Inorganic Contaminants

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1 Inorganic Contaminants Dr Stephen R Smith Dept Civil & Environmental Engineering Page 1

2 Scope of Presentation Sources and concentrations Qualifying environmental limits Phytotoxicity Dietary intake Grain quality Soil fertility and microorganisms Recent research and other issues Conclusions Page 2

3 Metals in UK Sludge Recycled to Land Total concentration (mg kg -1 DS) Year Zn Cu Pb Cr Ni Total concentration (mg kg -1 DS) Cd Hg Year Page 3

4 Summary of Metal Inputs to UWW in European Countries (ICON 2001) Page 4

5 Metals in Livestock Waste Compared to Sludge (mg kg -1 DS) Zn Cu Poultry Pig Sludge Page 5

6 PTE Limits in Soil (mg kg -1 ds) PTE EU (86/278/EEC) EU (proposed) UK D DK US Zn Cu Ni Cd Pb Page 6

7 Environmental End-Points Phytotoxicity (Zn, Cu, Ni) Conservative soil limits highly protective Zootoxicity Humans via plant uptake (Cd) Humans via offal meat (Pb) Animals (Cu) Soil fertility (Zn) focus of debate Soil microbial biomass Nitrogen-fixing bacteria Page 7

8 Phytotoxicity Zn, Cu, Ni Pot trials No evidence of phytotoxicity in long-term sludge-treated soils Page 8

9 Zn Phytotoxicity - Pot Trials (Davis and Carlton-Smith, 1984) 8 Page 9 Yield of ryegrass (g pot -1 ) Critical soil content 320 mg kg Imperial Total CollegeZn Londonin soil (Log 10 mg kg -1 )

10 Phytotoxicity Sludge-Amended Field Soils No effect on yield Ryegrass = sensitive accumulator crop Long-term, well equilibrated soils Zn <2700 mg kg -1 Cu <1350 mg kg -1 Ni <540 mg kg -1 Minimal risk to crops from PTEs Page 10

11 PTE Availability Historic site soils Soil-crop transfer PTE pl = PTE S + ph Assess risk relative to HNOAEC in plants Page 11

12 PTE Content in Soil that Equates to the HNOAEC in Plant Tissue PTE HNOAEC in leaves Max tissue content Max soil content Predicted soil content Page 12 Zn Cu Ni (ph = 6.0) mg kg -1 leaf content }<HNOAEC

13 Soil PTE Concentrations that Protect Against Phytotoxicity PTE Predicted or *max soil content in historic site soils UK soil limits (ph 6-7) Soil lmit derived from US EPA 503 cumulative application Typical content in sludge (mg kg -1 ) Zn *Cu *Ni Page 13 *For Cu and Ni the soil content is for the highest recorded plant Imperial tissue College London concentration

14 Dietary Intake Cd (Carlton-Smith 1987) Cd in crop (mg kg -1 dm) Page EU and UK limit Lettuce (b = 0.75) Red beet (b = 0.22) Wheat (b = 0.1) Potato (b = 0.04) Cabbage (b = 0.03) Total Cd Imperial in College soil London (mg kg -1 )

15 Dietary Intake Model for Cd at 3 mg kg -1 in Soil (Carlton-Smith, 1987) Non-impacted food groups 3.7 µg d -1 Bread and cereals 6.66 µg d -1 Cabbage and legumes 1.01 µg d -1 Root vegetables 12.9 µg d -1 Leafy salads 0.68 µg d -1 Potatoes 8.8 µg d -1 Total intake = 34 µg d -1 at 3 mg Cd kg -1 in sandy loam soil, ph 6.5 Page 15 WHO provisional daily intake limit = 70 µg d -1

16 Dietary Cd Intake Steering Group on Chemical Aspects of Food Surveillance, 1993 Soil Cd content (mg kg -1 ) % Food from sludged land Dietary intake of Cd (µg day -1 ) % Increase on background intake of 18.8 µg Cd day -1 Page 16

17 Metal Uptake and Soil ph Value UK Statutory Soil Limits According to Soil ph Value >7 Zn Cu Ni Page 17

18 Grain Cadmium EU Regulation (EC) No 466/2001 of March 2001 setting maximum levels for certain contaminants in foodstuffs Max EU limit in wheat grain of mg kg -1 dry matter (0.2 mg kg -1 FW) Chaudri et al. (2007): Page 18

19 Total Topsoil Cd Concentrations (mg/kg) (Chaudri et al., 2007) Soil ph Predicted with 95% confidence to meet EU limit Present topsoil limit value (SI, 1989) Page 19

20 Grain Cadmium Grain Cd (mg kg -1 ) Wheat Cassington y=0.097x Barley Woburn y = 0.015x Soil Cd (mg kg -1 ) Page 20

21 Soil Fertility Sustainable soil management Microbial end-points Microbial biomass Soil respiration* Nitrogen transformations* Nitrogen fixation Enzyme activity Microbial community structure *Premi & Cornfield 1969, 1971; Cornfield et al., 1976) Page 21

22 Soil Microbial Biomass Total weight of microorganisms in soil Chloroform fumigation-extraction Microbial biomass Mediates biochemical transformations Repository of labile nutrients Relatively sensitive ecotoxic indicator Measured in sludge-treated soil Page 22

23 Woburn Market Garden Experiment (Brookes & McGrath, 1984) Soil metal concentration and microbial parameter FYM Sludge Zinc (mg kg -1 ) Copper (mg kg -1 ) Nickel (mg kg -1 ) Cadmium (mg kg -1 ) Microbial biomass (mg kg -1 ) Soil respiration (mg CO 2 -C kg -1 ) Biomass respiration (mg CO 2 -C g -1 biomass) Page 23

24 Soil Fertility Microbial Biomass Historic site soils High Zn 450 mg kg -1 Significant effect of Zn detected at >500 mg kg -1 - High Zn soil contained 450 mg Zn kg -1 - Uncontaminated control had 45 mg Zn kg -1 Page 24

25 N 2 -fixation in Woburn Soil (McGrath et al 1988) Page 25

26 Rhizobium leguminosarum bv. trifolii (Chaudri et al 1993) Page 26 Log 10 cell number g -1 soil UK limit ph Total Zn content in soil (mg kg -1 )

27 Rhizobium in Long-term Sludge-Amended Soils (Smith 1997) PTE Mean total soil concentration (mg kg -1 ) Rhizobia absent Zn Cu Rhizobia present Cd Page 27

28 Long-term Sludge Experiments Independent Scientific Committee Reviewing the Soil Fertility Aspects of Sludge Application to Land (1993) recommended: further research was needed to examine the effects of heavy metals from sewage sludge on soil microorganisms Effects of sewage sludge applications to agricultural soils on soil microbial activity and the implications for agricultural productivity and long term soil fertility Aim to improve the robustness of scientific information for setting soil metal limits not possible to identify unequivocally which (if any) heavy metal in sewage sludge was responsible for adverse impacts Page 28 of soil microbial activity (Zn, Cu and Cd)

29 Location of Sites in Britain 1. Gleadthorpe (GLE) 2. Woburn (WOB)* 7 3. Watlington (WAT)* 8 4. Pwllpeiran (PWL) 5. Rosemaund (ROS)* 1 6. Bridgets (BRI) 7. Hartwood (HAR) Auchincruive (AUC) 9. Shirburn (SHI) - calcareous Focus on Zn, Cu and Cd as principal elements of concern * Metal-amended liquid sludge and metal salt experiments Page 29

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31 Overview of Results Sludge cake sites: Microbial biomass and rhizobia decreased at some of the sites with increasing soil Cu and Zn Liquid sludge and metal salt sites: Decrease in microbial biomass due to Cu similar to cake sites No consistent microbial biomass response to Zn No consistent rhizobia response to Zn, Cu or Cd Page 31

32 Why are Cakes Toxic but not Salts?? Localised high metal concentration Sludge particles contain toxic amounts of metals? 6000 mg kg -1 Zn (x10 normal value) 5000 mg kg -1 Cu (x16 normal value Average soil concentration does not represent exposure to soil microbes Metal salts/liquid sludges blend more uniformly in soil profile Average soil concentration does represent exposure to soil microbes Page 32

33 Overall Conclusion from Long-Term Trials Current operational practice is safe and nonhazardous to the soil microbial community Apparent toxicity of the cake sludges explained by localised exposure to high metal concentrations On basis of salt and liquid sludge plots, current EU maximum soil limits protect soil microbial processes and long-term soil fertility Setting the results of SP0130 in a policy context (Defra 2007) Immediate regulatory changes as a result of the experimental results are unwarranted Page 33

34 Other Issues and Recent Research Other elements eg Pt group, Ag Predicting metal availability: Variability of extractable and soil solution measurements may limit prediction of metal availability Total soil metal preferred + other soil variables Model parameters include: ph, organic matter, clay minerals, CEC Australian National Biosolids Research Programme proposals Renewed interest in CEC (eg Oorts et al 2006; Rooney et al 2007) Surrogate for other soil parameters Thoroughly investigated and dismissed as reliable predictor by earlier research for biosolids-amended soil Microbial biomass questioned as a robust indicator (Broos et al 2007) Problems due to small-scale spatial heterogeneity Use of metal salts to determine affects of soil properties on toxicity (eg Smolders et al 2004; Oorts et al 2006; Rooney et al 2007) Careful interpretation in terrestrial risk assessment and relevance to biosolids Page 34

35 Conclusions Reduce diffuse metal inputs as far as practicable Research defined metal limits are realistic and pragmatic Protect crop yields, human diet and soil fertility Excessively precautionary approach is not justified by available scientific evidence Current EU Directive controls on metals offer flexibility and appear safe and precautionary Page 35