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


    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



  • 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 (0.1g/l for a given pesticide)

    Advanced treatment processes required for efficient removal

    Acid herbicides whats 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


    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


    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:


  • Passive sampler uptake profile

    Deployment Time

    Equilibrium Phase Linear Phase





    Taken up




    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 1g/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









    0.0 100.0 200.0 300.0 400.0 500.0

    Chemcatcher With PES Membrane

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











    0.0 100.0 200.0 300.0 400.0 500.0

    Chemcatcher Without PES Membrane

    Mass on Disk (g) 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


    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

  • 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