where art thou, elbow spring? (updated)

Back in 1973, none other than Gunnar Brune, the King of Springs in Texas added a spring at Beverley Northwest Park in Austin, Texas, into the Texas Water Development Board’s water well database (on my birthday no less!). The spring flowed at about 10 gallons per minute, had a temperature of 70 degrees Fahrenheit, and earned the state well number of 58-43-105. Gunnar took a water sample which came back with a total dissolved solids of 420 parts per million.

The Texas Water Development Board places the spring along the dike that separates the park from Shoal Creek:

However, Gunnar’s field notes place the spring up against a cliff to the south:

It’s not unusual for water wells in the state database to not be exactly where they are supposed to be. These wells were surveyed back in the day when we didn’t have GPS (or, later, accurate GPS). So hydrogeologists back then plotted the location on a hard-copy topographic sheet back in the office and then, using rulers, estimated the coordinates in latitude and longitude.

Additional information that supports Brune’s location as shown in his field notes is a survey of a cave in this same area named Elbow Cave (hence me naming this spring Elbow).

The top image shows the cave relative to the properties behind the cave, namely 2712 and 2716 Pegram (and you can see the plan view of the cave and why they named it Elbow!). The area to the left of the entrance with the overhang (and the associated cross section) corresponds to this photograph I took of the area last weekend while picking up litter in the area:

And here is the locale stepping back from from the outcrop:

I took the image above this image from the area right behind that brush pile looking toward the right side of the image. The solid fence in at the top of the hill marks the property at 2716 Pegram.

I could see piping into the subsurface into darkness as well as evidence of past discharge from this area, so I am fairly confident this is where Elbow Cave is; however, the entrance is now blocked by debris. This places the cave here (see red dot):

Which pretty much sits right where Brune placed his location for the spring. It makes sense that this is where the spring would be since the cave is direct evidence of preferential flow that, over time, dissolved the limestone into the cave and its associated conduits. Therefore, the actual location of the spring should be 30°20’52.8″N 97°44’31.0″W.

This change in location might seem trivial, but it has become somewhat important as the neighborhood has engaged with the city on the fate of the pond you can see on the map above. Not shown on the map is a swimming pool to the east-northeast of the pond. The city is currently refurbishing the pool and, during renovations, found and fixed leaks in the pipes used to top off the pool. After those leaks were fixed, the pond began to dry up. One thought is to tap into Elbow Springs and direct the flow into the pond. Because the current location is from a state database and reported by a big-name city consultant, the city is reluctant to believe neighborhood riffraff that the spring in the state database is mislocated.

Sadly, there is no evidence of the spring today. One hypothesis is that springflow was destroyed or redirected when the city buried a large drainage pipe to direct runoff from Burnet Road into Shoal Creek. However, I don’t think the construction interfered with the spring at all. The cave (and thus spring) are located at the bottom of the Austin Chalk where the chalk is in contact with the underlying Eagle Ford Shale. If you look at the previous photo, you can clearly see where the chalk ends (all that competent limestone) and the shale begins (all that dirt below the bottom of that limestone). The construction is at a lower level completely in the Eagle Ford Shale. These field observations agree with the geologic map that shows the chalk ending at the edge of the old creek with the shale below. This can be seen all around the edges of the park, which started off as a gravel (?) quarry:

The vivid green is Austin Chalk, the light brown up against it is Eagle Ford Shale, and the yellow is alluvium. In short, the cave (and thus spring) is at too high of an elevation to have been interfered with. Furthermore, based on the cave map, it appears the rock formations outside and around the cave entrance remain untouched.

I am also thinking that the springs weren’t all that as indicated in Gunnar’s survey. The folks that surveyed Elbow Cave in 1964 did not find the cave discharging any water. They found water in the cave (the lines that hash completely across the cave indicate water; the numbers in the boxes indicate depth of water, ranging from 3 to 6 inches) and surmised (probably based on smell) that the water was sourced from irrigation or sewage. The cave map uses a number of symbols (and numbers!) based on cave mapping symbology. The circles with numbers are the number of feet of the air passage. Where the cave shows hashed lines clear across the cave bottom is where the cavers found standing water they surmised was leakage (sewage?) from the property above. The number with a bar above it indicates the depth below datum (?). A number in a square indicates the drop of the floor. I’m not sure what the naked numbers mean. (any cavers out there?)

Gunnar and the agency only measured this spring once, so we don’t know what the variation of flow might be. The agency has the water pegged as from the Edwards Aquifer, but I’m not convinced. This area is east of a major fault in the Balcones Fault Zone, and water levels along the fault zone in this area are nowhere near the land surface. It makes sense that the flow would be more local since the local geology is limestone sitting on top of shale/clay.

In the 1. 3, 6, and 12 months before the cavers surveyed the cave on February 2, 1964, there had been 2.58, 6.15, 9.47, and 20.03 inches of rainfall (at Camp Mabry nearby), respectively. In the 1, 3, 6, and 12 months before Gunnar surveyed the spring on February 16, 1973, there had been 1.93, 5.33, 13.22, and 28.31 inches of rainfall (at Camp Mabry nearby), respectively. So it had been rainier for over the previous three months for the cavers but rainier for Gunnar over the previous 6 to 12 months. Not sure what that says (but I said it).

Given the sewage connection (and the prevalence of leaking water and wastewater discharge in urban settings), rainfall may not matter as much as how the infrastructure is responding, which is difficult to assess.

update!

The good folks at the Texas Water Development Board have updated the location, added a name to the spring (Elbow Spring), and updated some other info on the database entry. Yay!

A caver bud noted that cave-mapping symbology wasn’t standardized until 1979 (and this cave was mapped in 1964). He thought that the numbers without boxes represented slope changes. That makes sense since there isn’t much headroom from the cave entrance to the land surface in this area. Ultimately, someone needs to go back in to see what the different numbers on that map might mean (unless the Texas cavers have a historical Rosetta Dripstone?).