MODELING INVESTIGATION ON THE SUSTAINABLE GROUNDWATER YIELD FOR WIANG PA PAO AQUIFERS SYSTEM

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1 MODELING INVESTIGATION ON THE SUSTAINABLE GROUNDWATER YIELD FOR WIANG PA PAO AQUIFERS SYSTEM Phatcharasak Arlai 1*, Manfred Koch 2, Sitisak Munyu 3, Kriangsak Pirarai 3 and Arun Lukjan 1 1 Research Unit for Sustainable Water and Environmental Resources Management, Nakhon Pathom Rajabhat University, Nakhon Pathom, Thailand 2 Department of Geohydraulic and Hydrological Engineering, Faculty of Civil Engineering, the University of Kassel, D Kassel, Germany 3 Department of Groundwater Resources, Jatujak, Bangkok, 10900, Thailand *Corresponding author. arlai_p@mail2.npru.ac.th Abstract The Kok river basin situated in the Golden Triangle delta in northern Thailand is becoming to serve as an important regional trade hub for the Yunan Province, China, Myanmar, Laos P.D.R and Thailand, as laid out in the quadrangle economic cooperation policy. As a consequence of the latter, the Kok river basin is expected to experience some major economic growth in the near future which, despite its positive social impacts, should exert some environmental stress on the natural resources of the region, namely, its water resources. Imminent climate change across the region as a whole may further exacerbate these adverse effects. Although groundwater is still at present time abundant in the Kok river basin and is supposed to support the water needed to sustain the envisioned future economic growth, no thorough investigation with regard to the quantity (yield) and the future sustainability of the groundwater resources in the basin exists up-to-date. Here we investigate this issue by means of a 3D numerical groundwater flow model (MODFLOW) for the Pa Pao aquifer basin situated within the Kok river basin -, whereby the focus of the study is on the estimation of the future sustainable groundwater yield under external stresses on this aquifer, namely, groundwater pumping. Based on first observational results of a recent exploratory hydrogeological investigation of the Thai Department of Groundwater Resources (DGR), the aquifer system is modeled with a top unconfined aquifer layer and three underlying confined layers, each of them separated by an aquitard. The groundwater flow model is calibrated in steady-state and transient mode using observed piezometric heads for the various aquifer layers for the year The sustainable (safe) yield of the Pa Pao aquifers system is then computed, based on a definition of the DGR, whereby sustainable yield is the maximum total pumping rate that ensures that the average piezometric head in each layer does not fall below a vertical distance of 20 meter from the land surface in the next 20 years. Employing these constraints for the future heads, the transient MODFLOW computations result in a total sustainable yield of 168,219 CMD for the Pa Pao Basin as a whole. Using a zone budget module within the groundwater model, the sustainable yields have then been calculated also sub-districtwise. Although still preliminary, the present modeling study should give policy makers a first tool at hand for future sustainable groundwater resources management in the Kok river basin. Keywords: Pa Pao aquifers basin, sustainable yield, groundwater modeling Introduction The Kok river basin is a northern border basin of Thailand and located at the golden triangle delta. The region has become the regional trade center among the southern part of Republic of China, Burma, Loas P.D.R and Thailand through Mae Khong river. Consequently, the economy in the Kok basin area has a large tendency of ongoing growth. As a matter of fact, as the Kok river region will continue to be developed, more and more infrastructure to serve this economic growth will be needed in the future. One of the critical infrastructures is water resources. As groundwater is an available and abundant water resource in the Kok river basin (DGR, 2009), its withdrawal could support the projected economic growth in the region in the future. However, up-todate, there is yet no groundwater investigation in the Kok river basin. This is the reason why the Thai Department of Groundwater Resources (DGM) has been starting to investigate the hydrogeology and the groundwater potential of the aquifers in the Kok river basin, in order to properly plan for the future sustainable groundwater resources management in the basin. Groundwater modeling is one of the indispensable tools to achieve this goal. For this reason the present authors have been setting up a

2 comprehensive groundwater flow model for the Kok river basin in general and for several of its sub-basins, in particular, in recent years (e.g. Arlai et al., 2012; Koch et al., 2012), whereby the emphasis has been on the estimation of the so-called sustainable yields of the various aquifer systems comprising the basin. The present paper will merely deal with analysis of the sustainable yield for the Pa Pao aquifers system which is part of the Kok river basin (Figure 1). The sustainable yield here is defined as the yield under the permitted groundwater level by DGR. Thus the sustainable yield informs on the available groundwater quantity that can still be extracted under the permitted groundwater constraints. With this information, a policy maker can make a decision on to whether permit a request of groundwater development in the area or not. Figure 1 Hydrogeological map of the Kok river basin with the red area delineating the Pa Pao aquifers (modified from DGR,2009) Study area and conceptual model The Pa Pao aquifers system is mainly elongated in the north-south direction and encompasses an area of 152 km 2. The regional direction of the observed groundwater flow is from the west, east and south towards the flat area in the mid of basin, where also most of the groundwater pumping occurs. The Pa Pao aquifers system is conceptualized, as shown in Figure 2. The main surface water is the Mae Nam Lao river that flows from the south to the north. It is specified as a river boundary in the model. As the Pa Pao basin is completely enclosed by ridges of mountains (Figure 2), no-flow boundaries are set up here in the conceptual model. The basin flanks are old pervious terraces which are specified as recharge boundaries. The geological investigation (DGR, 2009) unveils that the aquifer system is comprised of 6 layers, namely, one unconfined and 5 confined aquifers. The 1 st, 3 rd and 5 th layer are low-hydraulic conductivity layers with thicknesses of 2, 4 and 4 m, respectively, whilst the 2 nd, 4 th and 6 th aquifer layer that consist of gravel, sand and sandy clay have average thickness of 53, 36 and 40 meters, respectively. Figure 2 Conceptual model of the Pa Pao aquifer basin Model setup A fully 3D- FD groundwater model of Pa Pao aquifers is set up which conforms with the conceptual model, as well as with the various hydrogeological and hydrological data. Thus, the 3D model is comprised of one unconfined, 3 confined aquifers and 2 intervened aquitards. The horizontal grid spacing has been is optimized by checking the misfit errors as a function of the grid spacing. Figure 3 shows that the optimal grid spacing is 400x400 m 2 as a further reduction of the grid size will not lead anymore to a further decrease of the error. Therefore, this grid spacing has been used in all subsequent modeling tasks. Finally, all geological, geophysical and hydrological data obtained from the investigations of the DGR (2009) have been imported into the model. The net recharge into the aquifer system has been estimated based on precipitation and land use Figure 3 Graph of grid spacing versus the error Figure 4 FD grid (left and Mae Nam Lao river (right in the Papao groundwater model

3 Groundwater model calibration Groundwater model calibration is carried out thereafter, in order to make sure that the model can mimic the groundwater system reasonably well. Since the groundwater levels had not yet been measured in the study area before, the DGR started a install monitoring wells and measure the piezometric head in 2008 (Figure 5). The piezometric heads used in the present models have been in the time period January to June, Using this data steady-state and transient calibrations of the groundwater model have been carried out. Figure 6 Scatter-plot of observed versus calculated head for three aquifer layers in steady state calibration Figure 5 Monitoring wells in Pa Pao aquifers basin: the locations of monitoring wells pumping wells and The steady-state calibration serves to calibrate the set of hydraulic conductivity and recharge. The error evaluation measures are assessed both quantitatively and qualitatively (Anderson and Woessner, 1992). A quantitative assessment is obtained from the scatter-plot of the observed over the calculated heads (Figure 6). This plot shows that the parameters are well-calibrated, as all correlated points lie within the 95% - confidence band. On the other hand, a qualitative assessment is gained from Figure 7 which shows the spatial isolines for the observed- and the calculated heads. One may notice that that the spatial distributions of observedand calculated heads conform reasonably well to each other. In any case, these results indicate that the set of calibration parameters is satisfactorily well calibrated. The transient calibration aims to calibrate the storage and recharge parameter. In this transient case the quantitative assessment of the calibration result is obtained from the comparison of the time series of the monthly observed- and calculated piezometric head (Figure 8). The figure shows agien that the model is reasonably well calibrated, as there is an acceptable agreement between these two data sets. Figure 7 Isolines of observed and calculated heads Figure 8 Time series of observed- and calculated piezometric heads in layer two of the model Sustainable yield Each aquifer system has a unique definition of its sustainable yield which depends on the particular local hydrological and environmental conditions. For the Pa Pao aquifers system, the DGR has defined the sustainable yield as the maximum total pumping rate which ensures that the average piezometric head in each layer does not all below to - 20 meter from the land surface in the next 20 years". In this paragraph this sustainable yield concept will be introduced into the groundwater model. This is done

4 by attributing to each active cell of the groundwater model a groundwater pumping well and augmenting the pumping rates during the sustainable yield computation trials until the aforementioned headcondition is violated. The corresponding pumping rate denotes then the maximum sustainable yield for that active cell. Furthermore, the pumping rates in the active cells are divided into two zones, namely, a zone #1 which comprises the model area where the aquifer thickness is not bigger than 50 m (mainly the western flank of the Pa Pao basin) and zone #2 with aquifer thicknesses bigger than 50 m (mainly in the center of the basin) (see Figure 9). As the reactions of the hydraulic heads in the aquifer depend also on the thickness of the aquifer layer, - for the same pumping rate a thin layer exhibits a larger drawdown than a thick layer - different, but constant across the corresponding zone, base pumping rates are also applied in these two aquifer zones. The initial pumping in each active cell is calculated from the averaged value of the currently used total pumping rate in the zone. This results in an initial pumping rate of CMD for zone #1 and of 0.24 CMD for zone #2. Using these starting pumping rates, the groundwater simulations have been repeated many times by slowly increasing the pumping rates, until the sustainable yield constraint - piezometric head no more than -20 meters below the surface in the next 20 years - is attained. As a final result of these computations a total sustainable yield of 169,794 CMD is obtained for the Pa Pao basin. With present-day groundwater withdrawal of just 1,575 CMD, this would allow for an additional 168,219 CMD to be extracted in the coming 20 years. In the next step, the zone budget module embedded in the groundwater model has been used to calculate the sustainable yields individually for the 8 subdistricts which make up the Pa Pao basin (see Figure 10). The numerical results are listed in Table1. Figure 10 Sub-districts of the Pa Pao Basin Table 1 Sustainable yield in each sub-district of the Pa Pao aquifers basin Subdistrict Sansalee Viang Ban Pong Pa Ngew Viang Kalong Jedi District Yield in the next 20 years Sustainabl e Yield (CMD) Pa Pao 25,835 25,345 Pa Pao 33,573 33,153 Pa Pao 15,993 15,993 Pa Pao 29,308 29,183 Pa Pao 29,785 29,285 Pa Pao 24,502 24,477 Jedi Mai Pa Pao 10,798 10,783 Total 169, ,219 Summary Figure 9 The two pumping zones in the Pa Pao aquifers system The Pa Pao aquifers system has been conceptualized by a 3D hydrogeological model which comprises 6 modeled layers, i.e., top unconfined aquifer- and 5 confined aquifer layers. A 3D-FD groundwater flow model is then used to simulate the groundwater flow within this conceptual model. The model has been calibrated in both steady-state and transient mode and the modeled heads show good agreement with the observed ones.

5 The sustainable yield concept is then applied to the calibrated model. A sustainable yield of 168,219 CMD for the next 20 years is obtained for the Pa Pao basin as a whole. The sustainable yield is then also computed sub-districtwise. Eventually, these results should give the DGR officers a holistic protocol in hand for near-future sustainable groundwater resources management in that basin. Acknowledgments The authors express there sincere thank to the Department of Groundwater Resources to support their research budget and for valuable assistance. References [1] Anderson, M.P and W.W. Woessner (1992), Applied Groundwater Modeling: Simulation of Flow and Advective Transport, Academic Press, Orlando, FL. [2] Arlai, P., A. Lukjan and M. Koch (2012), Numerical Investigation of the Groundwater Balance in the Mae Sai Aquifer, northern Thailand, Procedia Engineering, 32, [3] Department of Groundwater Resources (2009), Groundwater Resources Assessment in the Kok river basin. [4] Koch, M., P. Arlai, P. and A. Lukjan (2012), Modeling Ivestigation of the future permissible Yield in the upper Chiang Rai Aquifer System, Procedia Engineering, 32,