The impact of on-site domestic wastewater effluent on karst springs: An evaluation of contamination fingerprinting and tracing techniques

Abstract

Groundwater from karst aquifers is a major source of drinking water worldwide, but these complex aquifer systems are exceptionally vulnerable to pollution and may be impacted by multiple contamination sources. In rural and suburban areas, human wastewater effluent (from on-site domestic wastewater treatment systems - DWTSs) and agricultural sources are mostly considered among the most significant threats to groundwater quality. The potential impacts of domestic wastewater effluent are microbial and chemical pollution of aquifers. This study examined flow cytometric (FCM) fingerprinting of bacterial cells in groundwater together with faecal indicator bacteria (FIB) at nine separate karst springs of varying catchment size over a 14 month period in order to assess whether such a technique can provide faster and more descriptive information about microbial pollution and the dynamics of karst aquifer systems. Moreover, the data has also been evaluated with respect to the potential of using turbidity as an easy-to-measure proxy indicator of microbial pollution in a novel way. It seems that FCM can provide additional information from which enhanced insights into faecal pollution sources and its fate and transport in such karst catchments can be gained. This study provides valuable new information on the potential and limitations of turbidity as an indicator of faecal groundwater contamination in karst. Additionally, this study has evaluated whether a historically used microbial tracer (yeast) which is readily available, inexpensive and environmentally friendly, but usually overlooked in modern karst hydrogeological studies due to challenges associated with its detection and quantification in the past, can reemerge as a valuable tracer using the latest technology for its detection and quantification in karst systems. Two field-based studies on separate karst systems were carried out in low-flow conditions using portable particle counters as well as flow cytometry to monitor the recovery of the yeast at the springs. Soluble fluorescent dyes were also injected as tracers simultaneously with yeast particles for comparison of transport dynamics. On one tracer test, through a karst conduit of much higher velocities the injected yeast particles and fluorescent dye arrived at the same time at karst spring, in comparison to the tracer test on conduit system with lower groundwater velocities in which the yeast particles were detected before the dye at the sampling site. Portable particle counter and flow cytometry successfully detected yeast tracers during both tracing studies. Despite the fact that no significant advantages of flow cytometry over the portable counter system can be reported on the basis of the presented results, this study has shown that flow cytometry can be successfully used to detect and quantify introduced microbial tracers in karst environments with extremely high precision. Importantly, in this study, a range of chemical contamination fingerprinting techniques were evaluated in terms of their ability to determine human wastewater pollution impacts on karst aquifers. The results suggest how fluorescent whitening compounds, microplastic particles and faecal sterol and stanols can be used together to cover different detectability chances and provide additional information about DWTSs pollution impacts on karst springs. This study also provides an important benchmark for microplastic contamination in low-lying karst aquifer systems. Moreover, for the first time in the scientific literature, a link between changes in fluorescent whitening compounds signals and changes in concentrations of microplastic particles in karst groundwater has been reported, which indicates that the majority of microplastic particles originated from human wastewater sources. The highest detection rates of fluorescent whitening compounds and high concentrations of microplastic particles were found in karst catchments with very high densities of DWTSs and high percentages of DWTSs in the catchment that are within 200 m of at least one karst feature, indicating a direct pathway into the underlying aquifer. Despite clear limitations of Cl/Br ratios as a cheap and easy-to-perform approach, the results still suggest some, although very low, potential for applicability at springs in karst environments in a multiple-tracer (tiered) approach that should be investigated in the future. Furthermore, a fluorescence-based multi-parameter approach was trialed in order to determine whether these methods can be used to predict rainfall-induced rapid changes of microbial water quality at two karst springs and link those changes to human effluent contamination. The results from three monitoring periods at two karst springs revealed marked responses to rainfall events for all of the microbial parameters measured. Total cell counts (TCC) measurements using flow cytometry (FCM) showed a very strong positive correlations with monitored faecal indicator bacteria (FIB) and total coliforms (TC), therefore, this fluorescence-based and cultivation-independent technique can be very useful to track rapid changes in microbial water quality at karst springs. Furthermore, a very strong positive correlations were also found between tryptophan-like fluorescence (TLF) measurements and concentrations of all monitored microbial parameters, hence, this study also demonstrated that such fluorescence-based approach as well can be useful for detecting rapid changes in concentrations of traditional faecal indicators. Interestingly, it was found that fluorescent whitening compounds (FWCs) signals do not necessarily follow temporal variations of microbial indicators, however, the frequency of detection of positive FWCs signals may still reveal useful information about the magnitude of human wastewater effluent impacts on karst aquifer systems.