Wells

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 and the rate at which the ponding water infiltrates, install a shallow well with a water level logger:

• Use a perforated pipe (a piezometer) that will allow surface water to enter the well but will help prevent sediment from accumulating within it.
• Locate the well where ponding occurs first - near the inlet or the lowest point in the feature.
• Measure where the ground surface is relative to where you will be taking your water level measurements.
• Ensure there is a small reservoir in the well to keep your level probe submerged in dry conditions. This practice will also help with calibration.

Deeper wells

Some LID features which are designed to retain water during precipitation events store stored 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.

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. 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 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.

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).

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.

Any form of rain or snow.

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).

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.

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