In 1994, three research Institutes along with two private companies set up a number of field experiments at nine field sites across the UK (pdf file) [1]. The sites were chosen to reflect the different soil types found in agricultural and land uses suitable for application of waste water sludge thereby identifying those soil types more vulnerable to heavy metal contamination.
Host Institute | Field Site | Site Location |
---|---|---|
The James Hutton Institute | Hartwood [2] | North Lanarkshire |
Rothamsted Research [3] |
Watlington |
Oxfordshire Bedfordshire |
Scottish Agriculture College [4] | Auchincruive [5] | Ayrshire |
ADAS [6] |
Bridgits Rosemaund Shirburn Gleadthorpe Pwllpeiran |
Hampshire Herefordshire Oxfordshire Nottinghamshire Cereigion |
The project was divided into three phases: Phase I: 1994 - 1998, Phase II: 1999 - 2001 and Phase III: 2002 - 2006. In Phase 1, sludge cake with high levels of a single heavy metal (Cd, Cu or Zn) was applied annually so as to bring the soil concentrations up to the target level after four years. Subsequently and with the exception of some long-term build up plots, no more sludge was added and the plots were maintained for regular sampling and analysis. Treated waste water sludge was applied at the start and periodically within each phase up to the level of metal determined by the rates as given above.
Soils were sampled periodically throughout the three phases from each site and analysed for heavy metal content, soil chemical properties and biological properties - respiration (measurement of soil microbial activity), microbial biomass (measurement of total microbial population) and rhizobial nodulation (measurement of soil nitrogen fixation).
Site | Topsoil texture | pH | % organic carbon | % total nitrogen |
---|---|---|---|---|
Hartwood | Sandy clay loam | 5.9 | 4.32 | 0.25 |
Auchincruive | Sandy clay loam | 6.2 | 3.39 | 0.20 |
Watlington | Sandy loam | 6.8 | 1.29 | 0.17 |
Woburn | Loamy sand | 6.9 | 1.12 | 0.14 |
Bridgits | Silty clay loam | 6.3 | 1.91 | 0.23 |
Rosemaund | Silty clay loam | 6.2 | 1.69 | 0.25 |
Shirburn | Calcareous clay loam | 8.1 | 2.23 | 0.35 |
Gleadthorpe | Sandy loam | 6.0 | 1.42 | 0.18 |
Pwllpeiran | Clay loam | 6.4 | 3.71 | 0.41 |
Although waste water sludge could potentially contain a large number of heavy metals it was impossible to test all the metals which may be present and which could be responsible for the toxic effects noted on crops and soil microbial processes. An initial choice had to be made based on previous experimental evidence from sites around Britain. It was unlikely to be chromium, lead and mercury as these are largely insoluble in sludge treated soil of near neutral to neutral pH, and hence are less likely to be bioavailable. However, zinc and copper had been implicated from trials done at Rothamsted Research and at ADAS so were chosen. Cadmium was also chosen as there was some evidence that this might affect the Rhizobium population and hence the capacity to fix nitrogen in soil.
Metal | Rate 1 | Rate 2 | Rate 3 | Rate 4 | Upper UK permissible limit (at time of experiment) |
---|---|---|---|---|---|
Zinc | 150 | 250 | 350 | 450 | 300 |
Copper | 50 | 100 | 150 | 200 | 135 |
Cadmium | 1 | 2 | 3 | 4 | 3 |
The rates which were chosen reflected on current legislation and the maximum permissible amounts which could be applied to soil.
More details about the waste water sludge and the project are below.
Links:
[1] https://www.hutton.ac.uk/sites/default/files/files/map of LTSE feild sites(1).pdf
[2] http://www.hutton.ac.uk/about/facilities/hartwood
[3] http://www.rothamsted.ac.uk/
[4] http://www.sac.ac.uk/
[5] http://www.sac.ac.uk/contactsac/officelocation/
[6] http://www.adas.co.uk/
[7] https://www.hutton.ac.uk/background/benefits-problems
[8] https://www.hutton.ac.uk/background/contacts
[9] https://www.hutton.ac.uk/background/acknowledgements
[10] https://www.hutton.ac.uk/staff/colin-campbell
[11] https://www.hutton.ac.uk/staff/pat-cooper
[12] https://www.hutton.ac.uk/staff/duncan-white
[13] https://www.hutton.ac.uk/about/facilities/hartwood/LTSE
[14] https://www.hutton.ac.uk/about/facilities/hartwood/LTSE/background/contacts
[15] https://www.hutton.ac.uk/about/facilities/hartwood/LTSE/background/acknowledgements