ABS: The design of environmental monitoring well shall include the well casing, well screen, screen filter, annulus seals and well head protection.
Well casing and well screen:
Today, most of the monitoring wells will be constructed of polyvinyl chloride (PVC) or stainless steel, followed by low carbon or galvanized steel. Other materials (ABS, PE, PP) are rarely suitable for the construction of environmental monitoring wells, or the size of production is usually not suitable for the construction of monitoring wells.
In selecting the appropriate well material, the installation of a monitoring well requires consideration of potential sources of chemical interference. In general, the interference to groundwater quality caused by wellbore and well screen includes adsorption, desorption and leaching, corrosion and chemical degradation of well material, production residue resulted from the production, storage and processing of well material. In the installation process, there is also potential interferences as result of adhesion of plastic and plastic materials, heat welding.
The diameter of the well is mainly dependent on the purpose of the well construction, and other factors may include the following:
- Equipment in the monitoring wells: sampling, measurement, other experiments;
- The installation method of the well: hollow auger or direct push type;
- Expected depth and well strength: the deeper the construction, the more strength required;
- Difficulty in well purging: the smaller the diameter, the easier it is to purge well;
- Volume of the well: the amount of liquid required to pump in the well;
- Groundwater recharge rate: the smaller the diameter of the wells in the water level recovery time shorter;
- Unit cost for pipe material and drilling: the smaller the diameter, the lower the cost.
Design of the screen
The design of the well screen requires two important design parameters, effective dimensions and uniformity coefficient. The effective size means that the mesh diameter of the sieve that can retain 90% (or 10% passing) of fine particulate matter in groundwater. The uniformity factor refer to the ratio of the mesh size to the effective particle size of the particulate matter that can be retained by 40% (or 60% through) the groundwater.
The design of the length of the screen requires consideration of whether its length affects the acquisition of representative groundwater samples and data measurement. A shorter length of the well screen separates each discrete water depth from each other. This allows the monitoring wells to collect measurements of the groundwater samples and hydrological data from isolated zone (ie, the target monitoring area). The length of the screen (position) should also take into account the fluctuation of the groundwater, which makes easier for water level measurement and collection of LNAPL samples.
Well Development:
For most cases in monitoring well development, coarse and permeable artificial materials are used as the filter layer around the well screen. When the natural well completion material is finer and the particle size gradation is uniform, or the particle size distribution is well graded, it is recommended to use special artificial monitoring well filter pack. At the same time, artificial monitoring well filter pack also need to filter the surrounding soil particles with effective particle size less than 0.01 inches, the average uniformity coefficient is less than 3.
Pre-packed well screen refers to filter material that is pre-installed with the well screen. Pre-packed well screen can be deployed using direct push drilling rig during the drilling process. This method has little perturbation to the natural soil. ASTM D6725 standard for direct push rig installation of pre-packed filter monitoring wells have a detailed description of the each step.
Annulus Seal:
Use a grouting pump to fill the sealage into the annular space around the well. Its main use has the following:
- Prohibit the longitudinal transfer of water or contaminants in the annular space around the well by isolating the sampling area from other external environments in terms of hydraulics and chemistry
- Prevent penetration of surface water and potential surface contaminants
- Prevent external corrosion or chemical degradation of the well pipe and increase the life of the well
- Provide more structural integrity
Well Head Protection:
The last step is the construction of the wellhead. The ASTM D5787 standard provides a design step for monitoring well’s protection mechanism. There are two types of structure for wellhead: 1) higher than the ground type; 2) embedded. The main purpose of the construction is to prevent the surface runoff from entering and descending into the annular gap of the monitoring well and infiltrating into the monitoring well. In addition, the protection mechanism can also prevent the accidental damage and damage to the monitoring wells.
