip: Conceptual models are important. Be sure the one you’re going by is reasonable, based on readily available data.
Dr. Stone has more than 30 years of experience in hydro- science and is the author of numerous professional papers as well as the book, Hydrogeology in Practice – a Guide to Characterizing Ground-Water Systems (Prentice Hall). Feel free to argue or agree with him by email (wstone04@
gmail.com).
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