Monitoring of polar pollutants in surface waters using Chemcatcher-based passive sampling

methods

Anthony Gravell

Natural Resources Wales

Llanelli laboratory

About us

Natural Resources Wales brings together the work of the Countryside Council for Wales, Environment Agency Wales and Forestry Commission Wales, as well

as some functions of Welsh Government.

Our purpose is to ensure that the natural resources of Wales are sustainably maintained, enhanced and used, now and in the future

The principal adviser to the Welsh Government on the environment, enabling the sustainable development of Wales’ natural resources for the benefit of

people, the economy and wildlife

Who we are and what we do

• Overview of passive sampling

• Significance of acid herbicides

• Passive sampling

• Laboratory based studies

• Review of results

• Summary

Overview

• Compounds such as mecoprop, MCPA, tricolpyr and clopyralid widely used to control weeds in grassland

• Highly water soluble and fairly persistent leading to frequent run-off

• Regularly detected in major drinking water catchments, above PCV level (0.1µg/l for a given pesticide)

• Advanced treatment processes required for efficient removal

Acid herbicides – what’s the big deal

• To reveal tributaries responsible for significant input of acid herbicides

• To provide ‘proof of concept’ – will catchment monitoring reduce loadings of pollutants?

• Avoids ‘hit and miss’ nature of spot sampling

• Passive samplers can be deployed for extended periods (weeks) and can reveal time weighted average (TWA) concentrations of pollutants

Why use passive sampling?

• Various configurations of sampler available: Semi-Permeable Membrane Device (SPMD) Polar Organic Contaminant Integrative Sampler (POCIS) Diffusion Gradients in Thin Films (DGT)

ChemcatcherTM (patented design by University of Portsmouth)

• All designs feature a receiving phase with a high affinity for the pollutants of interest

• May include a diffusion limiting membrane to reduce bio-fouling and/or extend deployment period

• Mounted in a suitable housing • Deployed in a protective cage

Passive Sampling Devices

ChemcatcherTM Components

Chemcatcher Parts

1 2 3 4 5

1. Base Plate

2. Receiving Disk

3. Protective Membrane (PES)

4. Retaining Ring

Fully Assembled Sampler

1

4 2 & 3

• Laboratory based studies identification of suitable receiving phase for recovery of

acid herbicides

assessment of which of the 16 currently monitored compounds are amenable to passive sampling

derivation of procedure to interface passive sampling with current routine method for analysis of acid herbicides in spot samples

performance of an acid herbicide uptake study from river water to investigate uptake kinetics

Project Outline (1)

Acid herbicides – what do they look like?

Mecoprop (pKa 3.7) MCPA (pKa 3.1)

Triclopyr (pKa 4.0)

Clopyralid (pKa 2.0)

Acid Herbicides – how to extract them?

• Almost completely ionised and therefore highly water soluble at typical environmental pH values (~6.5 to 8.5):

• Cannot be extracted by conventional means without reducing

pH to around 2. • Need to make use of negatively charged carboxylate group to

effect recovery, i.e. use an anion exchange process. • Filtration experiments showed that all 16 acid herbs can be

effectively captured using a commercially available Empore anion exchange disk.

Acid Herbicides – how to extract them?

Quaternary ammonium group on anion exchange

disk

Ionised acid herbicide molecule in water

Acid herbicides are recovered from the watercourse by interaction with the positively charged surface of the

anion exchange disk:

IIIIIIIIIIIIIIIIIIIIIIIIIII

Passive sampler uptake profile

Deployment Time

Equilibrium Phase Linear Phase

Curvi-

Linear

Phase

Mass

Taken up

onto

Passive

Sampler

Sampling Rate (Rs) derived from this gradient.

Time Weighted Average Concentration

Time weighted average concentrations (CW in ng/l) can be derived from a simple equation:

where: MS = mass of pollutant on Chemcatcher disk (ng) M0 = mass of pollutant on field blank Chemcatcher disk (ng) RS = sampling rate of pollutant (l/day) t = Chemcatcher deployment period (days)

Acid Herbicides Uptake Trial (1)

• Carried out over 16 day period in Mar 2013 in two 20 litre glass tanks containing R.Exe water spiked at 1µg/l.

• 14 Chemcatchers fitted with anion exchange disks in each tank. Mounted on a rotating carousel.

• Naked disks in one tank, disks covered with PES membrane in the other.

• Tanks refilled with freshly spiked water every 24hrs to avoid significant depletion.

• Chemcatchers removed at regular intervals, disks dried, extracted and analysed.

• Uptake profiles generated for each pesticide allowing maximum deployment periods & sampling rates to be derived

Acid Herbicides Uptake Trial (2)

Uptake Trial Test Tanks Naked Disk Tank Carousel After 24hrs

Uptake Profiles for Dichlorprop

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.0 100.0 200.0 300.0 400.0 500.0

Chemcatcher With PES Membrane

Mass on Disk (µg) vs. Exposure Time (hrs)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0.0 100.0 200.0 300.0 400.0 500.0

Chemcatcher Without PES Membrane

Mass on Disk (µg) vs. Exposure Time (hrs)

Routine Acid Herbs Method at SWW

• Routine method used for water samples based on solid phase extraction (SPE), methylation and analysis by gas chromatography-mass spectrometry (GC-MS)

• 13 out of 16 compounds readily recovered from disks using 10% acetic acid in ethyl acetate

• Of these, three showed serious losses during evaporation to remove acetic acid (essential prior to methylation)

• Based on the above and bearing in mind known usage patterns and frequency of detections, nine out of 16 acid herbs selected for subsequent uptake studies

Link to NRW routine Acid Herbs method

• Routine method used at NRW for water samples based on direct aqueous injection and LCMS QQQ analysis

• Improved recoveries of Triclopyr and Clopyralid using formic instead of acetic acid

• Use of vacuum rotary evaporation instead of nitrogen blowdown apparatus - reduces losses of certain compounds

• Matrix effects such as ion suppression observed for several compounds but this can be easily compensated for using labelled surrogate standards

Uptake Trial - Conclusions

• Chemcatcher deployment without PES membrane not feasible – equilibrium reached too rapidly & problems with disk integrity

• Samplers give linear uptake for all nine acid herbicides over periods of around 10-14 days

• Measured sampling rates in the approximate range 50 to 100ml/day which is in good agreement with published values for sampling of corresponding pollutants with POCIS type passive samplers

Chemcatcher deployment trials

• Field based studies:

Deployment of passive samplers at selected locations in test catchment with known history of acid herb pollution at time of peak pollution risk

River Tywi chosen by NRW due to historical issues with acid herbicides

DCWW and NRW partnership to monitor River Teifi in West Wales due to exceedance of PCV of 0.1ug/l for MCPA at water abstraction site / treatment works

The Teifi and North Ceredigion

Management Catchment

Teifi

Chemcatcher field trial location - Teifi

Teifi at Cenarth Falls

Teifi at Llechryd WTW

Highly variable river levels can rise up to 15 feet above normal levels in winter. The platform was not even accessible in mid-January

Results of trial

TWA (ng/L)

Site name Dicamba Benazolin Bromoxynil 2,4-D MCPA Trichlorpyr Mecoprop

Llechryd WTW <2.71 <0.07 0.28 1.57 11.62 0.98 0.46

Glanteifi <2.71 <0.07 0.37 2.09 20.54 0.61 0.72

Cenarth <2.71 <0.07 0.31 1.68 13.35 0.87 0.52

Henllan <2.71 <0.07 0.23 2.00 15.96 0.70 0.58

Llanfair <2.71 <0.07 0.55 2.07 13.03 0.48 0.47

Llanbedr STW <2.71 <0.07 <0.07 1.76 1.81 0.31 0.09

Tregaron <2.71 <0.07 <0.07 0.08 0.37 0.16 0.07

Port Einon <2.71 <0.07 <0.07 6.96 2.42 0.47 0.11

Llechryd WTW 34.24 35.10 0.12 64.73 425.25 0.32 101.55

Glanteifi 43.72 45.04 0.16 85.36 512.95 0.56 119.19

Cenarth 53.68 58.14 0.15 101.67 693.51 0.41 147.58

Henllan <2.71 <0.07 0.27 0.29 2.56 0.36 0.11

Llanfair <2.71 <0.07 0.32 0.28 1.84 0.41 0.11

Llanbedr STW <2.71 <0.07 <0.07 0.24 0.76 0.38 0.08

Tregaron <2.71 <0.07 <0.07 0.08 0.17 <0.09 0.07

Port Einon <2.71 <0.07 <0.07 0.62 9.66 0.10 0.07

Summary

• A Chemcatcher based passive sampling method, using an anion exchange receiving phase has been developed for the monitoring of nine acidic herbicides in surface waters

• Quantitative data can be obtained over deployment periods of at least two weeks

• Field trials have shown the technique to be capable of detecting spikes of pollution missed by spot sampling

• Concentrations of pollutants below the LOD of the routine acid herbicide spot sampling method can be detected by passive sampling

• Ian Townsend from South West Water

• Phillippa Pearson, Tara Froggatt, Julia Haslett & Sarah Jones from Dwr Cymru Welsh Water

• Graham Mills & Richard Greenwood – University of Portsmouth

• Melanie Schumacher from NRW Llanelli lab

Acknowledgements

Thank you for listening