Rainfall-driven E. coli transfer to the stream-conduit network observed through increasing spatial scales in mixed land-use paddy farming karst terrain

Sarah J Buckerfield,Richard S Quilliam,Susan Waldron,Larissa A Naylor,Siliang Li,David M Oliver

doi:10.1016/j.wroa.2019.100038

Sarah J Buckerfield, Richard S Quilliam + Show 4 more

Open Access

https://doi.org/10.1016/j.wroa.2019.100038

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Abstract

Karst aquifers have distinctive hydrology and supply 25% of the world’s population with drinking water, making them a critical geological setting for understanding and managing microbial water pollution. Rainfall causes elevated concentrations and loading of faecal microorganisms, e.g. E. coli, in catchment surface and groundwater systems, increasing the risk of human exposure to faecally-contaminated water. However, effective management of microbial water quality in complex karst catchments is constrained by limited understanding of E. coli - discharge responses to rainfall. We analysed how rainfall events of varying magnitude (2.4–100 mm) control E. coli-discharge dynamics at increasing spatial scales in a mixed land-use karst catchment in southwest China. During the wet season, hourly water sampling was undertaken throughout five storm events to characterise in high detail E. coli emergence with resulting flow across multiple sites of varying catchment area, stream order, and land-use. E. coli concentration was found to increase by 1–3 orders of magnitude following rainfall events. Maximum E. coli concentration and speed of E. coli recession were influenced by rainfall (amount, intensity), timing of agricultural activities, and position in the hydrological system. For high intensity events ∼90% of the cumulative E. coli export occurred within 48 h. E. coli concentration increased with increasing discharge at all sites. E. coli concentration at low discharge was higher in the headwaters than at the catchment outlet, while the rate of increase in E. coli concentration with increasing discharge appears to follow the opposite trend, being higher at the catchment outlet than the headwaters. This was attributed to the decreasing flow path gradient and increasing degree of development of the fissure network, but further event monitoring at varying catchment scales is required to confirm this relationship. The results provide novel insight into how rainfall characteristics combine with land-use and catchment hydrology to control E. coli export in karst landscapes.

Highlights

Karst aquifers provide 25% of the global population with drinking water (Hartmann et al, 2014)
High resolution characterisation of in-stream E. coli concentration in response to storm events is challenging; this study addresses this challenge and reports on a series of novel datasets of E. coli -Q relationships observed across a karst catchment over successive rainfall events
This study identified rainfall characteristics, land-use, and karstic hydrology to be fundamental controls on E. coli-Q dynamics and export in mixed land-use karst catchments typical of this terrain

Summary

Introduction

Karst aquifers provide 25% of the global population with drinking water (Hartmann et al, 2014). Microbial water quality response to different combinations of faecal inputs and hydrological drivers (such as rainfall) is poorly understood, in karst terrain (Vermeulen et al, 2015). This is partly because, unlike with nutrient pollution, in-situ high-resolution sampling combined with on-site analysis has not yet translated into a standard operating procedure for the quantification of E. coli, other faecal indicator organisms (FIOs), or human pathogens. Monitoring of FIO-discharge (Q) relationships through storm events is challenging It is further compounded by regulatory requirements to monitor microbial pollution at end-point receptors and locations of likely human exposure risk, such as bathing or shellfish harvesting waters, rather than quantifying FIOs distributed across catchment drainage networks (Oliver et al, 2016)

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