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The J-17 Index Well Recent J-17 index well readings are available from the San Antonio Water System.
The figure 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 rises 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. 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 650 to the top of the Edwards limestone formation.
The graph below illustrates the relationship between J-17 levels and springflows. There is a good relationship between the level of the J-17 well and flows at Comal Springs. What happened in July of 1983 is typical - when the J-17 level increased by about six feet, there was an obvious increase in 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 (see Flowpath Map). This is why the relationship between the J-17 well and San Marcos Springs is not as pronounced. We should note that on the graphs below, the y-axes do not start at zero, so the reader should be aware the areas under the curves are exaggerated and are not proportional to actual changes in the numbers which the curves represent. In other words, these graphs are a lie, because a truthful chart will always have axes that starts at zero. People use the axis trick all the time to make their graphs tell the story they want. It is sometimes useful to look at data that way, but it is always a lie. The only way to mitigate the lie is to disclose that you have done it, that's why I'm telling you this.
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 an elevation of 618 feet. During the '50s drought, the Springs were dry from June to November of 1956. In a repeat of the 1950's drought, Comal Springs would be dry for a number of years. History does not record a time when the San Marcos Springs have ceased to flow. The lowest recorded flow rate was 46 cubic feet per second in August of 1956. San Marcos Springs would cease to flow with a water elevation of about 574 feet at the Springs. The chart below shows average monthly J-17 levels since January 1933. Droughts in 1966, 1970, 1984, 1988, 1996, 2006, and 2009 are apparent in J-17 levels. It's a little hard to see here, but in many years the J-17 level recovers quickly around October....these rapid rises correspond to declines in use at the end of summer and soaking rainfalls associated with tropical storms and hurricanes. In Texas, climatologists use October as the beginning of the "water year", a division based on a general pattern of annual wet and dry periods rather than a calendar year.
The Good Friday Earthquake at J-17
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