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Bugs’n’mud
E. coli, turbidity and flow relationships for the Motueka
River
Lucy McKergow and Rob Davies-Colley
Outline
• background• research questions• methods• results• conclusions
Background
• E. coli bacteria– indicator for freshwater recreation – source= faecal contamination from warm-
blooded animals– transport = surface runoff, subsurface flows,
direct deposition, re-entrainment of bed sediment
• MfE & MoH (2003) guidelines– <260 cfu/100ml acceptable
• in small streams turbidity can be used as a surrogate for E. coli
Research questions
• can turbidity be used as a surrogate for E. coli in large rivers?
• how many E. coli are exported to Tasman Bay?
Motueka River
• At Woodmans Bend– 2047 km2
catchment– native + exotic
forest 60%, pasture 20%
– mean flow 82 m3/s– median flow 47
m3/s
Dataset
• flood event samples – June 03-June 04– sample interval 10 to 30 minutes –auto
sampler– continuous turbidity - OBS– lab turbidity – NTU– E. coli – Colilert, most probable number/100
mL
• monthly sampling – May 03 – Dec 05
1
10
100
1000
10000
1May03 9Aug03 17Nov03 25Feb04 4Jun04 12Sep04 21Dec04
Date
E.
coli
(M
PN
/100
ml)
0
200
400
600
800
1000
1200
Flo
w (
cum
ecs)
event
monthly
flow
Monitoring period
event
monthly
flow
Concentrations
baseflow rising falling
E.
coli
(MP
N/1
00
ml)
100
101
102
103
104
105
• concentrations high during events – particularly on rising limbs of hydrographs
Kolmogorov-Smirnov p=0.000
E. coli vs flow
1
10
100
1000
10000
1 10 100 1000 10000Flow (m3/s)
E.
coli
(MP
N/1
00 m
l)
baseflowrisingfalling
E. coli vs turbidity
1
10
100
1000
10000
1 10 100 1000Lab turbidity (NTU)
E.
coli
(MP
N/1
00 m
l)
baseflowrisingfalling
0
1000
2000
3000
4000
5000
18Sep03 12:00 19Sep03 12:00 20Sep03 12:00 21Sep03 12:00 22Sep03 12:00
Date
E.
coli
(M
PN
/100
ml)
0
100
200
300
400
500
600
700
800
900
1000
Flo
w (
m3 /s
) a
nd t
urbi
dity
(N
TU
)
E. coli Flow Field turbidity (NTU)
18-22 Sep 03
E. coli
Turbidity
Flow
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
18Jun04 19Jun04 20Jun04 21Jun04 22Jun04 23Jun04 24Jun04 25Jun04 26Jun04
Date
E.
coli
(MP
N/1
00 m
l)
0
200
400
600
800
1000
1200
Flo
w (
m3 /s
) an
d tu
rbid
ity (
NT
U)
E. coli Flow Field turbidity (NTU)
18-21 June 2004
Turbidity
Flow
E. coli
Loads
• LOADEST – USGS model– log-linear regression– lnQ, lnQ2, seasonality, decimal time
(centred to eliminate collinearity)
LOADEST
• E = 0.55• r2 = 0.69
• mean Ld = 1.4 x 107
#/day
• max Ld = 9 x 108 #/day
Inst loadobs
Daily loadpred
Conclusions
• bugs and mud are from different sources
• turbidity may not be a consistently useful surrogate for E. coli in large rivers– alternative is to use flow
