ISO 05667-22:2010 pdf free download – Water quality一Sampling- Part 22: Guidance on the design and installation of groundwater monitoring points.
Introducing materials into a previously undisturbed environment can result in chemical and biological alteration of both the materials and the groundwater. This in turn may reduce material performance, e.g. through clogging or failure through weakening and collapse. If the groundwater is altered, the samples obtained may not be representative, which may compromise the objectives of monitoring. Consequently, materials and method of installation are major considerations in the design process. Some considerations are:
i) the chemical environment in which the installation is placed — aggressive environments (saline, free-phase, low or high pH) rapidly degrade or corrode some materials;
ii) effect of materials on contaminants, such as sorption, oxidation, reduction;
iii) effect of contaminants on materials: corrosion, solution, strength, leaching;
iv) effect of materials on groundwater: leaching, oxidation, pH;
v) effect of flushing fluids on the environment: aeration, mixing, clogging, reducing environment;
vi) economic considerations.
The likely concentrations of key determinants should also be considered and can be divided into the following broad categories:
i) gross — present at high concentrations or as free-phase;
ii) low — substances thought to be present as minor constituents but at concentrations well above their detection limits;
iii) trace concentrations — substances assumed to be absent or present at concentrations close to detection limits.
The use of degreasants, lubricants, drilling muds, and oils during drilling should be avoided if at all possible, particularly when considering sampling for organic compounds.
4.5.2 Casing materials
Components and materials in the borehole need to survive for the projected lifespan of the installation. Pressures on materials can come from corrosion, gravitational forces, water pressure differentials (especially during development and sampling) and lateral pressures from ground movement and swelling clays.
Metals other than stainless steel are not generally considered suitable for use in a GQMP as they are subject to corrosion. Metal corrosion reduces the lifespan of an installation, through weakening and possible collapse, and may also affect groundwater chemistry in the GQMP and the surrounding aquifer, e.g. through the release of iron and trace metals.
The choice of casing material is therefore limited to plastics (HDPE, PTFE, ABS, uPVC), stainless steel and, in some cases, unusual materials such as fibreglass, silica or ceramics. The relative strengths of different materials are shown in Table 6.
When joining lengths of casing, the use of glues and welds should be avoided because of the risk of introducing additional chemicals into the environment. Screw threads are the most commonly used and generally the most appropriate joining mechanism, although the threads represent weak points in the casing length. All joints should be flush on the inside of the casing to reduce the risk of snagging or trapping of sampling equipment. They should also be flush on the outside to permit the insertion of a tremmie pipe down the annulus and to avoid bridging of backfill materials.
Threaded joints between casing lengths should not be lubricated by any material that may compromise the GQMP. Greases containing metals or hydrocarbons should never be used.
4.5.3 Filter packs
A filter pack is required to prevent the influx of fines into the borehole and to stabilize the flow to the sampling point. Turbid samples or samples with a high suspended solids content can affect analysis. Silt entering the borehole can clog the screened section.
The filter pack should be matched to the aquifer and to the size of the screen openings (usually referred to as slot size). An appropriately designed filter pack and screen prevent or limit the entry of fine material into the casing. Accurate filter design requires a particle size distribution (PSD), preferably from sieve analysis, but an estimate of the PSD can be made from visual inspection for the geological formation.