Water Management, Environmental Challenges, and Rehabilitation Strategies in the Khyargas Lake–Zavkhan River Basin, Western Mongolia: A Case Study of Ereen Lake

Abstract

The depletion of water resources caused by climate change and human activities is a pressing global issue. Lake Ereen is one of the ten natural landmarks of the Gobi-Altai of western Mongolia is included in the list of “important areas for birds” recognized by the international organization Birdlife. However, the construction of the Taishir Hydroelectric Power Station, aimed at supplying electricity to the western provinces of Mongolia, had a detrimental effect on the flow of the Zavkhan River, resulting in a drying-up and pollution of Lake Ereen, which relies on the river as its water source. This study assesses the pollution levels in Ereen Lake and determines the feasibility of its rehabilitation by redirecting the flow of the Zavkhan River. Field studies included the analysis of water quality, sediment contamination, and the composition of flora. The results show that the concentrations of ammonium, chlorine, fluorine, and sulfate in the lake water exceed the permissible levels set by the Mongolian standard. Analyses of elements from sediments revealed elevated levels of arsenic, chromium, and copper, exceeding international sediment quality guidelines and posing risks to biological organisms. Furthermore, several species of diatoms indicative of polluted water were discovered. Lake Ereen is currently in a eutrophic state and, based on a water quality index (WQI) of 49.4, also in a “polluted” state. Mass balance calculations and box model analysis determined the period of pollutant replacement for two restoration options: drying-up and complete removal of contaminated sediments and plants vs. dilution-flushing without direct interventions in the lake. We recommend the latter being the most efficient, eco-friendly, and cost-effective approach to rehabilitate Lake Ereen.

1. Introduction

Mongolia is home to thousands of lakes that serve as vital ecosystems, freshwater resources, and cultural landmarks. However, many of these lakes have undergone significant shrinkage or even complete desiccation in recent decades due to a combination of climatic stressors and human activities [1]. While rising mean annual air temperatures and shifting precipitation patterns have contributed to hydrological instability, the most pressing concern today is the degradation of lake systems and the loss of their ecological functions [2]. One area under considerable pressure is the Valley of the Great Lakes—a vast endorheic basin in western Mongolia, bordered by steppe vegetation and several major mountain ranges, some of which are glaciated. This valley plays a critical role in the region’s hydrology and ecology, supporting numerous lakes and wetlands. Ereen Lake, the focus of this study, lies in the Khyargas Nuur–Zavkhan Gol river basin within the Valley of the Great Lakes and has experienced significant hydrological and ecological changes in recent years, making it a valuable case for examining lake degradation and restoration potential.

While the Khyargas Nuur–Zavkhan Gol river basin performs well across most water security indicators such as rural and environmental water security—both in comparison to the Mongolian national average and the Central Asian Internal Basin—it falls behind in terms of “resilience to water-related disasters” (Figure 1) [2]. This vulnerability has significant implications for the long-term sustainability of lakes such as Airag Lake and Ereen Lake, which are increasingly threatened by prolonged droughts, shifting precipitation patterns, and mounting anthropogenic pressures. The historical degradation of Ereen Lake, along with similar water bodies in the Valley of the Great Lakes, underscores the fragility of these ecosystems under the combined effects of climatic and socio-economic stressors. In recent decades, Ereen Lake has undergone periodic desiccation, shoreline retreat, and alterations in sediment composition—clear indicators of a disrupted water balance and ecological decline. These changes not only mirror broader regional trends in lake shrinkage across western Mongolia but also raise serious concerns about the sustainability of traditional land use practices and the feasibility of restoration. Effectively addressing the lake’s degradation therefore demands an integrated understanding of both natural and human-induced drivers, alongside thoughtful consideration of local livelihoods and land management practices in any future rehabilitation strategies.

Lake restoration efforts are becoming increasingly urgent, particularly in the arid and semi-arid regions of Mongolia, where water bodies are especially vulnerable. Despite the growing need, comprehensive data and scientifically grounded restoration strategies remain limited. This study examines the challenges facing one of western Mongolia’s lakes and investigates both ecological indicators and rehabilitation strategies aimed at restoring its ecological integrity. The research on the Khyargas Nuur–Zavkhan Gol river basin is guided by three key questions:

(i)

What environmental challenges impact hydropower generation in the region?

(ii)

What is the current ecological status of Ereen Lake?

(iii)

What strategies are most effective for the rehabilitation of Ereen Lake?

Effective restoration of Ereen Lake requires integration within Mongolia’s broader water governance framework. Since 2005, Mongolia has adopted integrated water resources management (IWRM) approaches, creating river basin organizations and councils to oversee local water use, conservation, and planning [3,4]. These institutions play a key role in shaping how restoration projects are designed, implemented, and monitored, particularly in the Khyargas Nuur–Zavkhan Gol river basin where Ereen Lake is located.

In the following, we use the original Mongolian words as follows: “aimag” = (political) province, “els” = sand desert, “gol” = river, “nuur” = lake, “soum” = (political) district, “uul” = mountain.

2. Study Region

Western Mongolia’s landscapes include the Altai Mountains at the border with China in the south and the Tannu Ola Mountains with Russia in the north (Figure 2). In between these lies the “Valley of the Great Lakes” with steppe vegetation and other mountain ranges, of which some are glaciated.

The glacier-covered high mountains of western Mongolia are significant “water towers” that are of vital importance for ecosystems and economic activities. Glaciers in the Mongolian Altai covered an area of 372 km² in 2011 and 334 km² in 2016 [5,6]. The only glacier outside the Altai is at Otgon Tenger Uul in the western Khangai Mountains. Its meltwater drains into the study area, the Khyargas Nuur–Zavkhan Gol Basin, via the upper reaches of the Zavkhan Gol and contributes a small part of the runoff. In contrast, the neighboring Khar Us Nuur-Khovd Gol Basin is connected to the highest glaciers of the Altai, and meltwater runoff is collected in the Khovd Gol. The drainage system also includes Zavkhan Gol and Tes Gol as well as smaller endorheic streams.

With a total area of 122,315 km², the Khyargas Nuur–Zavkhan Gol Basin is one of the largest river basins in Mongolia, and Zavkhan Gol is one of its largest rivers with a discharge of 20–100 m³/s. The river flows through several “soums” including Taishir with its HPP; after crossing Airag Nuur, it eventually enters Khyargas Nuur. With a share of 45%, livestock was the largest source of income in the Khyargas Nuur–Zavkhan Gol Basin in 2018 [7]. Crop and manufacturing production are comparable to the national average, while the mining sector is almost completely absent.

3. Hydropower

Strengthening economic development and urban infrastructure in western Mongolia requires increased water utilization. Hydropower plants provide water resources as water supply for the population, irrigation of agricultural land, pasture farming, and mining. In 2013, the FAO identified six challenges for Mongolia’s integrated water resources management (IWRM) plans [8]: (1) improving water conditions in and around urban centers; (2) mining, water supply, and safeguarding environmental impacts; (3) supply of water for herders; (4) developing the national hydropower potential; (5) institutional framework for water management; and (6) policy development for water management. Of these, water supply for pasture farming and the expansion of hydropower to improve power supply in the western aimags should be emphasized in the Khyargas Nuur–Zavkhan Gol Basin.

Two larger hydropower plants (HPPs), Durgun and Taishir, exist in western Mongolia, and a third one, Erdeneburen, is under construction and expected to have a capacity of 90 MW, while the construction of a fourth one in the Tavaltain Khavtsal (Tavaltain Canyon) of the Khovd Gol has been postponed indefinitely. The Durgun HPP sits on the Chono Kharaih Gol within the Khar Us Nuur–Khovd Gol Basin and was completed in 2008, supplying 11 MW. The Taishir HPP was completed in 2007 and generates 10 MW of energy and supplies drinking water for the region. It had a significant impact on the downstream Zavkhan Gol ecology, particularly in the first ten years after it was commissioned in 2008. Our study area was and still is directly affected by these impacts.

Other smaller HPPs are connected to the grid for local power supply (e.g., Munkhkhairkhan since 2003, generating 150 kW; Zavkhan Mandal at Galuutai Gol since 2009, generating 110 kW) [9]. The increasing energy demand in the western “aimags” is still dependent on imports, primarily from Russia and less so from China, accounting for 76.5% in 2016. Some negative environmental impacts from these smaller HPPs were reported [10]. For example, Durgun Dam raised the water level of the nearby Khar Us Nuur, and Taishir Dam reduced the runoff in the downstream part of the Zavkhan Gol, which then negatively affected fish populations as well as pastureland and hay fields in the floodplain. For example, the controversial Khovd Dam project is expected to destroy valuable spawning habitats for salmonids [11,12].

4. Climate and Lake Level Oscillations

During the past seventy years, Mongolia experienced a 2.13 °C increase in mean annual air temperature (MAAT) and large variations in annual precipitation that has tripled since 2021. Precipitation patterns changed, and extreme precipitation events and droughts are more frequent. These changes indicate the onset of intense withering and drying processes [1], putting water resources at risk of severe depletion [2].

After years of a negative water balance, precipitation increased in both western and central Mongolia [13]. While mean annual precipitation (MAP) fluctuated around a relatively high stable level in Uliastai (1753 m a.s.l.) at the western edge of the Khangai Mountains between 2009 and 2022 (Figure 3), it significantly increased in Khovd (1395 m a.s.l.) east of the Altai Mountains (Figure 4). The mean annual air temperature (MAAT) increased by more than 1 °C in both Uliastai and Khovd.

A Mongolian water inventory found that 17% of its rivers and 32% of its lakes had dried up between 1990 and 2000 [14]. In the lower semi-arid elevations of the steppe in Mongolia, 68 lakes larger than 1 km² disappeared between 1990 and 2010 [15]. In contrast, glacier-fed lakes in the Altai experienced only slight water level variations between 1995 and 2015, remaining a consistent and reliable water resource for local herders [16]. However, these higher-elevation lakes probably act increasingly less as buffers, leading to lake level drops and even total drying-up in lower elevations.

While it is generally agreed that fluctuations of lake levels in western Mongolia are mainly influenced by precipitation, air temperature, and evaporation, they can be controlled by geomorphologic conditions such as natural damming through dune stripping that affects water inflow and outflow or HPP-related damming.

For example, Airag Nuur is the terminal lake of the Khyargas Nuur–Zavkhan Gol Basin and experienced significant lake level fluctuations during the last almost four decades (Figure 5). A period of lower precipitation resulted in a low lake level before 1994. The lake level then rose to a maximum in 2000, after which it fell to a very low level by 2015 and eventually a minimum by 2024. Interestingly, the period of a higher lake level between 1994 and 2005 coincides with higher temperatures and lower precipitation, which, one might argue, should have decreased water influx and increased evaporation. This pattern might reflect on a lag time between changes in climatologic conditions and lake level feedback. However, further investigations are needed to explain the lake level fluctuations. Without doubt, the abrupt drop in lake level after 2008 is a result of the commissioning of Taishir Dam. The lake level fell to a historic low by 2024 despite a relatively wet period from 2018 to 2021.

5. Ereen Nuur

Ereen Nuur (1459 m a.s.l.) is in the middle reach of the Zavkhan Gol, about 260 km upstream from Airag Nuur and about 110 km downstream from the Taishir Dam (Figure 6). The lake is 3.8 km long, 1.2 km wide, 2.1 m deep on average, and has a surface area of 4 km². It is surrounded by dunes at the northeastern edge of the Gobi Desert. The lake is one of the ten natural monuments of the Gobi-Altai Province and an important tourist attraction [14]. It is also a vital habitat for migratory birds in the region and included in the list of “Important Areas for Birds” by the international organization Birdlife [1].

After the commissioning of the Taishir HPP in 2008, the Ereen Nuur almost disappeared (Figure 7). The lake expanded by 2020, but then abruptly lost almost two-thirds of its area by 2024. This evolution occurred despite slightly increasing MAP and MAAT at Otgon Soum in the upper reaches of Zavkhan Gol between 1981 and 2024 and is a result of water flow regulations at the Taishir HPP. The reservoir reached its maximum by 2016, after which it was kept at around the same level. Before the construction of the Taishir HPP, the average annual flow of Zavkhan Gol at the site was 8.6–8.9 m³/s, but it dropped to 5 m³/s after the commission with an upstream flow of 4.1–5.44 m³/s and a downstream flow of 1.2–1.5 m³/s [1].

5.1. Water, Sediment, and Diatoms Analyses

In July 2022, samples were collected at twelve sites for water analyses, thirty-one sites for sediment analyses, and three sites for hydrobiological analyses (Figure 8). At three sites, water samples were taken at different depths. Water samples were analyzed on site for pH, redox potential, dissolved oxygen, electrical conductivity, total dissolved solids, alkalinity, and water temperature. They were then analyzed for anions and cations as well as heavy metals using chromatography and spectrometry methods and assessed using the Mongolian National Standard 4586: 1998 Water Environment Quality Indicator Standard [18].

Sediments were analyzed with a digital burette [19], and solid components in solution were measured with a hydrometer according to ISO 11277:2020 [20]. Elemental composition of the sediment samples was determined using inductively coupled plasma mass spectrometry (ICP-MS), a highly sensitive technique for detecting trace elements. Results were evaluated according to the Canadian Biological Effects Database for Sediments (BEDS) standard [21].

The Canadian Council of Ministers of the Environment Water Quality Index (CCME WQI) [22] was calculated based on seven parameters including water temperature, pH, total dissolved solids (TDS), dissolved oxygen (DO), oxidation-reduction potential (ORP), electric conductivity (EC), and turbidity.

We analyzed three sediment samples from three distinct sites for diatom assemblages, with the primary objective of supporting water quality assessment through these well-established bioindicators of ecological conditions. The samples were collected using standard periphyton sampling techniques, targeting submerged substrates to ensure consistency and comparability across locations. Diatom samples were prepared according to the standard methods by placing 0.2 g of the sample in a 200 mL beaker, adding three drops of 10% HCI and 25 mL of 33% H₂O₂, boiling in a fume hood on a heater for two hours to decompose the organic fraction, and mounting in Naphrax (R.I. = 1.7) [23]. Diatom identification followed applicable taxonomic standards [24,25,26]. To evaluate community structure and ecological integrity, diversity and evenness metrics were calculated, including the Shannon diversity index (H′) and Pielou′s evenness (J′). The resulting assemblage patterns provide valuable insights into nutrient dynamics and potential anthropogenic influences on the lake’s ecological status.

5.2. Water Properties

During the field campaign in July, the surface water temperature had a relatively high mean of 21.3 °C. The concentration of dissolved oxygen (DO) was 3.2–9.7 mg/L, with a mean of 7.7 mg/L and a lowest value of 3.2 mg/L near the lake’s outlet, suggesting oxygen deficiency in some parts of the lake, which can negatively impact aquatic life (

Table 1. Water properties for Ereen Nuur. (Temp.: temperature, TDS: total dissolved solids, DO: dissolved oxygen, ORP: oxidation-reduction potential, EC: electric conductivity, Turbid.: turbidity, Hard.: hardness).

Site ID	Sample ID	Temp.	pH	TDS	DO	ORP	EC	Turbid.	Hard.	Ca2+	Mg2+	NH4+	Na+	K+	Cl−	SO42−	Alkalinity (CO3− + HCO3−)
		[°C]		[mg/L]	[mg/L]	[mV]	[S/m]	[NTU]	[mEq/L]	[mg/L]
1	3	20.1	8.5	0.5		135	0.8	0.9	4.6	36.2	33.3	1.9	89.7	7.5	491	272	222
2	4	20.0	8.4	0.5	8.2	140	0.8	0.7	4.9	26.0	42.7	1.8			76.2	215	231
3	9	20.4	8.4	0.5	8.6	167	0.8	0.8	4.7	37.5	33.9	2.0	90.4	7.1	93.2	269	219
4	9 (1)		8.2	0.5		139	0.8	0.5
5	9 (1.5)		8.1	0.5		148	0.8	0.5
6	9 (2)		8.1	0.5		147	0.8	0.6
7	9 (3)		8.1	0.5		136	0.8	1.2
8	10	21.9	8.4	0.5	9.7	165	0.8	1.1
9	13	21.1	8.4	0.5	9.5	170	0.8	1.2	4.4	35.1	31.6	2.0	4	13	4.4	35.1	31.6
10	13 (1.5)		8.1	0.5		159	0.8	1.0
11	13 (3)		8.1	0.5		148	0.8	1.1	4.7	38.7	33.0	2.4	90.8	7.7	99.4	269	241
12	F13	23.4	8.1	0.6	6.8	147	0.9	2.9
13	16	21.3	8.1	0.5	7.7	180	0.8	1.1	4.6	37.2	33.4	2.1	92.4	7.7	101	277	224
14	20		8.1	0.6		163	0.9	1.1
15	20 (1)		8.1	0.5		162	0.8	3.6
16	20 (1.5)		8.1	0.5		169	0.8	3.5
17	22	21.8	8.1	0.5	7.1	193	0.8	0.9	17.4	145	120	2.8			91.9	260	236
18	23	23.6	8.2	0.8	9.0	89	1.2	1.4	9.4	41.7	87.5	3.6			198	545	181
19	25	20.4	8.2	0.5	3.2	182	0.8	1.2	15.6	31.3	168	3.2			116	305	187
20	29		8.3	0.7		51	1.0	2.9	3.8	16.6	35.6	2.3	133	10.2	144	368	155
21	33 (outlet)	21.6	8.4	0.2	6.9	126	0.3	13.1	2.0	27.8	6.9	2.0	10.8	1.6	9.2	37.2	92.6
22									4.3	19.4	40.2	2.6	113	9.7	106	333	157
23									7.3	41.7	62.5	2.1			116	406	145
24	38 (inlet)	20.1	8.4	0.1	8.1	189	0.3	21.1	2.1	28.9	7.6	1.4	12.2	1.6	7.9	27.8	104
25	T1 (Zavkhan Gol)		6.3	0.2		248	0.2	1.5	1.8	26.7	5.5	1.5	8.3	1.8	6.3	23.0	103

). The highest oxidation-reduction potential (ORP) of the fifteen sites downstream of Taishir HPP was 248 mV, with a mean of 148 mV. The pH was 6.2 in the same downstream stretch and 8.4 at the lake’s inlet. When photosynthesis occurs during summer, decomposition of carbon dioxide in the lake accelerates, resulting in a rise in pH levels [27] that can harm aquatic life. At both inlet and outlet locations, the electrical conductivity (EC) was 0.2 mS/cm, while it was 0.8–1.2 mS/cm in the center, pointing at a limited turnover and high evaporation rates [27]. Water turbidity was 0.7–21.1 NTU (Nephelometric Turbidity Unit) with a mean of 1.35 NTU in the lake’s center and other locations far from the shore. Downstream of the HPP, turbidity was 1.5 NTU, while the highest values were found at the inlet (21.1 NTU) and outlet (13.1 NTU), suggesting pollution with suspended particles that could originate in soil erosion or runoff from agricultural fields, which again can adversely affect aquatic life [28].

The lake water exhibited an anion dominance in the order of SO₄²⁻ > HCO₃⁻ > Cl⁻ and a cation dominance of Na⁺ > K⁺ > Mg²⁺ > Ca²⁺ > NH₄⁺ (Figure 9). This ionic composition was consistent at both the inlet and outlet sampling sites and has remained largely unchanged since 2013 [29]. The average ammonium content of the lake water was 2.3 mg/L, which is six times higher than the tolerance allowed by the Mongolian standard. High levels of ammonium are harmful to fish and aquatic invertebrates and cause an imbalance in aquatic life [30]. The average sulfate content was 260 mg/L, which is 2.6 times higher than the tolerance. At some sites, the DO concentration was below the tolerance, while nitrite, chlorine, and fluorine were above it.

5.3. Lake Sediments

The texture of a sediment helps to identify its source [31]. In our samples, the share of clay varied between 4.1% and 39.7%, typical for lake sediments, while the surrounding dunes are the origin of the relatively high sand content (

Table 2. Content of clay fraction and organic matter in Ereen Nuur’s sediments.

Sample ID	Clay Fraction [%]	Organic Matter [mg C/kg]
2	22.0	4.2
3	20.5	2.1
6	28.4	3.0
10	7.3	0.7
11	36.4	3.4
13	30.1	3.8
14	31.2	2.5
16	21.5	5.1
17	24.0	3.8
18	39.7	2.6
19	8.2	0.0
21	36.7	2.9
23	9.5	0.3
24	27.1	1.3
25	30.7	2.6
26	30.8	1.4
27	20.2	0.1
28	20.4	0.2
29	16.6	0.8
F1	4.1	0.2
F2	8.7	0.8
F5	6.1	0.1
F6	12.9	1.7
F7	20.0	2.1
F8	19.3	0.7
F9	15.8	0.5
F10	13.2	0.0
F11	13.0	0.1
F12	11.1	0.7
F13	6.9	0.5
F14	9.2	0.2

). Organic matter concentrations were higher and ranged from 2.1 to 5.1 mg C/kg with a mean of 1.6 mg C/kg in the northern, western, and central parts of the lake (Figure 10). Such increased levels signal an overgrowth of aquatic plants that can eventually result in a drying-up of the lake. Low contents were observed in the east, where both inlet and outlet are located, and south. Inaccessible sand dunes border the lake except in the northeast.

Sediment composition, revealed by element analysis, provides crucial insights into environmental processes. Copper concentrations with a mean of 59.9 mg/kg were above the Canadian standard tolerance at all sampling sites [32] (

Table 3. Metal concentrations in Ereen Nuur’s sediments. (* Thresholds after [32,33,34,35,36,37], PEL: permissible exposure limit.)

Element	Threshold *	PEL	F9 (0–2)	F9 (20–22)	F13	13	F3 (0–2)	F3 (20–22)
Arsenic (As)	5.9	17.0	34.0	21.0	15.0	25.0	26.0	8.0
Copper (Cu)	35.7	197	38.8	55.3	87.2	49.1	70.7	58.3
Chromium (CR)	37.3	90	78.0	59.0	54.0	43.0	80.0	52.0
Zinc (Zn)	123	315	35.0	36.0	66.0	36.0	88.0	53.0
Cadmium (Cd)	0.6	3.5	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0
Lead (Pb)	35	91.3	12.0	12.0	19.0	13.0	23.0	19.0

). At almost all sites, arsenic concentrations with a mean of 21.5 mg/kg were above the threshold for having adverse effects on aquatic invertebrate species, aquatic life cycles, and organisms’ behavior [33]. Chromium concentrations with a mean of 61.0 mg/kg were above the tolerance at all sites, although they did not exceed the threshold for having adverse effects on organisms [34]. The content of zinc, cadmium, and lead did not exceed the standard tolerances [35,36,37]. While organic carbon and nitrogen, nutrients that promote aquatic plant growth, were found, phosphorus was below the detection limit of 0.02 μg/L, which points at natural chemical and biological processes such as algae decomposition within the lake sediments rather than a discharge into the lake. The produced phosphorus is then released back into the water, creating conditions favorable for algae growth [38]. Any additional external phosphorus is probably of fluvial and aerial origin.

5.4. Diatoms

Diatoms are widely recognized as reliable bioindicators in freshwater ecosystems due to their rapid response to environmental changes, well-defined ecological preferences, and cosmopolitan distribution [39,40]. Their taxonomic composition, diversity, and abundance are strongly influenced by key limnological variables, such as nutrient concentrations, substrate type, hydrological regime, and anthropogenic disturbance [41,42]. Consequently, diatom-based assessments have become fundamental tools for evaluating water quality and ecological integrity in both lotic and lentic systems [43,44].

The diatom community in the three sediment samples comprised twenty-two species across nineteen genera, yielding a relatively high genus-to-species ratio of 0.86 (

Table 4. Occurrence of diatom species in three sediment samples from Ereen Nuur.

Diatom Species	Number of Individuals
	Sample 1	Sample 2	Sample 3
Achnanthes minutissima var. minutissima	46	12	32
Amphora fogediana	0	0	3
Amphora pediculus	6	3	0
Cocconeis pediculus	0	16	0
Cocconeis placentula var. euglypta	0	14	0
Cocconeis placentula var. lineata	157	30	5
Cocconeis placentula var. placentula	59	0	0
Cyclotella radiosa	0	16	0
Diatom tenuis	0	2	0
Encyonema minutum	9	0	0
Epithemia adnata	10	94	55
Epithemia sorex	19	68	175
Fragilaria tenera	31	22	0
Fragilaria capucina var. mesolepta	16	0	0
Gomphonema acuminatum	0	5	0
Gyrosigma sciotense	1	0	0
Navicula radiosa	16	5	8
Navicula reinhardtii	0	2	2
Rhopalodia gibba	5	0	13
Staurosira contruens	3	3	0
Staurosirella pinnata	3	5	0
Synedra ulna	4	0	0

). A lower ratio (typically around 0.6–0.7) is often indicative of greater specialization and a more complex habitat structure [45]. In contrast, higher values may suggest lower overall diversity, with a few dominant species possibly reflecting environmental stress, recent disturbance, or an early successional stage [41,46,47].

Diversity and evenness metrics further indicate that diatom community of Ereen Nuur is dominated by a limited number of taxa (

Table 5. Community diversity and evenness of diatoms in Ereen Nuur.

Indexes	Sample 1	Sample 2	Sample 3
Shannon diversity index (H′)	1.965246	2.005607	1.367419
Pielou’s evenness (J′)	0.725705	0.740609	0.65759

). While Samples 1 and 2 exhibit moderate diversity and relatively balanced evenness, Sample 3 is characterized by dominance of fewer, less evenly distributed species. A reduction in Shannon diversity (<2.0) and relatively low evenness suggest a community structure dominated by pollution-tolerant taxa. This pattern likely reflects the effects of environmental pollution, nutrient enrichment, or stress [41,43,48,49].

The overall diatom community, based on all three samples, was characterized by the co-dominance of taxa with relative abundances exceeding 10% in at least two samples. These included Achnanthes minutissima (11% and 12%), Cocconeis placentula var. lineata (10% and 41%), Epithemia adnata (19% and 32%), and Epithemia sorex (23% and 60%) (Figure 11). The relatively high abundance of Achnanthes minutissima, a small, adnate taxon tolerant of a wide range of environmental conditions, often reflects moderate stress or disturbance, such as physical instability or organic enrichment [50]. Its presence suggests early to mid-successional stages on hard or macrophyte-associated substrates [42]. Cocconeis placentula var. lineata, typically found in epiphytic habitats [43], is indicative of moderate nutrient levels [51]. The presence and abundance of Epithemia species, particularly Epithemia sorex at 60% in Sample 3, further suggest elevated phosphorus availability combined with potential nitrogen limitation, as these taxa harbor nitrogen-fixing endosymbionts and thrive under such conditions [45,51].

Collectively, this assemblage points to a moderately eutrophic aquatic environment with stable, likely vegetated substrates, possibly experiencing mild nutrient imbalance and moderate anthropogenic pressure. In particular, the dominance of Epithemia species—especially Epithemia sorex at 60% in Sample 3—and the significant presence of Cocconeis placentula var. lineata strongly suggest a nutrient-rich, possibly phosphorus-enriched, eutrophic setting.

The species composition reflects adaptation to either alkaline or acidic water conditions, fresh or slightly salty water, high nutrient levels, and moderate oxygen supply. This indicates that the lake has been supplied with considerable amounts of organic matter (manure) from farm animals and/or soil nutrients released after surface disturbance caused by trampling.

5.5. Water Quality Index

The water quality of lakes is subject to seasonal variations resulting from changes in temperature, biological activity, precipitation, and inflows. Therefore, the WQI presented here may not fully reflect these seasonal fluctuations or account for any short-term or long-term trends. The calculated WQI revealed that Ereen Nuur was “polluted”, harming the aquatic ecosystem and making it unsuitable for most human uses without substantial pollution control or treatment. The water poses risks to human health for those relying on it for agricultural, domestic, or recreational purposes. Additionally, the aquatic ecosystem is under significant stress, with the high nutrient levels, low dissolved oxygen, and elevated turbidity all contributing to the deterioration of habitat quality for aquatic life. In comparison to other lakes and rivers in western Mongolia, Ereen Nuur’s water pollution must be called severe.

5.6. Lake Rehabilitation

Lake restoration methods can be divided into two categories: preventive or indirect, and ameliorative or direct [52]. Based on water balance estimates using inflow and outflow as well as precipitation and evaporation data, two options for the lake’s rehabilitation were proposed [53].

Option 1 includes an initiated complete drying-up of the lake that then allows for the mechanical removal of polluted lake sediments and plants. Concerns about this treatment exist as it involves a destruction of the lake’s flora and fauna. Afterwards, the lake basin is flooded again. Furthermore, the contaminated sediments need to be disposed in a safe landfill. The entire measure is expected to take a period of 317 days including 177 days for the drying-up and 140 days for the refilling of the lake.

As Ereen Lake continues to dry up, the exposure of lakebed sediments poses serious environmental and public health risks, particularly if these sediments are dispersed by wind. Uncontrolled dust emissions from desiccated lakebeds have been well-documented in other cases, most notably at the Aral Sea and Owens Lake, where large-scale drying has led to chronic dust storms [54]. Such events have been linked to increased rates of asthma, cardiovascular disease, and other respiratory disorders in local populations, in addition to broader ecological degradation. One of the most practical and effective short-term strategies for dust suppression is the maintenance of a shallow water layer over exposed sediment surfaces [55]. A thin film of water—typically ranging from 5 to 15 cm—acts as a physical barrier by increasing the weight and cohesion of surface particles, thereby preventing their entrainment by wind. This method has the advantages of being low-tech, cost-effective, and rapidly deployable, making it particularly well-suited for the early stages of restoration at Ereen Lake.

Effective management of sediments contaminated with heavy metals and excess nutrients requires a comprehensive, environmentally responsible approach to prevent secondary pollution and protect surrounding ecosystems [56]. In this context, mechanical dredging is identified as the preferred method for sediment removal due to its precision and suitability for shallow water bodies. Best practices recommend hydraulic dredging, which limits sediment disturbance, combined with silt curtains [57] or barriers to control turbidity and pollutant spread. Once dredged, sediments contaminated with heavy metals should undergo stabilization or solidification using materials such as cement, fly ash, or other binders to immobilize contaminants [58,59]. Alternatively, sediments may be disposed of in waste landfills near Jargalant Soum following national and international regulatory standards for hazardous material handling. Continuous monitoring, including leachate testing and site inspections, is essential to ensure long-term containment and protection of surrounding environments.

To enhance the environmental safety and biogeochemical stability of dredged sediment prior to final disposal, the incorporation of organic carbon-rich materials—such as sawdust or biochar—is recommended [60]. These amendments offer multiple benefits: they stimulate microbial activity, support the formation of stable organic complexes, and, most importantly, reduce the mobility and bioavailability of nutrients. Additionally, the application of locally available mineral amendments, such as lime and zeolites, is advised. These materials have demonstrated effectiveness in various sediment remediation studies due to their capacity to immobilize heavy metals through adsorption, ion exchange, and pH buffering mechanisms [61]. The combined use of organic and mineral amendments not only promotes in situ stabilization of contaminants but also supports low-cost, locally appropriate, and sustainable remediation strategies.

Option 2 is a just dilution-flushing that includes only minor construction measures to divert a share of the Zavkhan Gol water to Ereen Nuur, while direct interventions in the lake and its sediments are avoided (Figure 12). An approximately 1 km-long spillway will connect the two original riverbeds with Ereen Nuur and Ajig Nuur, and flow diversion dams and weir crests will eventually control the water diversion. Four entrance jetties serve to stabilize inflow points and prevent erosion and sediment clogging. As a result, the lake’s water level is projected to increase by 2 m, effectively doubling its surface area, which is critical for restoring habitat connectivity, improving water quality, and supporting the lake’s biodiversity. The implementation of the measure is expected to take 373 days.

Among the regulatory interventions that could enhance the effectiveness of lake restoration efforts, the artificial creation of a flood hydrograph emerges as a particularly promising strategy. This approach involves the controlled release of water to mimic natural flood conditions, thereby increasing infiltration in downstream areas and supporting the re-establishment of hydrological connectivity. When strategically implemented, managed flooding can contribute to groundwater recharge, lake restoration, and sediment redistribution—each of which is critical for the long-term sustainability of lake systems.

In conjunction with artificial flooding, the construction of a dedicated channel to facilitate direct water transfer from the river to Ereen Lake during high-flow periods is recommended. Such infrastructure would enable more consistent water delivery, reduce evaporation losses, and enhance ecological resilience during dry seasons. This study concludes that dilution-flushing (Option 2) represents the most effective pathway for rehabilitating Ereen Lake under current environmental conditions.

However, it is also recommended that the managed flood hydrograph approach be incorporated into the broader suite of restoration strategies. While it may not serve as a standalone solution, it offers significant complementary benefits and warrants further investigation through hydrological modeling, pilot testing, and comprehensive impact assessment in future research.

6. Conclusions

This study assessed the current environmental conditions of Ereen Nuur in western Mongolia and explored potential rehabilitation methods to improve water quality and ecological health. Since the commissioning of the Taishir HPP in 2008, the lake’s water quality declined because of pollution, making the aquatic environment unhealthy for living organisms and the water unsuitable for human purposes. Nutrient pollution, high levels of ammonium, turbidity, oxygen deficiency, and internal phosphorus accumulation in the sediments are critical issues that require immediate attention. Furthermore, the drastic regulation of the water flow in the Zavkhan Gol downstream from the HPP bears the risk of a drying-up of the lake, which could result in increased salination of surrounding soils and sediments, contributing to desertification. These developments not only threaten the lake ecosystem but also pose significant risks to communities that rely on the lake for water, agriculture, and livelihoods.

The dilution-flushing technique has emerged as the most efficient, eco-friendly, and cost-effective rehabilitation approach for Ereen Nuur. Its ability to reduce external nutrient loads makes it the most promising solution for reversing the eutrophication process and avoid algal blooms. Given Ereen Nuur’s roles as an important habitat for migratory birds and a cultural landmark, its rehabilitation is crucial for maintaining biodiversity and socio-ecological balance in the region. Future efforts must ensure that monitoring and management strategies are in place for long-term success.