Exploring Small Sinkholes
Jacobs Well Natural Area
Poison ivy growing inside a small sinkhole at Jacob’s Well Natural Area. Photo: Betsy Cross
Tom Jones
Karst is first and foremost an aquifer water system, and not just a landscape.
Karst is well known and often described as a landscape, a geological formation occurring in limestone and carbonate rocks. More importantly, it is an aquifer and often a remarkable groundwater reservoir. Karst consists of surface and underground voids resulting from the dissolution of limestone by water made acidic by carbon dioxide in the soil and air. The voids allow for easy flow and storage of groundwater. It is a process that transforms a region of simple limestone into a karst landscape, with “karst features” such as sinkholes, chasms, caves, and underground rivers. Karst is also an aquifer water system.
Jacob’s Well Natural Area (JWNA) is located in one of the most highly naturally fractured landscapes in Hays County. These faults lay the foundation for karst forces to dissolve and erode the limestone. They create connected passageways for water to enter and flow deep into the bedrock.
This fault trace discovered at JWNA is an example of how faults look when they intersect the land surface.
Wimberley Valley fault map illustrating the high concentration of faults in the Wimberley Valley (Hydrogeologic Atlas of the Hill Country Trinity Aquifer, July, 2010)
During a recent visit to JWNA, Betsy Cross discovered several small sinkholes in the northern section. She sent a photo asking if I wanted to see them. She already knew my answer, and we met the following weekend to explore the area.
Tree growing inside a small sinkhole. The tree is rooted onto a small soil-covered ledge adjacent to large solution channels in the limestone.
The rock surface is part of the uppermost zone of weathered limestone. Note the absence of soils.
Click to enlarge the photo of the tree growing inside the sinkhole void. It’s easy to see the solution channels and the tree trunk rooted on a small soil ledge. I have only seen something similar a few times. Typically, there is an absence of vegetation inside sinkholes. This is due to the lack of sunlight, the thin soil, and little or no water near the surface. Since rainfall easily flows into the solution channels, it quickly moves downward and deeper into the aquifer. This results in little or no retained water near the surface. For the tree roots to intercept the groundwater, they would need to extend down about 50 to 60 feet. That’s a long way for a small tree. So where is the tree getting its water? There are two likely pathways, which are illustrated by the schematics below.
The vadose zone is an important part of the water cycle and stores at least 8.5 times more water than is held by rivers (Oki and Kanae, 2006).
Schematic of a perched aquifer. Perched aquifers are limited in size and are situated above the primary aquifer.
One possibility is that the tree roots capture water from near the surface in the vadose zone. Vadose water is subsurface water between the land surface and the saturated groundwater zone below the water table. The vadose (or unsaturated) zone includes water which is immediately available to the biosphere. A second possibility is the presence of a “perched aquifer.” A perched aquifer is formed by water confined on the top of an impervious to semi-pervious geologic layer of limited extent. It is located between a large aquifer (Cow Creek Limestone) and the ground surface. The perched aquifer is separated from the primary aquifer by unsaturated material.
My opinion is that the water supplying the tree at JWNA comes from the vadose zone as rainfall enters the limestone. I also believe that small perched aquifers are present throughout the Hill Country as part of the complex water cycle of this area.
A visit to JWNA would not be complete without a visit to the famous well itself. In this summer of drought, the impact on the well is easy to see. In mid-July, 2022, the water flow from the well is diminished but still flowing at 224 gallons per minute.
Karst is well known and easily spotted in both urban and rural areas of the Hill Country. It transforms a simple limestone into karst landscapes. However, its most important role is as an aquifer water system.
Headwaters of Cypress Creek: Jacob’s Well with a discharge water flow estimated at 224 gallons per minute (gpm). This photo was taken on July 16th, 2022. For reference, one year earlier (7/16/21), JW flow was 2244 pm—10 times greater.
Update July 28, 2022
JACOB’S WELL SPRING - ZERO FLOW
Jacob’s Well with near zero flow. There is a very small volume of water being discharged from the spring, but for the fourth time in recorded history, there is essentially zero flow. The extended drought is impacting the aquifers throughout the Texas Hill Country.