Source and Management of Water Colour in the River Tees

Is the blocking of peat drains an effective means of reducing water colour from upland peats at catchment scales?

Abstract

Extensive drainage of UK peatlands has been associated with dehydration of the peat, an increase in water colour and a loss of carbon storage. Water colour has been found to be proportional to the concentration of fluvial dissolved organic carbon (DOC) (McKnight et al. 1985). It has been considered that the blocking of drainage channels represents a means of peat restoration and a way of reducing DOC losses to surface waters. This study aimed to assess the effectiveness of drain blocking at both an individual drain scale and at a larger catchment scale (up to 1km2). The effect of external parameters become more pronounced as the DOC record is examined at larger scales. The catchment is an open system and water chemistry will be influenced by mixing with water from other sources. Also it is likely that at some point the drains will cut across slope leading to the flow of any highly coloured water down slope, bypassing the blockages, and entering the surface waters downstream. Degradation of DOC will occur naturally downstream due to the effects of light and microbial activity. There is, consequently, a need to examine the wider effects of drain blocking at a catchment scale to ensure that what is observed for one drain transfers to the whole catchment.

A series of blocked and unblocked catchments were studied in Upper Teesdale, Northern England. A detailed sampling programme of stream water, soil water and run off was undertaken in which a series of drains were studied in the 12 months prior to and post blocking. Water table depth, flow and weather parameters were also monitored.

This study could not find a significant decline in DOC concentration at zero or first order scale post blocking; however a small yet significant decline of 2.5% in DOC concentration relative to the control catchment was recorded at the first order scale. A decrease in DOC concentration is recorded as water flows from the zero to the first order in the same catchment. The study found that the effects of DOC degradation in the catchments were very small and that DOC degradation could not solely explain the decrease in DOC concentration seen from zero to first order drains indicating the importance of dilution effects in the catchments. The blocking of peat drains does significantly decrease the export of DOC which is largely achieved by decreasing water yield. The size of the DOC export reduction caused by drain blocking is seen to decrease as scale increases providing evidence for the existence of bypass flow around the zero order drain blockages. Blocking was found to have little impact on the level of the catchment water table. This can be explained by the peat bog being naturally very wet before intervention such that when blocking did occur the soil had little capacity to take in additional water. Water yield, however, is seen to decrease post blocking indicating that water and potentially DOC is being lost from the system. Principle component analysis and event analysis were performed on the hydrological and chemical data in order to trace and define this missing component of the water balance yet the analysis found that the water chemistry in the study catchment can be defined by a relatively simple mixing trend. As such this missing water remains undefined. The presence of bypass flow and water mixing will reduce the efficiency of any drain blocking and have wider implications for upland management and its practitioners.