Salinization or Depletion First? Two Clocks Compared

The aquifers of the Middle East are exploited faster than they are replenished, and the groundwater they hold is becoming more saline. Both statements are supported by a large literature. What that literature does not say is which of the two failures arrives first for a given aquifer, and the omission is not academic: a monitoring network with a salinity trigger protects nothing in an aquifer whose water will be pumped dry while it is still fresh, and a volumetric cap set on recharge protects nothing in a coastal aquifer whose fresh lens will be lost long before its volume is spent. The two literatures are separate, and the instruments documented in them are each written for one failure mode.

Geologic® put the question to the published record alone. For every system whose two records the method could use, the time to loss of the usable class and the time to volumetric exhaustion were computed on one method and compared draw by draw, with every value graded by how it was obtained and the sources' disagreements carried into the result as uncertainty. The expected answer, written down in advance together with the rule for rejecting it, was that usability is lost first in most systems. The rule rejected it: across nine systems, usability is lost first in three, the volume in three, and three are indeterminate. The clearest case is the Quaternary aquifer of the United Arab Emirates, where the volume of groundwater below 1,500 mg/L fell from 238 km³ in 1969 to 10 km³ in 2015, giving 2.7 years to loss of the fresh class against 430 for the total storage. The practical conclusion is that governance should match whichever clock runs faster in that aquifer, which cannot be known until both are computed.