Mobile sediments in a karst aquifer
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In karst aquifers, mobile sediments may play a fundamental role in determining water quality by concentrating and transporting contaminants. This study investigates spatial characteristics of sediment in a karst aquifer, examines temporal characteristics of sediment discharging from a karst spring, and describes a new method for tracing sediment in karst terrane. Sediment samples were collected from different compartments of a karst aquifer (the Edwards Aquifer, Central Texas) and analyzed for mineralogy, grainsize distribution, organic carbon content, and specific surface area. Suspended sediment samples discharging from a karst spring in response to two storms were analyzed for mineralogy and grainsize distribution. To test the hypothesis that allochthonous sediment can move through karst systems, a sediment tracer--montmorillonite clay homoionized to the lanthanide form--was developed. Statistical analysis of sediment characteristics separated the sampling sites into three groups: 1) streambeds, sinkholes, and small springs; 2) wells; and 3) caves. Characteristics of sediments from Barton Springs (the main spring in the region) showed a mixture of the characteristics of these three groups. The mineralogic signature of sediments discharging from Barton Springs in response to storms was initially allochthonous and then became more autochthonous. Temporal changes in aqueous chemistry of Barton Springs varied in response to rainfall, seasonal changes in aquifer level, and draining of the pool over the spring. The results indicate that some sediments, containing calcite and a high organic carbon content, are allochthonous, and others, containing dolomite and a low organic carbon content, are autochthonous. Sediments issuing from Barton Springs contain a mix of both allochthonous and autochthonous sediments. Based on geochemical characteristics, allochthonous sediments have a greater potential to sorb and transport contaminants than do autochthonous sediments. Development and testing of the sediment tracer verified that allochthonous sediments can be transported through karst. Sediments, particularly allochthonous sediments, can play a crucial role in contaminant transport in karst; the volume of these sediments will increase with increasing urbanization. Because those sediment characteristics which determine contaminant transport potential vary both spatially and temporally, determination of sediment volume and type is critical to evaluating its impact on water quality.