Groundwater HIA post edit - FreshwaterLife

A small domestic supply on the other hand, pumping several hundred litres
per day, may only require 1 m or so. The basic details of typical PWLs, and
existing pump intake levels are essential for assessment of derogation.
• Derogation impacts should be judged by considering both typical conditions
and dry or drought conditions. For dry conditions, considering the water
levels that would occur in a drought of 1-in-10-year return period would be
reasonable, rather than the worst case conditions. This frequency of
drought is classified as a moderate drought by the British Hydrological
Society classification system (Mawdsley et al 1990). Also, the annual
groundwater recharge during the 1-in-10-year drought was recommended
for use as part of an indicator of hydrological severity, developed during an
R&D project for the National Rivers Authority (NRA 1995). Using a
moderate drought assumes that all abstractors will accept some restrictions
on their ability to abstract in serious or severe droughts, and so derogation
does not apply to these conditions.
• The effects of the uncertainties in drawdown estimates should also be
considered before making a decision about derogation (see Box 2.1 for an
example). If the drawdown at which derogation will occur is greater than
the predicted drawdown but falls within the range of possible drawdowns
established by the sensitivity analysis, then the uncertainty may be too
great to make a clear decision at this stage. Further monitoring may be
required, during a pumping test or during the period of a time-limited
licence.
If derogation is predicted or occurs, then there may have to be negotiation between the
applicant and the potentially affected abstractors, before a licence can be granted.
Options include lowering the pumps, deepening the borehole, providing an alternative
supply, or even paying compensation.
Environmental impacts on water bodies and wetlands: The local conceptual
model, and information gained during water features surveys and any other
investigations are used to estimate the potential environmental impacts of the
abstraction on the water levels within any ponds, wetlands, meres, fens and springs.
There is currently limited capability for predicting the ecological impacts of water level
reductions on wetlands, and more research is urgently needed. The drawdown at
water bodies and wetlands can be estimated in a similar fashion to the estimation of
the potential for derogation, and the following factors should be considered:
• Some wetlands may be perched on a low-permeability substrate, and may
not be in hydraulic continuity with the aquifer beneath. For example, this is
thought to be the case for many upland blanket bogs on low-permeability
hard-rock terrain.
• The Thiem and Theis equations actually predict the drawdown in the
aquifer under the wetland. This is not necessarily the same as water level
changes in the wetland itself, because of the equivalent effect to there
being lower-permeability river-bed sediments in a river. For most wetlands,
there is some resistance to flow between the wetland and the underlying
aquifer, due to the build-up of sediment and organic material. A wetland
leakage factor 'C' is sometimes used (C = K/b, where K is the vertical
hydraulic conductivity and b is the thickness, of a semi-confining layer
beneath the wetland). Flux of water vertically between the wetland and the
groundwater depends on C and on the hydraulic head difference, but C is
very rarely known with any certainty. This subject is discussed at length in
Williams et al (1995).
36 Science Report – Hydrogeological impact appraisal for groundwater abstractions