Pogue Springs Dye Trace Report, Western Travis County, Texas

This technical report details a dye trace study conducted in and around Milton Reimers Ranch Park, western Travis County, Texas, initiated in April 2024. The study’s primary objective was to refine the existing conceptual hydrogeologic model of the Middle Trinity Aquifer, specifically within the Pogue Creek Watershed. This effort is crucial for developing regulatory and conservation strategies to preserve the diminishing groundwater levels and springflow in the region, which are threatened by persistent drought and excessive groundwater pumping. We injected three distinct fluorescent dyes—fluorescein, rhodamine WT, and eosine—at three sites within the Pogue Creek watershed: a natural recharge feature (swallet) and two wells. Dye monitoring occurred at 12 spring, stream, and well locations across four watersheds, primarily using activated charcoal detectors and, in some cases, field fluorimeters. The study aimed to delineate local springsheds by testing if groundwater from the Pogue Creek Watershed flows exclusively to Pogue Spring or also to springs in neighboring watersheds. Results indicate successful detection of only one dye, eosine, injected into the Reimers Park Abandoned Windmill Well. This dye was detected at Pogue Spring, located approximately 594 feet away, between 116 and 156 days post-injection, and has persisted for at least 150 days. Therefore, the estimated average groundwater velocity between the injection well and Pogue Spring is 4.5 ft/day. Dye was only detected via cumulative absorption on charcoal samples, as concentrations in water samples remained below the method detection limit. Field fluorimeters did not detect dye, showing false positives correlated with rainfall and likely sources of natural fluorescence. The observed groundwater velocity, while slower than typical conduit flow in karst systems, is faster than estimated matrix flow alone through limestone, suggesting components of conduit and fracture flow in addition to matrix flow within the Cow Creek Limestone. This study validates the existing conceptual model of a karstic Upper Cow Creek Limestone and highlights the hydrologic heterogeneity between the Upper and Lower Cow Creek units.