Temporal variability of tryptophan-like fluorescence as a surrogate for microbial contamination and its association with flow components of karst spring discharge.

This study investigated the relationship between temporal variability of microbial contamination at a karst spring and the different flow components of a karst groundwater system in western Ireland. A high frequency tryptophan like fluorescence (TLF) concentration time series was collected over 12 months as a surrogate indicator of microbial contamination. The study evaluated whether temporal variations in TLF were associated with the dominant spring flow components and assessed the relative influence of internal aquifer characteristics and rainfall intensity. Flow duration curves, master recession curves, Fast Fourier Transform spectral analysis, and continuous and discrete wavelet analyses were applied. Results characterize the spring as a triple-porosity karst system comprising fast conduit, intermediate fracture, and slow diffuse matrix flow domains. TLF variability was weakly associated with slow/baseflow but more strongly associated with intermediate and fast flow components. TLF was also strongly correlated with E. coli abundance (Pearson r = 0.703; Spearman r = 0.806; Kendall r = 0.644; p < 0.05), supporting its use as a surrogate indicator of microbial contamination. TLF temporal patterns were associated with the aquifer's internal flow architecture, although rainfall intensity or recharge alone did not fully explain the observed variability. Collectively, these findings suggest that TLF variability, and by extension microbial contamination signatures, is more closely associated with shorter timescale groundwater flow responses and internal flow partitioning than with rainfall forcing alone, highlighting the value of flow component-based monitoring for managing contamination risk in karst groundwater systems.