- Email: [email protected]
Scenario Modelling of Climate Change's Impact on Salinization of Coastal Water Resources in Reclaimed Lands
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 162 (2016) 25 – 31
International Conference on Efficient & Sustainable Water Systems Management toward Worth Living Development, 2nd EWaS 2016
Scenario Modelling Of Climate Change’s Impact On Salinization Of Coastal Water Resources In Reclaimed Lands Nicolò Colombania, Micòl Mastrociccob,* b
a Research Institute for Hydrogeological Protection, National Research Council, Via Amendola 122, 70126 Bari, Italy Department of Environmental, Biological and Pharmaceutical Sciences and Technologies, Second University of Naples, Via Vivaldi 43, Caserta 81100, Italy
Abstract A numerical model accounting for variable density flow and transport was built up to quantify the actual and future (2050) salinization trends of a coastal aquifer in the Po delta (Northern Italy). SEAWAT 4.0 was employed to model the interaction between the surface drainage system and the underlying aquifer. PEST was employed for inverse parameter calibration using hydraulic heads and groundwater salinities as constraints. The calibrated model was used to predict the behavior of the coastal aquifer using a multiple scenario approach: increase in evapotranspiration induced by temperature increase; increase in the frequency of extreme high rainfall events; extreme drought conditions; and irrigation canals dewatering due to salinization of the Po River branches. For each scenario, two sub-scenarios were established to account for the projected local sea level rise. The first three scenarios have only minimal effects on the aquifer salinization, while the fourth forecasts a severe aquifer salinization due to enhanced upward fluxes of saline and hypersaline groundwater. The scenarios quantified the possible future salinization trends of groundwater and could be useful to identify adaptation strategies which allow to better manage water resources of this and similar areas. Results show that the Po delta will experience a significant salinization by 2050 and that the major cause is autonomous salinization via seepage of saline groundwater rather than enhanced salt-water intrusion due to sea level rise. The enhanced autonomous salinization will increase the salt export into the drainage canals that are also employed for irrigation, posing serious treats to the local flourishing agricultural economy. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the EWaS2 International Conference on Efficient & Sustainable Water Systems Management toward Worth Living Development
* Corresponding author. Tel.: +39 0823 274609; fax: +39 0823 274605. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the EWaS2 International Conference on Efficient & Sustainable Water Systems Management toward Worth Living Development
doi:10.1016/j.proeng.2016.11.006
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Nicolò Colombani and Micòl Mastrocicco / Procedia Engineering 162 (2016) 25 – 31 Keywords: numerical modelling; reclaimed land; saline groundwater; autonomous salinization; worst-case scenario.
1. Introduction Climate change will increase the actual sea level at the global scale and especially in the Mediterranean region [1]. The projected future rates of forced warming and drying over the Mediterranean are predicted to be higher than in the past century [2]. This will turn into forced changes in surface air temperature, Mediterranean Sea annual-mean evaporation and surface freshwater fluxes, which will challenge the current water supply management in areas close to the coast. Thus, with the progressive loss of surface water resources due to anthropogenic pollution and the intensive agriculture water demand, groundwater resources will be gradually more stressed, especially in reclaimed coastal areas [3, 4, 5]. In these fragile environments, the relative sea level rise (RSLR) and changes in recharge and evapotranspiration patterns, will probably accelerate water resources depletion both quantitatively and qualitatively [6]. Thus, an improved understanding of the spatial distribution of groundwater salinity fluxes and of the factors controlling these fluxes is urgently required to support the sustainable management of coastal resources in the near future [7]. As many low-lying coastal areas, the Po delta hosts a coastal aquifer which was found to be affected by autonomous salinization caused by the upward flux of hypersaline groundwater eluted from peat lenses and from the underlying silty clay aquitard [8]. The agricultural land drainage system used to maintain these low-lying areas dry was identified as the main mechanism responsible of the upward flux [9]. Since the extensive canal network in the Po delta acts as a strong head control on the local unconfined aquifer and given that the head-controlled systems were demonstrated to be more prone to the effect of RSLR [5], a detailed representation of the drains/aquifer system is required to numerically simulate both actual and future seawater intrusion and saline seepage. For these reasons, to quantify the foreseeable impacts of climate change on the shallow unconfined aquifer of Po delta, a conceptual model was developed based on detailed topography and bathymetry, stratigraphic information from analysis of well logs and driller’s reports, hydrodynamic information from heads monitoring and pumping tests and hydrogeochemical information from multi-level sampling. Based on the conceptual model, a three-dimensional density-dependent groundwater flow model coupled with solute transport was developed and calibrated [10]. In this paper, the calibrated model was employed to create a series of different scenarios to investigate the effects of projected climate changes on groundwater salinity by 2050. The projected scenarios allowed identifying the zones of influence of RSLR and of extreme drought events; additionally they allowed to quantify the increase in salinization of the groundwater system, the salt loads discharged towards the surface water system and the changing volumes of freshwater. 2. Methodology 2.1. Site location and its geomorphological evolution The study area is located in the coastal floodplain of the Po River (Northern Italy) (Fig. 1). It covers 860 km2 (from 44°32’ N to 44°58’ N, from 12°00’ E to 12°17’ E), of which 76% is farmland and woodland, 21% is marsh lagoon (predominantly salt marshes) and 3% is urban area. The surface water system is complex since it is constituted by a dense network of drainage and irrigation canals, brackish coastal lagoons and rivers, by which the water is diverted by gravity following the seasonal agricultural and environmental needs. During the whole year, the excess water collected by the drainage network is pumped out into the main collector channels and discharged to the sea. The total mean annual volume of water that is pumped out of the study area to maintain this lowland dry is about 430 Mm3/y. To have a clear picture on how the reclamation network, and in general the surface waters, interact with the subsurface, an overview on the geomorphological evolution of this area is required.
Nicolò Colombani and Micòl Mastrocicco / Procedia Engineering 162 (2016) 25 – 31
Fig. 1. Grid discretization and boundary conditions applied to the model domain: no flow cells (white), constant heads cells (light blue), drain cells (yellow) and river cells (cyan). The monitoring network used to calibrate the models is constituted by multi-level samplers here named MLS (black circles), pit lakes (black triangles) and shallow monitoring wells here named SMW (black crosses).
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The alternation of glacial and interglacial periods had a significant impact on the evolution, and thus on the stratigraphy, of this coastal area [11]. During the last glacial maximum, the study area was characterized by the sedimentation of well-drained middle alluvial-plain sands. During the Flandrian transgressive phase, these sediments were replaced, with sharp contact, by fine sediment of alluvial-plains and delta-plains. During the early high-stand period, large sand spits and barrier islands grew, turning the previous bays into confined marsh lagoons, with widespread organic clay intercalations and peat horizons [12]. Afterwards, the depositional dynamics were modified not only by climatic change but also by anthropic events (infrastructures, reclamation works and fluvial diversions). The first hydraulic constructions are attributed to the monks of the Pomposa Abbey (VI–X century A.D.), but the first real reclamation is ascribable to the Duke of Ferrara (XV-XVI century A.D.). However, these reclaimed territories became marshy again because of negligence or strong sea storms and disastrous flooding that characterized the “Little Ice Age” (XVII-XIX century A.D.). Finally, an extensive reclamation began in 1870 and by 1960 nearly 100000 ha were reclaimed. The reclamation activity, such as the construction of artificial embankments, and the consequent simplification and rectification of canals and rivers induced the stiffening of the hydrographic network and increased the subsidence rate. Nowadays, the entire coastal area should be considered a critical area in which the average values of ground subsidence are around 2-8 mm/year, with localized peaks of 10-16 mm/year [13]. Due to subsidence most of the coastal territory results to be at altitude close to or lower than the actual sea level, from 9 m to -8 m above sea level (a.s.l.); the only topographic heights consist of ancient barrier islands, paleo-dunes, abandoned riverbeds and other river structures related to the Po River delta accretion, modern dunes and river banks. 2.2. Numerical model A non-reactive density-dependent transport model, implemented with SEAWAT 4.0 [14], was used to investigate the freshening and salinization processes within the unconfined aquifer. The model domain of the study area extends over 860 km2, 25 km in the East-West direction and 48 km in the North-South direction. The grid was defined by 125 columns and 243 rows, and vertically discretized into 10 layers of variable thickness. The top of the grid is represented by the local ground surface; the bottom of the grid, representing the top of the underlying aquitard (the silty-clay unit), was deduced from the stratigraphic logs available in the database of the Emilia-Romagna Geological Survey. A complete description of the boundary conditions employed in the model construction and its calibration using hydraulic heads, fluxes and concentrations can be found in Colombani et al. [10]. 2.3. Numerical Scenarios Four scenarios were simulated and compared to each other. For the first scenario, a 3% increase in the precipitation amount and an increase in evapotranspiration induced by a temperature increase of 2°C over the next 35 years [1] were simulated. In the second scenario, the frequency of extreme high rainfall events was increased. In the third scenario, an extreme drought condition was simulated. The fourth scenario simulated the interruption of canal usage for irrigation purposes. As regards the first scenario, the average annual precipitations recorded from 2001 to 2012 (676 mm/year) and climatic data used to compute the Penman-Monteith equation were downloaded from the Emilia-Romagna climatic database. In addition to precipitations, irrigation was also applied since the territory is mainly agricultural land; a mean annual value of 254 mm/year was used according to a previous study [15]. The daily climatic data were then mediated over the period 2001-2012 to get mean values for each day of the year. Then, the mean daily values were used as input for a 365 days simulation with Hydrus-1D [5] to estimate the actual evapotranspiration and recharge rates. The average annual precipitations were increased by 3% and the future actual evapotranspiration was calculated relying on the equation proposed by Hanson et al. [16]. Prediction of sea level rise were found in Lambek et al. [17], where the subsidence rate of 1.5 mm/year summed to a minimum projected Adriatic Sea level rise of 1.8 mm/year will produce a net RSLR for the next 40 years of 0.13 m in the proximity of the Po River delta. While the maximum projected Adriatic Sea level rise of 14 mm/year plus the subsidence rate of 1.5 mm/year will produce a net RSLR for the next 40 years of 0.62 m. In the second scenario, the frequency of extreme high rainfall events was increased (triplicated). The average annual recharge amount was assumed to occur during winter and spring periods (using stress periods of six months each), instead of being distributed over the entire year. The same sub-scenarios for the predicted RSLR
Nicolò Colombani and Micòl Mastrocicco / Procedia Engineering 162 (2016) 25 – 31
were run as explained above. The third scenario simulated an extreme drought condition by setting null recharge for the last year of simulation, while maintaining the other boundaries as in the first scenario. The same sub-scenarios for the predicted RSLR were run also for this third scenario. To account for the large uncertainties connected with the climate forecasts, the first three scenarios were run forcing the input values of plus and minus one standard deviation of the predicted variables. The fourth scenario simulated the interruption of canals usage for irrigation purposes. Here freshwater recharge from the canals was set equal to zero (canals dewatering) for a total of 6 months in order to simulate an interruption due to degradation of water quality or salinization of watercourses. This last simulation was important to understand the effect of no freshwater recharge from canals. The same sub-scenarios for the predicted RSLR were run also for this fourth scenario. 3. Results The average thickness of the shallow freshwater lens (TDS<2.5 g/L) throughout the model domain in the actual conditions was compared with the average thickness of the shallow freshwater lens calculated for the four scenarios. This enabled to quantify the change in the brackish-freshwater interface that could be induced by climate changes at the aquifer scale. The assessment of the shrinkage/enlargement of the shallow freshwater lens is of particular interest in these intensively exploited agricultural lands, since a small change in the freshwater thickness could largely impair the crop yield of plants sensitive to salinity stresses [18]. Future effects of climate change on surface water salinization are chiefly important, since the reclamation network is also used for irrigation purposes, thus even the irrigation water could possibly enhance the soil salinization. In addition, the sub-scenarios exemplified the impact of the projected low RSLR (LSLR) and high RSLR (HSLR) on saltwater upward flow (Scenario 1 in Figure 2).
Fig. 2. Results from the LSLR scenarios (left plot) and for the HSLR scenarios (right plot) expressed as change of freshwater thickness integrated over the entire model domain.
In the first scenario, the increase of precipitation and temperature based on the climate change projections for the Mediterranean region [1] caused only minor effects on the brackish-freshwater interface. Indeed, the thickness of the freshwater lens increased on average through the entire domain of about 0.12 m (Scenario 1 in Figure 2), especially in the portions of the aquifer close to the reclamation canals. A small difference is observable between the LSLR and HSLR sub-scenarios (Scenario 1 in Figure 2), with a less pronounced effect on the freshwater lens in the HSLR case with respect to the LSLR case. The consequent upward movement of the saline groundwater in some cases can be as high as 70 g/l [8]. In addition, the second scenario (Scenario 2 in Figure 2) did not affect much the thickness of the freshwater lens and the aquifer salinization compared to the actual situation. The salinity distribution throughout the aquifer did not
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differ essentially from the salinity distribution of the calibrated model, meaning that extreme high rainfall events will not increase the natural recharge of the aquifer and will not preclude the already existing upward flux of the saline groundwater from the underlying aquitard. Once more, a more marked shrinking of the freshwater lens is appreciable in the HSLR case rather than in the LSLR case (Scenario 2 in Figure 2). This point was already stressed in a previous study at the local scale [5], where the impact of the projected RSLR was found to be much more important than the projected climate changes for this zone. The scenarios of the model at the whole aquifer scale, confirm this hypothesis. The same considerations apply also for the third scenario, where extreme drought conditions were simulated. This finding supports the results found in the first two scenarios, since even without recharge for a prolonged period the thickness of the freshwater lens was only marginally changed with respect to the base case (Scenario 3 in Figure 2). The major variations in the thickness of the freshwater lenses are shown in the fourth scenario. The interruption of the water flow within the canals network during the irrigation season, due to the hypothetical salinization of the nearby watercourses, could create an upward movement of the high salinity groundwater, which actually resides at the bottom of the aquifer. In figure 2, the scenario 4 shows an average decrease of 1 m of the freshwater lenses over the whole aquifer. This mean that in the most depressed zones, with topographic altitudes of even -4 m a.s.l., an almost complete salinization of the aquifer could occur. In contrast with the other scenarios, the LSLR and HSLR sub-scenarios show very similar results, confirming that the canals network is the main driver of aquifer freshening. These results highlight the key role of the canals network in low-lying reclaimed lands, which could enhance soil salinization in the near future, either actively or passively. 4. Conclusions The study shows that the current irrigation scheme and the related drainage system, consisting of a dense canals network, govern the groundwater fluxes in the coastal aquifer of the Po River delta, which was previously demonstrated to be affected by autonomous salinization. The upward saline flux from the underlying aquitard slowly salinizes most the surface water bodies, which are for the majority hydraulically connected with the aquifer. The scenario modelling points out that an increase in temperature or in the extreme high rainfall events will have only a minor influence on the aquifer salinization rate, while interruptions of the canals usage could create serious upward movement of the saline groundwater currently residing in the bottom part of the coastal aquifer and in the underlying aquitard. The impact of the projected sea level rise seems much more important than the projected climate changes; thus, future studies will need to better delineate the forthcoming sea level rise at local scale. The numerical scenarios developed in this study calculated the potential climate change effects on aquifer salinization and could be useful not only to find strategies for water management of this region but also as example for other similar low-lying areas affected by autonomous salinization.
Acknowledgements We gratefully thank the Geological, Seismic and Soil Survey of Emilia-Romagna Region for the aquifer’s base map and the ARPA SIMC for the meteorological data. References [1] S. Gualdi, S. Somot, L. Li, V. Artale, M. Adani, A. Bellucci, A. Braun, S. Calmanti, A. Carillo, A. Dell'Aquila, M. Déqué, C. Dubois, A. Elizalde, A. Harzallah, D. Jacob, B. L'Hévéder, W. May, P. Oddo, P. Ruti, A. Sanna, G. Sannino, E. Scoccimarro, F. Sevault, A. Navarra, The CIRCE simulations: regional climate change projections with realistic representation of the Mediterranean Sea, Bull. Amer. Meteorol. Soc. 94:1 (2013) 65-81. [2] A. Mariotti, Y. Pan, N. Zeng, A. Alessandri, Long-term climate change in the Mediterranean region in the midst of decadal variability, Clim. Dyn. 44:5-6 (2015) 1437-1456. [3] G.H.P.O. Essink, E.S. Van Baaren, P.G. De Louw, Effects of climate change on coastal groundwater systems: A modeling study in the Netherlands, Water Resour. Res. 46:10 (2010). [4] P. Rasmussen, T.O. Sonnenborg, G. Goncear, K. Hinsby, Assessing impacts of climate change, sea level rise, and drainage canals on saltwater intrusion to coastal aquifer, Hydrol. Earth Sys. Sci. 17:1 (2013) 421-443.
Nicolò Colombani and Micòl Mastrocicco / Procedia Engineering 162 (2016) 25 – 31 [5] N. Colombani, M. Mastrocicco, B.M.S. Giambastiani, Predicting salinization trends in a lowland coastal aquifer: Comacchio (Italy), Water Res. Manage. 29:2 (2015) 603-618. [6] L. Benini, M. Antonellini, M. Laghi, P.N. Mollema, Assessment of water resources availability and groundwater salinization in future climate and land use change scenarios: a case study from a coastal drainage basin in Italy, Water Resour. Manage. 30:2 (2016) 731-745. [7] P. Ranjan, S. Kazama, M. Sawamoto, Effects of climate change on coastal fresh groundwater resources, Global Environ. Chan. 16:4 (2006) 388-399. [8] B.M.S. Giambastiani, N. Colombani, M. Mastrocicco, M.D. Fidelibus, Characterization of the lowland coastal aquifer of Comacchio (Ferrara, Italy): hydrology, hydrochemistry and evolution of the system, J. Hydrol. 501 (2013) 35-44. [9] A.D. Werner, C.T. Simmons, Impact of sea-level rise on sea water intrusion in coastal aquifers. Ground Water 47:2 (2009) 197-204. [10] N. Colombani, A. Osti, G. Volta, M. Mastrocicco, Impact of climate change on salinization of coastal water resources, Water Resour. Manage. 30:7 (2016) 2483-2496. [11] A. Amorosi, M.C. Centineo, M.L. Colalongo, G. Pasini, G. Sarti, S.C. Vaiani, Facies architecture and latest Pleistocene-Holocene depositional history of the Po Delta (Comacchio Area), Italy. J. Geol. 111:1 (2003) 39-56. [12] M. Stefani, S. Vincenzi, The interplay of eustasy, climate and human activity in the late Quaternary depositional evolution and sedimentary architecture of the Po Delta system, Marine Geology, 222-223 (2005) 19-48. [13] P. Teatini, M. Ferronato, G. Gambolati, M. Gonella, Groundwater pumping and land subsidence in the Emilia-Romagna coastland, Italy: modeling the past occurrence and the future trend, Water Resour. Res. 42 (2006) 1-19. [14] C.D. Langevin, D.T.Jr. Thorne, A.M. Dausman, M.C. Sukop, W. Guo, SEAWAT Version 4: A computer program for simulation of multispecies solute and heat transport. U.S. Geological Survey Techniques and Methods 6-A22, (2008) 39 pp. [15] V.G. Aschonitis, M. Mastrocicco, N. Colombani, E. Salemi, G. Castaldelli, Assessment of the intrinsic vulnerability of agricultural land to water and nitrogen losses: case studies in Italy and Greece. In Evolving Water Resources Systems: Understanding, Predicting and Managing Water-Society Interactions. IAHS Red Book 364 (2014) 14-19. [16] R.T. Hanson, L.E. Flint, A.L. Flint, D.M. Dettinger, C.C. Faunt, D. Cayan, W. Schmid, A method for physically based model analysis of conjunctive use in response to potential climate changes. Water Resources Research, 48:2 (2012) W00L08. [17] K. Lambeck, F. Antonioli, M. Anzidei, L. Ferranti, G. Leoni, G. Scicchitano, S. Silenzi, Sea level change along the Italian coast during the Holocene and projections for the future, Quarter. Internat. 232 (2011) 250-257. [18] N. Katerji, J.W. Van Hoorn, A. Hamdy, M. Mastrorilli, Salt tolerance classification of crops according to soil salinity and to water stress day index, Agric. Water Manage. 43:1 (2000) 99-109.
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ScienceDirect Procedia Engineering 162 (2016) 25 – 31
International Conference on Efficient & Sustainable Water Systems Management toward Worth Living Development, 2nd EWaS 2016
Scenario Modelling Of Climate Change’s Impact On Salinization Of Coastal Water Resources In Reclaimed Lands Nicolò Colombania, Micòl Mastrociccob,* b
a Research Institute for Hydrogeological Protection, National Research Council, Via Amendola 122, 70126 Bari, Italy Department of Environmental, Biological and Pharmaceutical Sciences and Technologies, Second University of Naples, Via Vivaldi 43, Caserta 81100, Italy
Abstract A numerical model accounting for variable density flow and transport was built up to quantify the actual and future (2050) salinization trends of a coastal aquifer in the Po delta (Northern Italy). SEAWAT 4.0 was employed to model the interaction between the surface drainage system and the underlying aquifer. PEST was employed for inverse parameter calibration using hydraulic heads and groundwater salinities as constraints. The calibrated model was used to predict the behavior of the coastal aquifer using a multiple scenario approach: increase in evapotranspiration induced by temperature increase; increase in the frequency of extreme high rainfall events; extreme drought conditions; and irrigation canals dewatering due to salinization of the Po River branches. For each scenario, two sub-scenarios were established to account for the projected local sea level rise. The first three scenarios have only minimal effects on the aquifer salinization, while the fourth forecasts a severe aquifer salinization due to enhanced upward fluxes of saline and hypersaline groundwater. The scenarios quantified the possible future salinization trends of groundwater and could be useful to identify adaptation strategies which allow to better manage water resources of this and similar areas. Results show that the Po delta will experience a significant salinization by 2050 and that the major cause is autonomous salinization via seepage of saline groundwater rather than enhanced salt-water intrusion due to sea level rise. The enhanced autonomous salinization will increase the salt export into the drainage canals that are also employed for irrigation, posing serious treats to the local flourishing agricultural economy. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the EWaS2 International Conference on Efficient & Sustainable Water Systems Management toward Worth Living Development
* Corresponding author. Tel.: +39 0823 274609; fax: +39 0823 274605. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the EWaS2 International Conference on Efficient & Sustainable Water Systems Management toward Worth Living Development
doi:10.1016/j.proeng.2016.11.006
26
Nicolò Colombani and Micòl Mastrocicco / Procedia Engineering 162 (2016) 25 – 31 Keywords: numerical modelling; reclaimed land; saline groundwater; autonomous salinization; worst-case scenario.
1. Introduction Climate change will increase the actual sea level at the global scale and especially in the Mediterranean region [1]. The projected future rates of forced warming and drying over the Mediterranean are predicted to be higher than in the past century [2]. This will turn into forced changes in surface air temperature, Mediterranean Sea annual-mean evaporation and surface freshwater fluxes, which will challenge the current water supply management in areas close to the coast. Thus, with the progressive loss of surface water resources due to anthropogenic pollution and the intensive agriculture water demand, groundwater resources will be gradually more stressed, especially in reclaimed coastal areas [3, 4, 5]. In these fragile environments, the relative sea level rise (RSLR) and changes in recharge and evapotranspiration patterns, will probably accelerate water resources depletion both quantitatively and qualitatively [6]. Thus, an improved understanding of the spatial distribution of groundwater salinity fluxes and of the factors controlling these fluxes is urgently required to support the sustainable management of coastal resources in the near future [7]. As many low-lying coastal areas, the Po delta hosts a coastal aquifer which was found to be affected by autonomous salinization caused by the upward flux of hypersaline groundwater eluted from peat lenses and from the underlying silty clay aquitard [8]. The agricultural land drainage system used to maintain these low-lying areas dry was identified as the main mechanism responsible of the upward flux [9]. Since the extensive canal network in the Po delta acts as a strong head control on the local unconfined aquifer and given that the head-controlled systems were demonstrated to be more prone to the effect of RSLR [5], a detailed representation of the drains/aquifer system is required to numerically simulate both actual and future seawater intrusion and saline seepage. For these reasons, to quantify the foreseeable impacts of climate change on the shallow unconfined aquifer of Po delta, a conceptual model was developed based on detailed topography and bathymetry, stratigraphic information from analysis of well logs and driller’s reports, hydrodynamic information from heads monitoring and pumping tests and hydrogeochemical information from multi-level sampling. Based on the conceptual model, a three-dimensional density-dependent groundwater flow model coupled with solute transport was developed and calibrated [10]. In this paper, the calibrated model was employed to create a series of different scenarios to investigate the effects of projected climate changes on groundwater salinity by 2050. The projected scenarios allowed identifying the zones of influence of RSLR and of extreme drought events; additionally they allowed to quantify the increase in salinization of the groundwater system, the salt loads discharged towards the surface water system and the changing volumes of freshwater. 2. Methodology 2.1. Site location and its geomorphological evolution The study area is located in the coastal floodplain of the Po River (Northern Italy) (Fig. 1). It covers 860 km2 (from 44°32’ N to 44°58’ N, from 12°00’ E to 12°17’ E), of which 76% is farmland and woodland, 21% is marsh lagoon (predominantly salt marshes) and 3% is urban area. The surface water system is complex since it is constituted by a dense network of drainage and irrigation canals, brackish coastal lagoons and rivers, by which the water is diverted by gravity following the seasonal agricultural and environmental needs. During the whole year, the excess water collected by the drainage network is pumped out into the main collector channels and discharged to the sea. The total mean annual volume of water that is pumped out of the study area to maintain this lowland dry is about 430 Mm3/y. To have a clear picture on how the reclamation network, and in general the surface waters, interact with the subsurface, an overview on the geomorphological evolution of this area is required.
Nicolò Colombani and Micòl Mastrocicco / Procedia Engineering 162 (2016) 25 – 31
Fig. 1. Grid discretization and boundary conditions applied to the model domain: no flow cells (white), constant heads cells (light blue), drain cells (yellow) and river cells (cyan). The monitoring network used to calibrate the models is constituted by multi-level samplers here named MLS (black circles), pit lakes (black triangles) and shallow monitoring wells here named SMW (black crosses).
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The alternation of glacial and interglacial periods had a significant impact on the evolution, and thus on the stratigraphy, of this coastal area [11]. During the last glacial maximum, the study area was characterized by the sedimentation of well-drained middle alluvial-plain sands. During the Flandrian transgressive phase, these sediments were replaced, with sharp contact, by fine sediment of alluvial-plains and delta-plains. During the early high-stand period, large sand spits and barrier islands grew, turning the previous bays into confined marsh lagoons, with widespread organic clay intercalations and peat horizons [12]. Afterwards, the depositional dynamics were modified not only by climatic change but also by anthropic events (infrastructures, reclamation works and fluvial diversions). The first hydraulic constructions are attributed to the monks of the Pomposa Abbey (VI–X century A.D.), but the first real reclamation is ascribable to the Duke of Ferrara (XV-XVI century A.D.). However, these reclaimed territories became marshy again because of negligence or strong sea storms and disastrous flooding that characterized the “Little Ice Age” (XVII-XIX century A.D.). Finally, an extensive reclamation began in 1870 and by 1960 nearly 100000 ha were reclaimed. The reclamation activity, such as the construction of artificial embankments, and the consequent simplification and rectification of canals and rivers induced the stiffening of the hydrographic network and increased the subsidence rate. Nowadays, the entire coastal area should be considered a critical area in which the average values of ground subsidence are around 2-8 mm/year, with localized peaks of 10-16 mm/year [13]. Due to subsidence most of the coastal territory results to be at altitude close to or lower than the actual sea level, from 9 m to -8 m above sea level (a.s.l.); the only topographic heights consist of ancient barrier islands, paleo-dunes, abandoned riverbeds and other river structures related to the Po River delta accretion, modern dunes and river banks. 2.2. Numerical model A non-reactive density-dependent transport model, implemented with SEAWAT 4.0 [14], was used to investigate the freshening and salinization processes within the unconfined aquifer. The model domain of the study area extends over 860 km2, 25 km in the East-West direction and 48 km in the North-South direction. The grid was defined by 125 columns and 243 rows, and vertically discretized into 10 layers of variable thickness. The top of the grid is represented by the local ground surface; the bottom of the grid, representing the top of the underlying aquitard (the silty-clay unit), was deduced from the stratigraphic logs available in the database of the Emilia-Romagna Geological Survey. A complete description of the boundary conditions employed in the model construction and its calibration using hydraulic heads, fluxes and concentrations can be found in Colombani et al. [10]. 2.3. Numerical Scenarios Four scenarios were simulated and compared to each other. For the first scenario, a 3% increase in the precipitation amount and an increase in evapotranspiration induced by a temperature increase of 2°C over the next 35 years [1] were simulated. In the second scenario, the frequency of extreme high rainfall events was increased. In the third scenario, an extreme drought condition was simulated. The fourth scenario simulated the interruption of canal usage for irrigation purposes. As regards the first scenario, the average annual precipitations recorded from 2001 to 2012 (676 mm/year) and climatic data used to compute the Penman-Monteith equation were downloaded from the Emilia-Romagna climatic database. In addition to precipitations, irrigation was also applied since the territory is mainly agricultural land; a mean annual value of 254 mm/year was used according to a previous study [15]. The daily climatic data were then mediated over the period 2001-2012 to get mean values for each day of the year. Then, the mean daily values were used as input for a 365 days simulation with Hydrus-1D [5] to estimate the actual evapotranspiration and recharge rates. The average annual precipitations were increased by 3% and the future actual evapotranspiration was calculated relying on the equation proposed by Hanson et al. [16]. Prediction of sea level rise were found in Lambek et al. [17], where the subsidence rate of 1.5 mm/year summed to a minimum projected Adriatic Sea level rise of 1.8 mm/year will produce a net RSLR for the next 40 years of 0.13 m in the proximity of the Po River delta. While the maximum projected Adriatic Sea level rise of 14 mm/year plus the subsidence rate of 1.5 mm/year will produce a net RSLR for the next 40 years of 0.62 m. In the second scenario, the frequency of extreme high rainfall events was increased (triplicated). The average annual recharge amount was assumed to occur during winter and spring periods (using stress periods of six months each), instead of being distributed over the entire year. The same sub-scenarios for the predicted RSLR
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were run as explained above. The third scenario simulated an extreme drought condition by setting null recharge for the last year of simulation, while maintaining the other boundaries as in the first scenario. The same sub-scenarios for the predicted RSLR were run also for this third scenario. To account for the large uncertainties connected with the climate forecasts, the first three scenarios were run forcing the input values of plus and minus one standard deviation of the predicted variables. The fourth scenario simulated the interruption of canals usage for irrigation purposes. Here freshwater recharge from the canals was set equal to zero (canals dewatering) for a total of 6 months in order to simulate an interruption due to degradation of water quality or salinization of watercourses. This last simulation was important to understand the effect of no freshwater recharge from canals. The same sub-scenarios for the predicted RSLR were run also for this fourth scenario. 3. Results The average thickness of the shallow freshwater lens (TDS<2.5 g/L) throughout the model domain in the actual conditions was compared with the average thickness of the shallow freshwater lens calculated for the four scenarios. This enabled to quantify the change in the brackish-freshwater interface that could be induced by climate changes at the aquifer scale. The assessment of the shrinkage/enlargement of the shallow freshwater lens is of particular interest in these intensively exploited agricultural lands, since a small change in the freshwater thickness could largely impair the crop yield of plants sensitive to salinity stresses [18]. Future effects of climate change on surface water salinization are chiefly important, since the reclamation network is also used for irrigation purposes, thus even the irrigation water could possibly enhance the soil salinization. In addition, the sub-scenarios exemplified the impact of the projected low RSLR (LSLR) and high RSLR (HSLR) on saltwater upward flow (Scenario 1 in Figure 2).
Fig. 2. Results from the LSLR scenarios (left plot) and for the HSLR scenarios (right plot) expressed as change of freshwater thickness integrated over the entire model domain.
In the first scenario, the increase of precipitation and temperature based on the climate change projections for the Mediterranean region [1] caused only minor effects on the brackish-freshwater interface. Indeed, the thickness of the freshwater lens increased on average through the entire domain of about 0.12 m (Scenario 1 in Figure 2), especially in the portions of the aquifer close to the reclamation canals. A small difference is observable between the LSLR and HSLR sub-scenarios (Scenario 1 in Figure 2), with a less pronounced effect on the freshwater lens in the HSLR case with respect to the LSLR case. The consequent upward movement of the saline groundwater in some cases can be as high as 70 g/l [8]. In addition, the second scenario (Scenario 2 in Figure 2) did not affect much the thickness of the freshwater lens and the aquifer salinization compared to the actual situation. The salinity distribution throughout the aquifer did not
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differ essentially from the salinity distribution of the calibrated model, meaning that extreme high rainfall events will not increase the natural recharge of the aquifer and will not preclude the already existing upward flux of the saline groundwater from the underlying aquitard. Once more, a more marked shrinking of the freshwater lens is appreciable in the HSLR case rather than in the LSLR case (Scenario 2 in Figure 2). This point was already stressed in a previous study at the local scale [5], where the impact of the projected RSLR was found to be much more important than the projected climate changes for this zone. The scenarios of the model at the whole aquifer scale, confirm this hypothesis. The same considerations apply also for the third scenario, where extreme drought conditions were simulated. This finding supports the results found in the first two scenarios, since even without recharge for a prolonged period the thickness of the freshwater lens was only marginally changed with respect to the base case (Scenario 3 in Figure 2). The major variations in the thickness of the freshwater lenses are shown in the fourth scenario. The interruption of the water flow within the canals network during the irrigation season, due to the hypothetical salinization of the nearby watercourses, could create an upward movement of the high salinity groundwater, which actually resides at the bottom of the aquifer. In figure 2, the scenario 4 shows an average decrease of 1 m of the freshwater lenses over the whole aquifer. This mean that in the most depressed zones, with topographic altitudes of even -4 m a.s.l., an almost complete salinization of the aquifer could occur. In contrast with the other scenarios, the LSLR and HSLR sub-scenarios show very similar results, confirming that the canals network is the main driver of aquifer freshening. These results highlight the key role of the canals network in low-lying reclaimed lands, which could enhance soil salinization in the near future, either actively or passively. 4. Conclusions The study shows that the current irrigation scheme and the related drainage system, consisting of a dense canals network, govern the groundwater fluxes in the coastal aquifer of the Po River delta, which was previously demonstrated to be affected by autonomous salinization. The upward saline flux from the underlying aquitard slowly salinizes most the surface water bodies, which are for the majority hydraulically connected with the aquifer. The scenario modelling points out that an increase in temperature or in the extreme high rainfall events will have only a minor influence on the aquifer salinization rate, while interruptions of the canals usage could create serious upward movement of the saline groundwater currently residing in the bottom part of the coastal aquifer and in the underlying aquitard. The impact of the projected sea level rise seems much more important than the projected climate changes; thus, future studies will need to better delineate the forthcoming sea level rise at local scale. The numerical scenarios developed in this study calculated the potential climate change effects on aquifer salinization and could be useful not only to find strategies for water management of this region but also as example for other similar low-lying areas affected by autonomous salinization.
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