KARST SPRING MONITORING USING ACOUSTIC DOPPLER VELOCITY DATA

Temporal hydrologic data such as discharge, specific conductance, and turbidity from springs have been measured to identify, analyze and interpret the characteristics of karst aquifers. Recent technological advances of acoustic Doppler velocity (ADV) meters now allow for collection of high-precision, continuous water velocity data applied to calculate discharge from springs and underwater caves. The U.S. Geological Survey has recently (2003-06) conducted several investigations in central Texas using ADV meters in Comal Springs (Edwards aquifer), San Marcos Springs (Edwards aquifer), Barton Springs (Edwards aquifer), and Jacob's Well spring (Trinity aquifer). ADV data from these springs reveal previously undocumented patterns in spring flow and enhance discharge monitoring during intense recharge events. These investigations have identified some hardware limitations of ADV meters.

ADV monitoring in Barton Springs began in April 2004, and data collected for a 2-year period reflect two important findings. (1) In June 2004, discharge for 10 days was calculated from empirical ADV data when the primary variable (stage) could not be measured because of extreme flooding. By applying the ADV data during this period, calculated discharge was about 25% more than a value estimated using previous methods. (2) Regular, repetitive diurnal fluctuations in base spring flow of about 10 percent became resolvable in the ADV data when discharge was below 45 cubic feet per second. These patterns might reflect daily pumping from high-capacity water wells, although no data have been collected to test this hypothesis. Data from an ADV meter installed at Jacob's Well spring in April 2005 show a correlation between discharge and the velocity measured in a cave passage 60 feet into the spring, which allows for spring-flow calculation during storm events. These data also indicate a limitation of ADV meters in exceptionally clear water with low velocities: Below 0.06 feet/second and acoustic signal-strength values of 48 decibels, measurements become erratic. In April 2006, a pilot study at Comal and San Marcos Springs began to test the application of ADV meters to improve the discharge records of these springs and evaluate aquifer dynamics from the numerous flow orifices associated with these springs.