Possible causes of high
manganese concentrations in
Scottish groundwater
Sally Homoncik
Mountain Environments, Callander, Scotland
Alan MacDonald, Brighid Ó Dochartaigh
British Geological Survey, Edinburgh
Kate Heal, Bryne Ngwenya
School of GeoSciences, The University of Edinburgh, Scotland
1
Why interested?
• Naturally occurring Mn common in drinking water supplies and is
a micronutrient
• EC MAC 0.05 mg Mn l-1 for aesthetic reasons
• WHO health-based limit guideline is 0.4 mg Mn l-1 but calls for it
to be revised downwards
• Groundwater increasingly important for water supply in Scotland,
but some new boreholes abandoned due to excessive Mn
Groundwater Use
Volume Ml d-1
Public Water Supply Boreholes
100
Public Water Supply Springs
70
Industry
80
Agriculture
40
Private Water Supplies
40
Total
330
Clogging of well screen in
borehole by Mn oxides (image
credit: Derek Ball)
2
Factors affecting Mn concentrations in
groundwater
• Rock
geochemistry
• Water
chemistry
• Microbiological
activity
Eh-pH diagram for Mn in the
presence of CO2 and H2O at 20oC
(produced in PHREEQ)
3
Mn in groundwater database (1)
• Baseline Scotland since
2005
• High quality data: field
filtration and acidification
Image credits: Derek Ball
4
Mn in groundwater database (2)
• Data collected prior to 2005 in other BGS projects
and by other organisations
– Same quality assurance as Baseline Scotland data
– Limit of detection < 0.003 mg Mn l-1
• Removal of duplicate samples for same site
• Final database:
– Mn concentrations for 475 sites across Scotland
– Other physicochemical parameters: pH, Eh, dissolved
oxygen (DO), specific electrical conductance (SEC),
dissolved organic carbon (DOC), HCO3, Ca, Mg, NO3-N, Fe
• Analysis of complete database and Devonian
samples only (to minimise effect of rock
geochemistry)
5
N
Moray
Mn
concentrations
in groundwater
in Scotland
Central Belt
Aberdeenshire
Strathmore
Mn (mg l-1)
Dumfries and
Galloway
6
Cumulative frequency plot of Mn
concentrations for complete database
28% of samples
exceeded EC MAC
Mn concentration mg l-1
7
Distribution of
groundwater
samples by
geological
category
100 km
8
Summary statistics for Mn concentration by
geological category
10
Mn mg l-1
1
0.1
EC MAC
Max
0.01
75th percentile
0.001
Median
25th percentile
0.0001
76
0.00001
48
46
52
48
52
50
35
68
Min
9
Northern
Devonian
Analysis of
Devonian samples
to minimise
influence of rock
geochemistry
Strathmore
Devonian
Southern
Devonian
10
Stacked bar chart of Mn concentrations by pH category
100
n=35
n=46
n=48
90
80
Evidence of pH control: %
of samples with elevated
Mn concentrations
decreases as pH
increases
% of samples
70
Mn mg l-1
60
>0.05
50
0.005-0.05
0.001-0.005
40
<0.001
30
20
10
0
<7.0
7.0 - 7.5
>7.5
11
pH category
Multiple linear regression analysis of Devonian samples
Predictor
Predicting log10Mn
Predicting log10Mn
Predicting log10Fe
Predicting log10Fe
(all predictors)
(4 predictors)
(all predictors)
(4 predictors)
pH
Eh
0.011 (-)
0.043(-)
DO
0.001 (-)
0.961 (+)
0.011 (-)
0.037 (-)
<0.001 (-)
0.608 (-)
SEC
DOC
0.047 (+)
0.007 (+)
---
---
HCO3
Ca
Mg
NO3-N
log10Fe
0.077 (+)
log10Mn
---
---
---
# Samples
34
59
34
41
R2 Adjusted
65%
62%
53%
58%
12
High Mn
concentrations
distributed
across pH and
Eh conditions
Mn concentrations (mg l-1) for the complete database plotted on
Mn predominance diagram (produced with PHREEQC v.2)
13
Mn vs. Fe concentrations for complete database
Mn mg l-1
P< 0.001, R2 = 25%
Co-occurrence of high
Fe and Mn
concentrations –
mainly due to
mobilisation in similar
pH and redox
conditions, but also to
Mn release when Fe
oxides reduced
High Mn and low Fe
concentrations when
suitable pH-redox
conditions for Mn to be
mobilised but not Fe
Co-occurrence of low
Fe and Mn
concentrations – due
to sorption of Mn to
Fe precipitates
Fe mg l-1
14
Conclusions and implications
• Further research required to predict Mn
concentrations in groundwater
– Role of rock geochemistry
– Improve understanding of Mn-Fe interactions
• Excessive Mn concentrations may be
undetected/underestimated in private water
supplies in Scotland
– Mn rarely measured even though excessive
concentrations common: 28% sites > EC MAC
• Mn concentrations may be underestimated if
samples not filtered and acidified
15