1 Possible causes of high manganese concentrations in Scottish groundwater Sally Homoncik Mountain...

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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