Variability of E. coli in streambed sediment and its implication for sediment sampling
Introduction
Fecal indicator bacteria (FIB), including E. coli, are the leading cause of impairments in rivers and streams within the United States. Elevated E. coli concentrations alone are responsible for impairments in over 100,000 miles of rivers and streams in the U.S. (USEPA, 2016). Conventional sources of water quality impairment include runoff from manure applied agricultural fields, direct fecal deposition, leaching from failed septic systems, and wastewater treatment plant outflows (Garzio-Hadzick et al., 2010). The presence of FIB in streams is used to indicate fresh fecal contamination (Garzio-Hadzick et al., 2010; Wade et al., 2003), and thus the risk of pathogen exposure (Haack, 2017). However, FIBs have the ability to survive and persist in the environment for a long time (Anderson et al., 2005) meaning that their presence does not always indicate fresh contamination. FIB can live in the environment for a few hours (Haack, 2017), days (Haack, 2017; Anderson et al., 2005), weeks (Haller et al., 2009; Jamieson et al., 2005), or even months (Haack, 2017; Garzio-Hadzick et al., 2010; Czajkowska et al., 2005) depending on the suitability of the environmental conditions.
Different environmental factors influence the survival and persistence of FIB in the environment including temperature, nutrient availability, salinity, oxygen levels, predation, and UV radiation (Craig et al., 2004; Hughes, 2003; Thomas et al., 1999; Davies et al., 1995). Sediment is one such environment that provides favorable conditions for FIB survival (Jamieson et al., 2005; McElhany and Pillai, 2010), with increased nutrient and carbon availability (Davies et al., 1995), reduced exposure to sunlight (Koirala et al., 2008), and protection from predatory protozoans (Jamieson et al., 2005; (Decamp and Warren, 2000). Survival times of FIB increases from days or weeks in the water column (Haack, 2017; Garzio-Hadzick et al., 2010; Czajkowska et al., 2005) to weeks or months in sediments (Garzio-Hadzick et al., 2010; Haller et al., 2009; Anderson et al., 2005; Czajkowska et al., 2005).
Higher concentrations of FIB are often observed in sediment when compared to the overlying water column (Kovacic et al., 2011; Edge and Boehm, 2011; Jamieson et al., 2005; An et al., 2002). For example, Van Donsel and Geldreich (1971) found fecal indicator bacteria concentrations were up to 1000 times greater in the sediment than in the water column. Similarly, Pandey et al. (2018) reported the average E. coli concentration in sediment was 47 to 386 times higher than the average E. coli concentration in the water column.
Sediment disturbance results in the resuspension of particles along with bacteria in the sediment. Disturbances can be caused by storm events (Pandey and Soupir, 2014; Fries et al., 2006; Jamieson et al., 2005; Crabill et al., 1999), recreational activities (Abia et al., 2017; An et al., 2002), animals crossing the river or stream (Abia et al., 2017; Sherer et al., 1988) and commercial activity (Pettibone Irvine et al., 1996; Irvine and Pettibone, 1993; Liou and Herbich, 1976). FIB concentrations in the water column can increase 8 to 88-times due to sediment disturbances (Abia et al., 2017; Pandey and Soupir, 2014), thus substantially contributing to water quality impairments (Abia et al., 2017; Nagels et al., 2002; Sherer et al., 1988).
To accurately predict FIB concentrations in the water column, the resuspension of FIB from streambed sediments must be considered (Rehmann and Soupir, 2009) as these reservoirs of FIB can substantially increase FIB concentrations in the water column. Artificial flood studies have been performed to isolate the impact of sediment disturbances on microbiological water quality (Muirhead et al., 2004; Nagels et al., 2002). For example, Muirhead et al. (2004) released water from a supply reservoir to generate an artificial flood and exclude runoff sources of fecal contamination. They measured E. coli concentrations in the water column up to two orders of magnitude higher than background concentrations, increasing from 102 to 104 CFU 100 mL−1, indicating that bed sediments alone can result in concentrations that exceed water quality standards when disturbed. The single sample maximum standard for E. coli in limited contact recreation waters is 1178 CFU 100 mL−1 (USEPA, 2016). This release of FIB from the sediment to the water column may be incorrectly interpreted as recent fecal contamination rather than resuspension which may lead to an inaccurate perception of water quality.
To better understand sediments as a source of FIB, sediments must be quantified and monitored for concentrations of FIB; however, little information exists to help inform sampling design. Therefore, the goal of this study is to assess the variability of E. coli in streambed sediment and the implications for sampling and reporting results. The main objectives of the present work are to (i) assess the spatial variability of E. coli in sediment across the stream cross-section, (ii) determine the number of samples required for a representative E. coli concentration in sediment, and (iii) evaluate some of the uncertainty in sampling and processing samples. This information will be helpful to design sediment sampling regimes and to understand the impact of unconventional sources of water quality impairments, which are often overlooked.
Section snippets
Study area
This study was conducted on two tributaries of the Big Sioux River located in eastern South Dakota (Fig. 1): Skunk Creek and Sixmile Creek. Skunk Creek covers a total drainage area of 73,000 acres and the land use is dominated by agricultural production. Approximately 65% of the land is cropland where 38% corn, 26% soybeans, and the rest are miscellaneous crops. The grassland and pastureland cover 29% of the land. Total 213 animal feeding operations (AFO) are in this sub-watershed including 66%
E. coli concentrations and variability
The measured sediment E. coli concentrations were highly variable, which was demonstrated by the high standard deviation relative to the arithmetic mean concentrations (Table 1). Sediment E. coli concentrations had a three order of magnitude range. The minimum concentration measured at all five sites was 4 CFU g−1 located at Sk4 and the maximum concentration was 997 CFU g−1 at Sk1. Sk2 and Sk3 showed similar concentrations and SM had the second-lowest concentrations. Similarly, the highest and
Conclusion
The current study focuses on understanding the variation of E. coli concentration in bed sediment and its implications for sediment sampling. To achieve the project goal sediment samples were collected from five monitoring sites by using a five-by-five grid to understand the E. coli variability across stream cross-section, the samples were processed two times to analyze E. coli stability in sediment samples and determine the sample size required for getting representative data from a particular
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was funded by the USEPA, South Dakota Department of Environment and Natural Resources, and USDA Hatch projects SD00H604‐15 and SD00H452‐14 courtesy of the SDSU Agricultural Experiment Station. We appreciate the support for this work including the Moody County Conservation District and East Dakota Water Development District who assisted with site access as well as Miranda Lebrun and Abhinav Sharma who helped with sample collection and processing.
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