The Yellow River basin in China – Part 2
This is the second of four posts presenting my research into the ways in which climate change is impacting the environment within the Yellow River basin. Having described the geography and geology in Part 1 (yesterday), this second part looks in detail at the hydrogeology of the three distinct geographic zones within the surface water catchment.
The Tibetan Plateau
Based on observational data and extensive modelling, the IPCC (AR4 2007) has concluded that temperature changes induced by anthropogenic global warming (AGW) have already been – and will continue to be – most pronounced at higher latitudes.
Nevertheless, studies at lower latitudes in China have found evidence of AGW-induced temperature changes at high altitude; where conditions are similar to those nearer sea level at higher latitudes. However, Tibetan mountain permafrost is not as thick as that at high latitudes; and its distribution is highly dependent on slope aspect. Furthermore, irrespective of location, the presence of permafrost – unlike glaciation – is not always readily apparent because it is overlain by a seasonally thawed layer (the active layer) usually less than 3 metres thick (MATSUOKA et al. 2009: 39-40).
A variety of data collected at the Geological Environmental Monitoring Station of Qinghai Province in 2002 suggests groundwater levels are falling; these include the downward migration of spring lines and discharges within alluvial fans; the reduction in valley-bottom areas covered by moorland; the disappearance of thermal springs; and the drop of groundwater levels in densely-populated areas (cited in HAN et al. 2009: 59).
According to Mori et al., who have undertaken a detailed three-dimensional modelling of the entire basin, groundwater resources are limited in the Tibetan Plateau region because there are few sedimentary basin structures to contain them and, therefore, surface water is the main source of water for agricultural and domestic use (Mori et al. 2009: 131).
Major ion studies of the hydrochemistry of groundwater throughout the Yellow River basin have established that bicarbonate type groundwater dominates beneath the Tibetan Plateau; whereas isotope studies (of hydrogen and oxygen) indicate that, in general, most groundwater has been subject to minimal surface evaporation prior to sub-surface percolation (HAN et al. 2009: 66-7). Exceptions to this general rule are highlighted in subsequent sections of this essay. Groundwater in the Yellow River source area (i.e. the Tibetan Plateau) is calcium-bicarbonate type, except for sodium-sulphate type thermal spring water at the provincial capital of Xining (Uchida et al. 2009: 89).
The degradation and/or disintegration of permafrost leads to the deeper percolation of subsurface water. Furthermore, the fact that lake shrinkage has been observed implies that the subsequent reduction in interflow to lakes is greater than any increase in surface runoff from melting glaciers. Based on the results of a two-year intensively instrumented study, it has been concluded that, at current rates of change, the shallow Tibetan mountain permafrost (i.e. where it is currently less than 15m thick now) could thaw completely within 50 years (MATSUOKA et al. 2009: 40-2).
At high altitude, therefore, groundwater circulation is affected by the presence and/or seasonal thawing of permafrost. As such, two separate groundwater systems have been identified; unconfined groundwater in unconsolidated strata; and deeper groundwater in well-fractured bedrock (HAN et al. 2009: 75).
Water balance calculations undertaken by the Geological Survey of Qinghai Province, from 1956-67 and from 1977-99, show that there is only a positive change in water storage in years of high rainfall and low evaporation. Notwithstanding the absence of data for 1968-76, there appears to be a long–term drying trend; with only 4 out of 23 years since 1977 recording a surplus. Furthermore, droughts lasting 2 or 3 years were sufficient to cause no-flow events in 1961, 1979, and 1997 (HAN et al. 2009: 80-1).
The Loess Plateau
In the area around Yinchuan, Quaternary deposits are typically in excess of 1700 metres thick, with at least 3 separate aquifers (one unconfined and two confined) being widely recognised. Downstream of the most arid climatic area (i.e. in the Hubao Plain below Baotou), the occurrence of unconfined groundwater is more sporadic and only a single confined aquifer has been identified (HAN et al. 2009: 60-1).
In the Guanzhong basin (i.e. the Wei catchment of the northern half of Shaanxi Province, around the city of Xi’an), groundwater is relatively deep. As such, it should be less vulnerable to pollution than elsewhere, which may be just as well given that this is a relatively densely-populated area. In the Taiyuan basin (in the extreme eastern part of the deeply-incised Loess Plateau) the recharge areas are mainly limestone outcrops; with abstraction mainly occurring from Quaternary strata in the valley bottom of the River Fen tributary. Here again, however, there are two distinct groundwater bodies; unconfined and confined (HAN et al. 2009: 62-5).
Sulphate-bicarbonate waters are dominant beneath the Loess Plateau; and isotope studies indicate that evidence of evaporation, mineralisation, and/or salinisation are widespread within the shallow and/or unconfined aquifers of the Yinchuan and Hubao Plains. Furthermore, within deeper aquifers here – and/or with increasing distance from recharge areas elsewhere – hydrochemistry becomes complex; with a wide variety of groundwater types having been identified due to the large range of rock types present (HAN et al. 2009: 67-73).
However, in general, the same two groundwater types predominate here; with a clear division between shallow calcium-bicarbonate groundwater deeper sodium-sulphate groundwater (Uchida et al. 2009: 89).
Two circulation systems have been identified in the area of the Yinchuan Plain; local (shallow) and regional (deep); with typical residence times (i.e. carbon-14 ages) of less than 10 years and greater than 5000 years respectively. In the Taiyuan basin, two groundwater circulation patterns have also been identified. Whereas shallow groundwater flow is determined by topography, deeper groundwater flow and/or discharge his heavily affected by artificial pumping. Where unconfined groundwater is present, surface discharges are generally due to vertical flows induced by evaporation; causing salinisation (HAN et al. 2009: 76-8).
Data from 2000 to 2004 for the Yinchuan Plain area suggest that typically 80% of groundwater recharge is artificially induced by irrigation methods; whereas evaporation and abstraction account for 47% and 22% of groundwater losses respectively. It is believed that current annual abstraction is probably equivalent to at least 33% of the mineable resource beneath the plain. Equivalent data for the Habao Plain suggest overall abstraction is equivalent to 65% or total recharge; but with groundwater mining (i.e. unsustainable abstraction) occurring in densely-populated areas. In the Taiyuan basin, the situation is much worse; with abstraction already greater than recharge and groundwater levels continuously falling. No comparable data are available for the Guanzhong basin (HAN et al. 2009: 81-3).
The North China Plain
The water level in the Yellow River is typically 3 to 8 metres higher than the groundwater level beneath the surrounding alluvial plain, which makes the Yellow River an important source of groundwater recharge in the area; mainly as a result of large-scale irrigation schemes: As such, the zone of influence of the Yellow River extends between 13 and 26 km on the north bank; and up to 20km on the south bank. Within the surrounding alluvial deposits, groundwater is believed to circulate to a depth of 350 metres and can be found in four separate Quaternary units Q4, Q3, Q2, and Q1 (HAN et al. 2009: 65-6).
Within the lower reaches of the Yellow River, shallow bicarbonate type groundwater is mostly of good quality; with low overall mineralisation and a typical hardness of less than 450 mg/l (HAN et al. 2009: 74). In Shandong Province, many shallow groundwater samples have been found to be sodium-bicarbonate type; with some resembling the composition of sea water (Uchida et al. 2009: 89). However, deeper fossil groundwater has been found to be of meteoric origin; between 10,000 and 25,000 years old (Uchida et al. 2009: 101-2, and Tamanyu et al. 2009: 110).
Annual rainfall is typically between 600 and 700 mm, which would appear to have been equivalent to 87% of long-term groundwater recharge in the area (i.e. after evaporation) due to the unconsolidated nature of the fine clay and silty-clay soils. However, recharge direct from the river and via irrigation systems are also important (HAN et al. 2009: 79).
Water balance data for the lower reaches of the Yellow River suggest that infiltration from precipitation represents 60% of recharge, with artificially-induced infiltration and direct leakage from the Yellow River accounting for 26% and 11% respectively; whereas pumping and evaporation account for 37% and 60% of groundwater losses respectively (HAN et al. 2009: 83).
Average groundwater levels in confined Quaternary aquifers beneath the Yellow River (up to 400 m below sea level) have fallen from less than 5m below ground level in 1980, to greater than 30m in 2002. Furthermore, comparative piezometric (contour) maps for these confined aquifers beneath the North China Plain as a whole indicate level reductions of up to 80m, in the same time period, in densely populated areas such as Dezhou and Canzhou.
However, in proximity to the Yellow River, little change has been observed along much of its length (from Xingxiang down to the Provincial Capital of Jinan); whereas increased abstraction would appear to have caused a 60m drop in the area around Binzhou (Tamanyu et al. 2009: 110-1).
Tomorrow, in Part 3 of this essay, I pull all of this information together to look at the relationship between economic development and water pollution; and to look at how groundwater modelling is being used to help assess and predict problems.