A biological classification scheme to assess the sensitivity of Scottish and Northern Ireland Soils to heavy metals.

June 2001


The Scottish Environment Protection Agency (SEPA) and the Northern Ireland Environment & Heritage Service (EHS) have duties placed on them by Government to regulate polluting discharges to the environment and to monitor for the effects of such discharges and are both members of SNIFFER (Scotland & Northern Ireland Forum for Environmental Research). The duties extend to monitoring of the quality of air, marine waters, surface waters including rivers and lakes, groundwaters, land and soil.

An increasing amount and also a wider range of organic wastes are being applied to land under new waste management procedures. This is driven primarily by the perceived benefit accrued from the addition of organic matter which is presumed to result in ecological improvement. Many wastes, however, contain potentially toxic elements such as heavy metals. SEPA/EHS require guidance on which soils are likely to be most suitable for this purpose and where environmental risks are minimised. This report details a project funded by SNIFFER to examine the feasibility of developing a biological classification to identify soils that might be sensitive to heavy metal pollution.

An existing methodology for assessing the sensitivity of soils to heavy metals (Blume and Brümmer, 1991), based on easily measured physico-chemical attributes of soils, was tested to determine how well it predicted biological effects. The validity of the approach was determined by testing 20 different soils, covering the range found in Scotland and Northern Ireland, in a dose response experiment using copper (Cu), zinc (Zn) and cadmium (Cd) sulphate salts at 0, 0.5, 1, 2 and 4 times their respective UK guideline limits. Biological sensitivity was assessed using three bioluminescent bacterial biosensors (Pseuodomonas fluorescens, Escherichia coli and Rhizotox C) and three microbial community parameters (microbial biomass, basal respiration and Biolog community level physiological profiles, CLPP). Measurements were made after 1 month equilibration of the metals and again after 8 months to determine if there was evidence of either recovery or chronic toxicity in the microbial community. Effective concentrations (EC50), causing a 50% reduction in the biological parameters, were calculated and used to compare soils and rank them in order of biological sensitivity.

The bacterial biosensors were most sensitive to Zn and showed a clear relationship with the amount of metal found in solution after 1 month. Most of this response was due to pH differences between the soils. The Biolog CLPP was the most sensitive of the community measures, especially to Cu, and showed toxicity to Cu increased over time.

The overall results showed that while the Blume and Brümmer classification was generally good for most soils there were also exceptions with soils predicted to have low sensitivity clearly having evidence of toxicity. In addition to soils with low pH already identified by the Blume and Brümmer method, arable soils, with low organic matter and relatively high pH were found to be more sensitive than predicted. The soil property that most influenced the toxic response was soil pH value (in CaCl2). The % C and soil texture also explained a proportion of the toxic response in some cases. Taking into account the biological sensitivity of soils with low organic matter a new biological sensitivity classification was proposed which matched the ranking of sensitive soils more closely than the physico-chemical method.

The new sensitivity classification was mapped for the region of Fife to demonstrate how such an approach could have strategic value for determining which soils may be most vulnerable to inputs of heavy metals such as those derived from waste recycling. The main difference between the biological and physico-chemical approaches was that the former resulted in more soils being identified as sensitive to Cu and additionally resulted in greater differentiation of Cu sensitivity. While further validation of this new classification would be required it demonstrates how easily measured attributes may be used to predict the potential ecosystem sensitivity to heavy metals. This could be used at a strategic planning level and at a field level for site evaluation prior to giving consent to recycle heavy metal containing wastes.


heavy metals, risk assessment, Blume and Brümmer, bioassays, soil microorganisms, soil sensitivity, spatial representation, Fife, mapping

Copies of this report are available from the Foundation, price £35.00, less 20% to FWR members