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Comparable SS removals have been reported in other systems, but the lower
nutrient removal rates at Langside probably reflect a number of factors: the onstream
nature of the system and short residence time (1 day compared with the 14–21 days
preferred for offstream ponds); the often organic nature of the pollution; the presence
of large amounts of sediment which releases nutrients as the water warms in spring;
the residual effect of large winter loadings; and the likelihood that the vegetation is
no longer acting as a sink. In terms of pond design, this means that offstream pond/
wetland systems are preferable because, even if nutrient remobilisation occurs, input
to the receiving watercourse would be small during low flow conditions.
The results emphasise the need to evaluate new ponds carefully and to establish
maintenance protocols that ensure maximum retention capability is retained.
Continuous flow and water quality monitoring is essential for assessing treatment
efficiency as performance varies over short timescales (hours and days) and infrequent
water quality analyses may miss rapid changes in concentration during storm events
that can impact on receiving watercourses. Vegetation and macroinvertebrate survey
results show that considerable variability in biodiversity and conservation value exists
between systems, probably due to the interaction of many factors, including: system
age, pollutant and hydraulic loading, habitat heterogeneity, microclimate, localisation
within the landscape and proximity of other wetlands/ponds for colonisation. There is
a need to investigate the relationships between faunal and floral diversity and water
treatment efficiency, and therefore quality, in order to propose recommendations for
design and maintenance that maximise both pollutant removal and habitat value in
farm ponds/wetlands.
FURTHER WORK
The issues identified above will be investigated in systematic assessments of
established and newly-constructed farm ponds/wetlands in Scotland during the next
2 years. The research will include continuous flow and water quality monitoring of
systems of different ages, representing the main farm types in Scotland that present
a risk of diffuse pollution from steading run-off. Sites will include ponds designed in
accordance with current best practice and constructed with funding from the Scottish
Executive and SEPA’s Diffuse Pollution and Habitat Enhancement Initiatives. Factors
relating to pond maintenance requirements and costs (e.g. vegetation growth,
sediment accumulation) and their habitat and surroundings will also be considered.
Although these pond/wetland systems are highly complex, it is hoped that the key
processes and influencing factors can be identified and modelled to provide answers
to questions, including:
• What are the relationships between hydraulic and pollutant loadings and treatment
performance?
• How does design of a pond/wetland (e.g. water depth, extent of margins, presence
of a treatment train, treatment of first flush) affect treatment performance and
habitat value?
• How compatible are water quality treatment and habitat functions of ponds, and
do trade-offs need to be made?
• How does pond/wetland treatment performance and habitat value change over
time?
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