Agricultural sources for lake pollution : soil erosion in Slapton Ley catchment.

The purpose of this study was to investigate the role
of soil erosion as a factor in lake pollution and in
particular the transport of phosphorus from field soils to
streams and lakes in association with mobile sediment.
Four land uses were selected as representative of the
Slapton Ley catchment area to investigate the levels of
phosphorus in the soil. The surface soil samples from the
selected land use areas were analysed to determine the
water-soluble phosphorus level in solution and exchangeable
phosphorus level in sediment.
Twelve experimental plots were studied in order to
assess the erosional effects of overland flow and thus to
determine the level of phosphorus from different land uses
which may be influencing the eutrophication of the lake. It
was concluded that slope angle, vegetation cover, surficial
soil properties, animal influence and agricultural practice
are the main factors influencing sediment transportation by
overland flow.
Estimated results for the agricultural fields indicated
that the actual phosphorus loss to the Ley is always greater
in sediment than solution and actual phosphorus loss in
sediment is greater in arable (root) and cereal than in
grass.
The point water samples (136) from 13 different sources
were grouped. Mean value of phosphorus concentration from
the point sources indicated that the agricultural land uses
such as arable and cereal provide 2 times more exchangeable
phosphorus attached to sediment than the other land uses
whereas farm and sewage provided 5 times more soluble
phosphorus in water than other sources.
Phosphorus concentration during peak discharge was
examined for the Cara catchment. The results indicated that
the ratio of phosphorus concentration in suspended sediment
to phosphorus concentration in water is 240: 0.3 and that
there is a linear relationship between phosphorus in water
and phosphorus in sediment during the peak discharge.
Sediment phosphorus levels in the marsh area were also
examined. The results indicated that the top layers of the
marsh sediment particularly at the surface, have higher
phosphorus concentration than the lower layers and that
there are higher levels in sediment than in water.
From these results the conclusion was drawn that the
soluble phosphorus in water is at highest concentration in
sewage works effluent. However this effluent contributes a
small proportion of phosphorus load to the Ley compared with
the arable (root), cereal and grass lands in the catchment.
Agricultural sources, particularly arable sources such as
root and cereal play an important role on soil erosion as a
factor in lake pollution and in particular in the transport
of phosphorus from field soils to streams and lakes in
association with mobile sediment in the Slapton Ley
catchment.