River Sampling Regime

The primary goal of sampling was to identify the pollutants which are prevalent in the river under different conditions. Other studies examining riverine water quality (Davies and Neal, 2007; Neal et al., 2011; Stapleton et al., 2008) that used a sampling regime to collect primary water quality data, have overwhelmingly used macro level sampling, or they have used existing EA water quality data.

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The land use for the whole catchment was identified from the 2007 Land Cover Map (CEH, 2007) Figure 10. The overall area of the catchment considered in the study, from the farthest downstream sample location, was 190 km2, also shown on Figure 10.

Figure 10 - Land use across study catchment (CEH.,2007)

The selection of sampling locations was influenced by several criteria. Firstly, it was preferred to use existing EA sampling locations, as this facilitated a comparison with historical data. Unfortunately there was only a single existing sample location in the study

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area, meaning multiple new locations needed to be selected. Secondly, new sample point locations were limited by the physical access to the river and its tributaries, particularly in central Wigan, where there are several culverted sections which made gaining safe access difficult.

Initial sample locations were identified to provide a holistic coverage of all sections of the river course passing through the Wigan urban area, including all major tributaries. Suitable siting was not always possible, due to both the culverted nature of some tributaries and/or lack of an accessible location where samples could be easily taken. A high density of sample locations was desired to observe the downstream fluctuation of pollutants in response to different conditions. Using these considerations, along with local knowledge and the ordnance survey data of the area, a list of potential sample locations was generated, and subsequently site visits were carried out to confirm their suitability.

Figure 11 - Map of known CSO and WWTWs Discharge locations

Key Combined Sewer Overflows Waste Water Treatment Works Outfall

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Existing CSO and WWTWs discharge locations were also identified, however as the primary interest was to monitor pollutant load generated from surface water runoff sample points were not sited to seek to observe the effect of CSO/WWTWs discharge on river water quality. A map of known CSO locations was obtained from the EA and is included as Figure 11. Further details of the adjacent Horwich WWTWs are included as Appendix VIII. Further CSO details were also sought from UU but it was not possible for them to be supplied. They indicated that most CSOs were built to an “asset standard” but this would be dependent on when they were installed these will have varied.

It was desirable, where possible, to locate sample points in close proximity to existing SWD. However, this was not always possible, due to the limited access to the river banks. Despite these constraints 25 site locations were identified which are shown on Figure 8 (a full list of sample sites giving locations details are included as Appendix II). Finally, in addition to the points above, several other practical considerations also came into effect to determine the suitability of sample locations. These were:-

 Can the site be easily accessed on foot?

 Is there appropriate parking close by? (samples were decanted at the vehicle)

 Can samples be safely collected?

 Is it possible to take a sample from the centre of the river channel?

 Geographical proximity – the total number locations that can feasibly be visited by a single researcher within a day.

 What is the land use of the area surrounding the sample location?

Using these criteria, the most suitable sample points were concentrated in the lower portion of the catchment area, where the land use surrounding the Douglas’s course was primarily urban and suburban, as it was the contribution of runoff from these areas that was of most interest.

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Figure 12 - Proposed WQV for Testing

The physical operation of sampling followed the EA’s standard method of sample collection, as described in EA document “Chemical and microbiological sampling of water - Operational instruction 19_09” (Anon, 2010). Specifically this involves dropping a tethered, clean, stainless steel can into the river either from a bridge or using a telescopic sampling pole, and drawing a water sample. This process is repeated three times at each site. When a sample is collected, the first two samples are discarded preventing cross contamination with the previous location sampled. The third sample is decanted into 4 separate sterilised bottles, before being transferred into a cool box for transport to the local EA depot, where it was stored in a fridge before transfer to the EA Laboratory. All laboratory testing was conducted by the EA National Lab Service and is completed within 24 hours of samples being collected from the river. Spot samples were collected from the centre of the river and considered to be representative of the channel cross sectional water quality at the time and location they were collected.

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The frequency at which samples were collected considered the episodic nature of diffuse pollution. Ten separate sets of samples were collected and samples were taken in sequential order from upstream to downstream. It was attempted with each sample collection to capture different conditions in the river. For example, some sample sets were collected during base or low discharge conditions whereas with other sets a specific attempt was made to collect samples throughout the duration of a storm event. Details of the total numbers of samples collected, and the dates of sample collection are covered in Chapter 4 (section 4.4).

Finally, the water quality variables (WQV) for which samples were tested were identified through a literature search for typical urban pollutants, as well as historical sample data indicating pollutants that had been previously observed in high concentrations. Figure 12 displays the range of WQV which were identified through this process, including bacteria which are associated with faecal contamination to rivers, and several other chemicals which are identified as priority control substances under the WFD. All the variables selected are important water quality indicators, and most have EQS limits set under the WFD/RBWD.