Abstract
The creation of ponds and wetlands has the potential to alleviate stream water quality impairment in catchments affected by diffuse agricultural pollution. Understanding the hydrological and biogeochemical functioning of these features is important in determining their effectiveness at mitigating pollution. This study investigated sediment and nutrient retention in three connected (on-line) ponds on a lowland headwater stream by sampling inflowing and outflowing concentrations during base and storm flows. Sediment trapping devices were used to quantify sediment and phosphorus accumulations within ponds over approximately monthly periods. The organic matter content and particle size composition of accumulated sediment were also measured. The ponds retained dissolved nitrate, soluble reactive phosphorus and suspended solids during baseflows. During small to moderate storm events, some ponds were able to reduce peak concentrations and loads of suspended solids and phosphorus; however, during large magnitude events, resuspension of deposited sediment resulted in net loss. Ponds filtered out larger particles most effectively. Between August 2019 and March 2020, the ponds accumulated 0.306 t ha−1 sediment from the 30 ha contributing area. During this period, total sediment accumulations in ponds were estimated to equal 7.6% of the suspended flux leaving the 340 ha catchment downstream. This study demonstrates the complexity of pollutant retention dynamics in on-line ponds and highlights how their effectiveness can be influenced by the timing and magnitude of events.
Highlights
This paper provides further evidence on how the trapping efficiency of in-stream pond features is often dependent on the magnitude and frequency of storm events they experience, with high discharge and sediment inputs leading to a rapid reduction in storage capacity and causing ponds to overflow
This paper demonstrates how the effectiveness of on-line ponds for the mitigation of diffuse agricultural pollution on clay soils with a 2.5% slope can be highly variable due to the different retention capacities of sediment and nutrient fractions under different hydrological conditions
Concentrations by averages of 29% and 5%, respectively. Despite their small size (
Summary
Diffuse agricultural pollution in the UK is estimated to have an annual cost of £238 million, resulting from reduced water quality and associated treatments costs [9]. These costs consider the wide-ranging negative impacts on biodiversity, ecosystem services, landscape value, rural public access and enjoyment, water and air quality, and natural resources. Soil erosion can lead to increased sedimentation in watercourses and a reduction in channel capacity, thereby increasing flood risk which is already an increasing concern as a result of climate change [10].
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