Selenge watershed assessment: Sediment loading and concentration
Two broad-scale assessments were performed during high-and low-flow conditions. During the 1st assessment, river levels were high for western rivers. The Eg River (E) was at flood stage, and the Selenge River (S1) and the upper Orkhon River (O2) carried high loads of TSS (Table 3). The northern taiga rivers, Yeroo and Kharaa, remained low. During the 2nd assessment, rainstorms in the NT region raised river levels, offering the opportunity to assess conditions under higher flow levels (Table 3). In the NT region, TSS loads doubled between the 1st and 2nd assessments for the Tuul (T1), Kharaa (K1), and Orkhon (O1) rivers. The total daily mass of TSS moved by the Yeroo River (Y1) increased 9-fold during this time, increasing from 16 tons per day (t·d−1).
Within the context of large shifts in loading rates as a result of regional storm systems, a few patterns emerged. We found that the Sharyn and Kharaa rivers appear to be small contributors to the overall sediment budget of the NT region. Of the rivers draining from the NT and CS regions into the Orkhon River, the Tuul River (T1) consistently had the highest TSS concentrations and carried the largest suspended sediment loads. On the 2nd sampling date, the Tuul River was transporting larger sediment loads than the upper Orkhon River (O2), despite having only approximately half the discharge. One exception to the predominance of the Tuul River loading was observed for the Yeroo River drainage (also an active mining region) during the latter half of August. The Yeroo River (Y1) doubled in volume as a result of rainstorms before the 2nd sampling period, resulting in a 9-fold increase in TSS load. A comparable increase in volume was not observed for the Tuul River during the period of the study. TSS loading from the main stem of the Selenge River (S1) was 29 times greater than the TSS loading from the Orkhon River (O1) during the 1st assessment.
The size of the Selenge River in relation to its tributaries (540 m3·s−1, or ˜5–7 times the discharge of the Orkhon River) shows its importance to overall sediment loading. These results, however, do not indicate that the W region has worse water quality or watershed conditions then the NT and CS regions. A table of typical flow volumes for August provided by the Mongolian Institute for Meteorology and Hydrology (excerpted in Table 1) indicates the Selenge River had normal discharge for this time of year; however, the NT and CS rivers were only flowing at 20 to 30% of their August means. If the Tuul, Orkhon, and Yeroo rivers had been flowing at higher levels, it is likely that the TSS load would have been much higher because higher flows mobilize instream stores of sediment (Knighton 1998). For example, the Yeroo River experienced a 9-fold increase in TSS load with a doubling in discharge between the 1st and 2nd assessments (Table 3). Furthermore, the ratio of the Selenge River TSS load to that of the Orkhon River dropped from 29 to 3 between those periods.
The water quality of the W region appears to be excellent. Sampling conducted at Hualgant, upstream on the Selenge River (S2), indicated exceptionally low turbidity and TSS (Table 3). The TSS load at 52 was 33% of the downstream load (S1) measured 1 week prior. Further evidence of the water quality of the upstream portions of the western region comes from the single assessment of the Eg River. At the confluence with the Selenge River (E), the Eg River is roughly the same size with a mean August discharge of 256 m3·s−1, as compared to 292 m3·s−1 for the Selenge (Table 1). Discharge was not measured during the 10 August sampling because the river was at flood stage (flowing over its banks) and there was no bridge. For this reason, loads could not be calculated; however, the turbidity and TSS were quite low at 24 nephelometric turbidity units (ntu) and 27 mg·L−1 respectively, especially considering the river was at flood stage. The Eg River drains out of Lake Huvsgol, a subalpine lake with exceptional water clarity (Goulden et al. 2000) and provides a strong indication of low TSS loading in the west-central region. For the NT region, the regional assessment showed high TSS loading for the Tuul River and the Yeroo River, particularly in the 2nd half of the month.
An evaluation of the accuracy of the loading measurements can be performed by summing TSS loads from upstream stations and observing whether this load is measured at downstream stations. For the 1st assessment, the loading sum of 222 t·d−1 for the Yeroo (Y1), Kharaa (K), Tuul (T1), and Orkhon (O2), corresponds closely with the 236 t·d−1 measured for the terminus of the Orkhon (O1). Similarly, for the 2nd assessment, river loadings sum to 316 t·d−1 can be compared with 401 t·d−1 for the terminus.
A 44-year record of sediment loading from the Russian reaches of the Selenge at Mostovoy gives an average daily load of 5,545 t·d−1 (Korytny et al. 2003). This is comparable to the 1,211 to 6,800 t·d−1 reported here.
Selenge watershed assessment: Nutrient loading and concentrations
Total acid-hydrolyzable phosphorus was determined for 2 locations in the 1st assessment and 8 locations in the 2nd assessment (Table 3). The rise in TP levels from the 1st to the 2nd sampling trip matches the rise in sediment concentrations for this time period. TP is usually found adsorbed to particulate matter, so TP concentrations increase with increased sediment concentrations (Wetzel, 1983). In the Selenge watershed overview provided by the late August sampling, the Tuul River had TP concentrations that were 200 to 300% of concentrations found on the other sites. Because floodplain soils of the Selenge basin have been found to be high in TP (Ubuganov et al. 1998), it is likely the elevated levels of TP observed in this study are a result of floodplain disturbance caused by mining operations. Other sources of TP could be municipal sewage inputs from Ulaanbaatar, 500 km upstream. The Sharyn River was unexpectedly high (128 nl/L) given its location between 2 similar rivers with lower levels. The Sharyn River had a higher temperature (21°C) and much lower flow rates than the Yeroo (15°C) and Kharaa rivers (18°C, Table 3), suggesting that the high concentrations are an effect of reduced flushing in slow-moving stagnant water. Additionally, high densities of livestock were observed in and around the channel.
Dissolved oxygen and salinity showed fewer differences between regions and between sampling dates than was seen for sediment and turbidity. Dissolved oxygen levels ranged between 6 and 10 μl/L. Trends followed typical patterns of being higher in headwaters (Berner and Berner 1987). The highest oxygen level (8–10 μl/L) was recorded for the Eg River, followed by the Kharaa River (8 μl/L) and the headwaters of the Yeroo River (Y4), above mining or human habitation (8 μl/L). The lowest levels were recorded for the Tuul (T1) and Orkhon rivers (O2) during the 1st sampling trip (6 and 5–6 μl/L, respectively).
Salinity was in the range of 21 to 171 mg·L−1. The lowest values were found in the headwaters of the Yeroo River (Y4), and the highest values were observed for the Tuul River (140 mg·L−1), Kharaa River (148 mg·L−1), and the Sharyn River (171 mg·L−1). Salinity did not show a correlation with TSS loading. For example, Yeroo River (Y1) sediment loading increased substantially over the month (16–140 t·d−1), but salinity fell slightly (54–43 mg·L−1). Salinity can act as a measure of groundwater interactions with surfacewater, the residence time of water in the stream, and weathering conditions in the watershed (Wetzel 1983). Our results suggest that the high mountain areas of the Yeroo headwaters have less weathering and less groundwater influence than lowlying floodplain reaches such as the Tuul and Orkhon rivers.
The Selenge watershed assessment conducted during 2 periods in 2001 shows that the rivers of west-central Mongolia (W) are generally lower in TSS loading and TP concentrations than the central regions (NT and CS). Higher central-region loading may be the result of increased land disturbance as a result of the placer mining in the area. As a result, we focused on finer-scale impacts of mining in 2 focus areas in eastern Mongolia (Bugant and Yalbag rivers) where active mining occurs, to determine the direct impacts of mining on the water quality of Mongolia's large and small rivers.
Taiga Focus Area
Elevated turbidity, TSS, and TP concentrations in tributary drainages undergoing active mining were found in both 2001 and 2003. In contrast, sites above mining activities showed very low values for these water quality parameters, indicating pristine conditions (Figures 3 and 4).
Site Y4 was located above all mining activities (Figure 3). During the 2001 expedition, TP and TSS were not measured; however, turbidity was extremely low. Because TSS and TP are strongly correlated with turbidity (Kronvang et al. 1997), it is likely these values were also extremely low at Y4. Samples taken from the Yeroo River just above the Yalbag River (Y3) indicate very low TP and TSS concentrations. Similarly, in 2003, very low concentrations of turbidity, TSS, and TP were recorded at Y3 and Y4.
Figure Figure 4.. Northeast Taiga Focus Area: water quality assessment. Dark bars for sampling 16–18 August 2001, Light bars for sampling 20–24 September 2003. Discharge for Yeroo River just above Yalbag Tributary (Y3) estimated from site Yeroo River in Khan Khentii Ecological Reserve, above mining activities (Y4). Phosphorus = total acid hydrolyzable phosphorus; SS = total suspended sediment concentration; Q = discharge.
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In contrast, tributaries with mining activities showed high turbidities, TP, and TSS. In 2001, the Yalbag tributary (YA) had 15 times higher turbidity and 7 times higher TSS than the Yeroo River at their confluence (Y3, Figure 4). In 2003, YA had 8 times higher turbidity, 5 times higher TP concentration, and 11.5 times higher TSS than the Yeroo River (Y3). On the Bugant tributary (B), the TSS concentration for 2001 (135.2 mg/L) was 13 times higher than the Yeroo River at their confluence (Y2, 11.9 mg/L). TP was 3.6 times higher. In 2003, Bugant tributary (B) was 4 times more turbid and had 5 times more TSS and 3 times more TP.
In some mined watersheds, settling ponds are constructed by damming up the mouth of tributaries with the excavated fill. In the short run, the dams reduce sediment loading to the main stem and, therefore, lower the turbidity, TSS, and TP. However, by blocking the entrance to the channel, the dams result in the loss of the stream habitat for spawning fish and other aquatic organisms. Without maintenance, these loose earthen dams will fail and release large quantities of sediment to the Yeroo River.
Steppe Focus Area
Sampling in the Zaamar region indicates a decline in the water quality of the Tuul River as it flows through the mining region (Figure 5). In 2001, minor increases in suspended sediment concentrations (from 92 to 117 mg/L) and turbidity (from 61 to 85 ntu) were observed moving downstream through the mining region and below. However, TP more than doubled in concentration (increasing from 74 to 185 nl/L). Below the mining region (T25, T35, Table2), turbidity levels remained the same, and TP and TSS concentrations dropped slightly. Dissolved oxygen increased slightly with downstream direction (from 7.4 to 8 mg/L) as did salinity (99 to 106 mg/L). However, the river discharge in 2001 was less than 23% of mean flow recorded by a long-term gauging station (Table 1). It was suspected that during higher flows, the effects of floodplain mining would be more pronounced. This was borne out by the 2002 sampling, indicating pronounced spikes in suspended sediment downstream of the dredge pits used for mining placer deposits (Figure 5). Higher flows would be expected to cause larger TSS concentrations, especially if the dredge pits were to be breached. Discharge was not measured during the 2002 monitoring.
Figure Figure 5.. Southern central Steppe Focus Area: water quality assessment. Zamaar Region of Tuul River. Unfilled triangles and filled triangles indicate total suspended sediment concentration (mg/L) in 2001 and 2002, respectively. Circles indicate total phosphorus (μg L−1) in 2001. Asterisk indicates locations of placer mines at km 133, 116, 103.
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Ulaanbaatar, the capital of Mongolia, with a population of more than 700,000, is on the banks of the Tuul River. Batima (1998) reports that the Tuul River was the most polluted in Mongolia. Batima found the water quality degradation to be largely biological rather than chemical—a result of waste-water discharge. Although it is likely that the city is impacting water quality in the Tuul River, the large spikes in TSS values observed immediately downstream of mining activities in Zaamar (Figure 5) point to a local impact rather than loading from Ulaanbaatar 500 km upstream.
The other major environmental impact, although not measured quantitatively, is immediately obvious to any visitor to this region. The river terrace, extending to the visible horizon, is extensively disturbed, with 30 m-high tailing piles, cavernous excavations, dredge pits full of turbid water, and the complete loss of floodplain soil horizons over many square kilometers. Because of elevated soil moisture and floodplain deposition, low-lying floodplain areas often form the most productive grazing lands. Currently, more than half the population of Mongolia is supported directly or indirectly by the pastoral economy (Fenandez-Himenez 2000). Mining companies are legally required to rehabilitate the landscape; however, there has been very little effort to do so. As of 1999, the Zaamar region had 9,000 ha of disturbed land, of which only 29 ha had any restoration measures (Dallas 1999). Although the Tuul River can be expected to flush out fine sediment particles eventually, the loss of the rich floodplain pastures may pose a long-term threat to the sustainability of human settlements in the region.