Wells

Cross-section view of a bioretention cell with a shallow monitoring well. For further details click here for STEP's, Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide (Pg. 195 - 204). A note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

This article gives tips on the design and installation of monitoring wells into LID practices. It does not address the much deeper wells used to monitor groundwater levels. Monitoring wells are frequently installed on infiltrating systems such as bioretention, bioswales, infiltration trenches and hybrids thereof. The two questions most frequently addressed are:

1. How quickly is surface ponding being infiltrated beneath the surface?
2. How quickly is the facility draining through exfiltration to native soils and/or through an underdrain?

Shallow wells

To measure the depth of ponding on the filter media bed surface, and rate at which the ponded water drains, install a shallow well with a water level logger:

• Use a section of perforated pipe or well screen with a geotextile sock around the perforated section that will allow ponded water to enter the well while preventing sediment from accumulating within it.
• Locate the well in the deepest portion of the surface ponding area or where ponding most frequently occurs.
• Measure the depth of the water level sensor in the well, relative to the filter media surface to translate sensor readings into surface ponding depth measurements.
• Ensure there is a solid cap on the bottom of the well to form a small water reservoir (e.g., 100 mm deep) to keep the water level sensor submerged during dry conditions. This practice also helps with checking calibration of the sensor.

Deeper wells

Some LID features are designed to retain water within the media that makes up the LID feature (e.g. permeable pavement will be on top of a gravel bed where water can be stored). To measure the amount of water retention in a monitoring well, install a water level logger:

• Install the well while the feature is being constructed or undergoing major maintenance.
• If there is an underdrain in the feature, make sure the bottom of the well reaches below it.
• Install the well so that surface water will not impact the water level within the well. A standard monitoring well casing over the well pipe should be sufficient.
• If possible, measure the depth of the underdrain. Having this measurement helps determine when water is percolating into the underdrain.
• Ensure there is a small reservoir in the well to keep your level probe submerged in dry conditions.
• Where possible, wells for measuring water levels within permeable pavement systems should be outside of trafficked areas.

Cross-section view of a bioretention cell with a deep monitoring well. For further details click here for STEP's, Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide (Pg. 195 - 204). A note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

Monitoring Well Specification

The monitoring well should be installed during construction in a pervious area within the trench between tree planting areas. The well should be vertical from top to bottom.

Feature	Notes
Material	The pipe should be rigid to prevent bending (e.g. PVC).
Diameter	minimum 100 mm
Perforations	The perforations may be similar spacing similar to underdrain pipes. Perforations are only required on the bottom third of the pipe. The well should be sealed at the surface to prevent short circuiting of surface water flow along the sides of the well. Bentonite clay is often used for this purpose.
Sock	The perforated portion of the pipe should be wrapped with a geotextile sock. Standard geotextile provided with perforated pipe may be used.
Bottom/sump	The bottom 100 mm of the well should be capped and free of perforations to provide a small reservoir of water to keep the sensor wet during dry weather.
Length	The well should extend into the native soil below the facility by a minimum 150 mm. Top should be above the surface overflow elevation or above the maximum ponding depth to prevent surface water from entering the well. If the perforated drainage pipe is installed in a v-notch below the bottom of the facility, the well should extend 150 mm below the invert of this drainage pipe.
Cap	The well should be finished with a lockable cap to prevent vandalism.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.

Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed).

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

A perforated pipe used to assist the draining of soils.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

The natural ground material characteristic of or existing by virtue of geographic origin.