Water conservation measures are imposed when the Edwards Aquifer falls to a certain level. However, I've noticed that the aquifer's level can rise even though there has been no rainfall. How do you explain this?
To answer the question, we need to first understand what the "aquifer level" means.
The aquifer level is measured at the J-17 index well, which is located near the national cemetery at Fort Sam Houston in San Antonio. The level is actually an indication of the relative amount of pressure exerted upward in the index well from water higher up in the Edwards Limestone out to the west, similar to the rise of mercury in a barometer. The top of the J-17 index well has an elevation of 730.8 feet above mean sea level, so that a reported level of 640 feet means that the water is 90.8 feet below the surface. The elevation at the top of Edwards Limestone at J-17 is approximately 250 feet above mean sea level, so a reported level of 640 feet means that the water has risen at least 390 feet in the well from the top of Edwards Limestone.
Four factors control the relative height of water in the index well, and the interplay among them can cause the relative level of water in the J-17 index well to change up or down.
- As more water enters the aquifer from the west from recharge after a rainfall, the amount of pressure in the aquifer increases and the water level in the index well rises. Depending on where the recharge occurred, it may take up to several days after the rainfall event before this pressure increase results in a rise in the water level at the J-17 index well.
- When a lot of water is removed from the aquifer during periods of excessive pumping, the pressure the upstream (i.e., western) portions of the aquifer exerts on the index well diminishes. This decrease in pressure not only reflects the overall withdrawal of water (actual drawdown) but also a localized and transient pressure effect. The aquifer water level nearest the actively pumping wells is depressed, and it may take some time after pumping rates are reduced for the water table to recover to a stable level - one that is indicative of the overall water level in the aquifer. Many rises in the index well level occur on weekends, when the demand for water from the producing wells is reduced and the water table near the index well can recover.
- Because the aquifer is confined, barometric pressure influences the reported water level of the aquifer. High or low barometric pressure can change the water level of the aquifer by as much as half a foot.
- Earth tides, small changes in the Earth's gravity affected by the moon and planets, can also change the water level in the index well by as much as half a foot.
This month's Whizard is Dr. John Stamatakos, a senior research scientist in the Center for Nuclear Waste Regulatory Analyses (Div. 20).
The Lighter Side
February 27, 2014