Soluble salts

Conductivity meter

All soils, filter media, and green roof media contain some water soluble salts which include essential nutrients for plant growth. When the concentration of water soluble salts exceeds a certain level, harmful effects on plant growth occur. A soil containing a high concentration of soluble salts is referred to as a saline soil. Salt-affected soils often result from the flow of salty water onto an area, either laterally (e.g., intentional infiltration of deicing salt laden runoff in LID BMPs; de-icing salt laden runoff splashed onto roadside soils) or by artesian flow of salty groundwater onto topsoil.

The soluble salts design specification for the soil component of LID BMPs is intended to ensure its fertility and suitability for maintaining healthy vegetation cover. Where concentration of soluble salts deviates from design specification, vegetation cover may be spotty or uneven, growth may be stunted, or in extreme cases, plantings may not survive and vegetation cover becomes dominated by weeds. To ensure the soil will support the growth of plantings, which contributes to the drainage and water treatment performance of the BMP and adds aesthetic value, testing of soluble salts should be done as part of construction, assumption and verification inspections.

Testing

Soluble salts concentration in soil can be assessed by measuring the ability of a soil and water mixture to conduct an electrical current, referred to as electrical conductivity (EC). The common unit for measurement of EC is milli Seimen per centimetre (mS/cm). The official international unit of measurement is Seimen per metre (S/m). One mS/cm is equal to one deciSiemen per metre (dS/m) or 0.1 Seimen per metre (S/m).

There are several methods available for preparing the soil and water mixture for EC testing. The method recommended for use in testing the soil component of LID BMPs for EC is using a 2:1 (v/v) water/soil slurry . Other laboratory methods for measuring EC in filter media include the Saturated Paste (SP) method or Saturated Media Extract (SME) method .

For green roof media, soluble salt concentration should also be measured using EC but with application of the SME method to prepare the soil and water mixture . In this extraction procedure, a sample of the media is brought to saturation with deionized water containing a small amount of Pentetic acid (i.e., DTPA) to enhance extraction of micro-nutrients . The SME method should also be used to prepare soil water extraction solutions for measuring concentrations of extractable phosphorus and nitrogen for green roof media.

Inspections

Construction inspections

If laboratory testing indicates soil soluble salts concentration is not within the design or product specification range, notify the media or topsoil supplier, issue a “do not install” order to the construction site supervisor and contact the design professionals and property owner or project manager to determine corrective actions.

Assumption and Verification inspections

If laboratory testing indicates soil soluble salts concentration is higher than the design or product specification, or acceptance criteria, corrective actions are only needed if problems with vegetation cover , condition or composition (i.e., dominance by weeds) are also detected through visual inspection. Where vegetation cover is poor, unhealthy or dominated by weeds and soluble salts are higher than the design or product specification or Acceptance Criteria, schedule investigative work to do further sampling and testing to determine the affected area and depth and decide on corrective actions. Depending on the findings, corrective action could involve flushing the soil area with fresh water or removal and replacement of an uppermost portion of the media with material that meets the design or product specification. Corrective actions to address soluble salts exceedance in green roof media should be prescribed by the designer, product vendor or media supplier.

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The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

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.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

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.

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.