HYDRODYNAMICS OF SMALL SPRINGS ALONG THE SOUTHERN EDGE OF THE EDWARDS PLATEAU, EDWARDS-TRINITY AQUIFER, BANDERA COUNTY, TEXAS

Numerous small springs discharge along the southern edge of the Edwards Plateau in Texas. These springs provide a locally important source of water and have been considered prime sources for bottled spring water. Springs issuing from alluvium / colluvium found in valley bottoms are common but may be more susceptible to surface water influence than bedrock springs. If surface water contributes to spring flow, it increases pollution risks to the spring and makes the water less desirable for some uses. Many alluvial / colluvial springs originate from bedrock aquifers then continue to flow through alluvium / colluvium before discharging. Locally spring management consists of extracting woody vegetative cover, spring-water quality tests for surface water influence, and spring development by removal of overlying Quaternary deposits. The springs in this area are more complex than originally conceived and lack of understanding may lead to erroneous management strategies. Fluctuations in flow patterns and chemistry helped distinguish bedrock from alluvial / colluvial springs, recharge zones and flow paths. Multi-parameter data loggers collected pH, temperature, dissolved oxygen, conductivity, and weir heights (discharge) for approximately one year. Fractures dominate the carbonate bedrock porosity, while Quaternary deposits are composed of porous media with irregular grain sizes. Springs behaved differently depending on the stratigraphic units within their flow system, the contribution of bedrock versus alluvium / colluvium to flow systems and the seasons in which monitoring took place. As an example, springs in the Devils River Formation responded almost immediately to precipitation events with increased flow and decreased conductivity. Results provide insight to spring complexity and baseline data that may be representative of the quantity and quality of springs along the southern edge of the Edwards Plateau.