Hydrogen sulfide is heavier than air and so tends to settle in depressions and may reach toxic levels within excavations and in confined spaces. Therefore, it is strongly recommended that onsite gas monitoring and occupational health and safety measures are implemented to deal with this contingency during the disturbance of ASS materials, particularly when carried out in urban environments. Quality control/quality assurance (QC/QA) The accuracy and reproducibility of field data is essential to ASS assessment and management. Quality control (QC) measures should be described and implemented to provide data quality assurance (QA). Field data, and consequently laboratory analysis, cannot be considered representative of site conditions without appropriate QC/QA. Minimum required QC/QA procedures include: collection of one field duplicate for every 20 investigative samples, use of standardised field sampling forms, methods and Chains of Custody, and documented calibration of field instruments. Field duplicates are used to assess small-scale variability at a single sampling point. To measure repeatability, field duplicates must be taken from the same soil sample after mixing. Field duplicates should be submitted to the laboratory as blind samples with no indication to the laboratory that these samples are duplicates. Minimum soil sampling density The necessary soil sampling density is dependent on a number of factors, including the nature and variability of the soil materials, both laterally and vertically on the site, and the nature, depth and size of the proposed development. Sufficient soil sampling should be undertaken to create sufficiently detailed three-dimensional maps and cross-sections of soil materials at the site especially including the spatial extent of Net Acidities by depth to allow the development of adequately detailed management plans. Guidance in this section is provided on the minimum soil sampling requirements for an ASS investigation. The minimum number of sampling locations varies depending on the volume of soil to be disturbed and the nature of the disturbance. Table 6.1 summarises the minimum number of sample locations required for different types of ASS disturbances. Table 6.1 Minimum soil sampling densities for ASS investigations. Type of disturbance Extent of site Sample point frequency Small volumes (≤ 1000 m3) – prior to disturbance Volume of disturbance (m3) Number of boreholes < 250 251–500 501–1000 2 3 4 Project area (ha) Number of boreholes
LC Light clay plastic smooth to touch; slight resistance to shearing between thumb and forefinger; will form ribbon of 50–75 mm. 35–40% LMC Light medium clay plastic bolus; smooth to touch; slight to moderate resistance to ribboning shear; will form ribbon of about 75 mm. 40–45% MC Medium clay smooth plastic bolus; handles like plasticine and can be moulded into rods without fracture; has moderate resistance to ribboning shear; will form ribbon of 75 mm or more. 45–55% MHC Medium clay smooth plastic bolus; handles like plasticine; can be moulded into rods without fracture; has moderate to firm resistance to ribboning shear; will form ribbon of 75 mm or more. 50% or more HC Heavy clay smooth plastic bolus; handles like stiff plasticine; can be moulded into rods without fracture; has firm resistance to ribboning shear; will form ribbon of 75 mm or more. 50% or more Source: NCST (2009).