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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:
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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. |
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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. |
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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.
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:
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Whether past contaminative
processes undertaken at the site have had significant
impact upon the groundwater. |
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The level of contamination that
maybe present within the groundwater. |
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The hydraulic gradient and direction
of groundwater flow beneath the site. |
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Provide baseline data for the
groundwater before any development or remediation
commences. |
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To monitor the short term and
longer term impact of remediation excavations and
/ or installation of foundations such as deep piles
on the groundwater. |
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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.
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.
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 EA specification.
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.
Voelcker Consultants 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:
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Production of Phase
1 Desk Study reports through extensive
information gathering, analysis and interpretation;
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Designing, commissioning and
supervising Phase 2 Intrusive Site
Investigations, to meet the requirements of both
the client and statutory authorities; |
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Undertaking groundwater monitoring
exercises; |
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Using collected data to undertake
groundwater risk assessments; |
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Design & Implementation of
remedial schemes and undertaking post remediation
monitoring (Phase 3); |
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Validation of remediation (Phase
4) and production of Post Remediation Validation
Reports and remediation statements; |
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Liaison with statutory authorities
(e.g. the Environment Agency, local authorities,
HSE etc.) |
Voelcker Consultants undertake site
investigations and reporting in accordance with relevant
guidelines including BS 10175: 2001 and Environment
Agency guidance. We have extensive experience in undertaking
site investigations and groundwater monitoring on a
wide range of sites.
Please contact
for a discussion of your requirements.
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