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Groundwater pollution remediation and protection 267

environmental pathway is to determine the con- model are combined to provide contaminant source
centration of the contaminant at an abstraction site data that are then used in a pollution risk model to
while the exposure pathway assesses the effects of calculate a probability distribution for the concentra-
exposure to contaminated water. tion of a contaminant at a selected pumped borehole
To gain the most efficient use of time and financial for comparison with water quality standards.
constraints, tiered approaches to risk assessment are
widely applied. Increasing tiers equate to increasing
levels of sophistication with respect to site-specific 7.3.4 Spatial planning and groundwater
risk assessment. In general, four levels of assessment protection
can be identified:
• Tier 1: Preliminary investigations: qualitative desk The problem of nitrate and pesticides leaching from
study; determination of source–pathway–target regions of intensively managed arable farming and
chains; limited intrusive investigation and sampling. cattle grazing affects wide areas of western Europe
• Tier 2: Site characterization: semiquantitative; some and other regions of the world and it has become
intrusive site investigation and sampling; prioritiza- clear that more stringent controls on land-use activ-
tion and screening methods. ities, integrated into local and regional spatial planning,
• Tier 3: Generic risk assessment: qualitative/semi- are required if groundwater quality is to improve.
quantitative; comparison of estimated contaminant Such an approach has been applied in the United
concentrations with generic guidelines and standards; Kingdom and Denmark (Box 7.5) and provides
assessment of environmental pathway; intrusive experience for the future formulation of strategies
site investigations; computer simulations/modelling; for reducing the impacts of diffuse groundwater
stochastic approaches. contamination.
• Tier 4: Quantitative risk assessment: use of derived In England, the Pilot Nitrate Sensitive Area (NSA)
contaminant concentrations for exposure pathway scheme was started in 1990 and by the time of
assessment; exposure assessment models; integrated the Main NSA scheme in 1998, 80% of the land area
approaches to risk assessment. comprising 35,000 ha in 32 catchments was included
This type of tiered approach is the basis of the in this voluntary, compensated agri-environment
American Society for Testing and Materials (ASTM scheme. Two levels of payment were offered to
2001) standard guide to RBCA (risk-based correc- farmers entering into the scheme with higher com-
tive action) for site investigation and remediation pensation given under the Premium scheme for
at petroleum-contaminated sites, although the pro- arable land conversion to grass with total nitrogen
−1
cess can be applied to any contaminant and release inputs of less than 150 kg ha . Under the alternative,
scenario. The framework has a site characterization Basic scheme, farmers were paid less for sowing
stage followed by a three-tier approach where the winter cover crops, restricting organic manure inputs
human health and environmental risks are equally and their timing, and limiting nitrogen fertilizer
accounted for. As each tier is completed there is an inputs to below the recommended optimum. Table
evaluation to determine if more information is 7.3 summarizes the results of long-term monitoring
required. If so, then the next level of assessment is of the effectiveness of the scheme in the first 10 catch-
conducted; if not, then an assessment of the correc- ments included in the Pilot NSA scheme with respect
tive action is required. A tiered approach is also used to measured soil nitrate losses prior to and during
by the Environment Agency in England and Wales in the scheme. The results indicate an overall reduction
the hydrogeological risk assessment of landfills of about 50% in nitrate concentrations and 28% in
(Leeson et al. 2003) as outlined in Fig. 7.10. nitrogen fluxes leaving the root zone compared to
A practical example of a Tier 3 risk assessment is baseline values. Corresponding values for the further
provided by Davison et al. (2002), who presented a catchments included in the Main NSA scheme are
management tool to identify the best use for urban 34% and 16%, respectively, between 1994 and 1996
groundwater pumped from a user-defined location. and 1998 and 2000 (ADAS 2003). Take-up and con-
A probabilistic catchment zone model and land-use version of arable land under the Premium scheme