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Chapter 13 - Ash PFA storage
13.1 Potential constraints
associated with PFA storage areas
PFA is composed largely of fine particles (2-20Ám) of aluminosilicates. Although the precise chemistry of PFA varies according to the coal from which it is derived, when freshly deposited it is typically both highly alkaline (pH 9-11) and high in salts, including borates at levels that are toxic to most plants. PFA has a low nitrogen content but reasonable levels of phosphate. The availability of this phosphate to plants varies according to the pH of the ash, becoming increasingly available as the PFA weathers. After 10 to 20 years of weathering, PFA is a fairly neutral and inert substance. It is generally a rather impermeable material because of the fine particle size and its tendency to form a stable cement 10-20 cm below the surface. This encourages pools to form in any hollows left in the surface of a PFA deposit.
Many areas used for the storage of PFA have been noted for their wildlife interest. Although the slurry is lifeless, when stored in lagoons next to the coast it can provide the bare open landscape that many waders select for roost sites. The inert nature of weathered PFA enables any deeper hollows to develop into clear pools suitable for many wetland plants and animals. Large areas of damp or wet PFA have been colonised by reedbeds and their associated bird-life. PFA has improved the wildlife value of some gravel pits, reducing the depth in deeper pits and producing shallowly shelving shorelines in others. However, PFA is best known in the conservation world for the huge populations of orchids that it can support. Further discussion about PFA and its value to wildlife is to be found in Ref. 81. (Appendix 11 - Selected references and further reading).
Fresh PFA can remain virtually barren of vegetation for up to 5 years. The problem results from the combination of chemical properties exhibited by PFA: high boron content, low nitrogen levels, salinity, and high alkalinity. Salt tolerant plants (halophytes) can cope with this combination however, and, consequently, are often the first colonists. While a variety of species, including saltmarsh grasses (Puccinellia spp.) and sea-spurrey (Spergularia spp.), are often to be found at coastal sites, Spear-leaved Orache is typically the only such species to be found colonising inland sites. Within a few years, however, PFA weathers to become a distinctly inert substance on which a rapid succession of plant communities takes place; young woodland has developed on PFA only 30 years old. Plants growing on PFA have been shown to accumulate molybdenum, selenium and arsenic, which has implications for anyone considering the use of grazing stock on such sites.
The lack of nitrogen in the substrate is a significant inhibitory factor to plant growth. Once the alkalinity of the PFA has declined to pH 8 through weathering, leguminous plants start to colonise, their ability to fix atmospheric nitrogen briefly allowing them to become the dominant component of the plant community before more rapid succession gets under way. The slow plant colonisation is mirrored by earthworm colonisation; studies indicate that it may be 3-4 years before the first earthworms appear and 20 years before a mature population is established (22 (Appendix 11 - Selected references and further reading)).
Where PFA has reacted to form a cement, the depth to which plant roots are able to penetrate is likely to be severely limited. Consequently, trees and shrubs will develop very shallow root plates, and are thus prone to wind-blow, desiccation, and drowning.
The fine composition of PFA means that it is good at retaining water. The slurry is slow to dry out and consolidate, which makes it behave like quicksand. Vehicular access, land-forming works and even pedestrian access may be virtually impossible for several years following deposition.
A small proportion (typically 1-2%) of PFA is made up of small hollows known as cenospheres. These particles are also known as 'floaters' as they float on the surface of water. Although more inert than fresh bulk PFA, there have been a few observations suggesting that the scum formed by floaters can sometimes dry to leave an encrustation on birds and other wildlife that come into contact with it. While many waterbirds seem to avoid this scum, both Shoveler and waders have been observed feeding in it (see Features 11.3 (Chapter 11 - Water storage reservoirs) and 13.1 ). These light particles are perhaps more of a problem when they dry out, as they form a very fine dust which is not easily settled out.
This section considers situations where PFA has proved attractive to wetland wildlife and how features of value can be provided in the future. There are several good reasons for considering the creation or enhancement of wetlands on PFA storage and deposition areas:
1) the impermeable crusts that form on PFA deposits;
2) its deposition as a slurry;
3) the inert nature of weathered PFA.
The stressful environment presented by fresh PFA limits the wildlife value of active lagoons. The shiny, wet slurry can, however, prove attractive to waders, gulls and a few other waterbirds looking for a safe roosting or loafing site. The lagoons at Musselburgh near Edinburgh (Scottish Power) and West Thurrock in Essex are two examples where large numbers of roosting waders regularly gather. Relevant ideas for optimising lagoons for roosting birds are given in 12.2.1.(Chapter 12 - Silt storage lagoons). Although the unconsolidated nature of the material limits the potential for producing surface contours, it will be possible to get some bed variation, as with wet silt, by varying the positions of points of discharge and outflow. However, the range in PFA particle sizes is less than for most silts so that deposition tends to be more uniform.
Where a series of lagoons are used on a long rotation, as at Fiddlers Ferry Power Station (Feature 11.3 (Chapter 11 - Water storage reservoirs)), bed variation may have the added benefit of creating hollows in which pools remain while a lagoon is rested. The proposals for Fiddlers Ferry suggested a management regime for the three PFA lagoons to enhance the wildlife interest over their life (Figure 13.1).
Occasionally, lagoons are colonised by Common Reed after ash deposition has ceased. It is not clear what determines whether a lagoon is colonised, but it is assumed that reeds do best where there are high levels of surface moisture. The lagoons at Uskmouth in Gwent and West Thurrock (both National Power) contain substantial reedbeds, the latter even supporting breeding Bearded Tits. It should be possible to increase the wildlife value of these reedbeds by adding areas of open water, such as ditches and pools (see 10.5 (Chapter 10 - Wetland designs for wildlife) and 14.2.1(b) (Chapter 14 - Water treatment systems)). However, the soft nature of the material means that this is only realistic once it has completely dried out, by which time the reedbed may be going into decline.
PFA lagoons are typically constructed of clay and consequently hold water even when empty of ash. A number of lagoons have been left in this state following the cessation of coal burning on the site, providing yet further opportunities to encourage wetland wildlife. At Richborough in Kent (Powergen) two disused PFA lagoons have been adapted to take the site drainage water and, more recently, have been targeted as part of the wildlife enhancement scheme being implemented around the power station. The intention is to raise the water levels within the lagoons by improving the original sluices and pumping in additional water as the need arises.
Although it is widely recognised that flooded gravel pits can be of considerable wildlife interest, there are many pits, particularly some of the older ones, where this interest is limited owing to deep water and steep banks. To overcome such limitations, an inert material, such as PFA, can be used to backfill the pit. PFA slurry can be used to create a gently shelving beach, a feature often missing from such pits. Where more extensive deposition of PFA is proposed, it can be used to convert deep pits into shallow lagoons (see 6.1 (Chapter 6 - Water: the physical environment) and 16.1.4 (Chapter 16 - Reclaiming industrial land)). It may be preferable, in nature conservation terms, to part fill a whole series of pits rather than completely fill a few pits. The existing wildlife interest of a flooded pit should always be assessed before a decision is made to tip in it (see 3.4 (Chapter 3 - Deciding what to do)).
a) Floating islands. Where PFA has been dumped in a flooded pit, floaters often form a whitish scum on the surface of the water. Although at some sites this scum is removed for use in the production of an insulation material, elsewhere it has been colonised by a succession of plants to form the basis of floating islands. Typically the first colonist of this unusual medium is Floating Sweet-grass, while Common Reed, Common Reedmace and water-crowfoot are other plants commonly found during early development. Colonisation can be very rapid, the first plants sometimes appearing within a few months of deposition. A layer of dead plant material gradually develops on top of the floaters, providing a suitable substrate for more terrestrial plants. Willows, in particular, colonise these rafts, with Brambles and Birch being other regular colonists. A floating mat of willowbirch woodland developed right across a lagoon at Gale Common, South Yorkshire, in only 20 years. Where the covering of floaters is more patchy, discrete, ovalshaped, floating islands develop. It has been suggested that this material could be used specifically to create artificial islands within medium sized lakes (81 (Appendix 11 - Selected references and further reading)). Such islands would have a few potential drawbacks: they would be free-floating and difficult to secure by conventional means, while access for management would be hindered by their fragile nature. It might be possible to increase the integrity of these islands by laying strong nylon netting over a deep layer of floaters that have started to colonise with plants; this idea, however, has yet to be tested.
Many PFA mounds have developed interesting terrestrial habitats and it is important that these are assessed before consideration is given to converting areas into wetlands. However, owing to the impermeable nature of much PFA it is often possible to introduce wetland features simply by excavating hollows. An interesting large pond has developed on the PFA deposits at Rye House Power Station, Hertfordshire (National Grid). This rain-fed pond is crystal clear and supports substantial growths of stoneworts and other submerged and emergent plants, and huge numbers of breeding frogs, while Sand Martins and Kingfisher have nested in the surrounding banks.
Many sites where PFA has been allowed to colonise naturally have vast colonies of orchids, which have given this industrial landscape a high conservation value and aesthetic appeal. Three closely related species of orchid are usually involved: Southern Marsh, Early Marsh and Common Spotted, together with hybrids between these species. Such Dactylorhiza orchids are typical early colonists of PFA, their minute wind-borne seeds germinating in the rather bare alkaline deposits. The first Southern Marsh Orchids appeared at Ladywalk (see Feature 13.1) in 1974 and by the end of the decade there were tens of thousands present (including hybrids with Common Spotted Orchid). However, they have since declined steadily as other, more aggressive, plants have become established. The orchids require a good supply of soil water and their demise may partly result from competition for this resource, grasses and trees benefiting at their expense. By 1987 there were only 4-5,000 Southern Marsh Orchids left on this site.
Occasionally, other species of orchid are found on PFA sites. The very local Marsh Helleborine was discovered on the Ladywalk Nature Reserve in 1986. While other orchid species have declined on the reserve, the helleborines have increased and spread (over 1,000 stems in 1993), thanks to the work of volunteer wardens. As well as weeding out competing plants in the immediate vicinity of clumps of helleborine, and cutting down shading trees and shrubs, they have watered the plants during periods of dry weather. While there may be few sites where the resources are likely to be available to carry out such intensive management, this work does have implications for other comparable sites. These positive results suggest that the widely observed demise of orchids on PFA sites results from competition rather than a detrimental change to the underlying PFA, as has been suggested previously. If this is the case, it should be possible to recreate suitable conditions for orchids by occasionally stripping off the vegetation back down to the bare PFA. At Wigan Flashes in Greater Manchester, areas of fresh PFA are being introduced on an experimental basis in the hope that adjacent orchid colonies will spread across them.
The tendency of PFA particles to cement together means that sheer-sided cliffs can be excavated out of consolidated mounds. The consistency of this material seems to be ideal for burrow construction; both Kingfishers and Sand Martins have been accommodated on a number of sites. When the dried ash is being excavated from the lagoons at Fiddlers Ferry, suitable faces are maintained for breeding Sand Martins; over 100 pairs are present in some summers. Insects, such as solitary bees, also seem to find PFA a suitable substrate in which to build their burrows. The dark colour of bare PFA means that it warms up quickly, making it particularly attractive to these beat-loving creatures.
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