Possible causes of high manganese concentrations in Scottish groundwater

1. 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

2. 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 Why interested? Clogging of well screen in borehole by Mn oxides (image credit: Derek Ball)

3. 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)

4. Mn in groundwater database (1) • Baseline Scotland since 2005 • High quality data: field filtration and acidification Image credits: Derek Ball

5. 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)

6. Moray Aberdeenshire Central Belt Strathmore Dumfries and Galloway Mn concentrations in groundwater in Scotland N Mn (mg l-1)

7. Cumulative frequency plot of Mn concentrations for complete database 28% of samples exceeded EC MAC Mn concentration mg l-1

8. Distribution of groundwater samples by geological category 100 km

9. Max 75th percentile Median 25th percentile Min Summary statistics for Mn concentration by geological category EC MAC 76 48 46 52 48 52 50 35 68

10. Northern Devonian Strathmore Devonian Southern Devonian Analysis of Devonian samples to minimise influence of rock geochemistry

11. Stacked bar chart of Mn concentrations by pH category n=35 n=46 n=48 Evidence of pH control: % of samples with elevated Mn concentrations decreases as pH increases

12. Multiple linear regression analysis of Devonian samples

13. 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

14. Mn vs. Fe concentrations for complete database 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 Mn mg l-1 Co-occurrence of low Fe and Mn concentrations – due to sorption of Mn to Fe precipitates Fe mg l-1

15. 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