Pulverised fuel ash (PFA) also known as 'fly ash' is the waste product of burning pulverised coal for the generation of electricity. The use of coal is still a prime (and sometimes growing) method of energy production in many countries. In the past, power stations fuelled this way were all too common in the UK (some still remain). PFA was deposited into lagoons (shallow pits) or used to fill excavations, transported there either in a dry state or as slurry.
An example of defunct site is located at Barking Riverside (east London) where millions of tonnes of PFA were deposited on the former Barking Marshes, over a 30-40 year period (ending in the 1970s). The study below was conducted in the late 1980s and early 1990s. However, first the unusual nature of PFA is explored: its composition, properties, structure, colonisation by plants and finally, its use as a material used in conservation.
It is said [Frances 1961] that around 95% of coal ash consists of alumina, silica, iron oxide and lime. The remaining 5% being largely made up of: magnesia, sodium and potassium oxides, titanium oxide (basic constituents) and chloride, sulphate and phosphorous (acidic constituents). There are many rarer elements present in coal ash, e.g.: the platinum metals, rare earths, metaloids and natural radioactive heavy metals are all represented. Of particular interest are the latter two groups. The metaloids include the element boron, an essential trace mineral for plants but extremely phytotoxic in larger quantities. Radioactive isotopes of uranium, radium and to a lesser extent thorium naturally occur in coal and are significantly enriched in the ash [Environmental Resources Ltd. 1988]. The decay of these materials results in the emanation of radon gas. Under certain circumstances this gas can pose an increased risk to the health of people living in the area.
PFA is composed largely of spherical silica type particles more akin to sand than true soil. These may be separated into two distinct types:
THE PROPERTIES AND STRUCTURE OF PFA SOILS
Because of its unusual physical and chemical properties, deposits of PFA behave quite differently from most natural glacial or drift materials with regards to soil formation and subsequent colonisation by plants. A normal loamy soil may exhibit a 'crumb' structure. Conversely, PFA 'soils' are usually stuctureless and possibly cemented. When freshly laid PFA soils contain virtually no organic matter (except possibly a little residual carbon). Additionally, they are biologically sterile. Nitrogen (N) is quite absent, phosphorus (P) although of relatively high content is not readily available to plants, but potassium (K) is adequate. PFA soils show good moisture retention, however, drainage is poor and impermeability to water percolation may become a problem with time. Fertilisers have a poor response and retention, requiring twice the quantity of a loamy soil. The pH of PFA soils is high, usually within the range of 8 to 13, it will though decrease with weathering and increase in dry periods as moisture and solvated materials are raised to the surface via capillary action [Emberson 1990]. Further, plants may take up trace elements from their roots, via their sap then retain and concentrate the material(s) in question, in the leaves. When the leaves fall the trace elements(s) can be concentrated in the upper humic layer [Frances 1961]. A 'hard pan' effect may develop, usually between 60 and 80 cm below the surface, which may inhibit drainage [Emberson 1990].
LIMITING FACTORS AFFECTING THE COLONISATION OF PFA SITES BY PLANTS
Three major factors limit the growth, and therefore colonisation, of PFA soils:
Coal contains between 3 to 100 ppm boron. The element is enriched in the ash and thus the percentage considerably higher. Frances  shows that a rich ash may have a boron concentration of 600 ppm with a maximum reached at around 3,000 ppm. Deposits of relatively high boron content (about 5%) have been found on the boiler tubes of steam generation plants within power stations [ibid]. Thus it seems likely the boron content could be considerably reduced in PFA as opposed to ash proper (also, a proportion of it may not be in a form extractable and therefore usable by plants). Nevertheless, it seems the percentage of the element is still too high. The result, plant growth is limited within the first 2 to 3 years after PFA deposition.
Although less critical than boron, the concentration of soluble salts is still important, particularly sulphates. High concentrations of these salts lead to the colonisation first by halophytes, such as Atriplex spp. and certain mosses. These plants can cope with relatively high concentrations of electrolytes [Shaw 1990].
The pH of freshly deposited PFA may be as high as 13, as with other limiting conditions, it takes 2 to 3 years for values to fall to a point low enough to allow plant growth (say, less than pH 10). After 30 years the results of weathering are clearly evident. Two good examples come from the Lee Valley Park near Chesthunt, Herts. [BES, 1990]: The surface pH from two locations within the site (ie. 1 and 2) were found to be around 7; at 15 cm depth the pH for '1' was between 9.0 and 9.6, and for '2', 7.4 to 7.9; at 40 cm the pH values for '1' were recorded from 9.6 to 10.2, and at location '2', 8.9 to 9.7. The higher pH values for '1' at 15 and 40 cm shows the effect of the 'hardpan'. This layer was shallower in the first location, thus leaching was inhibited below its level. The pH of the original soil at both locations and the three depths was found to be within the range of 6.9 to 7.4 and exhibited no sign of stratification.
COLONISATION OF PFA BY PLANTS
Colonisation of Floaters
Because floaters contain less harmful phytotoxins and are generally of a less alkaline nature (ie the pH is lower) they are relatively quickly colonised by plants [Shaw 1990]: In flooded PFA lagoons these low density particles will float on water often forming a thick layer. Rapid colonisation by plants will take place, for instance by Ranunculus spp., Juncus and Typha. Within one year a floating bog is formed.
Colonisation of Bulk PFA
It has been demonstrated that the colonisation of bulk PFA will not proceed for the first 2 to 3 years. Succession is most likely take place as follows [ibid]:
THE TILBURY EXPERIMENT
During the 1980s and 90s National Power conducted a series of experiments on PFA soil at their power station, aside the Thames, at Tilbury, Essex. The aim of these trials was to examine whether succession can be accelerated by the introduction of seeds from plants which have colonised other PFA sites naturally. Seeds were not taken from the plants directly but via the topsoil. The buried seedbank would contain genetic material of most plants that had grown well on the site [Shaw 1991]. The trials made use of seeds, both separately and together, from the three main stages in PFA succession:
The topsoil was collected from selected saltmarsh and PFA sites in England and Wales. Some of the experimental plots were fertilised with (low ammonium) NPK and poultry manure whilst others were left as unfertilised controls [ibid].
Results indicate that plants of the saltmarsh performed particularly well (e.g. Chenopodium rubra and species of Atriplex). One surprise perhaps, was the success of Melilotus officinalis that appears to be well adapted to PFA colonisation.
A sharp humic upper soil horizon begins after around 25 years. In terms of soil formation this is a very short period. It seems, at least for the first 10 years PFA is toxic to earthworms, thus there is very little mixing of surface organic matter with the materials from the lower levels (ants are often the only source of vertical mixing). Thus the humic layer sometimes becomes acidic at the surface. Although conditions generally are calcareous (i.e. basic below), paradoxically, mosses of acid habitats have been found to occur at the surface [Shaw 1990]. The concentration and distribution of organic matter on the soil has, in fact, been demonstrated as being a reliable method of determining the age of a PFA site. Interestingly, succession on such a site was indicated as being a less reliable method of estimating the length of establishment [ibid].
CONSERVATION AND PFA
Use in Encouraging Grassland Diversification
PFA and other nutrient poor materials are now finding value in conservation [Hopkins 1990]: Consider 85% of Britain's grassland receives K, P, and N. The result is ’improved’ grassland where the delicate or rarer species are forced out by more aggressive plants. For example, Perennial Rye-grass Lolium perenne may proliferate at the expense of the less aggressive fescues Festuca spp. In situations such as these PFA has been added to the 'improved' soil to introduce environmental stress and with it the diversity of plant species [Hanley 1990]. Three main methods of application have been tried [ibid]:
It seems the last method was most successful. However, PFA may not be the best waste material for this purpose. There is some evidence that the relatively high concentration of phosphorus it contains may encourage the growth of certain legumes (e.g. clovers). Eventually soil nitrogen levels would rise via fixation and give unwanted improvement. Thus, in the long term, a waste material low in phosphorus would be much better like that obtained from crushed tower blocks.
Miscellaneous Conservation Uses
When wet, PFA has proved an ideal roost for waders; booms have proved useful to contain floaters and encourage the growth of floating bogs. Finally, PFA 'cliffs' have proved useful for Sand Martins to breed [Hanley 1990].
RELOCATING UNCOMMON SPECIES
With other industrial lime rich wastes at least 1.5 metres depth of material needs to be transported to an alternative site to give best conditions for transplantation [Cakebread 1990]: This ensures weeds from underlying strata do not find their way through and hinder the growth of any transplanted species. Plants (such as orchids) can be marked in the summer and dug up and moved to a new site in the winter. Increased germination and rejuvenation of certain plants has been noted, like the Valerian Valeriana officinalis and the Common Centaury Centaurium erythraea.
If PFA and soils and plants were to be moved in this way it seems likely the top humic layer containing most of the 'weed' species would have to be removed. Most viable seed is found within the first 2 cm of soil [Shaw 1991]. Further, PFA below the hardpan may not be immediately useful as pH, soluble salts and boron levels could all still be too high. Therefore there is a possibility that only the 40 to 60 cm between the humic layer and the hardpan would be suitable for transporting and the subsequent transplanting of more sensitive species. In addition, once the soil has been relocated over an alternative substrate to a depth of at least 1.5 m the height of the water table would need to be maintained (at least until an impervious hardpan had formed). Suggestions include the use of liners or possibly additives [Hanley 1990]. It must be remembered that plants such as marsh orchids Dactylorhiza spp. require damp conditions for survival.
PFA AT BARKING
The power station that was to dominate the latter history of Barking Marshes opened on the 19th of May 1925, the ceremony being conducted by King George V [CLESCL 1925]. With little doubt the dumping of ash began soon after (although until the opening of station 'C' probably not on a large scale). The ash was placed in nearby 'lagoons'. An illustration from 1934 [the Engineer] shows this practice was well established. The 'station 'A', as it was termed, capacity of electricity production was then 240,000 kW. In 1933 a new plant (station 'B') opened increasing generating capacity to 390,000 kW [The Engineer 1934]. Both pulverised fuel and stoker systems were then in use, fuelled by 'run of the mine' coal [CLESCL II 1931]. Bituminous coal from Northumberland and the Midlands was used. A proximate analysis was: moisture 10%, ash 14%, volatile matter 29%, and fixed carbon 47% [British Electric Authority]. By 1948, station 'B' was operating at its maximum capacity of 300,000 kW. Additionally, a third station 'C' was under construction in 1948. This would have an eventual capacity of 240,000 kW [ibid]. PFA from these dumps was quenched and transported in trucks possibly to the ash-dumps shown just north of the station (re: the Engineer 1934). It seems though, in the early years, a substantial quantity of the ash may have been removed from the locality: Some may have been dumped at sea. Conversely, it seems too that some sort of market existed for PFA that would have led to quantities being removed from site [CLESCL II 1931]. Dumping of fly ash (and coal) over Barking Marshes continued for 30 to 40 years until it was run-down between 1965 and 1975. The power station closed in 1981 [Howson c.1982].
Extent of Deposits
Over the 30 to 40 years dumping of PFA continued at Barking. A large proportion of the 271 ha of Thames Marshes at Barking Riverside (previously referred to as 'Ripple Levels' and later ‘Barking Reach’) was covered with the material. From a map of surface deposits a total of 49 ha were found to be overlaid with PFA (about 18%). Most of the ash, some 32 ha, lay west of Renwick Road, covering the majority of the section, the remaining 17 ha being located in the eastern section on the other side of the road. The actual quantity of dumped PFA deposited over the 30 to 40 year period is unclear. It seems good records were not kept, particularly in the early years. Estimates, calculated on the basis that a power station produces 1,000 tonnes of ash per megawatt per day, indicate that in the order of 10 million tonnes of PFA could have been deposited [Environmental Resources Ltd. 1988].
East Renwick Road West Renwick Road
The Floral Phoenix
How then does the model of recolonisation (outlined above) relate to the flora occurring on PFA at the site in Barking? In the section east of Renwick Road grazing by cattle holds back 'normal' succession. However, in the cattle free western side (Thameside Park) it is found to be particularly applicable: high boron, pH and sulphate levels certainly seem to favour the growth of mosses and halophytes in areas of more recent ash dumping and those freshly disturbed: Atriplex spp., Sea Beet Beta vulgaris ssp. maritima and Sea Aster Aster tripolium are all common. In longer established parts the Coltsfoot Tussilago farfara is of frequent occurrence. Invasion by the more common 'weed' grasses and other herbs is in progress e.g. False Oat-grass, ragworts, thistles Cirsium spp., Melilotus and Trifolium spp. In areas left undisturbed for 10 years or more calcareous weeds are evident in line with the model. In this category, in wooded areas two species of orchid occur the Common Spotted Dactylorhiza fuchsia and Southern Marsh Dactylorhiza praetermissa, and in patches with only sparse vegetation the Common Centuary Centaurium erythraea. Many other plants associated with calcareous soils occur in the PFA areas at Barking, examples are: Old Mans' Beard Clematis vitalba, Wild Mignonette Reseda lutea, Butterfly-bush, Buddleja davidii, Viper's-bugloss Echium vulgare, Yellow-wort Blackstonia perfoliata and Beaked Hawk's-beard Crepis versicaria ssp. haenseleri. Again in line with the model of PFA colonisation is the appearance of woody plants. The history of the copse site, as best it can be determined, shows that the last of the dumping and dereliction occurred in the early 1960's and the first woody plants appeared during the mid-1970s, within the 10 to 15 years suggested. Two species of willow are common: the Goat and Grey Salix caprea and S. cinerea. In roughly equal numbers with the willows is Birch Betula sp. The model states that Oak Quercus sp. has not been found on a site less than 20 years established and usually makes an appearance after 30 years or more. In 1991 the site had been subject to succession between 25 to 30 years, the Oak was absent. However a few small Oaks occurred in the eastern section, on areas where other forms of landfill had been practised.
It is said there is little vertical mixing of PFA soils and this can lead to the formation of an acidic surface layer of organic matter. Thus (perhaps paradoxically) plants often associated with more acidic soils can be found on the calcareous PFA. The result at Barking: within the ash lagoons east of Renwick Road, intermingled in tracts of bare soil, are various fescues Festuca spp. and the Cat's-ear Hypochaeris spp.
The lime rich hardcore and other rubble dumped on parts of this area also encourage a few plants of calcareous habitats. Most notable are the Vervian Verbena officinalis that Jermyn  reports as uncommon in Essex and the Musk Thistle Carduus nutans a plant of quite limited distribution in the London area [Burton 1983].
Two small pockets of land still occur where little intentional dumping has taken place and the marshland floor is exposed. These areas of original grazing marsh are flat, low-lying and as a result sometimes flooded in winter. Walls of PFA and other dumped materials rise up 2 to 3 metres virtually enclosing each. The flora of the smaller area (less than 1 ha) has not been surveyed in detail, however, the larger area (around 3.7 ha) is dominated by Cock’s-foot Dactylis glomerata intermingled with Creeping Thistle Cirsium arvense. The Ribwort Plantain Plantago lanceolata, Hoary Cress Lepidium draba, Creeping Cinquefoil Potentilla reptans, Bird’s-foot Trefoil Lotus corniculatus and Red Clover Trifolium pratense are also quite common. There are also extensive patches of Perennial Rye-grass and Twitch Elytrigia repens. Generally, though, there are few rarities or plants of great interest.
The examples given above clearly demonstrate that plant diversity is increased in areas of environmental stress on PFA soils. Yet the areas of original marshland may be described as 'species impoverished'.
Today (2014), most of this area is being developed for housing. Many of the floral and faunal rarities have long since vanished as succession, lack of management and adverse changes in hydrology (caused by the preparation of the land for housing) take their toll. Nevertheless, a small area of the original site remains undeveloped today*; the Ripple Nature Reserve. This might be a shadow of the land it was once part of, and the orchids long since gone, but it remains a green and pleasant oasis still worth visiting.
Finally, in compiling this work I was very fortunate to have the wonderful Barking Central Library as a resource – sadly this has been redeveloped and is no longer the place of learning about our past it once was.
* Please note that despite the description of the Ripple LNR given by this website, no orchids have occurred here since 2001
British Ecological Society (BES) Industrial Ecology Group, 1990 (30th of May). North Metropolitan Pit Orchid Sites.
British Electricity Authority (London Division) c. 1948. Barking Power Station.
Burton R., 1983. Flora of the London Area, London Natural History Society.
Cakebread, 1990. British Ecological Society Conference on PFA at Hatfield Polytechnic on the 30th of May.
Clapham, Tutin and Moore, 1987. Flora of the British Isles, 3rd edition. Cambridge.
Clapham, Tutin and Warburg, 1981. Excursion Flora of the British Isles, 3rd edition, Cambridge.
CLESCL, County of London Electricity Supply Company.
Emberson P. (from National Power), 1990. British Ecological Society, Conference on PFA at Hatfield Polytechnic on the 30th May.
Environmental Resourses Ltd., 1988. Barking Reach Environmental Assessment Appendix, interim report (Sept. 1988).
Francis W. Coal, Its Formation and Composition, London: Edward Arnold Publishers Ltd., 1961.
Hanbey, 1990. British Ecological Society, Conference on PFA at Hatfield Polytechnic (30th of May) from the Groundwork Trust.
Hopkins J., 1990. British Ecological Society, Conference on PFA at Hatfield Polytechnic (30th of May) from the NCC.
Jermyn S., 1974. Flora of Essex. Essex Naturalist Trust.
Shaw P., 1990. British Ecological Society Conference on PFA at Hatfield Polytechnic (30th of May). From National Power.
Shaw P. 1991. Experimental Revegetation of PFA by Plant Communities of Conservation Interest: The Tilbury Project. National Power.
The Engineer, 1934 (Feb. 2nd, 9th, and 16th). The New Barking Power Station.
If you are interested in the history of Barking and Dagenham in east London (and who is not?) why not visit the following website (compiled by my brother) where history is in the past: Barking & Dagenham Local History