2.1. FEFLOW (Finite Element Subsurface FLOW and Transport System) Transit Model with Ice Scenarios
Nowadays, groundwater simulation and modeling are some of the main tools for groundwater aquifers [12
], which visualize the situation and conditions of water in underground porous media for the protection of groundwater, as well as the restoration and development of aquifers. Groundwater level loggers collect more data, but because they cannot be manually controlled, only software modeling and processing provide an option that is both fast and accurate [13
Every spring over the last decade, during the months of March and April, the Tuul River has dried out or has not flowed [14
]. Therefore, we need to address on this problem by building complexes of hydraulic structures, establishing measures for flow control, and building artificial groundwater recharge systems like drainage or flooding areas near drinking water extraction wells. This study attempts to find suitable artificial groundwater recharging methods for the upper part of the central source groundwater aquifer, encompassing the water supply source area of the Tuul River valley inside Ulaanbaatar. The central source of drinking water supply system was established and put into operation in 1959. Drinking water is extracted from 93 deep well pumps with seven booster pumping stations; see Figure 4
The capacity of the source reserve is 114 × 103
/day. Nowadays, water extraction is from 70–80 wells, and its volume reaches 87–90 × 103
/day for supply to the capital city [4
]; see Figure 3
The central source of the A–A zone was simulated in FEFLOW simulation. The upper part of the central source intake area near Well 68 takes recharge from river surface water until January, when the Tuul River flows under ice cover. After that period, the Tuul River freezes while the riverbed bottom and groundwater continuously decrease until May. However, in the western side of Zone A, the intake wells area near Well 51 recharge comes not only from Tuul River but also from the Uliastai River; therefore, recharge takes place early in May. As shown in graph, the Uliastai River flow freezes early in November as a small river due to a groundwater decrease that begins in October, and the following recharge in May depends on melting water from both rivers; see Figure 5
The groundwater recharging process runs as follows:
In the initial phase in which surface runoff is non-existent, maximum groundwater declines are observed from the end of April to early May. If the value of the groundwater table is same and above the elevation of the riverbed, it becomes possible for the river to flow downstream without recharge.
The icing phenomena is one of the most important parameters for semi-arid, highly continental climate conditions and plays significant role in the hydrological cycle and regime of Mongolia. The icing or ice cover of rivers and lakes takes place in the cold season, which occurs over five to six months, when the ice cover thickness reaches to 0.8–3.2 m. However, big mountain rivers with a greater slope and bigger perturbation boulders in riffle sections stay open for the whole year and do not completely freeze along the length.
Generally, for surface ice cover to become firmly established, the mean (depth-averaged) temperature of water must be less than 2 °C, the daily average temperature must be less than −5 °C [15
], and the wind speed must be less than 5 m/s [11
The Uliastai River, one of the tributaries of the Tuul River, originates from the Khentii Mountains, flows from the north east side to the south through Ulaanbaatar, and contributes to the Tuul River through the water supply wells of the central source for Ulaanbaatar.
Icing in the aufeis accumulates during winter along the streams and the river valley in northern Mongolia, which is dominated by semi-arid, highly continental regions. In the Uliastai River, building ice lasts from the middle of October until the end of December, and the melting process starts from the end of March and ends in April.
The icing dynamics depend on the groundwater fluxes and discharges alongside of the main channel. The spring leakes through drainage channel, builds ice sheets over the frozen riverbed, where the main stream flows under ice cover. The ice generating process and icing dynamics are studied from the middle of October to the end of December.
The main stream flows by the main channel under ice cover. The side spring streams over the top of the ice sheets. This phenomenon is called icing or aufeis. Icing or aufeis consist of sheets of stratified ice formed by freezing consecutive water leaks [16
The water flows over existing ice layers. It forms through the upwelling of groundwater discharge or manmade drainage channels, where groundwater discharge is blocked by ice, perturbing the steady-state condition and causing a small incremental rise in the local water table until discharge occurs along the bank and over the top of the previously formed ice [16
In the beginning of November, river flow freezes from side benches, with the spring discharge no longer extending energy because it had frozen first. After the riverbed completely freezes and takes ice cover, the spring discharges leak from under the ice or ice hummock while the groundwater head and pressure flows increase over frozen ice sheets to create the next ice sheet.
A groundwater flux from drainage canal flows on surface as spring and creates the next ice sheet, which fills the lower ravines and smoothest horizontal by ice sheet.
The groundwater flux alongside the river drains through drainage canal-built icing phenomena, while springs’ flow beds are blocked by ice and the main stream in the main riverbed flows under the ice cover. In this section, the Uliastai River gains streams of almost one-third of the flow of the discharge abstracts to groundwater, and groundwater comes from springs.
Thus, phenomena create side spring leakage from under frozen soil, leakage which flows over frozen soil and ice-covered streams, fills ravines and lower lands, creates ice sheets on the ice until the river valley gains the same level of ice. After that, the average daily temperature decreases to under −20 °C, and then the groundwater leakage discharge decreases the head and pressure of groundwater flux in underground flows. Additionally, drained groundwater, spring discharge from the surface, and groundwater decreases influence the quantity of groundwater volume. The ice thickness measurements from the 15th to the 30th of December showed that the thickness of the aufeis sheets have not increased; see Figure 4
In this way, groundwater leakage, like drained water or springs over frozen soil and ice cover, creates stratified ice sheets over other ice sheets. In the Uliastai River, there are spring discharges of 32 L/s—a small amount of water that nonetheless build ice thicknesses in some places up to 1 m thick. Some of the rivers make ice sheets of thicknesses of several meters. The decline of the ice storage depends on the quantity of spring discharge. The end of building aufeis is usually in the last days of December or the first days of January—after the longest December night ends and the colder days of the year start. In a larger river, such as the Tuul, is possible to create an aufeis until February. An aufeis typically starts to melt during summer and finishes by the end of April (or sometimes, the beginning of May), and it will often form in the same place each year [7
From these icing ideas, a MATLAB code was written and used for the management of aquifer recharge for the central source of the A zone, which accumulates ice storage due to the use of melt water for recharge in the dry season [17
The aufeis code was written as one mole of water exchanging energy with cold air and ice, extending velocity of in both the x and y directions could widen in an ellipsoidal way due to the Darcy–Weisbach law. The aufeis spreading dynamics of one mole thick water sheet over ice sheets decrease the extension size while decreasing air temperature day by day from −5 to −30 °C [17
The result shows that length of ice spread is 2329 m, with a width of 458 m and thickness of 1.54 m. The code for ice storage was simplified with the same slope along flat area. The MATLAB code presents the ice extending dynamics of spreading water with pipeline levees, and from leakage point to pipeline levee of 1500 m. The length of ice storage is only 1500 m; see Figure 6
. The results show that the length of ice spread is 1500 m, with a width of 400 m and thickness of 2.72 m, thicker than the unlimited area. In the following figure, you can see two ice storages: unlimited and unlimited area with underground pipeline levee, see Figure 7
. These ice storages will change recharging boundary condition in FEFLOW model as a one-month early recharge in April by melt water from north side, where the Tuul River flows in May [8
The relation between the estimated quantity of the MATLAB ice code and the FEFLOW simulation characterizes the boundary condition change in the northwest side of simulated area. The groundwater recharge from the river starts in May, when the Tuul River flows again. However, in the simulation, the aufeis-stored ice was shown to melt earlier in April and to recharge groundwater. The aufeis changes the northwest boundary condition so that the recharge starts from the beginning of April. The drainage canal begins to flow with water from May, when the Tuul River flows again; see Figure 6
. The eastern boundary groundwater fluctuation taken from Well 68 was not found to change. However, the downwards flow direction of the northwest boundary condition was found to change with recharge from the ice melt water; see Figure 6
, blue line.
One of the artificial recharging groundwater resources is the temporary and spatial redistribution of surface runoff, which in the beginning of winter results in ice formation. The surface runoff flows over the northern side of the central section A zone through filtration channel will release water at the end of canal and, thus, it again creates more ice sheets over frozen ones.
There are three ways of promoting ice creation in cold regions in winter: on the ground surface, in the underground open pit or channel, and on the river bed. Of these, surface and underground ice creation were considered in the FEFLOW simulation scenarios. See Figure 7
The rate of water release is 1 m3/s at the beginning of November until the middle of December, and creates ice storage when the average temperature decreased under −5 °C.
From the above figures, it is shown that the northern drainage canal allows us to accumulate 1 m3/s flow water for a month, from November to the middle of December, and 3.9 × 106 m3 water can be stored on the surface. However, with losses from evaporation and winter fog over frozen ice sheets during the melting season, evaporation loss allowed only half of this quantity—about 2 × 106 m3—to accumulate for recharging groundwater. Here melted water from ice storage recharges groundwater in the central source A–A zone from the northern side, from the beginning of April until May.
Ice accumulation from November to the end of December also recharges groundwater while water flows through the drainage canal. It is then transferred to the artificial regime with the subsequent supply of water to canals, functioning together until the period of ice formation, provided there is ice accumulation from surface water on the end of the canal.
The underground dam will be built in the first layer until reaching the natural permafrost layer 5–10 m from the surface [18
], considered as an ice wall on the western boundary line.
2.2. FEFLOW Steady Model with Water Reservoir Scenario
A preliminary analysis, as presented in this study, was to identify low-cost MAR implementation measures adapted to the specific natural conditions of the Northern Mongolia. Thus, the coldness of cold weather can be used to keep water in ice form and as a water resource in the winter season in addition to being used during the dry season, characterized by low flow, by melting ice when the rivers have dried out.
The accumulated ice would recharge the groundwater in the dry season from March to May by melting, and the riverbed would be dry without water or cover ice.
To find the total additional water resource recharged by the northern drainage canal and ice storage, the abstraction rate should be increased until the available maximum rate, when groundwater drawdown decreases under the filter screen at the bottom of wells and soaks up air. For calculation of the potential maximum abstraction quantity in the central source A zone without MAR methods, it should be estimated using FEFLOW simulation until some well groundwater drawdown reaches to the bottom of the well screen, pumping air instead of water. In this way, groundwater fluctuation ranges can be established in FEFLOW.
The groundwater fluctuation graph of monitoring well N8 show that by increasing daily extraction to 60,986 m3/day, it was not possible to pump water while the groundwater drawdown was under the bottom screen. However, in the middle of group of wells, groundwater level is in extreme drawdown, and only 45,121 m3/day is possible. Therefore, the maximum abstraction rate of this area is 45,121 m3/day, which means 16,469,165 m3/year = 16.5 × 106 m3 of water per year.
Nowadays, the exploitation rate is 20,329 m3/day (average of 2009–2011), 7.5 × 106 m3/year corresponding to half of the possible maximum abstraction.
The recharge quantity with MAR method and then the maximum abstraction rate increase until 70,133 m3/day, which corresponds to 25.6 × 106 m3/year.
The difference between the above maximum abstraction simulations shows that it is possible to increase the groundwater resource extraction rate to around 25 × 103 m3/day using MAR methods. That means approximately 912,500 m3/year = 9.125 × 106 m3 additional groundwater during wet season could be kept as reserve in the upper part of this central source area A–A zone.
The percentage for recharge groundwater sources in this area was estimated by the sum of all methods of each MAR method, and is taken as 100% for all together.
The percentage of the increased amount of water reserve by each method, after combination of the MAR methods, from increased groundwater sources is as follows.
Drainage canal brings 73.5% of artificial recharge in 6.7 × 106 m3 water (1 m3/s discharge water through the drainage canal from May to November 86,400 m3/day × 184 day = 15,897,600 m3 = 15.9 × 106 m3).
Ice storage keeps water in ice form in 21% or 1.9 × 106 m3 (1 m3/s discharge water for ice storage would be 5.2 × 106 m3, but fog and evaporation during icing and melting bring about huge loss).
Ice walls delay groundwater flux and maintain stability in the groundwater table, leading to retention of an additional 5.65% of groundwater, which is about 516 × 103
in water amount [17
All of these FEFLOW simulations were simulated in a transient model, where the river surface water level increases in the wet season over the riverbed, and decreases in the dry season under the dry riverbed. The table of the eastern and western boundary groundwater fluctuates under the surface, depending on the recharge from the river for whole year.
Another alternative simulation scenario is for the water release from the surface water reservoir with a dam, with a constant q rate of 26.6 m3
/s and creation of table for constant groundwater water. In this case, the model is taken with the eastern and western boundary using a constant groundwater level and hydraulic head and, also, in the southern boundary as the Tuul River with a constant level value for groundwater recharge. In the FEFLOW simulation, the maximum abstraction rate reached 90,288 m3
/day. Following this, the simulation produced an error report indicating that the aquifer has no groundwater; see Figure 8
This means the flow control by surface reservoir accumulates flood water and, additionally, the recharge groundwater quantity for the central source of A zone is 16.5 × 106 m3/year.
The flow control upper reservoir releases a constant outflow throughout the year, and it means it also recharges the central source of B zone, occupying a larger area twice in size, with 50 intake wells. The constant release rate for the whole central source area that is recharged from the reservoir would be 49.5 × 106
/year. An additional 350 × 106
of accumulated water in the reservoir would be reserved for use as a freshwater resource [20
All these simulated calculations involved recharging water from only reservoir. When we are simulating a combination of artificial recharging groundwater sources and reservoir release for surface water recharge, then the additional recharged groundwater would be 167,700 m3
/day; see Figure 9
These MAR methods and the reservoir recharge system include the following simulation scenarios.
The northern drainage canal recharge groundwater resource from the northern side in wet season;
Ice storage for groundwater recharge by melting ice in the dry season in early April with Tuul River flow starting in May;
The underground ice dam which accumulates groundwater backside up to the permafrost layer 5–10 m [18
The surface reservoir with constant release discharge into Tuul River, which recharge this area from the southern side over the whole year.
In this combination of scenarios, the maximum possible abstraction rate is increased up to 167,700 m3/day after simulation. The maximum possible abstraction quantity per year reaches up to 44.7 × 106 m3/year.
The FEFLOW simulation was only run for the central source of A zone. When we take the second B zone with 50 wells, this maximum abstraction rate would be increased, and at least doubled or 89.5 × 106 m3/year due to water recharging because the B zone is twice as large as the A zone. If the Tuul River flows yearly with constant discharge, then river surface water will be recharged for industrial sources, and that leads to increases in the additional groundwater resource. This will increase the flow from groundwater sources to 149 × 106 m3/year and create natural conditions for discontinuous Tuul River flow to assist in avoiding a situation of shortages in the fresh water supply for Ulaanbaatar city.