EDWARDS'S J-17 EarthCache
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Your link to the Edwards Aquifer! Listed Coordinates are for Parking.
Read the following Earth Science about the Edwards Aquifer and the J-17 index well, and answer the following questions: Click on the two images below to get more information about the J-17.
1. When you are standing at the coordinates below, how far above the Aquifer are you?
2. What is the elevation above sea level where you are standing?
3. If the J-17 index well says the aquifer is at 700 feet, how far below you is the water level in the well?
4. There is a large building just East of the well building. There is a four digit building number on this building, what is it?
If the correct answers are not emailed to me I will regretfully delete your log, after notifying you. You are welcome to email me the answers before you log the Earthcache.
Earth Science:
I submitted this Earth Cache because of the importance of the geology of the Edwards Aquifer to the people, plants and animals of San Antonio, Bexar county, and surrounding counties. Without the Edwards Aquifer and the water it holds, there would not be much water for any of us to use. The J-17 well is a direct link to the aquifer and how it is doing so it is a vital tool for the aquifer. We are all stewards of the aquifer, and we all need to do our part to conserve it’s resources.
The J-17 index well is located in the small building at the base of the large water tower near the national cemetery at Fort Sam Houston in San Antonio, and is at 29 28.746 N, 98 25.952 W. It is on a major Edwards flow path and responds quickly to pump-age and recharge, so it has been used since 1956 to record changes in the level of the Aquifer in the San Antonio area. The level of the J-17 well has ranged from 612 feet during the 1950's drought to 703 feet after historic rains in 1991 and 1992. There is much confusion about what the reported Aquifer level means. When weather casters say the Aquifer stands at 650 feet, it does NOT mean there is 650 feet of water left or that it is 650 feet to the top of the Edwards formation. The number is simply an indication of relative pressure being exerted on water at the location of the test well.
The J-17 Edwards cross section image below illustrates this concept...the Edwards formation is between 300 and 700 feet thick, so it is about as thick as the Tower of the Americas is tall. Out to the west in the recharge zone, the Edwards outcrop at the land surface is higher than the top of the Tower. Water tends to flow downhill, and it so happens that "downhill" is directly under most of San Antonio. Water is heavy stuff, and as new water enters the formation in the recharge zone, it places tremendous pressure on water already deep inside, forcing water up through cracks and wells toward the land surface. So water rises in the test well because of pressure being exerted by water higher up in the Edwards formation out to the west. It does not rise all the way to the elevation of water to the west because of friction. When water does rise all the way to the top of a well in this manner, the well is called artesian and water flows out without pumping. A good index well such as J-17 is one in which pressure is never sufficient to cause the well to become artesian. To get water out of J-17, it would have to be pumped.
The land surface at the top of the J-17 well is at 730.8 feet above sea level, and the downtown area around the Tower of the Americas is around 650 feet above sea level. A reported Aquifer level of 650 feet, for example, would indicate the top of water in the well is about even with the bottom of the Tower. A J-17 reading of 650 also means that some water use restrictions will be implemented. The water in the well is still 80.8 feet below the land surface, so to extract water from the well it would have to be pumped that distance. It is still another 400 feet from the well to the top of the Edwards limestone formation.
The J-17 spring flow image below illustrates the relationship between J-17 levels and springflows from the Edwards Aquifer. There is a good relationship between the level of the J-17 well and flows at Comal Springs. Most of the water that becomes Comal springflow originates with recharge far to the west of the Springs and moves past the J-17 well on its way toward New Braunfels. In contrast, much of the water discharging at San Marcos Springs originates from recharge in the vicinity of the Springs and does not move past the J-17 well. This is why the relationship between the J-17 well and San Marcos Springs is not as pronounced. Flows at Comal Springs become intermittent when the level of the J-17 monitoring well drops below 620 feet. All flow at Comal ceases at a J-17 well reading of 618 feet.
Additional Hints
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