Assessing acid sulfate soil contamination: a new indicator for waterways

Transcription

1 Assessing acid sulfate soil contamination: a new indicator for waterways Funded by: Dr Kieryn Kilminster

2 Why are acid sulfate soils a problem? Pyrite» Potential acid sulfate soils (PASS)» Undisturbed, no ecological risk Tackling acid sulfate soils on the Western Australian Coast

3 Why are acid sulfate soils a problem? Pyrite Oxidising agent digging dewatering draining H 2 SO 4 H 2 SO 4 Drainage pathway low ph acidity nutrients metals & metalloids Tackling acid sulfate soils on the Western Australian Coast

4 Identified significant hazard Acid sulfate soil risk mapping project (undertaken from ) revealed: of 675 soil cores sampled, 48% contained PASS in the top 3 m and approximately 8% of sites showed evidence of oxidised pyrite from this survey, ~ square km in WA were classified as high risk

5 Why an indicator for impact on water? Much of the identified acid sulfate soils are close to water What happens when pyrite is oxidised? 2FeS 2 + 7O 2 + 2H 2 O 2Fe SO H + Acid (which can mobilise metals)

6 Why an indicator for impact on water? Much of the identified acid sulfate soils are close to water What happens when pyrite is oxidised? 2FeS 2 + 7O 2 + 2H 2 O 2Fe SO H + Trace this sulfate as early warning indicator

7 Tracing the sulfur source Sulfur isotopes can help us identify source Sulfur occurs naturally in four stable isotope forms, of which 32 S and 34 S are most abundant (95% and 4.2% respectively). Sulfate in water is precipitated with BaCl 2 to give insoluble BaSO 4, which is then analysed by Isotopic Ratio Mass Spectrometry. Microbial sulfate reduction

8 What do you need to measure?

9 { Categorising samples into iso-groups Probably { natural Indicator categorises water samples based on dominant sulfur influence and whether there is relatively more or less sulfate than would be expected in seawater Probably acidic { Sulfate-reducing but perturbed

10 Isotopic mass balance approach What do you need to measure?

11 What do you need to measure?

12 What do you need to measure?

13 Water quality Testing the indicator in south-west Western Australia Broad brush approach Perth to Albany 90 estuary sites (summer/winter) 150 catchment sites (spring)

14 What did we find? Iso-group 3 and 5 Iso-group 1 and 2 In ~300 water samples: Acidic: 5% of sites with clear signal, 15% mixed signal Iso-group 4a and 4b Sulfur pristine: 45% of sites Sulfate-reducing (but perturbed): 35% of sites Likely to see many more acidic sites if monitoring targeted first flush events

15 Evidence the indicator is useful Al and Fe were elevated in Acidic groups in estuarine samples Filtered Unfiltered A Filtered Unfiltered B 1.2 Aluminium (mg L -1 ) Iron (mg L -1 ) a 4b 5 Iso-group Al was also higher for acidic catchment water samples

16 Evidence the indicator is useful Peel Harvey Estuary biotic study Water quality data (including indicator) Metal content of wild caught fish and shrimp

17 Aluminium (max =500 g/g Iron (max = 1650 g/g) Fish metal content Two sites had high in Al and Fe Both sites were identified in water monitoring as having clear acidic drainage signals (iso-group 3)

18 How to apply isotope indicator?

19 If iso-group 3 or 5 How to apply isotope indicator?

20 How to apply isotope indicator?

21 Where could you use this? Broad scale surveillance for legacy problems (e.g. pre-guidelines) Test current best management practices for dredging, developments, dewatering Routine testing of water bodies where high likelihood of ASS being disturbed Works in estuaries Groundwater studies associated with drying climate or abstraction

22 About the indicator Early warning indicator, will show SO 4 2- signal, even when acidity is neutralised by existing buffering (in water or soil) Shows both subtle and obvious impacts of acid sulfate soils Sensitive enough to see issues in estuaries (where seawater SO 4 2- is so dominant) Turnaround time slow as no lab in WA Batches of samples usually necessary to run analysis

23 Case study Assessing the influence of coal mine drainage (Collie)

24 More information Please me for copy of excel spreadsheet for calculating the iso-group for water samples Paper: Kilminster K and Cartwright, I (reviewed) A sulfur stable isotope based screening tool for assessing impact of acid sulfate soils on waterways submitted to Journal of Marine and Freshwater Research

25 Acknowledgements Prof. Ian Cartwright (Monash University) Water Science Branch and regional staff of Department of Water (Mandurah, Bunbury, Albany) involved in field work Curtin University, Marine and Freshwater Research Laboratory and National Measurement Institute (analysis of samples) Funding from Australian and Western Australian Governments

26 Case studies Sulfur isotope indicator drain input downstream Site did show inputs of non-seawater sulfate, but sulfate-reduction dominant

27 Analysis showed localised impact only Case studies

28 Case studies Acidic wetland (Lake Mealup) Monthly water samples confirmed likely ASS influence Iso-group 5 approximately ~4000% extra sulphate

29 ASS risk maps Assumes disturbance (drainage, excavations, detwatering) typically do not extend deeper than 3 m. (1) High to moderate risk of ASS within top 3 m of natural soil surface (2) Moderate to low risk of ASS within top 3 m of natural soil surface (activities disturbing soils at depths greater than 3 m carry a high to moderate risk of disturbing ASS). (3) No known risk of ASS occurring within 3m of natural surface (or deeper)