Groundwater Monitoring and Risk Assessments

Background

Contamination generated by sites with a history of industrial use can impact upon a number of environmental receptors, one being groundwater.

Geological strata containing water that underlie a site may be classified as an aquifer. The presence of an aquifer beneath a site can have implications for the intrusive investigation and also the redevelopment and further use of the land.

An aquifer can be defined as a deposit of rock, permeable by groundwater that may be used to supply groundwater abstraction wells and may also support springs and be in hydraulic continuity with the water in rivers or other aquifers.

There are a number of different geological strata across the UK that are classified as aquifers. Abstraction wells from these aquifers are used for potable supply and varied industrial and agricultural uses. The Environment Agency classifies all aquifers as controlled waters but divides them into three categories:

Major Aquifer: Highly productive and used for potable supply on a regional scale.

Minor Aquifer: Maybe important for water supply locally.

Non-Aquifer: Negligibly permeable, containing insignificant quantities of groundwater.

Factors that will influence the vulnerability of an aquifer to contamination from industrial sites include whether the aquifer is classed as confined or unconfined; the depth of the aquifer; whether the Major Aquifer is overlain by a Minor Aquifer that is in hydraulic continuity with; and the soil vulnerability.

The ability of a soil to attenuate pollutants is dependent upon a number of factors related to its lithology. There are three classes and six sub classes of soil vulnerability classification groups for UK soils. The groups are based on the natural ability of the overlying strata to attenuate and adsorb contamination and is based on many factors including physical and chemical properties, texture, structure, grain size, soil water regime, rate of drainage and permeability, clay content and soil type.

Some strata have a high leaching potential and have very little ability to slow or halt the progress of contaminants and transmit them readily to the underlying aquifer. Other strata have a low leaching potential and are thus either impermeable or have a number of natural factors that can slow or stop the leaching of contaminants.

Confined Aquifer: An aquifer that lies between two aquitards - strata that do not allow water flow. The overlying impermeable strata can give a degree of protection to the aquifer from the leaching of contamination from the surface.

Unconfined Aquifer: The upper boundary represents the water table and there is no overlying capping layer of impermeable strata, also known as a water table aquifer.

Potable Supply

The Environment Agency divides the area surrounding an abstraction borehole used for potable supply (source) into Source Protection Zones (SPZ) that are defined as follows:

• Inner protection zone: Defined by a 50 day travel time for groundwater to reach the source (abstraction borehole) or defined by a minimum of 50 metres.
• Outer protection zone: Defined by a 400 day travel time from any point below the water table to the source, or the minimum time recharge area required to support 25% of the protected yield, which ever is the greater.
• Total Catchment: Defined as the area around the source within which all groundwater recharge is presumed to discharge at that source.

A contaminated site located within an SPZ for potable supply gives rise to concern and the need for investigation to assess the impact of the site upon the groundwater within the underlying aquifer.

Investigation of Groundwater

Determination of the potential impact of a contaminated site upon an underlying aquifer is required by the Environment Agency and local authority; especially if the site is underlain by an unconfined Major Aquifer or the site lies within an SPZ for a groundwater abstraction borehole. These issues should be investigated prior to a site being redeveloped to assess the following aspects:

• Whether past contaminative processes undertaken at the site have had significant impact upon the groundwater
• The level of contamination that maybe present within the groundwater
• The hydraulic gradient and direction of groundwater flow beneath the site
• Provide baseline data for the groundwater before any development or remediation commences
• To monitor the short term and longer term impact of remediation excavations and/or installation of foundations such as deep piles on the groundwater
• To ascertain whether remedial measures need to be used to decontaminate the groundwater.

Data collected from groundwater monitoring may be used to undertake groundwater Risk Assessments (Link to B4) in conjunction with a number of computer based modelling packages and Environment Agency Guidance. 

Installation of Boreholes

In order to obtain information about the quality of groundwater present, boreholes with groundwater monitoring standpipes usually need to be installed at the site. If the location of the source of contamination is not known, sufficient boreholes would need to be installed to give adequate coverage of the site up gradient and down gradient of the direction of groundwater flow.

If the source of contamination within the ground is known to be present; a minimum of three groundwater monitoring wells should be installed, both up gradient and down gradient of the direction of groundwater flow in the vicinity of the source of contamination.

Clean Drilling Techniques

Should a site be located near to a borehole used for potable supply within the Inner Source Protection Zone for a Major Aquifer it is likely that any groundwater monitoring wells will need to be installed using clean drilling techniques so that the risk of cross contamination between contaminated overburden and underlying aquifer is minimised. Boreholes will also need to be decommissioned to Environment Agency (EA)specification.

Groundwater Monitoring

Subsequent rounds of groundwater sampling are then undertaken, a typical schedule of groundwater monitoring visits usually comprises:

Pre–Remediation Monitoring: One or two rounds of sampling prior to commencing remediation excavation or installation of pile foundations, to assess the initial chemical content of the groundwater with respect to chemical contamination and to provide baseline data for comparison with subsequent results.

Monitoring During Remediation: The number of monitoring rounds carried out will depend on the size of the site; the expected duration of the remediation excavations; the levels of contamination encountered and the requirements of the Environment Agency.

Post-Remediation Monitoring: When remediation or installation of foundations is complete one or two rounds of post remediation / foundation installation monitoring are usually required to assess the long term impact of the work. One round is usually carried out a month after completion and a further round maybe undertaken three to six months after completion.

Capabilities

ESG has been involved in a wide range of development projects requiring site investigations of potentially contaminated sites and groundwater. We are able to provide a comprehensive contaminated land investigation service including:

• Production of Phase 1 Desk Study reports through extensive information gathering, analysis and interpretation;
• Designing, commissioning and supervising Phase 2 Intrusive Site Investigations, to meet the requirements of both the client and statutory authorities;
• Undertaking groundwater monitoring exercises;
• Using collected data to undertake groundwater risk assessments;
• Design & Implementation of remedial schemes and undertaking post remediation monitoring (Phase 3);
• Validation of remediation (Phase 4) and production of Post Remediation Validation Reports and remediation statements;
• Liaison with statutory authorities (e.g. the Environment Agency, local authorities, HSE etc.)

ESG undertake site investigations and reporting in accordance with relevant guidelines, including BS 10175: 2001 and Environment Agency guidance.