Final Report Summary - MEDEA (Advanced Methodologies for the Determination of the Lability of Trace Metals and Their Application to Contaminated Soils)
1. PROJECT OUTLINE Much of Western Europe has inherited soil contaminated with heavy metals from past mining, mineral processing and industrial activities. When performing risk assessment for eco-toxicological, human health or ground water vulnerability studies it is important to measure the labile fraction of metal that controls the solid-solution equilibria. Many simple extraction schemes have been devised to determine the fraction of available metal in soils but these generally do not give accurate measures of the geochemically reactive i.e. labile pool. One of the most advanced methodologies for accurately determining lability is ‘isotope dilution’ (ID). This technique simply reflects the pool of reactive metal in the soil and can be used to model solid-solution equilibria and the fixation of metal ions into less available forms. This Marie Curie fellowship aimed to (i) introduce and develop multi-element stable isotopic dilution assays into the British Geological Surveys laboratories, (ii) examine the lability and solution characteristics in a range of contaminated soil types (peats and alluvial soils), (iii) compare ID with other trace metal extractants in predicting soil pore water concentrations using the WHAM speciation model, (iv) look at the effects of anaerobic conditions on the labile pools of trace metal and redox sensitive elements such as Fe in alluvial soils and (iv) combine ID with Pb apportionment to examine the source of labile Pb in soils. 2. CASE STUDY I - PEAT SOILS FROM ROOKHOPE CATCHMENT (UK) 2.1. Outline: Rookhope catchment (Figure 1) in Weardale (NE England) has been a Pb mining area since Roman times. It has a legacy of contaminated (Zn, Cd, Pb) in soils. To complement existing data, contamination levels in peat cores were determined along with 206Pb/207Pb isotope ratios used in source apportionment in different pools of soil Pb and plants. 2.2. Major findings -Pb isotopes are unevenly distributed throughout the catchment: landscape position strongly influences the soil/plant exposure to atmospheric deposition -A simple Pb uptake model based on the free ion activity model provided the best description of grass uptake in contaminated soils (Figure 2) -Interpretation of paired plant (grass and heather) and soil Pb isotope data suggests that uptake of Pb was largely soil rather than atmospherically derived -Managed peat burning appears to be a major way of recycling Pb back to peat for plant uptake 2.3. Socio-economic impacts The impacts of this work are that it provides information regarding (i) concentrations of Pb in grass and heather that is relevant to owners of grazing cattle and the grouse industry, (ii) the paired soil and vegetation survey including ID on peat cores combined with isotope analysis shows that uptake into the grass and heather is predominately through the roots and can be modelled using a simplified Free ion activity model and (iii) managed heather burning is a way to recycle Pb within the soil-plant system. This work provides information with the Peatscapes (North Pennines Area of outstanding Natural Beauty) forum whom BGS has collaborations. 3. CASE STUDY II - ALLUVIAL SOILS FROM RIVER TRENT CATCHMENT (UK) 3.1. Outline The river Trent catchment (Figure 1) includes tributaries draining mineralised areas affected by extensive historical mining since pre-Roman times. Metal-rich sediments still pulse into the tributaries and therefore floodplains downstream the orefield act as a (temporary) repository of metals. In addition, several other sources of metal contamination to alluvial soils exist such as sewage works, power stations or traffic-related sources. A high resolution geochemical survey of the East Midlands confirmed enrichment of Pb, Zn and Cd. The first part of the work involved a survey of 24 top and sub soils from alluvial soils in the Trent catchment extending from the Peak District to Trent Falls. The aims of this work were to (i) determine spatial and vertical distribution of metal concentrations in top and sub soils, (ii) examine the labile pools of Cd, Zn and Pb, using multi-element ID (iii) extract soil pore waters, (iv) use WHAM 7 to model the solid–solution equilibria using the labile, total and 0.05M EDTA extractable pools of metals as inputs and (v) source apportionment of Pb in the total, labile and solution pools. In the second part of the work we studied the response of metals hosted in alluvial soils to an extended flooding event. To this end, we examined the effects of anaerobic conditions on alluvial soils and measured changes in the labile pools of Cd, Zn, Pb and Fe over a 40 day period by incubating soils in an O2 free environment. 3.2. Main results -In most sampling sites, the concentrations of Pb, Zn and Cd were considerably greater than the background levels (Figure 3). -We assessed two equilibrating electrolytes for multi-element ID assays in circum-neutral pH soils with high clay and organic C contents. Neither 0.0005M EDTA or 0.01M Ca(NO3)2 were appropriate for all soils and metals. -Labile pools of Cd, Pb and Zn varied between 45-95%, 9-56% and 11-37%, respectively. No greater fixation i.e. lower %lability of metals with depth was observed, possibly because soil characteristics such as pH, FeOx and clay content were generally similar; a result of the recycling of eroded and deposited soils within the river system (Figure 4). The lability of metals appears to be esentially controlled by soil pH (Figure 5). -The labile pool of metals provided much improved predictions of the solution metal concentrations using WHAM 7 compared to EDTA or total metal concentrations. This confirms that the labile pool measured by ID is the best representation of the pool of metal that is in equilibrium with the soil pore waters (Figure 6). -The labile pool of Pb is most prominently bound to Fe and Mn oxides across the pH range, whilst the labile Zn and Cd also show affinity for other sorptive surfaces such as organic matter or clays (Figure 7). -Statistically significant isotopic shifts (p<0.001) indicate that Pb from traffic-derived sources is enriched in the bioavailable pools i.e. labile and pore water pools, compared to the total soil pool (Figure 8). This suggests that, despite the withdrawal of leaded petrol in 2000, traffic-derived Pb is highly available in soils. Changing redox conditions caused by periodic flooding are likely to hinder fixation mechanisms, thus help maintaining more recently deposited petrol-derived Pb within the labile pool. Evidence for vertical migration of Pb from traffic-related sources down soil profiles was also found. -Under anaerobic conditions for 40 days there was very little change in the labile pool of metals measured. However, if an easily labile source of C was added, major increases in the size of the labile Pb and Fe pools were found indicative of the reduction of Fe3+ oxides, releasing Fe2+ and associated Pb (Figure 9). 3.3. Socio-economic impacts This work was a major study examining the lability of contaminants from various sources using ID and pore water concentrations in the UK. The paper will be of interest to the UK Environment Agency and to sustainable development groups such as the OnTrent organisation. The analysis of changes in the labile pools of metal under anaerobic conditions provides information in how contaminated alluvial soils in the Trent catchment may respond to periodic flooding events and, more specifically, their ability to fix the trace metals hosted. 4. CASE STUDY III - CAN ID ASSAYS BE USED IN EXAMINING ALKALINE COAL ASH? 4.1. Outline As a method development exercise we examined whether ID could be used to determine the isotopically exchangeable pool of metals in alkaline coal ash. This presents a potential source of trace element pollution to environments through leaching during storage or through being wind-blown. Whilst extraction and leaching tests are used for regulatory purposes, these provide little information regarding the mechanisms behind metal fixation and how it may change with the lagooning of fly ash and the processes associated with its weathering. In the first instance we used ID to determine labile pools of Cd, Pb and Zn in a range of fresh and weathered fly ash, where pH is generally much more alkaline (pH=9-12) than in soils. 4.2. Main results -Isotopic dilution assays worked effectively for Cd and Pb in high pH coal ashes using a 0.0005M EDTA equilibrating solution, whilst Ca(NO3)2 did not solubilise sufficient labile metal to provide robust ICP-MS isotope analysis. -At pH>11.5 ID method failed (Figure 10) as the spiked isotope appeared to be sorbed (presumably onto FeOx) or precipitated in minerals that may form under alkaline conditions such as ettringite. -The studied metals are poorly reactive in coal fly ash. Only 0.3-3% of the total Pb and Zn and 4-13% of the total Cd are labile. Fly ash weathering exerts little impact on the lability of these metals. -We examined different extractions and found that a 0.05M EDTA extraction can be used as a fast, cost-effective and simple proxy for isotope dilution assays to estimate metal lability (Figure 10). 4.3. Socio-economic impacts As coal use for power generation is certain to increase globally, the understanding of the weathering processes involved in coal ash storage and their impact on trace element availability are of increasing interest. This is of particular relavence in the Trent valley UK where large volumes of ash was dispossed in dumps over many years. ID provides an insight into the fixation and weathering processes that occur in both fresh and weathered coal ash. This is the first time ID has been used to examine the reactivity of elements hosted in an extensively used industrial by-product such as coal fly ash.