Air Quality Report 6
Diffusion tube monitoring
3.1 Hydrogen Sulphide monitoring
Diffusion tubes monitoring Hydrogen Sulphide have been set at twelve off-site locations at various compass directions around Withyhedge Landfill with additional tubes on-site, as shown on Figures 3-1 and 3-2.
The tubes are positioned to enable free air movement, safety during maintenance and consideration of potential damage, theft or vandalism. The suitability of the monitoring positions is being reviewed as the programme develops, hence, on 4 June 2024, two additional monitoring positions were added to the network; D11 in Prendergast and D12 in Crundale.
As highlighted in summary report 5, on 11 July 2024 an additional tube (referred to as SCH1) was located alongside the permanent monitoring station positioned by Pembrokeshire County Council (PCC) at Spittal School.
To assist with putting the results from D1-D12 into wider context, diffusion tubes were also positioned at four off-site positions during the latest monitoring period. These positions were:
- Pembroke Dock, between the Aldi supermarket and adjacent church (tube D13),
- St David’s, close to the primary school (tube D14)
- Saron Road, Ammanford, ~100m west of the primary school (tube D15)
- Ffordd Coed Darcy, Llandarcy off Junction 43 of the M4 (tube D16)
Details of each position are summarised in Table 3-1.
In addition to the off-site monitoring, diffusion tubes have also been placed on-site at Withyhedge Landfill. These positions are summarised in Table 3-2 and shown in Figure 3-2.
Table 3‑1 Off-site Monitoring positions
Community monitoring locations
Figure 3-1 & 3-2 reference |
Location description |
Position |
Height above ground/m |
D1 |
Spittal Cross cross-roads west of Spittal |
Street furniture at cross-roads |
0.6 (old & new positions) |
D2 |
Adjacent Spittal School |
Lamp post |
2.1 |
D3 |
Corner of spring gardens and Castle Rise, Spittal. Adjacent to farm |
Lamp post |
2.1 |
D4 |
Cross-roads of B4329 and Spring Gardens East of Spittal |
Street furniture |
2 |
D5 |
B4329 between Scolton and Bethlehem |
Street furniture |
2.2 |
D6 |
B4329 at Bethlehem |
Lamp post |
2.2 |
D7 |
On road heading west out of Poyston Cross |
Lamp post |
2.2 |
D8 |
Adjacent to properties at Poyston Water |
Lamp post |
2.1 |
D9 |
Rudbaxton Water Bridge |
Northern side of bridge |
1.2 |
D10 |
Adjacent to Junction with A40 near Corner Piece Inn |
Lamp post |
1.9 |
D11 |
Withybush Road, Prendergast |
Sign post |
1.8 |
D12 |
B4329 Crundale near junction with Cross Lane |
Sign post |
1.8 |
D13 |
Pembroke Dock, between Aldi and church |
Sign post |
1.8 |
D14 |
St David’s, adjacent primary school |
Sign post |
1.8 |
D15 |
Saron, Ammanford |
Sign post |
1.8 |
D16 |
Llandarcy |
Lamp post |
1.8 |
SCH1 |
Adjacent council monitor at Spittal school |
On fence next to Council monitor air intake |
1.8 |
Table 3-2 Monitoring positions
On-site monitoring locations
Figure 3-1 & 3-2 reference |
Location Description |
Position |
Height above ground/m |
Access ramp (WL1) |
Eastern side of access ramp |
Metal post |
2.1 |
Eastern fence posts (WL2) |
Fence post close to edge of permanent capping |
Fence post |
1.1 |
Litter skids (WL3) |
Metal post close to edge of permanent capping |
Metal post |
2.2 |
Western fence post (WL4) |
Fence post west of temporary capping |
Fence post |
0.9 |
CCTV tower (WL5) |
Metal post south of active Cell 8 |
Metal post |
2.2 |
IBC cell 8 (WL6) |
Metal post west of active Cell 8 |
Metal post |
1.65 |
Cell 7 IBC corner (WL7) |
Metal post south of Cell 7 |
Metal post |
1.9 |
Figure 3‑1 Community monitoring positions D1-D12 and SCH1
Figure 3‑2 On-site monitoring positions
3.1.1 Review of Hydrogen Sulphide results
The results of the Hydrogen Sulphide diffusion tube monitoring are summarised in Tables 3-3 and 3-4. The original laboratory certificates from the latest monitoring are included in Appendix 1. The term exposure period is used to define the sampling period when air was able to diffuse into the tubes before the tubes were taken down, sealed and returned to the laboratory for analysis. Analysis has been performed at Gradko International which is a UKAS accredited testing laboratory (No. 2187).
During the first exposure period wastes were being removed from the crest of the site, gas wells were being drilled into the waste mass and temporary capping of the west facing flank was in progress. During the second and third exposure periods capping works were continuing and gas extraction extending to newly capped areas. In the subsequent periods, capping works were completed and being actively maintained.
Table 3‑3 Hydrogen Sulphide results from Community Monitoring Positions
Description |
Exposure period: 5 Feb - 1 Mar |
Exposure period: 1 Mar - 3 Apr |
Exposure period: 3 Apr - 7 May |
Exposure period: 7 May - 4 Jun |
Exposure period: 4 Jun - 9 Jul |
Exposure period: 12 Jul - 13 Aug (1) |
Gas | Hydrogen Sulphide (H2S) |
Hydrogen Sulphide (H2S) |
Hydrogen Sulphide (H2S) | Hydrogen Sulphide (H2S) | Hydrogen Sulphide (H2S) | Hydrogen Sulphide (H2S) |
Location | ppb | ppb | ppb | ppb | ppb | ppb |
Laboratory blank | 0.05 | 0.04 | 0.05 | 0.06 | 0.04 | 0.03 |
Community monitoring positions |
Exposure period: 5 Feb - 1 Mar |
Exposure period: 1 Mar - 3 Apr |
Exposure period: 3 Apr - 7 May |
Exposure period: 7 May - 4 Jun |
Exposure period: 4 Jun - 9 Jul |
Exposure period: 12 Jul - 13 Aug (1) |
Junction west of Spittal - D1 |
<0.08 |
Removed |
0.12 |
<0.07 |
<0.06 |
<0.06 |
Spittal School - D2 |
<0.08 |
<0.06 |
0.14 |
<0.07 |
<0.06 |
<0.06 |
Spittal - D3 |
<0.08 |
<0.06 |
0.07 |
<0.07 |
0.06 |
<0.06 |
Upper Scolton - D4 |
<0.08 |
<0.06 |
0.14 |
<0.07 |
0.06 |
<0.06 |
Scolton Road - D5 |
<0.08 |
<0.06 |
<0.06 |
<0.07 |
<0.06 |
<0.06 |
Bethlehem - D6 |
<0.08 |
<0.06 |
<0.06 |
<0.07 |
<0.06 |
<0.06 |
Poyston Cross - D7 |
<0.08 |
<0.06 |
<0.06 |
<0.07 |
<0.06 |
<0.06 |
Poyston Water - D8 |
<0.08 |
<0.06 |
0.06 |
0.09 |
0.28 |
<0.06 |
Rudbaxton - D9 |
0.1 |
0.07 |
0.07 |
0.1 |
<0.06 |
<0.06 |
A40 Junction - D10 |
<0.08 |
0.07 |
removed |
<0.07 |
<0.06 |
<0.06 |
Withybush Road - D11 |
no data |
no data |
no data |
no data |
<0.06 |
0.16 |
Crundale - D12 |
no data |
no data |
no data |
no data |
<0.06 |
<0.06 |
Spittal School – SCH1 |
no data |
no data |
no data |
no data |
no data |
<0.06
|
Other off-site positions |
Exposure period: 5 Feb - 1 Mar |
Exposure period: 1 Mar - 3 Apr |
Exposure period: 3 Apr - 7 Apr |
Exposure period: 7 May - 4 Jun |
Exposure period: 4 Jun - 9 Jul |
Exposure period: 12 Jul - 13 Aug (1) |
Pembroke Dock - D13 | no data | no data | no data | no data | no data | <0.06 |
St Davids - D14 | no data | no data | no data | no data | no data | <0.08 |
Saron, Ammanford - D15 | no data | no data | no data | no data | no data | <0.06 |
Llandarcy - D16 | no data | no data | no data | no data | no data | <0.06 |
Notes:
(1) Exposure for each tube was D1-D12 exposed 12 July – 13 August, SCH1 11 Jul – 13 August, D13 and D14 17 July-13 August, D15 11 July – 14 August, D16 12 July – 14 August.
Table 3‑4 On-site Hydrogen Sulphide monitoring results
Description |
Exposure period: 8 Feb-1 Mar |
Exposure period: 1 Mar-3 Apr |
Exposure period: 3 Apr-7 May |
Exposure period: 7 May-4 Jun |
Explosure period: 4 Jun–9 Jul |
Exposure period: 1 Jul–14 Aug |
Gas |
Hydrogen sulphide (H2S) |
Hydrogen sulphide (H2S) |
Hydrogen sulphide (H2S) |
Hydrogen sulphide (H2S) |
Hydrogen sulphide (H2S) |
Hydrogen sulphide (H2S) |
Location |
ppb |
ppb |
ppb |
ppb |
ppb |
ppb |
Laboratory Blank |
0.05 |
0.04 |
no data |
no data |
no data |
no data |
Access ramp (WL1) |
1.48 |
Lost |
no data |
no data |
no data |
no data |
Fence posts (WL2) |
1.82 |
no data |
no data |
0.27 |
0.1 |
<0.06 |
Litter skids (WL3) |
2.04 |
no data |
no data |
no data |
no data |
no data |
Field fence post (WL4) |
0.29 |
1.38 |
0.31 |
0.12 |
<0.06 |
<0.06 |
CCTV tower (WL5) |
0.6 |
4.4 |
9.24 |
2.16 |
1.11 |
0.09 |
IBC cell 8 (WL6) |
1.04 |
no data |
no data |
no data |
no data |
no data |
Cell 7 IBC corner (WL7) |
1.8 |
6.54 |
3.97 |
no data |
no data |
no data |
Comparison of the concentrations detected using diffusion tubes in the community with the health-based evaluation criteria in Table 3-5 indicates that the concentrations continue to fall below these guidance values for intermediate/lifetime exposure.
The concentrations observed in the community around the landfill site are also the same as those detected at the other 4 positions in South Wales (D13-D16).
Table 3‑5 Referenced health based guidance values
(Values taken from references 1 and 2)
Gas |
Intermediate exposure criteria(up to 1 year) |
Lifetime exposure criteria |
Hydrogen sulphide concentration |
20 ppb (30 µg/m3) |
1 ppb (2 μg/m3) |
Concentrations of hydrogen sulphide reported from the tubes exposed on-site have continued to fall, with the most recent results being comparable to those recorded at the off-site monitoring locations.
These on-site concentrations are several orders of magnitude below the workplace exposure limit of 5000 ppb for an 8-hour time-weighted average reference period (Ref 3). This suggests that the ambient levels on site do not pose a long-term risk to site workers and help place the concentration levels detected into context.