Effect of Low Energy Waves on the Accumulation and Transport of

Feb 10, 2017 - The possibility that sand-associated E. coli that accumulated over the 5 day simulation period could lead to a water quality advisory d...
0 downloads 0 Views 1MB Size
Subscriber access provided by University of Newcastle, Australia

Article

Effect of low energy waves on the accumulation and transport of fecal indicator bacteria in sand and pore water at freshwater beaches Ming Zhi Wu, Denis Michael O'Carroll, Laura Jill Vogel, and Clare E. Robinson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05985 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36

Environmental Science & Technology

1

Effect of low energy waves on the accumulation and transport of fecal indicator

2

bacteria in sand and pore water at freshwater beaches

3 Ming Zhi Wua, Denis M. O’Carrolla,b, Laura J. Vogela, Clare E. Robinsona *

4 a

5 6 7 8

Department of Civil and Environmental Engineering, Western University, London ON, Canada N6A 5B9

b

School of Civil and Environmental Engineering, Connected Water Initiative, University of New South Wales, Manly Vale NSW 2093, Australia

9 10

Submitted to Environmental Science & Technology

11 12

*Corresponding author

13

Address for Correspondence:

14

Clare Robinson

15

Department of Civil and Environmental Engineering, Western University

16

London ON, Canada N6A 5B9

17

Tel: 519-661-2111; Fax: 519-661-3779

18

e-mail: [email protected]

19

1 ACS Paragon Plus Environment

Environmental Science & Technology

20

Abstract Art

21

22 23

2 ACS Paragon Plus Environment

Page 2 of 36

Page 3 of 36

24

Environmental Science & Technology

Abstract

25

Elevated fecal indicator bacteria (FIB) in beach sand and pore water represent an

26

important non-point source of contamination to surface waters. This study examines the

27

physical processes governing the accumulation and distribution of FIB in a beach aquifer.

28

Field data indicate E. coli and enterococci can be transported 1 and 2 m, respectively, below

29

the water table. Data were used to calibrate a numerical model whereby FIB are delivered to

30

a beach aquifer by wave-induced infiltration across the beach face. Simulations indicate FIB

31

rapidly accumulate in a beach aquifer with FIB primarily associated with sand rather than

32

freely residing in the pore water. Simulated transport of E. coli in a beach aquifer is complex

33

and does not correlate with conservative tracer transport. Beaches with higher wave-induced

34

infiltration rate and vertical infiltration velocity (i.e., beaches with higher beach slope and

35

wave height, and lower terrestrial groundwater discharge) had greater E. coli accumulation

36

and E. coli was transported deeper below the beach face. For certain beach conditions, the

37

amount of FIB accumulated in sand over five to six days was found to be sufficient to trigger

38

a beach advisory if eroded to surface water.

39 40 41

Keywords: Groundwater-surface water interactions, fecal contamination, bacterial transport, colloid transport, waves, Escherichia coli, enterococci

3 ACS Paragon Plus Environment

Environmental Science & Technology

42

1

Introduction

43

Fecal contamination diminishes the recreational and economic value of beaches.1,2 It

44

is estimated that human exposure to wastewater-polluted coastal waters worldwide results in

45

over 120 million cases of gastrointestinal disease and 50 million cases of respiratory disease

46

annually.3 Recreational water quality is assessed through routine monitoring of fecal indicator

47

bacteria (FIB), i.e., Escherichia coli at freshwater beaches and enterococci at marine beaches,

48

which correlate with the risk of water-borne illnesses to beachgoers. When FIB

49

concentrations in surface water exceed water quality standards (e.g., 100 colony forming

50

units per 100 mL [CFU/100 mL] based on a geometric mean for E. coli in Ontario, Canada4

51

and the United States5; 30 CFU/100 mL for enterococci in the United States5), a beach water

52

quality advisory may be issued. FIB are known to accumulate in foreshore sand and pore

53

water (herein referred to as the foreshore reservoir; Figure 1a), with concentrations

54

considered on a bulk volumetric basis often orders of magnitude higher than in adjacent

55

surface waters.6-10 Fecal contaminants in the foreshore reservoir pose a human health risk

56

through skin contact and ingestion,11,12 as well as being an important non-point source of

57

contamination to surface waters.6,13-16 Sources of FIB to the foreshore reservoir include

58

stormwater runoff, septic systems, sewer overflows and repeated seeding from bird and

59

animal feces.6,17-19 Infiltration of surface water across the beach face (Figure 1a) can also be

60

an important source of FIB to the foreshore reservoir.20-22

61

Prior research on the exchange of FIB between the foreshore reservoir and surface

62

water has largely focused on release of FIB from the reservoir to surface water.7,23,24 Few

63

studies have evaluated the delivery of FIB to the foreshore reservoir from surface water

64

infiltration. Due to the complexity of the beach environment (e.g., multiple forcing acting at

65

different temporal and spatial scales) it is difficult to quantify the exchange of FIB between

66

the foreshore reservoir and surface water via field measurements alone.6,24 Gast et al.25 4 ACS Paragon Plus Environment

Page 4 of 36

Page 5 of 36

Environmental Science & Technology

67

showed that plastic microspheres, used as a proxy for enterococci, were rapidly transported

68

about 0.5 – 0.8 m vertically and 6 m horizontally into the groundwater and unsaturated zone

69

of a beach aquifer in response to tide- and wave-induced surface water infiltration. More

70

recently, Brown et al.19 showed that while recirculating surface water may transport FIB from

71

bird feces deposited on the beach surface into the foreshore reservoir, subsequent FIB

72

transport from the reservoir to surface water (via groundwater flow) may be low due to FIB

73

attachment to sand grains. While studies have illustrated the importance of water exchange

74

(infiltration/exfiltration) across the beach face on the delivery of FIB to the foreshore

75

reservoir, improved understanding of the physical mechanisms controlling the transport of

76

FIB from surface water to the reservoir and their subsequent accumulation is needed.

77

Water exchange across the beach face and groundwater flow patterns are complex and

78

dynamic. At freshwater (e.g., Great Lakes) and micro-tidal marine beaches, waves are the

79

dominant coastal forcing and typically govern water exchange rates and beach groundwater

80

flow patterns. Averaged over a wave period, instantaneous waves produce an onshore upward

81

tilt of the sea or lake water surface (termed wave setup; Figure 1a). The hydraulic gradient

82

associated with wave setup drives groundwater flow recirculation that extends from the wave

83

run-up zone to offshore (Figure 1a).26,27 This causes significant quantities of surface water

84

and associated constituents, including FIB, to be delivered and transported through the beach

85

aquifer.26,28,29 Prior studies have quantified wave-induced water exchange and recirculation

86

for both steady and transient wave conditions as well as their impact on conservative and

87

reactive solute transport.26,28,30,31 Numerical modelling approaches are often used to identify

88

key mechanisms governing the effect of waves on groundwater flow and solute transport due

89

to the complexity of the environment.26,30,31

90

Understanding the processes governing bacterial transport in a beach aquifer is

91

important not only for FIB but also for other bacteria including pathogenic, sulfate-reducing 5 ACS Paragon Plus Environment

Environmental Science & Technology

92

and nitrifying bacteria.32 The transport of bacteria in groundwater is governed by bacteria-

93

sediment interactions including attachment to sand grains and straining.33-35 Bacteria also

94

experience inactivation and die-off, grazing, and possible replication.36-39 While prior studies

95

have examined the environmental factors affecting the persistence of FIB in beach

96

sand,10,38,40-42 the physical transport processes are less understood. For instance, we do not

97

understand the time scale at which FIB build up in the foreshore reservoir from surface water

98

infiltration, the partitioning of FIB between pore water and sand, and the physical factors

99

controlling the spatial distribution of FIB in the reservoir.

100

The objective of this study is to generate a mechanistic understanding of the effects of

101

low energy waves on the delivery, accumulation and distribution of bacteria, specifically FIB,

102

in the saturated portion of a beach aquifer. This study focuses on low energy waves which are

103

likely to lead to the accumulation of FIB in the beach aquifer, rather than higher energy

104

waves which may be erosive and thus associated with the release of FIB from the foreshore

105

reservoir to surface waters.43,44 Field data of FIB distributions in the beach aquifer at two

106

freshwater beaches with different groundwater flow conditions are first analysed. A

107

numerical model that simulates wave-induced groundwater flows combined with bacterial

108

transport is then presented with the numerical approach calibrated using the field data.

109

Finally, the model is applied to provide key insights into the physical controls on the

110

accumulation and distribution of FIB in a foreshore beach aquifer.

111

2

112

Materials and Methods 2.1

Field Sites

113

Field investigations were conducted at Burlington Beach (43°18’47”N, 79°48’02”W;

114

Lake Ontario) and Mountain View Beach (44°40’21”N, 79°58’58”W, Lake Huron).

115

Groundwater, surface water and sand surface levels were measured three times at each beach

116

from May – September 2013 to determine the groundwater hydraulic gradients and beach 6 ACS Paragon Plus Environment

Page 6 of 36

Page 7 of 36

Environmental Science & Technology

117

topography (see Supporting Information Section 1 for field methods). Burlington Beach is a

118

fine sand beach (median grain size diameter [d50] = 0.23 mm, uniformity coefficient [Cu] =

119

1.7, saturated hydraulic conductivity [Ks] = 19.4 m/d) with an average beach slope (β) of

120

approximately 0.05. The beach water table was relatively deep with groundwater flowing

121

landward (foreshore hydraulic gradient ranged from -0.005 to -0.01 from May – September).

122

Mountain View Beach had a shallower water table with lakeward groundwater flow

123

(foreshore hydraulic gradient ranged from 0.008 to 0.016 from May – September) and an

124

average beach slope of 0.03. This beach is comprised of fine sand (d50 = 0.23 mm, Cu = 2.3,

125

Ks = 17.8 m/d) that overlies a clay layer. Offshore the depth of the clay layer is approximately

126

1.5 m below the sediment surface.

127

Water and Sand Sampling and FIB Enumeration

2.2

128

Depth profiles of FIB concentrations in the pore water were measured at multiple

129

locations along a cross-shore transect at each beach (6 August and 9 September 2013 at

130

Burlington Beach, and 25 July 2013 at Mountain View Beach). Intact sand cores up to 0.8 m

131

deep and extending below the water table were also collected on 4 June 2013 at Mountain

132

View Beach. In addition, foreshore pore water and sand samples as well as surface water

133

samples were collected every 2 – 4 weeks at three and four cross-shore transects (50 m apart)

134

on Mountain View Beach and Burlington Beach, respectively, from May – September 2013.

135

E. coli and enterococci in water and sand samples were enumerated using standard

136

membrance filtration methods45 with bacteria extracted from sand using methods

137

recommended by Boehm et al.46 The sampling and FIB enumeration methods are further

138

described in Supporting Information Section 1. E. coli and enterococci concentrations are

139

expressed as log CFU/100mL for water samples and log CFU/g of dry sand (based on sand

140

moisture content). The log transformed FIB concentrations were used for the statistical

141

analysis. 7 ACS Paragon Plus Environment

Environmental Science & Technology

142

2.3

Numerical Model

143

Groundwater flows and bacterial fate and transport in a beach aquifer exposed to

144

steady low energy waves was simulated in the finite-element solver COMSOL Multiphysics

145

(version 4.4).47 Variably saturated groundwater flow in a beach aquifer was simulated using

146

the Richards equation.48 Bacterial fate and transport in the beach aquifer included die-off and

147

attachment of bacteria to sand grains simulated using colloid filtration theory (CFT).49 Given

148

parameter values are better defined for E. coli compared to other FIB, the transport of E. coli

149

was simulated with enterococci transport simulated for select cases. Key model parameter

150

values are shown in Table 1 with details of the mathematical model and additional parameter

151

values provided in Supporting Information Section 2. Note, some field results are described

152

in this section as they are important for explaining the model set-up.

153

Two-dimensional models were set up to simulate the beach aquifers at Burlington

154

Beach and Mountain View Beach (Figure 1b,c). The model simulates the delivery of FIB to

155

the beach aquifer by surface water infiltration driven by low energy waves. Once delivered to

156

the aquifer, FIB may be transported by the flowing groundwater, accumulate in the sand, die-

157

off or exfiltrate to surface water. The effect of waves was simulated by considering the phase-

158

averaged effect of waves as described by wave setup (eq S8 in the Supporting Information),

159

rather than simulating instantaneous wave action. Submerged nodes along the boundary BCD

160

(Figure 1b) were assigned a hydrostatic pressure corresponding to the wave setup profile.

161

Nodes landward of the wave setup point along BCD were unsaturated and represented as a

162

no-flow boundary. As there was no infiltration landward of the wave setup point, the majority

163

of FIB was transported in fully saturated pores in our domain.

164

The simulated lake E. coli concentration was determined as part of the model

165

calibration with a constant concentration applied for nodes along boundary BCD with

166

infiltration into the aquifer as determined by the steady state flow model (Figure 1b). A zero 8 ACS Paragon Plus Environment

Page 8 of 36

Page 9 of 36

Environmental Science & Technology

167

concentration gradient was applied for nodes with exfiltration. The first-order die-off rate

168

coefficient µdec for FIB was estimated based on an average value determined from microcosm

169

experiments conducted using foreshore sand from Burlington Beach.50 Given die-off rates for

170

both FIB in water and sediment often have similar ranges [O(0.01-1) 1/d],51-55 the same µdec

171

was assumed for FIB in the aqueous phase and those attached to sand. The FIB concentration

172

in the terrestrial groundwater (Qt) was zero based on negligible FIB detected in the landward

173

groundwater at each site (Figure S2). The initial FIB concentration in the beach aquifer was

174

set to zero. This assumes that all FIB in the beach aquifer were released to surface water by a

175

preceding period of high erosive wave conditions.44 Details of the model domain and the flow

176

boundary conditions including wave setup are described in Supporting Information Section 2.

177

A simulation time of 5 days was adopted. This corresponds to the average time between

178

periods of higher wave activity on Lake Huron.

179

The bacterial attachment efficiency (αtot) and lake E. coli concentration were

180

determined by fitting model simulations (after 5 days simulation time) to the observed E. coli

181

vertical travel distance (Figure S2) and the observed mean E. coli saturated sand

182

concentrations at the two beaches (Figures S3 and S4). The possible range of αtot for E. coli

183

was based on literature values (Table S3). The lake E. coli concentration was varied within

184

the range of concentrations measured in ankle-depth surface water from May – September

185

2013 (0.48-2.61 log CFU/100 mL at Burlington Beach; 0.30-2.38 log CFU/100 mL at

186

Mountain View Beach). Simulation results were consistent with the field results using αtot =

187

0.05 and a lake E. coli concentration of 1.81 log CFU/100 mL. Following calibration, a

188

sensitivity analysis was conducted to determine the physical controls on the accumulation and

189

distribution of FIB in the beach aquifer.

9 ACS Paragon Plus Environment

Environmental Science & Technology

190 191

3

Results and Discussion 3.1

Comparison Between Field and Simulated FIB Distribution

192

Consistent with prior studies,6,16,21,24 the highest concentrations of E. coli and

193

enterococci at Mountain View Beach and Burlington Beach from May – September 2013

194

were observed in the foreshore pore water, followed by surface water at ankle-depth, then at

195

waist-depth (Table 2; Figures S3 and S4). FIB concentrations were highly variable over the

196

sampling season at both beaches. E. coli and enterococci concentrations at ankle-depth and

197

waist-depth were generally similar between Mountain View Beach and Burlington Beach,

198

particularly in July through September (Figures S3c,d and S4c,d). The pore water and

199

saturated sand concentrations between the two beaches were not statistically different (pore

200

water: p = 0.34 for E. coli and p = 0.31 for enterococci; saturated sand: p = 0.11 for E. coli

201

and p = 0.15 for enterococci; Mann Whitney test). This may in part be due to the high spatial

202

heterogeneity in pore water and sand concentrations, coupled with the limited number of

203

sampling events at each beach (5 and 8 sampling events at Mountain View Beach and

204

Burlington Beach, respectively). Although the concentrations were not statistically different,

205

distinct trends are observed for saturated sand E. coli and enterococci concentrations between

206

Mountain View Beach and Burlington Beach, with concentrations typically larger at

207

Burlington Beach. At both beaches E. coli and enterococci pore water concentrations were

208

consistently higher than Ontario4 and U.S. EPA5 surface water guidelines (