HYDROGEOLOGIC CONTROLS ON THE OCCURRENCE AND MOVEMENT OF GROUNDWATER DISCHARGED AT MAGIC SPRINGS IN SPRING BRANCH, TEXAS

This study defines hydrogeologic controls, flow velocities, flow paths, groundwater delineation, and physical parameters of groundwater in a network of karst conduits in the lower member of Glen Rose Limestone near Spring Branch, Texas. The known conduit (C My Shovel Cave) includes 4475 meters of measurable passage with a surface entrance 1360 meters upstream from Magic Springs.

The field component of this study included a karst density survey, four dye traces, and continuous monitoring of specific conductance, pressure, temperature, water-level stage height, and flow rates at Magic Springs and other sites in the subsurface watershed. The general karst density identifies caves and dolines less than 6 features/0.17 km² but two areas have densities of 20 and 44 features/0.17 km².

Hydrographs of specific conductance and flow rates show patterns interpreted as pulses recharging dilute water through exposed caves, sinkholes, and sinking streams. These pulses have minimal reaction with the rock or matrix during recharge which then is superposed on base flow from the network conduit system in this karst terrane.

Four storm events were measured for dynamics and water quality. The maximum flow rate was 70 m³/min (41 ft³/s) with base flow conditions at Magic Springs below 0.9 m³/s (3 ft³/s). The autosampler or a charcoal packet, used as dye tracing monitor, detected fluorescence from all four injection sites. The measured groundwater velocity from dye tracing ranges between 1865 and 2930 m/day. All dye trace tests were conducted under base-flow conditions with recharge occurring at the high karst density areas and the sinking stream. Eleven sites were monitored but dye only appeared at Magic Springs. Modification of the groundwater drainage was required because two traces crossed the surface-water divide and Spring Branch Creek is a sinking stream.