Water plays an important role for life, especially groundwater—the largest freshwater source. It is a source of freshwater for human consumption, agriculture and industry, especially during periods of critical drought and disaster [1
]. However, climate change impacts every region, such as agriculture and water resources [2
]. In developing countries, groundwater may be facing shortages and contamination, because there is no plan to manage groundwater under climate change [5
Climate change has an impact on a global scale [7
]. The change in temperature and rainfall pattern are controlled by climate change [9
]. In addition, climate change has caused fluctuations in the hydrologic cycle, which impacted the sustainability of surface and groundwater. In particular, tropical climate changes strongly affect groundwater quality and quantity [10
], due to the strongly alternating hot and rainy conditions, indicating that climate change directly impacts both temperature and rainfall, which, in turn, affects recharge and hydraulic head [14
]. The change in groundwater resource under climate change has been studied; for example, Jackson et al. [16
] used the A2
emission scenario of the Intergovernmental Panel on Climate Change (IPCC) General Circulation Models (GCMs), which assumed a population growth of 15 billion by 2100 and a significant decline in fertility for most regions, but stabilizing above replacement levels [17
], to project climate by simulating the Chalk aquifer in the UK; they applied a distributed recharge in groundwater flow model and concluded that future groundwater recharge would range between −26% and 31% until 2080. Doll [18
] determined that groundwater recharge, under climate change in Northern Brazil, the southern edge of the Mediterranean Sea and southwest Africa, was decreased to 70% of groundwater recharge, under the A2
emission scenarios by 2050. Emelyanova et al. [19
] showed that the groundwater levels slightly rose in Southwestern Australia. Shrestha et al. [20
] described groundwater levels in the Mekong Delta, in Southern Vietnam, under climate change, which showed a declining trend. These studies showed that change in groundwater recharge and groundwater level depended on the selected emission scenarios and events [21
Climate change is a very important factor in projecting groundwater recharge [22
]. However, groundwater sensitivity to climate change is influenced with the aquifer type and depth of groundwater, with both shallow and deep, and small and large aquifers, varying with climate change [24
]. Then, the recharge and hydraulic head analysis was not sufficient to understand the impact of climate change on groundwater. The new indicator is groundwater age, because it can be analyzed by groundwater travel time and is not associated with aquifer type, depth and size.
Groundwater age, i.e., the time between water infilled to the aquifer until that water reached a position of interest [25
], has been shown to be a reasonable indicator of groundwater recharge [27
] and sensitivity of groundwater to the climate change and has been assessed in several complex systems [28
]. Torgersen et al. described the ideal groundwater age as the time elapsed between when water entered the saturated zone to the time the water was sampled at a specific location, i.e., a specific distance downstream, and defined old groundwater as groundwater that was recharged on a timescale from approximately 1000 to more than 1,000,000 years [35
]. The first approach to groundwater age modelling was described by Goode [36
], who opened many aspects in the field of numerical groundwater research. Initially, he used the mean over one basin, i.e., only one age. Ginn [37
] later showed that groundwater age was distributed over a region and transported in many directions, so the groundwater age is the age assigned to each particle of water. Vani and Carrera [38
] showed that when purely advection was considered, the age distribution increased drastically, because the age distribution needed a very fine grid to model the transport: they obtained a more realistic aquifer using a very high resolution (i.e., space, time and age dimensions). Woolfenden and Ginn [39
] modeled a real-world groundwater age distribution and showed the modeled age, measured to confirm the groundwater calibration, led to confidence in groundwater age distributions [37
] and mean ages [36
], calculated by the model.
] reviewed groundwater age dating and showed the advantage of using it to estimate recharge rate, flow rate and model calibration. Groundwater age can indicate groundwater recharge, i.e., young groundwater indicates high recharge, so groundwater can be pumped, but old groundwater indicates low recharge; pumping should be carefully managed because groundwater may not flow to the pumping location. Groundwater age can indicate groundwater flow direction, i.e., if we know the groundwater age, in many locations, we may plot groundwater contours. Groundwater flows from young to old age. Groundwater age can indicate sustainability of groundwater, i.e., if groundwater moves from a young and to an old age, groundwater will become inadequate, due to slower recharge. Then, groundwater should be conserved. In this research, groundwater age was applied to indicate the sustainability of groundwater.
Groundwater age was applied, in many numerical groundwater studies, for example, to estimate groundwater parameters. Sanford used groundwater age to improve overall calibration, such as variability in recharge, porosity and hydraulic conductivity [41
]. Michael and Voss used groundwater age to estimate the anisotropy, using MODFLOW and MODPATH in inverse modeling, and found that increasing hydraulic conductivity led to decreased age at each point, and found that in the zone of a discontinuous aquitard, or gap in an aquitard, groundwater was young [42
]. The groundwater age may also be combined with other parameters for calibrating groundwater modeling. Portniaguine and Solomon used groundwater age and head data to calibrate a groundwater model, because at some locations, where the head data were not available, the groundwater age streamlines were used to imply to groundwater flux, from which the recharge and hydraulic conductivity may be derived [43
]. Engdahl et al. used groundwater age distributions to estimate effectively homogeneous and heterogeneous aquifer: in a homogeneous aquifer, estimation of the properties at all scales was generally accurate, but in heterogeneous aquifers, it only provided reasonable orders of magnitude [44
] reviewed methods to measure groundwater age; for example, radioactive isotope tracking [35
] and groundwater models [39
]. The methods to measure groundwater were applied to aquifer recharge. Engdahl and Maxwell [49
] measured the change in age distribution under variable recharge (i.e., ±25% and −50%) and showed that the change in recharge and topography controlled the age distribution in steady-state conditions, because the recharge rate contributed to groundwater flux (velocity): low flux led to increased groundwater age, whereas high flux contributed to decreased age. Then, “old” groundwater (nonrenewable groundwater) indicated groundwater shortage, when groundwater was pumped, due to a low recharge rate, and it could not be recharged in human lifespans [50
However, in recent research, it was shown that climate change impacted surface water and groundwater, using mathematical model, for example, sensitivity analysis on climate, hydraulic head, water quantity and quality [2
]. The major parameters were rainfall and temperature. However, no one focused on the climate change impact on the distribution of groundwater age, which is the contribution of this study.
Thus, we developed a three-dimensional groundwater flow model to investigate the groundwater shortage affected by climate change, using groundwater age as an indicator. First, we describe the study location, including topography, hydrogeology and climate. Then, we detailed the materials and method for calibration and verification of the groundwater model. Since groundwater recharge predictions depend on the selected groundwater simulation model, discussion of the recharge rate in variable scenarios was included. Then we determined the groundwater age distribution to guide groundwater management under climate change.
4. Results and Discussion
4.1. Calibration and Verification
The calibration and verification by Tanachaichoksirikun et al. [60
] for the correlation of simulated and measured hydraulic head is shown in Figure 5
. In Figure 5
a, the simulated and measured hydraulic heads were compared, in 2009–2014, with an absolute residual mean error of 4.6 m and NRSE = 5.9% and R2
In the verification model (Figure 5
b), the simulated and measured hydraulic heads were compared in 2007–2008. The absolute residual mean error was 2.6 m, with NRSE = 3.8% with R2
= 0.95. So, our errors were less than those of Du et al. [70
], often used as a benchmark for numerical simulation of groundwater flow: for any node, the absolute residual mean error should be less than 5 m, with NRSE < 10%.
In addition, here, we calibrated using change in hydraulic gradient to anisotropy as in our previous work [60
]. The calibration using only the hydraulic head was not sufficient in a regional scale, because the LCP basin is alluvial. The aquifers are sedimentary units, with high anisotropy. the data showed that the optimal anisotropy was 104
, due to the high correlation between hydraulic head and hydraulic gradient (R2
> 0.8). Additionally, this model was analyzed for sensitivity to recharge and pumping and showed that groundwater flow was comparable with the real-world condition.
4.2. Simulated Versus Observed Groundwater Age
shows a close agreement between the observed 14
C groundwater ages from Sanford and Buapeng [45
] (over 11,000 years) and our simulation (over 10,733 years). The standard deviation was 3300 years, with R2
= 0.81. Thus, this model can be used for predicting groundwater age distribution: R2
> 0.8 and standard deviations in the range of 2000–4000 years [45
4.3. Predicted Groundwater Age Distribution
illustrates the groundwater age distribution in the LCP basin, for the base case and IPSL-CM5A-MR with RCP 2.6, 4.5 and 8.5. The groundwater age was shown separately for each aquifer and overall. In general, the younger groundwater was near the recharge and injection zones, where the groundwater age is zero and increased with gradient and depth. The groundwater age in the LCP basin was distributed widely, i.e., ages varied over a wide range; because of the regional basin, some groundwater was resident for a long time, travelled long distances and several sources of water were involved—as discussed in the previous section.
Under the base case (Figure 7
a), groundwater age in the PD aquifer covered 100 to 10,000 years, and the mean groundwater age was ~2800 years. The groundwater age in the NL aquifer covered 100–70,000 years, and the mean age was ~8000 years. The NB aquifer distribution covered 1000–100,000 years with a mean age of ~14,500 years. Overall, the average age was ~11,000 years. Thus, the study area was mainly old groundwater. Groundwater flowed slowly, because the basin was a sedimentary aquifer and clay layers between the aquifers hindered groundwater flow from the surface area to the aquifer.
Under RCP2.6 (Figure 7
b), groundwater age in the PD aquifer was 100 to 9000 years, with average age = 2988 years (i.e., 188 years longer than the base case). The average age distribution was 67% wider than the base case. Groundwater age in the NL aquifer was 200–80,000 years, with an average of 8986 years (+985 years from the base case). The age distribution was also wider, by 80.1%, compared to the base. Groundwater age in the NB aquifer was 1000–120,000 years. The average age = 18,253 years (+3745 years from the base). The age distribution was 62% wider than the base case. This showed that the lower rainfall contributed to increased groundwater age. The lower rainfall contributed to decreased recharge, low groundwater budget, which, in turn, led to lower hydraulic head and lower velocity. The shallow aquifer was affected by the climate change, due to lower distance and thinner clay layers.
Under RCP4.5 (Figure 7
c), groundwater age in the PD aquifer was 100–8000 years with average = 2850 years (+50 years). The distribution was 59% wider. Groundwater age in the NL aquifer ranged from 100 to 70,000 years. The average age was 7967 years (−34 years, i.e., less than the base case). The distribution was wider by 83.2%. In the NB aquifer, groundwater age covered 800–120,000 years with average = 14,911 years (+411 years). The distribution was 59% wider. This showed that the predicted rainfall was close to the base case. The groundwater age was similar to the base case.
Under RCP8.5 (Figure 7
d), groundwater age in the PD aquifer covered 100–8000 years with the average = 2746 years (−69 years). The distribution was 80% wider. Groundwater age in the NL aquifer covered 100–60,000 years. The average age = 8914 years (+913 years). The distribution was 89% wider. In the NB aquifer, groundwater age covered 1000–120,000 years, with average = 17,378 years (+2870 years). The distribution was 62% wider. This showed that the higher rainfall led to decreased groundwater age in a shallower aquifer, but in a deeper aquifer, groundwater age was apparently still increasing, but our simulation time extended only to 100 years. The rainfall had not flowed to the NB aquifer, so the groundwater age in NB was increased.
Additionally, the average groundwater age in a shallow aquifer was younger than in a deeper one. The groundwater age distribution in the PD aquifer covered 100–10,000 years, whereas, in the NL and NB aquifers, it extended to 100,000 years.
Although Engdahl and Maxwell [49
] suggested that climate change affected groundwater age, our work showed that groundwater age changed only slightly in transient groundwater flow conditions, because the climate change was not accompanied by high rainfall, at that time. In higher recharge scenarios, e.g., IPSL 8.5, the groundwater age distribution shifted slightly, because rainfall increased groundwater velocity and rain generated more surface water. In contrast, in lower recharge scenarios, e.g., IPSL 2.6, the groundwater age shifted significantly, because lower rainfall caused longer residence times and reduced flux (slower velocities).
Overall, changing groundwater age distributions, with differing climate change scenarios, reflected many changes of hydraulic head, recharge and velocity. The change in the age distributions showed the dynamics of the groundwater systems, as fractions of groundwater shifted slightly at various ages, from 10 to 100 years as shown in Figure 7
. The age distribution changes indicated changes in location and rate of recharge. Then, groundwater levels, that cause land subsidence, may be unstable. In climate change scenarios, with increased rainfall, making the regional system wetter, the upper aquifer was more completely filled and the excessive groundwater flowed to the surface water system. Groundwater age distributions were unaffected. Change in water budget between surface and groundwater was reduced and caused an increase in older groundwater, although the groundwater ages rarely changed.
4.4. Distribution of Groundwater Age under Climate Change Impact
The projected climate change, under the IPSL-CM5A-MR, was simulated to check for future groundwater age. More recharge, therefore, increasing groundwater levels, led to younger groundwater in shallow aquifers and older water in deeper aquifers. Thus, future groundwater recharge is the most significant feature that controls decrease or increase of age in each aquifer. Increased future groundwater age and the time to recharge was longer than human life spans, which indicated that groundwater in the LCP basin is nonrenewable groundwater. The groundwater recharge affected the groundwater fluctuation, which led to the groundwater problems, e.g., land subsidence and seawater intrusion.
In the shallower aquifer, groundwater ages decreased because, as older groundwater was pumped out of the lower aquifer, it was replaced by younger groundwater from the upper layer. The old groundwater that was pumped out of the aquifer cannot be replaced by recharge, because the effects of old groundwater were replaced by young water in the upper part of the aquifer and mixed with low velocity.
Nonrenewable groundwater will contribute to land subsidence and consequent detrimental effects, e.g., damage to infrastructure and increased flood damage and risk due to lower ground surface level [71
]. Soil is an elastic material, so if groundwater is recovered, the soil level will be increased. However, if groundwater level decreased for long periods and contributed to land subsidence for longer times, soil material will creep and become inelastic, so although groundwater level is restored, the land will not return to previous levels.
Since the LCP basin connects to the Gulf of Thailand, pumping groundwater has led to a progressive increase in salinity: decrease in hydraulic head, under the influence of pumping, especially in low recharge scenarios, led to rise of the sea–groundwater interface, leading to more saline groundwater resources. This effect was more obvious in the shallower aquifers, because of the higher sensitivity to hydraulic heads and to higher pumping, induced by lower costs.
The predicted increase in groundwater age from recharge will affect the groundwater level. Therefore, groundwater pumping must be carefully managed, because it may cause land subsidence, seawater intrusion and salinity. Moreover, pumping in nonrenewable groundwater may contribute to groundwater shortage, because rainfall cannot recharge it in human life spans. Here, we showed that the groundwater age distribution, in the deeper layers, i.e., the NL and NB aquifers, will gradually increase in the order of 100 years, indicating that recharge will not fill these aquifers; therefore, pumping in these aquifers must be carefully managed.
Variations in precipitation and recharge affect groundwater age distributions on regional scales. We assessed the distributions of groundwater age, under climate change in several scenarios, in Thailand’s Lower Chao Phraya (LCP) basin. The optimal climate scenario, IPSL-CM5A-MR, derived in work by Ruangrassamee et al. and Wattanasetpong et al. [67
], was used here, and we varied the Representative Concentration Pathway (RCP) models (labeled 2.6, 4.5 and 8.5), from 2020 to 2099. Simulations used MODFLOW-2000 and MODPATH under transient-state conditions. We found that groundwater age slightly decreased in the shallower aquifers, whereas the age increased in the deeper aquifers, under increasing recharge scenarios, because current rainfall did not move water to the deep aquifers. The overall average groundwater age gradually increased, due to groundwater age mixing in both shallow and deep aquifers. However, under decreasing recharge scenarios, groundwater age increased in both shallow and deep aquifers, because reduction of recharge caused groundwater to flow slower and for longer times, so that groundwater became older, which was also reflected in decreased in hydraulic heads.
The fluctuation of groundwater recharge led to change in groundwater age, which was an indicator of the groundwater level, which, in turn, allowed for land subsidence, seawater intrusion and increased salinity. Further, groundwater became short in the area, if groundwater was over pumped, especially in the NL and NB aquifers, where groundwater age increased in both increased and decreased recharge scenarios.