Introduction
Asia is the continent with the largest absolute surface area underlain by carbonate rocks. In Vietnam, carbonate rocks are present over an area of 99,300 km
2; more than a quarter of the total area of the country (Goldscheider et al.
2020). These carbonates formed during the Early Cambrian to Late Cretaceous (Tran et al.
2013; Van Nguyen et al.
2013). Dissolution processes, assisted by the subtropical climate, created a karst terrain characterized by sinking streams, caves, enclosed depressions, fluted rock outcrops, and large springs (Ford and Williams
2013).
Due to rapid percolation and concentrated infiltration through swallow holes, the filtration capacities of karst aquifers are weak. Karst aquifers, therefore, are highly vulnerable to pollutants. This vulnerability is intensified by high flow velocities in fissures and conduits, short residence times, and high hydraulic and hydrochemical variability after rain events (e.g. Pronk et al.
2007; Ford and Williams
2013; Jiang et al.
2015). Low- to middle-income countries, including Vietnam, are facing major concerns with water resource management in karst areas due to inadequate water supply, lack of source protection, and lack of wastewater disposal and treatment (Ender
2018). One of the main problems is fecal contamination of the water caused by human settlements, farming, and livestock, leading to a widespread occurrence of waterborne diseases (Montgomery and Elimelech
2007). Nguyet and Goldscheider (
2006a) found that poor microbial water quality is a common problem for subtropical Vietnamese karst regions, and poses a threat to human health as locals are dependent on the use of the poor-quality water sources. The analysis of fecal indicator bacteria (FIB) is commonly used to assess the hygienic quality of water. Analysis of total coliforms (TC), fecal coliforms (FC),
Escherichia coli (
E. coli), and enterococci are well-established indicators. As
E. coli has the highest sanitary significance, while TC and FC can partly originate from nonfecal sources and overestimate pollution (Cabral and Marques
2006; Doyle and Erickson
2006),
E. coli is used for this study as the indicator for fecal contamination. To determine the presence of
E. coli, standard culture-based techniques with incubation times between 18 and 24 h are commonly used. In remote areas, insufficient technical infrastructure and the time required for analyses often make it impossible to prevent contaminated water from entering the distribution system.
Suspended particles play an important role in the transport of pollutants because bacteria tend to adhere to particles (Dussart-Baptista et al.
2003; Pronk et al.
2006; Schillinger and Gannon
1985). In contrast, free-floating bacteria are less persistent within the aquatic environment. In karst aquifers, particulate substances are transported over long distances in short times and are influenced by processes such as sedimentation and remobilization. Due to the attachment of
E. coli to a mobile solid phase, they are strongly influenced by these hydrodynamic processes (Mahler et al.
2000; Goldscheider et al.
2010). Several studies have established particle concentration as an indicator of microbial pollution (Mahler et al.
2004; Pronk et al.
2007). To date, only a few studies measure particle concentration in-situ with a high temporal resolution, despite this being crucial for measurement accuracy and reliability (Wilkinson and Lead
2007). Previous studies mainly deal with the concentration of suspended particles at springs (e.g. Schiperski et al.
2015); only a few studies take into account the system’s input (swallow hole) (Pronk et al.
2007; Fournier et al.
2008; Goeppert and Goldscheider
2019).
The full range of factors influencing water quality in a complex surface water and groundwater system—contamination potentials, transport processes of pollutants, cave systems, hydraulic conditions, and daytime—has to be considered in its entirety. Especially in karst systems with high flow velocities and low filtration, it is crucial to take into account a comprehensive perspective and to use data with high temporal resolutions when assessing water quality in terms of sediment-contaminant interaction. High temporal resolution time series have so far primarily been event-based, collected during and after precipitation events or snowmelt (Pronk et al.
2009; Schiperski et al.
2015). In addition to hydro-meteorological parameters, local land-use practices (farming, sewage disposal) can have a major impact on water quality. Therefore, detailed time series under constant discharge conditions can be a valuable tool for the assessment of water quality—for example, Grimmeisen et al. (
2016) were able to identify the impact of leakages in an intermittent water supply system on urban groundwater by high-resolution monitoring.
In order to consider an appropriate and suitable number of local factors influencing water quality, the following methods were combined in this study: measurements across the catchment at high and low flow conditions; high temporal resolution simultaneous sampling with in-situ measurements under constant hydraulic conditions at selected paired sites; and GIS analyses of the anthropogenic land use in the catchment to highlight the contamination potentials for the water resources.
This study aims to (1) characterize seasonal fluctuations in water quality at karst springs, nonkarst springs, cave streams, and surface streams in a complex surface and karst groundwater system, (2) identify variations of water quality at springs—karst and nonkarst—and in a surface stream under constant discharge conditions, and (3) develop the basis for a pragmatic protection scheme based on a GIS analysis of the land use.
Conclusions
This study presents the basis for an adapted protection approach for a complex surface water and karst groundwater system for the Ma Le Valley, northern Vietnam. The characteristic contamination risks for the surface stream and the groundwater in the catchment were addressed by monitoring the spatial and temporal dynamics of suspended particles and E. coli. Additionally, a GIS-based assessment of the impacts of different land uses on the water quality in the catchment is presented and linked to the results of water-quality monitoring.
This study revealed significant seasonal differences in water quality at springs and in the surface stream, and fecal impacts at all sampled sites. In cultivated and agriculturally used regions, short-term and unpredictable deterioration of water quality in the surface stream is possible at any time, especially during the day. Constant discharge does not automatically imply constant water quality; even under stable conditions, a high variability of water quality can occur. The monitoring efforts were able to identify the sources of contamination and the likelihood of contamination in different parts of the catchments. Land-use activities in the study area generally have a higher influence on water quality than hydrologic conditions.
Suspended particle concentrations are a good low-cost proxy for bacterial contamination in catchments with high temporal and spatial variability of water quality. However, in the study site, particle concentrations are insufficient to capture all bacterial concentration spikes. Due to the different mobilization of particles and bacteria, only monitoring turbidity is not sufficient as an early-warning system for microbial pollution. Furthermore, the results of this study demonstrate clear differences in water quality depending on the time of day; at night the water is less polluted.
Based on the combination of GIS analyses of land use and the FIB data, several measures are recommended to reduce pollutant input:
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Treatment of the accruing sewage or equipping buildings with septic tanks
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Prohibition of burning garbage and littering
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Restricted use of agrochemicals, especially in the rainy season
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Reduced farming activities (particularly livestock breeding) in the valley basin
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Shift the timing of drinking water withdrawals to the nighttime
Besides, socio-cultural aspects such as education and awareness of the local population are essential for the downstream users of the river water. To respond to increasing water demand, the existing water supply system could be further enhanced by diverting water from springs with low microbial contamination and adapted protection strategies for their catchments.
This study highlights the seasonal variability of water quality and the large fluctuations in quality, even under constant discharge conditions, in a complex surface water and karst groundwater system. These fluctuations indicate the utility of advanced measurement techniques such as high-resolution particle measurements, to detect and predict the variability of contamination, and provides the basis for innovative and locally adapted water protection strategies. The outlined methodology is also transferable to other complex surface water and karst groundwater sites in subtropical environments.
Acknowledgements
The authors thank the whole sampling team, especially Patrick Keinarth, Nina Schwark, Joel Pompei, Oliver Dott, Niclas Danielzik, Ho Tien Chung, Doan The Anh, Tran Diep Anh, and Nguyen Van Dong for their help during fieldwork. We are grateful to Markus Klotz for the good cooperation and thank him for providing particle counters. Special thanks are given to Chloé Fandel for language checking the manuscript and for her valuable feedback.
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