A Monitoring Well is a well designed to detect, and monitor through time, trace levels of both inorganic and organic contaminants in groundwater systems.2 • Monitoring wells are installed to determine the ground-water quality at localities such as landfills, industrial facilities, service stations, Superfund sites, waste-water treatment facilities, mines, petrochemical plants, and areas of suspected or known ground-water contamination.1,2
Contamination from anthropogenic activities is a common problem for groundwater. For example, "BTEX" (benzene, toluene, ethylbenzene and xylene), which comes from gasoline refining, and MTBE - which is a fuel additive, and many industrial solvents are common groundwater contaminants, often the result of leaking subterranean storage tanks and dumping. 2 • Cleanup of contaminated groundwater tends to be very costly. Effective remediation depends on the detection and tracking of contaminants in the groundwater system. • As part of a comprehensive monitoring program, strategically placed Monitoring wells can help.
Correctly designed monitoring wells can allow the collection of: • Representative samples from a target monitoring zone to allow detection and monitoring of contaminant plumes 1,2 • Use of materials that don’t react with target analytes provide representative samples • Accurate hydraulic parameter data 2 • Hydraulic conductivity • Definition of preferential flow pathways • Calculation of ground-water flow velocity • Accurate ground-water level data at a specific location in the ground-water flow system 2 • Allows for the construction of potentiometric surface contour maps • Allow for definition of ground-water flow direction in the horizontal plane
Thousands of Monitoring wells are installed each year. • Many of these are designed and installed by contractors not aware of proper monitoring well design and construction practices. 2 • As a result, many monitoring wells have design flaws and were installed using materials and methods that may adversely affect the integrity and quality of samples retrieved from those wells.
The goal of the water monitoring system is to obtain groundwater samples that are representative of the groundwater system, retaining the physical and chemical properties of the groundwater, and that are minimally affected by the sample collection process. 2 Proper ground-water monitoring well design and installation techniques are necessary to minimize the chemical alteration of samples.
Monitoring Well Components • Bore Hole • Well Casing • Well Screen • Filter Pack • Annular Seal • Surface Protection
Surface Protection Protective Outer Cap Annular Seal WellCasing Bore Hole Filter Pack Water Table ↓ Well Screen
The well casing is a length of solid pipe and can be made of a variety of materials from PVC pipe to stainless steel, and isolates the well from the surrounding rock or soil, and is necessarily smaller in diameter than the borehole 1,2 • The well screen is used to allow water into the well and filter out the soil and sediment. It is often a piece of pipe with holes, slots, gauze, or a continuous wire wrapped around it. The top is usually installed above the water table. It is preferable that the well screen be professionally manufactured and not conjured in the field with a knife and makeshift materials. The screen is attached to the end of the casing by threaded joints. 2 • The Casings and screens are made from various materials that must be chosen carefully on the basis of cost, durability, and potential reactivity with the ground water. Teflon is the most costly, least durable, and most inert. Stainless steel is the most durable, is moderately costly, and is also essentially inert. PVC pipe is often used because of low cost. 1 • Only PVC pipe that is threaded should be used, as PVC pipe joined with solvents may add organic contaminants to the water. 2 • Due to high cost and structural weakness, Teflon is inferior to stainless steel as well casing material. Stainless steel may react negatively with acidic or saline ground waters. Under such conditions, PVC with threaded joints would be better. 2
The filter pack is often sand, and the grains must be necessarily larger than the filter screen slots. The filter pack surrounds the casing inside the borehole, and fills the annular space up past the well screen in the bore hole. The filter pack is often capped by a layer of fine sand. 2 • The annular seal is often comprised of a layer of expandable material such as bentonite pellets, capped by a layer of fine sand capped by a layer of grout which is pressurized in place. 1 • Capping the annular seal is the surface protection: a layer of concrete surrounding the casing, which can protrude above ground level, and a protective cover, which often includes a lockable cap. The layer of concrete effectively seals the area between the well and the borehole (the annular space) from movement of contaminants and pollutants 2
(4) External protective cap (5)
A basic requirement for proper and effective ground-water monitoring well design and installation is the working use of flexible guidelines that are adaptable to a range of chemical and geological environments. • To develop said guidelines, it is necessary to identify common problems in well design and construction.
Use of a well casings or screens that are not compatible with the hydrogeological environment or the known or anticipated contaminants 2 • Results in chemical alteration of samples or well failure
2. Use of a well screen that is not commercially produced. 2 • Well sedimentation or turbidity in collected samples during the life of the monitoring program
3. Use of a single well screen and-filter pack combination for all of the wells at a particular site, regardless of the geology or grain size distribution. 2 • Causes sample turbidity, invasion of overlying well construction materials, and lower than expected well yields.
4. Improper well screen length and placement2 • the retrieval of water quality data from discrete zones is impossible.
5. Improper selection of filter pack materials2 • Causes well sedimentation, well screen plugging, chemical alteration of ground-water samples, or well failure.
6. Improper selection and placement of annular seal materials 2 • Results in alteration of sample chemical quality, plugging of the filter pack and well screen, or cross-contamination from geologic units improperly sealed off
7. Inadequate surface protection measures 2 • Results in surface water entering the well, alteration of sample chemical quality, and damage to or destruction of the well.
Now That we Know Some Potential Pitfalls, What is the Role of Site Characterization in Monitoring Well Design?
Tools and Methods for Site Characterization Include: • Soil and rock sampling • Field analytical methods • Remote Sensing and geophysical methods
A thorough site characterization can provide a detailed knowledge of site specific geologic, hydrologic, geochemical, and microbiological conditions to aid in site specific well design. 2 • Aids in Monitoring well placement, well type, and determining desired monitoring well depth 2 • Aids in determining drilling method 2
Site specific design variables include: • Objective of the ground-water monitoring project • Water quality monitoring versus water level monitoring 2 • Surficial conditions such as drainage, topography, seasonal climate changes, and site access 2 • Geologic setting, flow pathways, degree of heterogeneity, porosity type, recharge or discharge conditions 2 • Ground-water / surface water interrelationships 2 • Ground water chemistry and microbiology 2 • Site specific contaminants and their chemistry, density, viscosity, reactivity and concentration 2 • Anthropogenic influences • Human induced changes in hydraulic conditions 2 • Regulatory requirements 2
I know about the site, I know Where My Wells Need To Go, now what? • Design the type of monitoring well needed based on the characteristics of the site and project objectives
Monitoring Well Completion Types • Single-Casing, Single-screen Wells • Multiple-casing, Single Screen Wells • Bedrock Completions • Cased • Cased with Conductor • Uncased • Monitoring Multiple vertically Seperated Zones • Well Clusters • Single-Casing, Multiple Screened Wells • Nested Wells • Single-casing, Long Screen Wells • Multilevel Monitoring Systems
Single-Casing, Single-Screen Wells • Simplest, most common type • This type of well is most useful for monitoring water table fluctuation or a single discreet interval, such as a thin sand seam within a matrix of silt and clay 2
Multiple-Casing, Single-Screened Wells • Referred to as Telescoping casing wells • Often used where it is necessary to drill through a one or more contaminated zones to complete a well in a formation below • A larger diameter bore hole is drilled to just below the contaminated zone, terminating at the top of a confining layer where a large diameter surface or conductor casing is installed and pressure grouted in place. 2 • A smaller diameter bore hole is then drilled from the bottom of the pilot zone down to the zone of interest 2 • The monitoring well is then completed with the surface casing in tact to prevent hydraulic communication between the upper zone and zone of interest 2
Sand Filter Pack (2)
Bedrock Completions • Cased with Conductor or surface casing • Cased • Open-bedrock bore holes
Cased with Conductor or surface casing 2 • Drill a hole through bedrock and complete as in the multiple –casing, single-screened method • Insures the zone of interest is isolated from the penetrated by the borehole, minimizing interzonal flow • Good for monitoring discrete zones beneath confining beds or beneath known contaminated zones • Good for monitoring zones directly beneath overburden (2)
Cased 2 • Drill a hole through bedrock to the zone of interest and complete in the same manner as a single-casing, single-screened well • Good for monitoring discrete zones (2)