Hydrogeologic controls on the occurrence and movement of groundwater discharged at Magic Springs in the Spring Branch Creek drainage basin: Spring Branch, Texas




Childre, Mark Tilman

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The hydrogeologic controls, flow velocities and paths, groundwater delineation, and physical characteristics in a joint controlled dendritic conduit-spring system have been characterized. The known conduit branches from C My Shovel (CM) Cave with 4475 meters (m) of measurable passages and tributaries. Surface entrance to CM Cave is located 1360 m upstream from discharge at Magic Springs.

Four storm events were measured characterizing the dynamics. The maximum discharge of these four events was 1.2 m3/s (41 ft3/s) with 0.08 m3/s (3 ft3/s) baseflow conditions at Magic Springs. The characteristic shape and response of discharge are well defined with a rise time between 5.5 and 6.5 hours (hr). The half flow period time (t0.5) ranges between 12.9 and 15.7 hr, depending on peak discharge. The rise time and t0.5 occur in less than one day and the conduit volume exceeds 0.5 x 106 m3. The conduit-spring system drains within 3.7 to 7.5 days after the storm event. The thermal effects are event driven, maintaining 85% of the temperature change over 1300 m. The spring discharge has total dissolved solids around 350 mg/L and is chemically stable.

The field component of this study include a karst density survey, four dye traces, and continuous monitoring of specific conductance, pressure, temperature, water-level stage height, and discharge at Magic Springs and in the conduit below CM Cave. The general karst density survey identifies caves and dolines within given area. There is a sinking stream that transfers flow from Spring Branch Creek into the conduit system and two focused regions in a karst plain having densities of 20 and 44 karst features/0.16 km2.

Hydrographs and chemographs show patterns interpreted as pulses of dilute water recharging through exposed caves, sinkholes, and sinking streams. These pulses have minimal reaction with the rock or matrix during recharge, which is superimposed on baseflow from the joint controlled dendritic conduit-spring system in this karst terrane.

The groundwater drainage basin has been defined. The dye tracing results identified groundwater piracy across surface water divides and helped define the groundwater drainage basin. Groundwater velocities were measured between 1800 m/d and 3000 m/d under baseflow conditions. The discharge at Magic Springs under these four storm events showed velocities between 8,700 and 15,120 m/d.

An autosampler and charcoal packets were both employed during dye tracing. Both detected fluorescence from all four injection sites. The measured velocities ranged between 1865 up to 2929 m/d under baseflow conditions. All dye trace tests were conducted under baseflow. Under baseflow conditions, dye was only traced to the Magic Springs locations from the eleven charcoal monitoring locations.


The author has granted permission for their work to be available to the general public.


caves, groundwater, karst hydrogeology, recharge, springs, Trinity Aquifer