The shrinking depths below

Fixing the water crisis in China's arid north begins with a proper understanding of the threats to underground supplies, says Matt Currell.

Water stress in northern China is driving some of the biggest engineering projects in human history: pipelines to transfer billions of cubic metres of water from south to north, desalination plants to supply the east coast, proposals to pump water across the country to open up coal production in the most arid regions. All of these schemes stem from the fact one of the most heavily populated, economically and agriculturally productive regions of the world is also gravely short of water.

Water may already be the key limiting factor for growth and development in northern China. Groundwater, which comprises approximately half of north China’s total water use, has in recent decades underpinned the region’s phenomenal agricultural productivity and provided potable and industrial water to tens, perhaps hundreds of millions of people. But the future is highly uncertain.

Aquifers across the board are stressed. Many areas now face essentially irreversible consequences of groundwater over-extraction: land-subsidence and, in the case of coastal aquifers, seawater intrusion. Others are seeing rising drilling costs as they send wells ever deeper to chase depleted resources, as well as groundwater quality problems caused by agricultural practices, industrial pollution and natural leaching of toxic elements.

I recently co-authored an analysis of peer-reviewed scientific literature reporting data on groundwater ages, replenishment rates, water quality and water-level declines across northern China. The study highlights major challenges for sustainable use of groundwater and raises serious questions about the future of this resource, on which the region so depends.

The review has four major findings. First, many of the heavily exploited, deep aquifers of northern China comprise palaeowaters, recharged thousands or tens of thousands of years ago, often under different climatic conditions to the present.

Second, the rates at which these aquifers are recharged are typically much smaller than extraction rates. In some cases, such as the far north-west of China, deep aquifers receive essentially no modern recharge. A major deficit between extraction and replenishment throughout many parts of northern China has resulted in major water-level declines that threaten to de-water the aquifers, trigger land subsidence and induce contamination.

Third, groundwater quality is deteriorating in many aquifers as spent irrigation water becomes the dominant source of groundwater recharge. This water is subject to high levels of evapotranspiration and mixing with agricultural chemicals. Naturally occurring fluoride and arsenic in many groundwater basins have also caused major health problems and are limiting the use of groundwater for potable supply in many areas.

Fourth, ecosystems which depend upon or interact with groundwater have been subject to substantial degradation, driven by factors such as loss of access to fresh groundwater as water tables have fallen. Problems have also been caused by salinisation in areas where water tables have risen under intensive irrigation, bringing salts near to the surface.

Northern China’s aquifers have allowed miracles of agricultural production to occur in otherwise dry, harsh climates. The drilling of major well-fields (areas containing dense concentrations of agricultural supply wells) tapping alluvial aquifers through the 1950s and 1960s facilitated a huge expansion in irrigated agriculture. This included the establishment of agricultural “oases”, vast swaths of green agricultural land in otherwise marginal desert terrains of the north-west, where people were encouraged to settle. Huge grain yields were also achieved in the North China Plain based on a dual-cropping rotation system producing wheat in winter and maize in summer.

The other major source of water for irrigation is surface water captured from dams, typically constructed in areas next to mountains, which in many cases are also the natural recharge zones for alluvial aquifers. Large amounts of water have been diverted from these recharge areas to the shallow sediments below fields. Unfortunately, the quality of the water by the time it reaches these sediments is generally poor. In most cases, confining layers also prevent it from replenishing deeper aquifer layers.

Particularly in north-west China, this infiltration of irrigation water, coupled with naturally high evaporation and transpiration rates, has caused widespread salinity problems – both for soil and shallow groundwater – making it difficult for any but the most salt-tolerant ecosystems to survive.

Meanwhile, in heavily populated urban centres, large localised declines in groundwater levels (called “cones of depression”) rapidly emerged and expanded as industries tapped into groundwater supplies at frenetic rates through the 1980s, 1990s and 2000s. In many cities – the worst affected examples including Cangzhou, Baoding and Hengshui – the rapid reduction in pore water pressures led to serious land subsidence and major fissures in the ground.

In Tianjin and other cities on the Bohai Sea coast, groundwater extraction for agriculture, drinking supply and salt harvesting has caused encroachment of saline water kilometres inland, turning increasing numbers of groundwater wells salty.

The widespread intensive use of groundwater has created both regional and local cones of depression, indicating that extraction rates are far greater than the capacity for aquifers to naturally recharge and release water from storage. It is true that aquifers generally have a capacity to “rebound” if pumping ceases or decreases to the point where it is exceeded by recharge. But the physical degradation of aquifer structures due to subsidence and water-quality problems like seawater intrusion are, in many cases, difficult or impossible to reverse.

Aquifers in northern China have in some cases been used not as continuing resources with renewable capacity, but rather “water mines”: once exhausted, they may be irreversibly degraded and never usable again, at least not to the previous extent.   

In large part, the over-exploitation of aquifers can be put down to the fact that estimation of aquifer recharge rates and water balances is an inherently difficult task and public data on water quality or groundwater level decline has historically been minimal. However, research conducted mostly in the last decade has produced a substantial body of information on many of the important aquifers of the region which can help to assess current levels of exploitation, sustainable extraction rates, water-quality issues and areas most at risk of salinisation and contamination. This data should be used as a basis for evaluating groundwater’s future role in supplying water to the parched north of China.

There are reasons for optimism in the quest for sustainable water usage in northern China. Progress is already being made in tackling groundwater over-extraction: the China Daily has reported that Beijing’s 40,000 or so private groundwater wells will be phased out in the next few years as water from the south-north transfer comes online. Beijing has also made enormous leaps in the fields of water recycling and water efficiency – as well as moving industries away from the city – meaning the capital now uses less water overall than it did in 1980, even though its population has doubled since then.

However, even with additional water arriving from the south-north transfer and continued efficiency improvements, it is predicted that Beijing will require inter-provincial water transfers to meet its demands, including diversions from the already highly stressed Yellow River. This highlights just how enormous the challenges are for water policy, planning and management.

Proponents of major engineering projects may boldly talk of mega-schemes pumping desalinated water from the Bohai Sea to Inner Mongolia in order to open up vast fossil-fuel resources, but the reality is that huge obstacles remain to the task of meeting existing water and food security challenges. These have to be met before such pipe dreams can be considered realistic.

Clearly, groundwater aquifers are an important part of doing so and, if used wisely, can continue to service irrigation and domestic water needs. But the mentality of the past decades must first be broken and a more intelligent approach to groundwater use and management established – one based on scientific data and understanding of aquifer recharge rates, storage capacity and susceptibility to contamination.

Additionally, proper understanding of the place of groundwater in the overall water cycle will be increasingly important as policymakers look to move water use onto a more sustainable footing. This includes recognition of its link to surface water, its role in supporting ecosystems and potential strategies for maintaining quality and quantity, such as managed aquifer recharge using re-cycled wastewater. If the north is to avoid being sucked dry both from above and below, this process must begin immediately.  

Matt Currell is a lecturer in environmental engineering at Australia’s RMIT University. He completed a PhD investigating groundwater quality and quantity issues in a heavily exploited aquifer in Shanxi Province in 2011. 

Homepage image by Matt Currell