Groundwater Tracing in a Density-Stratified Aquifer using a SCUFA Submersible Fluorometer

Cave-diving scientists from the Exploration Research Institute (ERI) have conducted annual research projects to study complex groundwater flows in Mexico's Yucatan peninsula. Recently, ERI utilized the unique capabilities of the Turner Designs SCUFA submersible fluorometer to aid in this study.

Along the eastern Caribbean coast of the Yucatan Peninsula, nearly all freshwater discharges to the sea through extensive cave networks and springs rather than by surface streams. This karst region is underlain by an extensive network of cave passages that carry fresh groundwater from the peninsula's interior to springs along the coast. The fresh groundwater "floats" atop deeper saline groundwater. Within 10 km of the coast, the seaward-flowing freshwater and underlying saline water begin to mix. The mixing of fresh and saline waters creates a complex "mixing zone" of brackish water. The mixing zone is often sharply stratified by the formation of haloclines and thermoclines that segregate water masses of various densities.

In 2003, ERI scientists worked in conjunction with the Grupo de Exploration de Ox Bel Ha (GEO) to begin testing advanced instrumentation that can quantify aquifer processes. Two applications for the SCUFA Submersible Fluorometer were evaluated during this project: (i) quantification of the rate of groundwater flow through a cave conduit, and (ii) a dispersion study in a very slowly flowing density strata.

Quantifying Flow Rate in a Cave Conduit
ERI deployed the SCUFA in conjunction with an acoustic Doppler current profiler instrument. The current profiler was placed at numerous points along a network of cave passages to quantify flow velocities at individual points in the conduits. However, as conduit cross-sections may change significantly over any distance, the flow velocities recorded by the current profiler only represent isolated "point velocities" and may not be representative of average flow rates on a more regional scale. For this reason, the SCUFA was utilized to quantify flow rates over longer distances.

In the field ERI programmed the SCUFA with a laptop computer prior to each dive. Once set up, the SCUFA was handed over to a project diver for transport into the cave system. The divers worked in teams of three to measure flow rates with the SCUFA. The procedure included the following steps: (i) deployment of the SCUFA, (ii) measurement of the distance between the SCUFA and the upstream dye release point, (iii) release of a slug of Rhodamine WT dye, and (iv) recording the time of dye release using a watch that was synchronized to the SCUFA during programming.

After release, the Rhodamine WT slug was carried by groundwater flow to the SCUFA, where it was recorded as a break-through type curve. Later, the SCUFA was recovered and the data downloaded on the surface. Flow rates were calculated according to Equation 1.

Equation 1
Groundwater flow rate = D / (TD - TR )

Where D = distance
TD = time of dye detection at SCUFA
TR = time of dye release

Dispersion Test in Very Slowly Flowing Water
ERI divers noted that certain saline water masses in the Yucatan study area do not appear to have visible flow. In order to determine if this was true, a simple test was devised using the SCUFA® and a slug of Rhodamine WT dye.

The SCUFA was programmed to log at 10 second intervals and was then taken into the cave system. Divers swam into the cave system in a slightly flowing brackish strata above the seemingly stagnant saline water strata. When at the study location, a diver reached through a halocline to place the SCUFA on the cave conduit floor and into the saline water body. The diver then injected a slug of Rhodamine WT dye under the halocline and dive team exited the cave system. Thirty one hours later the SCUFA was retrieved. The divers noted that the dye slug was dispersed, but did not appear to have moved significantly. When the SCUFA data was downloaded and subsequently reviewed it was obvious that the dye slug had moved.

Based on the SCUFA data, ERI researchers concluded that (i) the saline water body flows, (ii) the flow is oscillatory with periods of near stagnation punctuated by relatively rapid flow, (iii) flow is related to tide stage, and (iv) the periods of rapid flow cause significant mixing within the saline water body.

(pictures taken by Steve Auer)