Kilbury Report

Summary

High bacterial counts at Dartington Beach and Dartington Pontoon did not correlate with CSO spill data or rainfall. This suggested there was a continuous pollution load being added to the Dart. Current monitoring indicated that treated effluent from Kilbury sewage treatment works (stw) was the source. Sampling took place above and below the STW and at downstream sites on 6 occasions when no spills were recorded to isolate the impact of treated effluent on water quality in the Dart. Treated effluent from Kilbury was found to be a significant source of bacterial pollution in the Dart, the effect of this is greater when river levels are lower, which coincides with the dry summer period when recreational use is at its highest. UV (or other treatment to remove bacteria) from Kilbury effluent would noticeably improve downstream water quality at the heavily used sites of Dartington Beach and Dartington Pontoon.

Introduction

Friends of the Dart (FotD) are undertaking year-round water quality monitoring to the same standard as the Environment Agency bathing water monitoring programme. This involves monitoring E.coli (E.C) and Intestinal Enterococci (I.E) levels at 7 sites on the Dart. Four of these sites (Steamer Quay, Stoke Gabriel, Dittisham and Warfleet Creek) are designated bathing sites, the other 3 sites are community-designated (Holne Weir, Dartington Beach and Dartington Pontoon).

Table 1. Sites monitored by FotD and their bathing water classification. Classifications marked by * are not Defra designations but were calculated using the guidance in the Bathing Water Regulations (2013).

Mean bacterial concentrations for Holne Weir (E.C 342 cfu/100ml and IE. 112 cfu/100ml), Dartington Beach (E.C 2165 cfu/100ml and I.E 1675 cfu/100ml ) and Dartington Pontoon (E.C 1265 cfu/100ml and I.E 635 cfu/100ml) show a significant increase in bacteria concentrations between Holne Weir and Dartington Beach (6 miles downstream). High bacterial concentrations at Dartington often do not correlate with sewage spill data or rainfall observations recorded at the time of sampling. The town of Buckfastleigh lies between these two locations and is served by Kilbury Sewage Treatment Works (STW). Kilbury is the largest sewage treatment works upstream of Totnes and effluent is not subject to UV treatment (US EPA) before discharging directly into the Dart. SWW have stated that Kilbury can affect water quality at Dartington pontoon during low flows (Coomber 2022 unpublished).

To investigate the increase in bacterial concentrations between Holne and Dartington FoD wanted to investigate the exact impact of Kilbury on water quality. Alongside this, FoD wanted to assess the statement from SWW that the impact of Kilbury is only detectable in low flows. Finally, it is important to establish the impact installing UV treatment at Kilbury will have on downstream water quality

Methods

Six samples have been taken above and below Kilbury when there have been no spills recorded within 24 hrs on water fit live (https://www.southwestwater.co.uk/environment/rivers-and-bathing-waters/waterfitlive/storm-overflow-map). In addition to these samples, 3 were taken at Hood Barton Estate and Staverton to eliminate the possibility of other pollution sources between Kilbury and Dartington Beach. Sampling was taken over as wide a range of river and rainfall levels as possible.

Results

Above Kilbury

Bacterial concentrations above Kilbury and at Holne Weir are consistently below safe Levels (E.C <1000 cfu/100ml and I.E <400 cfu/100ml). When levels at Above Kilbury are elevated this pattern is mirrored at Holne Weir suggesting additional upstream pollution sources. There are multiple tributaries between these sites which is likely the driver of high counts above Kilbury (Figure 1).

Below Kilbury testing

Testing Below Kilbury has highlighted high bacterial counts which for E.coli were always above the limit of 1000 cfu/100ml and for Enterococci only one sample was below the limit of 400 cfu/100ml. The highest E.Coli Concentration was recorded at > 10,000 cfu/100ml and the highest Enterococci concentration was 4400 cfu/100ml (Figure 2).

Figure 2. Bacterial Concentrations of E.coli and Enterococci Below Kilbury

Downstream of Kilbury

Dartington Beach and Pontoon display elevated bacterial concentrations above safe levels (Quote results). In the full dataset for the 3 weeks that all downstream sites were sampled there were no obvious increases in bacteria except dartington Pontoon 1 week. (Bidwell Brook). (Figure 3).

Figure 3. Bacterial Concentrations at Hood Barton Estate (A), Staverton (B), Dartington Beach (C) and Dartington Pontoon (D).

Rainfall and River levels

Throughout sampling a range of river levels were observed varying from 0.43m to 1.12m. Rainfall on the day of sampling was always low to meet the sampling criteria of no spill recorded within 24 hours, however, rainfall levels in the week preceding sampling varied from 1mm to 74.3mm (Table 2).

Table 2. River levels, Daily total Rainfall and Preceding 7 days rainfall for Austins bridge.

Discussion
The impact of Kilbury STW on water quality

This study clearly highlights the release of treated effluent from Kilbury is a key driver of increased bacterial concentrations within the Dart. The highest recorded bacterial concentrations downstream of the works were >10,000 cfu/100ml E.coli and 4,400 cfu/100ml (Figure 2) these are way over the safe levels outlined in the bathing water regulations (2013). The tests downstream of Kilbury (Figure 3) revealed that it is a key contributor to poor water quality at Dartington Beach and Dartington Pontoon. It is important to acknowledge that there are other pollution sources, however, their impact appears to be a lot more short-term.

Downstream of Dartington Pontoon at Steamer Quay the freshwater input of Bacteria is likely to be one of the causes of it receiving a Poor Bathing status. The increase in bacterial concentrations at Steamer Quay suggests it is not a negatively impacted by Kilbury as the sites at Dartington.

How important are river levels

The effect of effluent on downstream water quality is most noticeable when river levels are low. Samples from 10/09/2024, 24/09/2024 and 05/11/2024 had the highest bacterial counts downstream of Kilbury STW (Figures 2 and 3) and the Lowest river levels of the sampling period (Table 2). This is very likely due to lower river levels reducing the dilution of effluent within the river.

However when river levels were higher on 01/10/2024 and 19/11/2024 treated effluent from Kilbury still negatively impacted downstream water quality. On both these weeks bacterial concentrations above Kilbury were elevated, this is likely to be attributed to reported spills within 24 hours at Holne Stw or other diffuse sources due to rainfall in the preceding week. The sample was taken on 19/11/2024 shows enterococci increasing by a factor of 4 downstream of the stw with concentration remaining at above safe levels (400 cfu/100ml) downstream (Figures 2 and 3). This sample also coincided with the highest river levels of the study (Table 2). This suggests that it is not during low flows that Kilbury negatively affects downstream water quality, however, its effect is most noticeable when river levels are low.

SWW’s commitment to UV treatment at Kilbury

SWW has committed to installing UV treatment at Kilbury by 2027. UV is effective at removing pathogens in waste water provided it is correctly installed and maintained (US EPA 1999). The installation of UV at Kilbury would lead to improvements in water quality at FotD community use sites (Dartington Beach and Dartington Pontoon). This would improve water quality at these sites. The disinfection of effluent from Kilbury could increase water quality to the point that these sites reach an equivalent of sufficient classification, with E.coli levels regularly below 1000 cfu/100ml and enterococci below 400 cfu/100ml. As there are points when bacterial counts have been low enough to achieve this. However the actual overall depends on other factors including reducing sewage spills and addressing more diffuse pollution sources.

Conclusions

Overall the installation of UV treatment at Kilbury would improve water quality downstream of the works. This effect would be most noticeable when river levels are low, which coincides with when the greatest number of people use the river recreationally. Therefore there would be a high benefit to installing UV treatment in reducing the risk these water users face.

It is not only in low flows that this benefit would be seen as even in the presence of pollution from other sources the signal from Kilbury is detectable.

References

Bathing Water Regulations 2013, UK Government. Available at https://www.ofwat.gov.uk/publication/the-bathing-water-regulations-2013/ Accessed 19/12/2024.

Coomber unpublished 2022, Our water quality study what we have learnt so far.

US EPA 1999, Wastewater Technology fact sheet. Available at https://www3.epa.gov/npdes/pubs/uv.pdf Accessed 19/12/2024.