| Very little is known about the connection between bacteria, groundwater geochemistry and water quality in karst terrains. The objective of this project was to determine if there was a correlation between bacteria types and water chemistry in two limestone bedrock springs draining an urban basin in Nashville, Tennessee. These springs, Tumbling Rock and Trough Springs are located in a karst terrain that is highly susceptible to contamination due to sinkholes and modified surface hydrology. The two springs were sampled every 1 to 4 weeks from June 2007 through March, 2009. The flow from the springs converged, and the combined discharge ranged from 150,000 gallons/day during a severe drought to 1 million g/d. An unexpected waste leak from a poultry-waste storage unit caused ammonia levels to rise to 40 mg/L for several weeks in the spring water. Additional water-quality parameters measured included temperature, specific conductance, dissolved oxygen, pH, sulfate, nitrogen, and bacteria Biological Activity Reaction Tests (BART). A continuous water-quality monitoring device was installed in Tumbling Rock spring to measure changes associated with different weather patterns. The pH values ranged from 6.5 to 8, temperature ranged from 16.5°C to 19°C, and specific conductance values were approximately 20% higher in the winter. There was a rapid rise in ammonia-oxidizing bacteria affiliated with the high ammonia levels, but both subsided after the leak was fixed. Additional analysis of bacteria types and geochemistry found a strong positive correlation between sulfur-related bacteria and sulfate concentrations (R-sq = 0.82). There was a moderate negative correlation between iron-oxidizing bacteria and dissolved iron concentration (R-sq = 0.25), which means when dissolved iron was low, iron-oxidizing bacteria were high. There was a moderate positive correlation between heterotrophic aerobic bacteria (HAB) and rain events, however it required a 10 day waiting period between rain and increased HAB (R-sq = 0.41). This 10 day delay probably reflects the time it takes for rain to enter the aquifer and stimulate bacteria growth with food. These data show there is a connection between geochemical conditions and bacteria populations in karst terrains... |
