Groundwater HIA post edit - FreshwaterLife

• Multiple abstraction points: Sometimes, a single licensed abstraction
may actually consist of multiple abstraction points, such as an array of
boreholes tens or even hundreds of metres apart. Again, judgement needs
to be used, as beyond a certain spacing, it becomes unrealistic to treat two
or more boreholes as a single borehole with larger effective radius, and
they need to be treated as separate boreholes. There are recognised
techniques, explained in most hydrogeological textbooks, for calculating the
combined effect of several boreholes, using the principle of superposition.
• Non-linear systems: Aquifers where the transmissivity varies with time
cannot be regarded as linear systems. The classic example is unconfined
chalk, where the transmissivity may vary, sometimes dramatically,
depending on the seasonal water level. As far as drawdown impacts are
concerned, the problem is the speed of the impacts and how this changes
seasonally. Again, professional judgement from a hydrogeologist is
needed, using the approach of performing two calculations, one ‘slow’
calculation using aquifer properties when water levels are low, and one
‘fast’ calculation for when water levels are high. The ‘slow’ case is probably
more important in terms of assessing drawdown impacts, as when water
levels are seasonally high, drawdown impacts are probably less likely to be
significant.
• Abstractions from karst and fractured crystalline rock: Care needs to
be taken when dealing with groundwater abstraction from karstic aquifers
and fractured crystalline rock. The assumptions inherent in analytical
equations such as those of Thiem, Thiem-Dupuit and Theis usually break
down, and it is no longer reasonable to pretend the aquifer is homogeneous
and isotropic (see Appendices 3 and 4).
Before moving to the next step, another issue needs to be discussed briefly, related to
the shape of the drawdown profile shown in Figure 4.3 (in reality a cross-section
through the cone of depression). It can be seen that assuming the drawdown is zero at
radius Ro, the radius ri can be dramatically less than Ro before the drawdown si
becomes significant, because of the shape of the curve. This introduces the possibility
of defining a more realistic radius for the search area, within which the drawdown is
likely to be significant, to reduce the number of water features that have to be
individually assessed for drawdown impact (the dashed red circle on Figure 4.2). With
a little rearrangement, the Thiem and Thiem-Dupuit equations can be used to define
that radius, having chosen an arbitrary value for significant drawdown (si).
This immediately raises the question of what is a significant drawdown, or what
justification is there for any particular value of drawdown? The answer is that it
depends on the nature of the water features that have been identified in Step 8. If the
water features include sensitive wetlands, for example, then a small drawdown would
be significant (perhaps of the order of 0.05 m). Specialist advice should be sought, if
necessary, on what is a reasonable figure to use, (see for example Brooks et al 2004).
Bear in mind that we are only setting a search radius, so err on the side of caution if
there is any doubt. Also, there is no reason why different search radii should not be
used for features with different sensitivities.
4.2.10 Step 10: Allow for mitigation of drawdown impacts
This step has been included because it is possible that discharges from the abstraction
can be used to mitigate drawdown impacts. It is much more common for discharges to
be used to mitigate flow impacts (considered under Step 5 above), but it is worth
34 Science Report – Hydrogeological impact appraisal for groundwater abstractions