Rend. Online Soc. Geol. It., Vol. XX (201X), pp. XX-XX, 1 fig., x pl., x tab. (doi: 10.3301/Rol.2012.xx) © Società Geologica Italiana, Roma 2012 (Stile: intestazione prima pagina)
Continuous monitoring of surface-groundwater interactions in a lowland coastal aquifer GIACOMO VOLTA (*) & NICOLÒ COLOMBANI (**)
RIASSUNTO Monitoraggio in continuo delle interazioni tra acque superficiali e sotterranee in un acquifero freatico costiero. La distribuzione delle acque superficiali nella zona costiera della Provincia di Ferrara è in gran parte controllata dalla rete di drenaggio gestita dal Consorzio di bonifica. Questa rete ha anche modificato le dinamiche esistenti normalmente tra acque superficiali e sotterranee. Il monitoraggio in continuo del carico idraulico, della conducibilità elettrica e della temperatura sia delle acque superficiali che dell’acquifero freatico costiero, a diverse profondità in prossimità dell’abitato di san Giuseppe di Comacchio, ha messo in evidenza quelli che sono i fattori che controllano ed influenzano le caratteristiche idrochimiche ed idrodinamiche dei corpi idrici considerati e dei rapporti tra essi intercorrenti.
KEY WORDS: Aquifer recharge, continuous monitoring, salinity, upconing. INTRODUCTION The Ferrara Province coastal area represents the result of a complex combination of numerous natural and anthropogenic processes. Since the Middle Age the request of cultivable lands led to the construction of a dense drainage network which reclaimed all the areas to the South of the Po River’s outfall called “Valli di Comacchio” (BONDESAN et alii, 1995; STEFANI et alii, 2005). This induced the stiffening of the hydrographic geometry and consequently increased the subsidence rate (TEATINI et alii, 2005). In these lowland coastal areas affected by saline and hypersaline groundwater (MASTROCICCO et alii 2012), fresh groundwater lenses are created by infiltration of rain water or from canals leakage. The freshwater thickness allows plant growth and thus exerts a strong control on both agricultural (MASTROCICCO et alii, 2010) and natural vegetation (ANTONELLINI et alii, 2010). The aim of this work is to study the hydrodynamic and hydrogeochemical interactions between the coastal unconfined aquifer and the canals network. _________________________ (*) Dipartimento di Fisica e Scienze della Terra, Università di Ferrara (**) Dipartimento di Fisica e Scienze della Terra, Università di Ferrara e Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università “Alma Mater” di Bologna;
[email protected]
Fig. 1 – Location of the study area with indication of the piezometers (upper plot). Also shown are the drainage network, the sandy aquifer thickness and the geomorphic evolution of the delta. Schematic representation of the monitoring configuration at the field site (lower plot).
In particular, in the area near San Giuseppe di Comacchio (FE), where the canals lay on permeable sandy sediments (AMOROSI et alii, 2003) and the unconfined aquifer is not stressed by pumping wells. MATERIAL AND METHODS The monitoring has been performed from July 2012 to November 2012, using a piezometerof the Geological Service of the Emilia-Romagna Region’s piezometric network (P8), located 10 m away from the “Canale della Gronda” in San Giuseppe di Comacchio village (Fig. 1). The Holocene geomorphic evolution of the area has been controlled by continental and marine deposition (AMOROSI et alii, 2003). Above the Pleistocene alluvional-plain deposits, the Flandrian transgressive phase (18–5.5 kyear) deposited back-barrier fine-
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grained sediments and transgressive barrier sands. Today the alternation of highs and lows in the topography (mainly below the actual sea level) corresponds to different coastlines and to different stages in the evolution of the Po Delta. The sedimentary sequence is characterized by transgressive bay and barrier deposits at the base and prograding sandy deltaic lobes and strand plains at the top; locally, coastal plain fine deposits overlay sand bodies making the aquifer semi-confined. A multi parameters probe “Hydrolab MS-5” was employed to detect salinity variations inside the piezometer at different depths and in the canal. Two dataloggers were inserted, at -4.5 m and -8 m above sea level (a.s.l.), inside the piezometer using Teflon packer rings to insulate each aquifer portion. While another datalogger was inserted in the “Canale della Gronda” (Fig. 1). The dataloggers were programmed to take records every 30 minutes, thus obtaining measurements of temperature, hydraulic head and electric conductivity (EC) of both surface and groundwater bodies. To quantify the fluxes between “Canale della Gronda” and the coastal unconfined aquifer, the approach proposed by POST et alii (2007), for the description of variable density groundwater flow direction, was chosen. The EC data were converted to salinity (S) using a linear relationship obtained during a previous study (MASTROCICCO et alii 2012), where 0.873 was found to be the slope between EC and S. These values were used to calculate the specific gravity of the fluid (ρ), essential for the definition of the variable density flow. The approach proposed by UNESCO (Technical papers in marine science, UNESCO, 1981) was adopted:
Fig. 2 – Continuous monitoring of the water level (m) in the drainage canal (black line) and of the equivalent fresh water head (m) in piezometer P8 in the bottom part (red line) and in the top part (blue line) of the unconfined aquifer; precipitation bars are also reported (mm).
The results showed that the hydraulic head increase in the channel during the summer season (when the “Canale della Gronda” is used for irrigation), was followed by a rise up of the equivalent fresh water head in the aquifer. On the other hand, in the autumn period, every time that the “Canale della Gronda” was dried up to drain the surrounding land, a sudden equivalent fresh water head drop was recorded (Fig. 2), even if rainfall events occurred. The horizontal flux during the summer period was directed from the channel to the aquifer with an average specific discharge of 0.14 m/d. While in autumn the drainage induced by the emptying of the channel leaded to a flow reversal, with groundwater seeping into the channel at a very low rate of approximately 0.01 m/d. This behavior involves a variation in the aquifer salinity,
where S is related to a coefficient depending from the variations of temperature; in the present case the coefficient was set equal to 0.0268 because the average temperature of groundwater in piezometer P8 resulted 16.5±0.3 °C. Applying the equivalent fresh water head definition and adapting the Darcy law for variable density waters, it was possible to calculate the values of horizontal and vertical flow:
where Kf is the freshwater hydraulic conductivity, hf is the equivalent freshwater head, ρ is the specific gravity of the saline groundwater and ρf is the specific gravity of the freshwater. RESULTS AND DISCUSSION All data collected by the dataloggers were processed using an Excel spreadsheet in which the analytical models described above were implemented.
Fig. 3 – Continuous monitoring of salinity (g/l) in piezometer P8 in the bottom part (red line and right axis) and in the top part (black line and left axis) of the unconfined aquifer.
which during the summer period decreased in both the monitoring points at -4.5 m and -8 m a.s.l. in piezometer P8. Clearly, the point located at -4.5 m a.s.l. showed a freshening trend, from 3 g/l to 1 g/l. While the monitoring point located in the transition zone at -8 m a.s.l. showed a smooth decrease from 40 g/l to 35 g/l until September. From September an increase of salinity values, especially in the lower portion of the aquifer was recorded (Fig. 3). In this last
CONTINUOUS MONITORING OF SURFACE-GROUNDWATER INTERACTION IN A LOWLAND COASTAL AQUIFER
period very large changes in salinity were recorded at -8 m a.s.l., this testify a movement of the transition zone in relation with the changes of vertical flow within the aquifer. CONCLUSIONS The study has highlighted what are the key factors that control the hydrodynamic behavior and the salinity in a lowland unconfined coastal aquifer with respect to a nearby drainage canal. The study showed that the drainage system consisting of a dense canals network, governs the groundwater fluxes. During the summer season the channel, used for agricultural irrigation, recharges the aquifer. This behavior leads to diminish the shallow groundwater salinity even in the absence of precipitation. On the contrary, during the fall season the channel is no longer used for irrigation and consequently flared to be used for purposes of the eaves. This conditions lead to a flow reversal and the underlying unconfined aquifer starts to recharge the channel, leading to a worsening, not only in the salinity of the aquifer, due to the rise of hyper-saline groundwater contained in the fine prodelta sediments, but also of the salinity in the surface waters bodies receiving the saline groundwater. The strong interaction between the surface waters and the groundwater, both in their hydrodynamic and hydrochemical aspects, is particularly evident, in the area near San Giuseppe di Comacchio (FE), where the canals lay on permeable sandy sediments, ensuring a good connection between the two water bodies. REFERENCES AMOROSI A., CENTINEO M.C., COLALONGO M.L., P ASINI G., SARTI G. & V AIANI S.C. (2003) - Facies architecture and latest Pleistocene–Holocene depositional history of
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the Po Delta (Comacchio area), Italy. J. Geol., 111, 3956. ANTONELLINI M. & MOLLEMA P.N. (2011) - Impact of groundwater salinity on vegetation species richness in the coastal pine forests and wetlands of Ravenna, Italy. Ecol. Eng., 36(9), 1201-1211. B ONDESAN M., FAVERO V. & V IGNALS M.J. (1995) - New evidence on the evolution of the Po-delta coastal plain during the Holocene. Quat. Int., 29/30, 105-110. MASTROCICCO M., COLOMBANI N., SALEMI E. & CASTALDELLI G. (2010) - Numerical assessment of effective evapotranspiration from maize plots to estimate groundwater recharge in lowlands. Agricul. Water Manage., 97(9), 1389-1398. MASTROCICCO M., GIAMBASTIANI B.M.S., SEVERI P. & COLOMBANI N. (2012) - The importance of data acquisition techniques in saltwater intrusion monitoring. Water Resour. Manage., 26(10), 2851-2866. POST V., KOOI H. & SIMMONS C. (2007) - Using hydraulic head measurements in variable-density ground water flow analyses. Ground water, 45(6), 664-671. STEFANI M. & V INCENZI S. (2005) - The interplay of eustasy, climate and human activity in the late Quaternary depositional evolution and sedimentary architecture of the Po Delta system. Marine Geol., 222223, 19-48. TEATINI P., FERRONATO M., GAMBOLATI G., BERTONI W. & GONELLA M. (2005) - A century of land subsidence in Ravenna, Italy. Environ. Geol., 47(6), 831-846. UNESCO (1981). The pratical salinity scale, 1978 and the international equation of state of seawater, 1980. Tenth report of joint panel on oceanographic tables and standards. Technical Papers in Marine Science, Unesco, Paris.
