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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
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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
Clogging of well screen in borehole by Mn oxides (image
credit: Derek Ball)
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
Factors affecting Mn concentrations in groundwater
• Rock geochemistry
• Water chemistry
• Microbiological activity
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Eh-pH diagram for Mn in the presence of CO2 and H2O at 20oC (produced in PHREEQ)
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Mn in groundwater database (1)
• Baseline Scotland since 2005
• High quality data: field filtration and acidification
Image credits: Derek Ball
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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)
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N
Aberdeenshire
Moray
StrathmoreCentral Belt
Dumfries and Galloway
Mn (mg l-1)
Mn concentrations in groundwater
in Scotland
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Cumulative frequency plot of Mn concentrations for complete database
28% of samples exceeded EC MAC
Mn concentration mg l-1
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Distribution of groundwater samples by geological category
100 km
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EC MAC
48 52 6835505276 4648
Summary statistics for Mn concentration by geological category
25th percentile
75th percentile
Median
Max
Min
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Northern Devonian
Strathmore Devonian
Southern Devonian
Analysis of Devonian samples
to minimise influence of rock
geochemistry
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Stacked bar chart of Mn concentrations by pH category
Evidence of pH control: % of samples with elevated
Mn concentrations decreases as pH
increases
n=35 n=46 n=48
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Multiple linear regression analysis of Devonian samplesPredictor Predicting log10Mn
(all predictors)
Predicting log10Mn
(4 predictors)
Predicting log10Fe
(all predictors)
Predicting log10Fe
(4 predictors)
pH 0.011 (-) 0.961 (+)
Eh 0.043(-) 0.001 (-) 0.011 (-) <0.001 (-)
DO 0.037 (-) 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%
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Mn concentrations (mg l-1) for the complete database plotted on Mn predominance diagram (produced with PHREEQC v.2)
High Mn concentrations
distributed across pH and Eh conditions
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Mn vs. Fe concentrations for complete database P< 0.001, R2 = 25%
Fe mg l-1
Mn
mg
l-1
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
Co-occurrence of low Fe and Mn
concentrations – due to sorption of Mn to Fe
precipitates
High Mn and low Fe concentrations when
suitable pH-redox conditions for Mn to be
mobilised but not Fe
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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