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

 

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.