158-9 Microbiological and geochemical assessment of water in a karst dominated system: South Central Texas

Session: Redox-Driven Nutrient and Contaminant Dynamics in Terrestrial Systems

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This study examines the biogeochemical and microbial interactions between groundwater and surface water in South-Central Texas' Edwards Aquifer system, with a focus on the relationships between water quality, metal chemistry, and microbial contamination across different hydrogeological environments. This study uses a multidisciplinary approach that incorporates chemical, physical, and microbiological data to elucidate the mechanisms and pathways governing contaminant fate and transport in a karst landscape characterized by rapid recharge, heterogeneous flow, and complex connectivity between surface water and groundwater. This study's core focus has been the comparative investigation of water samples from both riverine and well habitats, with representative locations being the Sabinal River, Nueces River, FRP House, FRP Western, and SARA Mud. The region's rivers and wells show clear and persistent disparities in water quality, which reflect their hydrological context, dominating recharge methods, and exposure to surface-derived contaminants. The findings demonstrate that river sites—directly connected to the land surface—are subject to pronounced temporal fluctuations in chemistry and microbiology, whereas groundwater wells exhibit more stable, buffered characteristics influenced by longer residence times and greater water-rock interaction. River locations had consistently higher and more variable E. coli and total coliform contents than groundwater wells. Elevated bacteria count in the Sabinal and Nueces rivers frequently surpassed recommended health levels for recreational and drinking water, particularly after storms. These spikes were strongly associated with rainfall, runoff, and non-point source pollution from agricultural, urban development, and malfunctioning septic systems. Groundwater samples from FRP House, FRP Western, and SARA Mud, on the other hand, had either undetectable or consistently low levels of indicator bacteria, supporting the significance of subsurface filtering and attenuation in mitigating microbial hazards. The findings underscore rivers' vulnerability to episodic contamination, particularly in karst terrains with little soil cover and direct hydraulic connectivity between the land surface and stream channels. An important takeaway from this work is that bacteria and metals are not independent pollutants in aquatic ecosystems and elevated concentrations of some metals (such as lead or arsenic) may exert selective pressures on microbial communities, suppressing some populations while allowing the establishment of metal-tolerant or even metal-transforming species. This interaction generates feedback loops that might increase pollutant mobility during recharge episodes, particularly when rapid transit disrupts the natural attenuation processes in the subsurface.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9491

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