Colour in upland water supplies is increasing, its levels have been related to droughts in 1976 and 1984, the highest level occurring in 1985. Records from treatment works indicate that colour varies between catchments which suggests that drought and its subsequenst oil moisture deficits, is not the only causal factor in colour production.
Research has shown that colour levels are related to both the physical and management characteristics of moorland. The most significant physical factor being the area of deep peat per catchment, as this represents the source of humic and fulvic acids which give rise to colour flows. Within these peat catchments, variations in colour level occur that cannot be explained by peat alone, leading to the
conclusion that moorland management strategies may also be responsible. Strategies studied included burning, ditching and liming, and consideration was given to the suggestion
that liming may reduce the amount of colour in raw waters.
These practices were found to increase soluble colour and in turn, levels of iron and aluminium in supply. Burning and ditching processes are linked to reductions in soil moisture, aerating soil conditions and in turn increasing levels of organic acids through decomposition. Liming increases p H and leads to increased decomposition,albeit by a slightly different set of processes, but has the same result.
Several research sites in North Yorkshire were used, the main ones being Howstean and Inmoor catchments, Scar House reservoir, in Upper Nidderdale, an area where discoloration had already been identified on a catchment scale and sites at the Washburn valley (Fewston and Thruscross reservoirs);
Glaisedale, North Yorkshire Moors and Gunnerside Moor, Upper Swaledale. Research involved large and small scale field and laboratory work, including extensive water sampling; isolation of four large peat blocks subsequently
drained and treated by burning and liming to assess the effects on soil moisture and colour production; and peat sampling in the form of cores, some of which were treated,
burned or limed and then leached using rainfall simulation. Resulting samples were analysed for water quality parameters, including colour, aluminium and iron.
Results have enabled those processes which potentially produce colour and the factors governing its magnitude to be addressed allowing management strategies for its reduction to be formulated. Emphasis is placed on the need
for good moorland management of heather, burning at a reduced temperature thereby minimising the effect on colour production; and the need to restrict or avoid
drainage, so keeping soil moisture deficits to a minimum. Finally, it is suggested that management of
colour by large scale liming is both inappropriate and ineffective as it appears to increase, rather than
decrease colour.