Journal of Environmental Protection and Ecology 13, No 3, 1600–1607 (2012) Computer applications on environmental information system
Simulation Models to Evaluate the Groundwater Resources in the Bahlui River Basin, Romania I. Mineaa*, I. Craciunb Faculty of Geology and Geography, ‘Alexandru Ioan Cuza’ University of Iasi, 11 Carol II Blvd., 700 502 Iasi, Romania E-mail:
[email protected] b Faculty of Hydrotechnical Engineering, Geodesy and Environmental Engineering, ‘Gheorghe Asachi’ Technical University of Iasi, 63–65 Prof. Dimitrie Mangeron Street, 700 050 Iasi, Romania a
Abstract. Today accelerated economic development of excessive exploitation of groundwater resources and their contamination with various pollutants have become a major issue that attracts the attention of many researchers. As in other areas, mathematical modelling has been used primarily for understanding groundwater flow systems and secondly, to predict their evolution. These methods are continuously developed efficiently and often integrated in the study of groundwater flow and transport of pollutants. Applying the mathematical modelling to the Bahlui river basin to evaluate groundwater flow in the river floodplain, in the west part of Letcani, to simulate a scenario of pollution of the phreatic water layer we obtained some results that allow us to evaluate a specific situation of pollution in an accident. Several situations have been simulated by positioning of drilling outside the mass movement of the pollutant or the direction of travel, with different pumping resulting in a series of values of pumping rate to prevent river water pollution of Bahlui by groundwater seepage. Keywords: mathematical modelling, drainage basin, groundwater flow, transport of pollutants.
AIMS AND BACKGROUND Today accelerated economic development of excessive exploitation of groundwater resources and their contamination with various pollutants have become a major issue that attracts the attention of many researchers. As in other areas, mathematical modelling has been used primarily for understanding groundwater flow systems and secondly, to predict their evolution1. These methods are continuously developed efficiently and often integrated in the study of groundwater flow and transport of the pollutants2. In this domain exists informatical application of these numerical models to solve the flow and mass pollutant transport in aquifers as follows – EM, GMS, AQUA3D, PLASM, SUTRA, RNDWALK. The MODFLOW software package contains some applications like MOC3D, MT3D, MT3DMS, PEST, UCODE, PMPATH, which offer the possibility to simulate the recharge of the aquifers, the *
For correspondence.
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flow to and from drilling, evaluating the evapotranspiration, transport of the pollutants or evaluate the change between the surface water and groundwater3,4. EXPERIMENTAL The modelling methodology for groundwater flow and mass transport purposes to follow some steps regarding to5: – establish the conceptual model to solve the flow and mass transport problems corresponding with the aquifers; – identify the best modelling model for flow and mass transport problems for the studied aquifer; – establish the parameters for numerical model; – exploit the model by repetitive flow and mass transport numerical simulation; – revalidation of the parameters (optimising the values for the some parameters, minimising the difference between simulation and observed values); – interpret the results and realising the quantitative and qualitative prognosis. The description of the aquifer with a porous medium, with a multilayer structure (2 water bearing beds with different hydrophysical characteristics), in which the process flow was believed to occur both in the horizontal plane, within each layer, and in the vertical plane, interstrat, seeping through an exchange or movement upward which make necessary the calculus of the vertical conductivity (VCONT) of the layers. The calculus formula is6: VCONT =
2 ΔVk (Kz)j,i,k
+
,
ΔVk+1
(1)
(Kz)j,i,k+1
where ΔVk and ΔVk+1 represent the thickness of the layer k, respectively, k + 1; (Kz)j,i,k and (Kz)j,i,k+1 represent the values of the hydraulic conductivity of the water bearing bed k, respectively, k+1. For the alluvial plain of the Bahlui river we can identify a different granulometry between the upper layer (by powdered clay) and lower layer (by sand and gravel) and exists the possibility to appear a semi-confining layer7. About these considerations the alluvial plain of the Bahlui river can be considered a quasi 3-dimensional model and the vertical conductivity is described by the following equation: VCONT =
2 ΔzU (Kz)U
+
2ΔzC (Kz)C
+
ΔzL
,
(2)
(Kz)L
where ΔzU, ΔzC, and ΔzL represent the thickness of the upper layer (U), semiconfining layer (C), and lower layer (L), respectively; (Kz)U, (Kz)C, (Kz)j,i,k and (Kz)L 1601
represent the hydraulic conductivity of the upper layer, semi-confining layer and, respectively, lower layer. HYDROGEOLOGICAL CONDITIONS The numerical simulation is placed in the alluvial plain of the Bahlui river, Letcani locality. This area is homogeneously regarding the hydrogeological aspects and the data can be extrapolated for a great area of the Bahlui alluvial plain and the tributary. Moreover, this area presents a great accidental pollution risk due the communication network (Fig. 1).
Fig. 1. Hydrogeological station Banu (geological profile and application area)
The area from a geological point of view presents 2 permeable rock layers (Fig. 1): 1st layer – by powdered clay (5.6–7 m depth), and 2nd layer – by sand and gravel (2.6–3 m depth). The powdered clay layer over the sand and gravel layer gives for this horizon an artesian character. The base lower layer is the compact impermeable marl clay8. The data from 4 drillings of the Banu hydrogeological station give an image of the yearly homogenously hydrogeological regime, with maximum values for June and July months and minimum values for the months December and January (Fig. 2) (Ref. 9). In qualitative aspect, the groundwater has sodium bicarbonate with a very high mineralisation generated by the presence of the clay. A comparative analysis of the Schöller –Berkaloff diagrams for 1974–2007 period showed the over-passing of the maximum concentrations for Mg2+, Na+ and SO42+ ions. Moreover, we observed the growth of nitrates concentration due the organic wastes from the zootechnical farms in the Letcani area. The local population use this groundwater like water supply source (from fountains and drilling) with a health risk and more, like irrigation source in the contrast with the restrictions regarding the hardness indicator (more of 25 German degree for all geological station drilling)8,10–12. 1602
Fig. 2. Yearly regime of the piezometric level at the Banu hydrogeological drillings F1, F2 and F3
NUMERICAL SIMULATION FOR GROUNDWATER FLOW AND MASS TRANSPORT IN THE ALLUVIAL PLAIN OF THE BAHLUI RIVER Applying the numerical modelling to solve groundwater flow and pollutant transport we made the following work hypothesis: – the aquifer is assimilated with hand porous medium with a multilayer structure (2 layers with different hydrophysical characteristics) where the process flow was believed to occur both in the horizontal plane, within each layer, and in the vertical plane, interstrat seeping through an exchange or movement upward; – hydrogeological conditions were considered to be a complete saturation of the aquifer layers (observed for the period between the October and April months); – the simulation of observation points corresponds to the position of the hydrogeological drilling of the Banu station; – the pollution scenario considers a point pollution source, advection transport for pollution substance and flow in a single phase13. The studied area of 2.25 km2 (30 rows and columns each of 50 m length in MODFLOW software). Two layers are analysed: – the 1st layer: 5 m depth in the external alluvial plain and 3 m depth near the Bahlui river, the height of hydrostatic level 9 m in the external alluvial plain and 8 m depth near the Bahlui river, 12 m horizontal conductivity and 1.2 m vertical conductivity, 20% effective porosity and 0.012 m2/day the optimum transmissivity obtained automatically by the recalibrations process (with PEST software); – the 2nd layer: 4 m depth in the external alluvial plain and 5 m depth near the Bahlui river, 15 m horizontal conductivity and 1.5 m vertical conductivity, 25% effective porosity with the same optimum transmissivity like in the 1st layer6. The coupling between the groundwater and surface water required the following parameters for the Bahlui river: hydraulic conductance of the riverbed 1000
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m2/day (hydraulic conductance of the alluvial depots of 4 m/day, 50 m length of the river for each cell, 15 m width and 3 m depth of the alluvial bed13. The repetitive groundwater flow simulation shows a relative uniform distribution of the phreatic surface level (Fig. 3). The flow direction confirms the general tendency from external alluvial plain to the Bahlui riverbed.
Fig. 3. Distribution of the threatic surface level for the studied area
To evaluate the economical exploitation potential of the groundwater it is necessary to do a hydrological balance (Fig. 4).
Fig. 4. Example of hydrogeological balance for a drilling with an exploitation rate of 250 m3/day
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Simulation of pollution was carried out under the same hydrogeological conditions, using the same hydrophysical parameters. In the natural conditions the pollution of the aquifer by advection processes can generate a pollution event on the Bahlui river for 3–3.5 days depending on the miscibility or non-miscibility of the pollutant (Fig. 5).
Fig. 5. Pollutant directions for an accidental event
In the case of an accidental pollution we can build an exploitation drilling on the direction of pollutant movement. It was simulating the situation by the emplacement of the exploitation drilling outside of the pollutant area or on the direction of the pollutant movement with different pump levels. For distance great of 200 m from the polluted area the pump solution on the exploitation drilling does not have a positive effect (Fig. 6, left side). The positioning of the exploitation drilling on the direction of the pollutant movement has a positive effect for the decontamination of the affected aquifer for a pump rate of 200 m3/day (Fig. 6, right side).
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Fig. 6. Simulation of a pollution situation and pumping of groundwater by the different placement of the exploitation drilling
CONCLUSIONS The pollution simulation of the aquifer from the alluvial plain of the Bahlui river, Letcani locality, applying the mathematical model MODFLOW shows the potential negative effect on the environment. The connection between the surface and groundwater can be evaluated by the tendency lines of the flow and finding technical solutions for decontamination of the aquifer. The different situation positioning of the exploitation drilling outside and on the direction of the pollutant to analyse the decontamination solutions was simulated. Acknowledgements. This work was supported by the European Social Fund in Romania, under the responsibility of the managing Authority for the Sectorial Operational Programme for Human Resources Development 2007–2013, grant POSDRU/89/1.5/S/63663: ‘Transnational Network of Integrated Postdoctoral Research in Science Communication. Institutional Development (Postdoctoral School) and Scholarship Program (CommScie)’.
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3. D. MYRONIDIS, P. STEFANIDIS, D. FOTAKIS: Quantitative Estimation of a Watershed 50year Peak Flood Discharge by Building a Custom GIS Application. J Environ Prot Ecol, 11 (3), 1183 (2010). 4. W. FETTER: Applied Hydrogeology. IVth ed. Prentice Hall Inc., New Jersey, 2001. 5. R. GIURMA-HANDLEY, I. CRACIUN, I. GIURMA, M. TELISCA: Modelling Techniques of the Groundwater’s Pollution Risk. In: VIth Int. Conf. on the Management of Technological Changes, Alexandroupolis, Greece, September 3–5, 2009, II, 73–75. 6. W. H. CHIANG, W. KINZELBACH: Processing MODFLOW. A Simulation System for Modelling Groundwater Flow and Pollution. User Manual, 1998. 7. V. BACAUANU, N. BARBU, M. PANTAZICA, A. UNGUREANU, D. CHIRIAC: Moldova Plateau – Environment, Economy. Science and Encyclopedic Press, Bucharest, 1980. 8. I. CRACIUN, I. GIURMA, C. R. GIURMA-HANDLEY: Quality Risk Evaluation of the Groundwater Resources on the Moldavian Area. Environmental Engineering and Management J., 8 (3), 391 (2009). 9. I. C. STANGA, C. RUSU, E. V. PANAITESCU: La dynamique du niveau phréatique dans le basin de Tutova et les risqué associés. Scientifical Annals of the Al.I.Cuza University of Iasi, LV, II – Geography, 63, (2009). 10. T. E. MAN, C.MODRA: Drought Impact of Environment and Agricultural Products. J Environ Prot Ecol, 9 (1), 70 (2008). 11. Fl. UNGUREANU, R. G. LUPU, A. STAN, I. CRACIUN, C. TEODOSIU: Towards Real Time Monitoring of Water Quality in River Basins. Environmental Engineering and Management J., 9 (9), 1267 (2010). 12. R. BENCHEA, I. CRETESCU, M. MACOVEANU: Monitoring of Water Quality Indicators for Improving Water Resources Management of Bahlui River. Environmental Engineering and Management J., 10 (3), 327 (2011). 13. I. MINEA: Bahlui Basin River – Hydrological Study. D. Th., Al. I. Cuza University of Iasi, 2009. Received 16 March 2012 Revised 30 April 2012
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