Title: Protection of the Water Environment using Balancing Facilities
Author: E Mulhall
Author: M Revitt
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_1499, Representation ID: 522, Object ID: 2646
This report describes the results obtained during two phases of an extensive monitoring programme (December 1998 to August 2001) of the water quality during dry and wet weather conditions within the balancing ponds, adjacent to the A34 Newbury By-pass. In addition, during Phase 1 (December 1998 to December 1999) the sediment and aquatic plant quality was monitored in one of the ponds. The ponds, along with several others, were constructed for the storage and treatment of surface water run-off from the newly constructed bypass. Pond B is a vegetated pond (predominantly populated by Phragmites australis) and Pond F/G is a constructed wetland incorporating a sub-surface flow system and initially two reed species (Phragmites australis at the inlet end; Typha latifolia at the outlet end). Phragmites established itself most successfully in the gravel substrate and because of problems encountered with the development of the Typha bed, this was eventually replaced by replanting with Phragmites. The temporal variability of monitored parameters, such as pH, dissolved oxygen, conductivity, BOD5 and suspended solids, are described and explained in terms of the main influencing factors. There is a tendency for pH to increase slightly across both ponds and towards the end of the monitoring period there was evidence of the occurrence of anaerobic outlet conditions. The measured nutrients during the 24 visits to Pond F/G and the 16 visits to Pond B were chloride, nitrate, phosphate and sulphate. Phosphate was rarely detected and chloride concentrations mirrored the changes in conductivity due to winter salting activities. Initially higher nitrate concentrations at Pond F/G and higher sulphate concentrations at Pond B were not maintained throughout the programme. Total metal concentrations have remained consistently low in the road runoff throughout the monitoring period with only zinc exhibiting levels in excess of 30AMicro/l. This appears to be consistent with the findings of other workers examining performance of wetland systems associated with other parts of the Newbury Bypass. The seasonal variations of metal levels in the plant biomass are explained in terms of the relevant biological processes. A general decrease in sediment metal concentrations throughout Phase 1 of the monitoring period is encouraging in terms of the long term viability of Pond F/G. The performances of both ponds are discussed for dry weather conditions and for storm events. Five storm events have been monitored for Pond F/G and for each of these the comparison of inlet and outlet pollutant loadings is based on the retention times determined through lithium dosing experiments. Following antecedent rainfall, the retention time of Pond F/G was of the order of 34 A 12 hours and that of Pond B, 60 A 20 hours. There is considerable variability in the parameters analysed in the collected inlet and outlet samples during the routine monitoring programme and this continues when dry weather only data is examined statistically. These results raise concerns about the use of analyses of grab samples for predicting treatment system performances. However, the results do suggest that the median overall removal efficiency for Pond F/G (8.6%) is more significant than the corresponding value (5.6%) for Pond B. The estimation of pollutant removal efficiencies for the 5 monitored storm events at Pond F/G gives more reliable data with good performances indicated for cadmium, chromium, nitrate and suspended solids and only slightly less positive performances for nickel, zinc and sulphate. A comparison of dry and wet weather removal efficiencies indicates that only copper demonstrated a higher removal during dry weather conditions with the release of copper from Pond F/G during 3 of the storm events being difficult to explain. The results suggest that accumulated salts arising from winter de-icing v activities may form complexes with certain metals and may be removed from the pond by increased flowrates associated with storm events. Based on both the observations during the routine visits and the results obtained from the comprehensive monitoring programme, a range of recommendations have been made. These recommendations are divided into three categories that relate to: a a a The future design and construction of wetlands for highway runoff treatment. Maintenance and management practices for preserving the operational integrity of existing systems. Future monitoring work which will extend our scientific understanding of the operational capabilities of wetland systems. It is proposed that future monitoring should continue to assess the performances of the ponds as they become more established. However, this monitoring should be on a reduced scale in terms of the analytes to be investigated and it is suggested that chromium and nitrate would be appropriate as indicators of performance, particularly during storm events. It is also recommended that such investigations be made on the basis of inlet and outlet loads to the reedbeds taking into account the pond retention time rather than on the basis of grab samples which were the basis for the work previously undertaken. vi 1.
Publisher: Environment Agency
Subject Keywords: Vegetated pond; Wetlandss; Reed beds; Surface flow system; Sub-surface flow system; Pond retention time; Removal efficiencies.
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