Groundwater HIA post edit - FreshwaterLife

Principle 1: The impact of a groundwater abstraction spreads until it has stopped
an equal amount of water leaving the aquifer. Imagine an aquifer system that is in
long-term equilibrium and where there are no abstractions. Inputs to the system may
include recharge from rainfall, regional groundwater flow, and infiltration from surface
water. Discharges from the aquifer may include springs, seepages, baseflow to rivers,
and regional groundwater flow. A simplified system of an island in a fresh water lake,
receiving rainfall recharge, is illustrated in Figure 2.3 (upper diagram). Now imagine
that a borehole is drilled, abstraction commences, and a cone of depression starts
spreading out from the borehole. Initially, the abstraction takes water from storage,
and the flow of groundwater into the lake is unaffected (Figure 2.3, middle diagram).
However, as the cone of depression spreads, it eventually reduces the gradient of the
water table where it intersects the lake, and groundwater flow into the lake will be
reduced (Figure 2.3, lower diagram).
Figure 2.3 Impact of groundwater abstraction on aquifer discharges
(after Bredehoeft et al 1982; note that the rainfall continues in the middle and lower diagrams)
The first principle to emphasise therefore, is that the effect of the abstraction will
spread until it has stopped an equal amount of water from leaving the aquifer (in both
confined and unconfined aquifers). This will usually be in the form of reduced
discharges (reduced springflow, reduced baseflow, or reduced seepage, for example).
In other words, all groundwater abstractions eventually have an impact, it is only a
question of where the impact will appear and how long it will take. This subject was
addressed over 65 years ago by Theis (1940), who said:
“Under natural conditions…previous to development by wells, aquifers are
in a state of approximate dynamic equilibrium. Discharge by wells is thus a
new discharge superimposed upon a previously stable system, and it must
be balanced by an increase in the recharge of the aquifer, or by a decrease
in the old natural discharge, or by loss of storage in the aquifer, or by a
combination of these.”
Principle 2: Impacts can be changes in flow as well as water level. It will be seen
later that the procedure for HIA differentiates between impacts due to changes in flow
and impacts due to changes in groundwater level. Impacts from changes in level are
easy to visualise, and include for example, lower water levels in neighbouring
boreholes or in groundwater-supported wetlands. Impacts from changes in flow are
10 Science Report – Hydrogeological impact appraisal for groundwater abstractions