of an overexploited aquifer (Prato, Italy)
5. turbidite sandstones
B
A
Figure 4. 1. sandy silt; 2. colluvium; 3. sandy-silty gravel; 4. clay and silt; 5. bedrock;
6. stratigraphic wells; 7. unconfinedgroundwater level; 8. semi-confined groundwater level.
Figure 3. Piezometric surface (April 2004).
Legend: 1. Water level measure points;
2. Water level contour lines; 3. Flow path direction ; 4. hydrographic network.
Combining these aspects with the rising impact of climate changes, this cyclic piezometric levels (Fig-ure 5) locally fall below the sea level reducing wells productivity, having a serious impact on the economical and social framework of the region.
O N G O I N G S T R AT E G I E S
In order to overcome this crisis a series of actions have been carried out to improve the hydrogeological balance terms over the last 25 years.
In a chronological order a first attempt to differentiate the resources for drinking purposes was to increase the use of surface water from the upper part of the river Bisenzio catchment-basin. Successfully reducing groundwater abstrac-tions for industrial use guided the creation of a dedicated industrial aqueduct for the re-use of treated waste water.
In parallel a series of small dams were built up to enhance the river losses into the aquifer, providing an important contribution to the groundwater balance. Most recentely more water is used from the Florence aqueduct supplied by the Arno river – whose discharge is regulated by a large dam placed on the upper part of the basin –- has been made in connection with the Prato aqueduct.
Despite the mentioned investments and commitments the Prato aquifer has not yet shown a significant and per-sistent recovery.
H Y D R O G E O L O G I C A L B A L A N C E
The groundwater balance of the Prato aquifer has been calculated for the hydro-geologic year 1st of October 2000 – 30thof September 2001, on the basis of an existing methodology proposed for the year 1988.
In the year of interest the amount of total rainfall has been about 25% higher than the long period average, thus an anomalous surplus of the balance has actually occurred.
The six terms of the water balance for the year 2000–2001 are: Hidden flow from the surrounding aquifers (As);
Well abstractions (Du); Rainfall infil-tration (Ia); River losses (If); Pipeline losses (Ri); Storage (dR).
1959 1966 1973 1980 1987 1994 2001
Water level [m.a.s.l.]
Figure 6. Hydrogeological scheme of the Prato urban aquifer.
All terms in millions of cubic meter
The equation is therefore structured as follow:
If + As + Ia + Ri = Du + dR
The conceptualization of an urban aquifer was adapted to our case study (Figure 6). The elements of complexity of the balance are mainly depending on several aspects such as the impact of well abstractions for civil and non civil uses, the variability of land use, with the dominance of low permeability areas obstructing infiltration, the presence of a complex system of pipelines and sewage channels, interfering with the groundwater level, locally recharging or draining the aquifer. The water balance structure follows the scheme of Figure 6, where the con-tribution of evapotranspiration is not taken into account, because it is negligible compared to the other terms such us industrial abstractions (error about 0.002 %).
The total volume of water stored rose from 97 Mm3 to 118 Mm3 during the hydrologic year. Rejuvenation time (Castany, 1985) of the aquifer is 1.7 to 2 years.
E X P E R I M E N TA L P L A N
A feasibility study of an artificial recharge program was developed including a series of studies and field inves-tigations. The geological and hydrogeological knowledge of the area was improved by geophysical investigations, along with core sampling and logging (Figure 7) and well tests on the aquifer properties. Additionally a continuous monitoring of the groundwater level nearby the location of the pilot plant was set up (Figure 8).
Transmissivity and hydraulic conductivity values were derived from a well pumping test (Figure 9a), while the same parameters for the non-saturated zone were provided by the recharge test (Figure 9b).
The values obtained for hydraulic conductivity are con-sistent throughout the saturated and the non-saturated zones, with a magnitude ranging between 4 to 9 m/d, according to the lithological composition of the core samples.
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Figure 7. Gamma Ray Well log and stratigraphy derived from core sampling
54.00 55.00 56.00 57.00 58.00 59.00
06/04 08/04 10/04 12/04 02/05
m. a.s.l.
Figure 8. Continuous monitoring of the water level at the plant location
As a tool to evaluate the effectiveness of an artificial recharge program of 400 l/s, corresponding to a recharge of 8 months a year, with a time duration of 10 years, a transient model of the aquifer was developed and calibrated for the time period 1960–2001. Although locally the model showed problems simulating single wells behaviour, likely because of poor reliability of industrial well abstractions rates, three different water management scenarios, for the next 10 years (2004–2014) were tested. Beside artificial recharge, a modulation of the river Bisenzio discharge was simulated, to predict the effects of improving river bed infiltration.
A third reference scenario was based on the actual data for the year 2001, keeping them constant during the 10-year simulation. Figure 10 shows the results on water levels for the three model scenarios at the monitoring well ‘Badie’.
The prediction model run with the values for 2001 (‘Status Quo’) indicates an attenuation on the effects of the over-exploitation with the consequent rising of the water level of 2 m over the 10-year period.
The influence of the river discharge modification (‘Rivermod’) provides an extra increment on the water level elevation of about 3 m the period to 2014. The most efficient scenario for the recovery of the aquifer is definitely represented by the artificial recharge (‘AR’). In particular the recharge begins to be effective 6 months after the activation of the injection well field.
Figures 11a and 11b show the spatial distribution of the effects resulting 10 years later respectively for an artificial recharge scenario and for a river discharge modification scenario. The latter substantially improves the portion of the river infiltration on the hydrogeological balance.
Monitor Badie
2001 2002 2004 2005 2006 2008 2009 2010 2012 2013 2014 2016
Time
Elevation (m a.s.l.)
Figure 10. Monitor Badie: simulated groundwater level trends for three different 2014 scenarios:
‘Status Quo’, ‘Artificial Recharge’, ‘River Head Modification’
- 9.8
0 15000 30000 45000 60000 75000 90000 105000 120000
Time [s]
Figure 9a. Well pumping test (constant discharge rate 3.6 l /s)
Figure 9b: well recharge test (constant recharge rate 2 l/s)
The highest impact of this scenario occurs in the centre of the urban area, where the cone reaches the maximum of depression.
The effects of the artificial recharge are depending on the hydraulic conductivity, therefore on the hydraulic gradient generated at the plant location, where we have the most effectiveness of the recharge.
However such increment, generated by a recharge rate of 400 l/s for 8 months, raises the water level above the topography of the area. For such reason an additional model simulation with a 30% reduced recharge rate proved compatible with the site elevation (Figure 12).
C O N C L U S I O N S
The hydrogeological studies and the groundwater balance of the Prato aquifer show that an artificial recharge by injection wells would provide a good strategy to reduce the water shortage and to increase water resouces in the area. A recharge plant in the north would not allow complete recovery of the aquiferin 10 years time at a maintain-able rate of 120 l/s. Enhancing river bed infiltration and simultaneously reducing groundwater demand are neces-sary additional measures.
R E F E R E N C E S
Castany G (1985). ‘Idrogeologia, Principi e Metodi’- Editore D. Flaccovio, Palermo 1985.
Dillon (Ed) (2002). Management of Aquifer Recharge for Sustainability, Proceedings of the fourth International Symposium On Artificial Recharge, ISAR 4, Adelaide, 22 – 25 Sept. 2002.
Landini F., Pranzini G., Puppini U., Scardazzi M. E., Streetly M. J., Valley S. (2004). An overexploited aquifer (Prato, Italy): a physical model for groundwater resources evaluation. Proceedings of the 32nd International Geological Congress (Firenze, 20 – 28 August 2004).
Landini F. (2005). Geological and Hydrogeological research for the design of an artificial recharge experimental plant in an overexploited porous aquifer in Middle Valdarno Basin (Tuscany, Italy). PhD Thesis, Department of Earth Sciences, University of Florence (Italy).
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0 100 0 2000 3000 4000
Figure 11. Water level increment contour lines simulated for 2014:
left) with a an artificial recharge;
right) with a river discharge modification
Figure 12