Deep groundwater is always extracted through a deep well screen and pump connected to the well head by a riser pipe. This deep extraction creates a hydraulic inflow field (Fig. 6.2) similar to that of a Ranney well (Nemecek, 1961). For a homogeneous aquifer with a groundwater abstraction
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amounting to 25% of the all-over groundwater recharge, apart from well head protection, requires a groundwater protection zone, ranging from 600 m to 1,850 m upstream of the production well head (Fig. 6.2). Pollutants entering either upstream or downstream from this zone will by-pass the deep-sited filter of the production well (Fig. 6.2). Considering an inhomogeneous aquifer system the boundary stream lines of the inflow parabola extend even further upstream than shown for the homogeneous aquifer. The area of inflow is in contact with shallow groundwater over extended areas of the upstream part of the catchment. This means in practice, that:
• a correctly constructed well with relevant deep screening does not require a protection zone close to the well head apart from physical protection of the well head installation;
• as deep exploitation alters the groundwater flow field within the inflow area and also downstream, production well protection should cover the catchment both upstream and downstream of the production well.
In the case of both homogeneous and inhomogeneous aquifers, the width of the inflow parabola (Fig.6.2) depends only on exploitation relative to the over-all groundwater recharge The axis of the inflow parabo-la moves further upstream with increasing filter depth.
This simple modelling example for steady-state conditions represents many model runs and underlines the difficulty of exactly defining special protection zones for deep/emergency groundwater exploitation.
This difficulty is compounded by:
• transient hydraulic responses of deep groundwater to all changes in hydraulic boundary conditions;
• potential vertical short cut pathways between shallow and deep groundwater (see below).
Therefore every protection measure for a deep-sited emergency water supply or deep groundwater exploitation in general must be associated with an early warning system.
Indicators for and application of an early warning system
Many early warning systems are focused on the unsaturated zone and aimed at detecting the Figure 6.2. Modeled streamlines representing the geometry and width of the inflow parabola
to a well in a homogeneous aquifer with rectangular boundaries, extracting 25% of the all-over groundwater recharge from 100 m depth. Natural groundwater flow is from left to the right. At distances 1,850 m and 600 m from the well head the two boundary stream lines are at land surface, defining the area from which pollutants would reach the point of extraction.
pollution plume before it reaches groundwater level and the saturated zone. In addition, so-called environmental or stakeholder tracers assist in defining, and indicate changes within, the groundwater flow and transport field. Such changes cannot be noticed in groundwater level observations, because they refer to rapid pressure equilibration and not to slow mass transport. Such largely non-reactive, environmental tracers/indicators, which significantly support emergency groundwater protection policy, are, i.e.:
• radioactive isotopes (3H, 39Ar, 14C), as well as
• stable isotopes (2H, 18O) and noble gases and
• chloride.
These occur in both shallow and deep groundwater at concentrations depending on their radioactive half-life, input (eg. thermonuclear) or processes reflecting past climate conditions during groundwater recharge (chapter 4.4).
These tracers can be used in both simple qualitative, and sophisticated quantitative, ways:
• Radioactive environmental tritium and carbon-14 are considered an excellent pair of non-reactive subsurface indicators to recognise ‘young’ groundwater infiltration to deep groundwater wells in both hemispheres. Under real or quasi-undisturbed groundwater conditions all tritium bearing water falls in the area of young and all tritium-free waters in the area of old groundwater (Fig. 6.3). Measurable tritium together with low 14C contents indicates mixing of young, recent recharge with old groundwater hence a hydraulic disturbance of the natural flow field, which may endanger water quality.
• In South Germany long term 14C measurements from deep groundwater extraction are available
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0 20 40 60 80 100
0 TU -~ 5 TU (southern Hemisphere)
0 TU -~ 50 TU (northern Hemisphere)
'fossil' old groundwater recent or 'young'
Figure 6.3. Schematic presentation of the occurrence of tritium (3H) and radiocarbon (14C) in a non-stressed groundwater body sampled at shallow depth (recent or ‘young’) and at depth (‘fossil’), both with a groundwater table close to the land surface. Shallow groundwater contains 3H (> 0 TU) and 14C> 60 pMC; deep groundwater is free of 3H and has 14C< 60 pMC (< 15 pMC for ‘fossil’ groundwater). Once the natural flow system originally with a quasi-stable 3H and 14C-distribution has been disturbed, groundwater with low 14C contents and 3H in the ‘mixed water’ zone. Note the different
tritium scales for the southern and northern Hemispheres. (see Chapter 4.4)
for both unconsolidated (Fig. 6.4) and consolidated (Fig. 6.5) rocks. If pollutants reach the deep, primarily tritium-free exploitation level, the 14C concentration would significantly increase with time under the given management practice; if 14C remained unchanged or decreased, the production well was not endangered. Both figures also show that in case of deep aquifers a sampling cycle of 3 to 5 years was sufficient to timeously assess well pollution or to refine sophisticated mathematical models to better predict pollutant transport. Such a survey would have been even more sensitive had it been executed downstream from the exploitation well rather than at the production well site itself.
Figure 6.4. Results of a long-term monitoring of 14C concentrations in groundwater from deep production wells in unconsolidated rocks of the Munich and Augsburg areas in south Germany. Similar results are obtained for consolidated rocks. Usually, only minor variations and trends are observed over time.
Figure 6.5. Results of a long-term monitoring of 14C concentrations in deep groundwater production wells in consolidated rocks of the area of Nurmberg in South Germany