Qiang Wu: Mining Impacts on Water Areas Explained

A leading researcher on mining impacts on water areas in coal mining is Qiang Wu (also written Wu Qiang in Chinese name order), a professor at China University of Mining and Technology (Beijing) who was elected an academician of the Chinese Academy of Engineering in December 2015, according to his Jilin University academician profile. His work deals with two sides of the same problem: water that floods into mines (water inrush) and the water that mines pump out, treat and release into rivers and aquifers.

This page sets out what his research actually found: the vulnerability index method for predicting floor water inrush, the “three maps and two predictions” method for roof aquifers, the risk thresholds used in Chinese mine hydrogeology codes, and his three-step plan for mine water (cut inflow, treat it, reuse it). It then puts those findings next to US data on mining water use and abandoned mine drainage, and ends with a checklist and calculator that a mining consultant can use on a real site.

Who is Qiang Wu? The leader behind the mine water research

His full name is Qiang Wu. Chinese sources usually write the family name first, so you will see both “Wu Qiang” and “Qiang Wu” on the same papers. The facts below come from his university profile and from papers he wrote himself.

• Born: October 1959 in Hohhot, Inner Mongolia (Jilin University profile).
• Training: bachelor’s degree from Hebei Geo University in 1982; master’s and doctorate at the China University of Geosciences (Beijing) between 1985 and 1991. The profile calls him the first doctor in mine hydrogeology in China.
• Current role: tenured professor at China University of Mining and Technology (Beijing) and director of the National Research Center for Water Hazard Prevention and Control in Coal Mines.
• Output: 10 books and more than 200 research articles, 50 of them indexed by SCI, per the same profile.
• International work: he organised the 12th International Mine Water Association (IMWA) Congress in 2014, was elected to the IMWA Executive Council and has chaired the China National Committee of IMWA since it was approved in March 2015 (Wu, Mine Water and the Environment, 2021). The IMWA council list for 2017-2021 names him as a member from Beijing.
• Government work: he has led State Council expert task forces investigating serious mine flooding incidents in China, according to his profile.

His main field is mine hydrogeology: the study of how groundwater and surface water behave around a mine. That covers two linked risks. The first is safety, because water bursting into workings can flood a mine. The second is environmental, because the water a mine drains, pumps and discharges changes aquifers and streams around it.

Mining impacts on water areas: the core findings

Across his papers, a few findings come up again and again. Each is stated here with the paper it comes from.

1. Water inrush is a large constraint on coal mining in North China. Citing figures from the former Ministry of Coal Industry, Wu and co-authors write that North China holds 60% of the country’s coal reserves, that more than 20 billion tonnes of those reserves are threatened by water from aquifers under the coal seams, and that about 40% of that coal cannot be mined normally (Wu, Zeng and Liu, IMWA 2014).
2. The old two-factor test is too simple. The standard Chinese test, the water inrush coefficient, divides aquifer water pressure by the thickness of the protective rock layer. Wu’s group argues it treats only two factors, weights them equally and misses the non-linear way inrushes happen.
3. Many factors, mapped together, predict risk better. His vulnerability index method combines a geographic information system (GIS) with weighting by expert judgement or a neural network, so that aquifer pressure, water yield, rock barrier thickness and faults are all mapped and scored together.
4. Faults and small structures are where water gets through. His IMWA 2014 paper focuses on “size-limited structures”, meaning small faults and collapse columns that act as water pathways from aquifers into coal seams.
5. The answer to mine water is to cut it, clean it and reuse it. His 2021 IMWA paper sets out a three-step approach: reduce the water entering the mine, treat what does enter, and use the treated water instead of discharging it.

The rest of this page takes each finding in turn.

What is a mine water inrush and why it matters

A water inrush is a sudden flow of water into mine workings. In coal mining, the water usually comes from a confined aquifer, a water-bearing rock layer under pressure, sitting below the coal (the floor) or above it (the roof). The rock between the coal and the aquifer is called the aquiclude or, in Wu’s papers, the aquifuge: a layer that does not pass water easily. When mining thins or cracks this barrier, or cuts a fault that links to the aquifer, pressurised water can break through.

Wu’s IMWA 2014 paper explains that floor inrush problems are worst when mining the lowest coal seams, because the aquifers below them have high water pressure and high water yield. Inrushes can raise mine discharge sharply or flood a mine completely. In China the main floor aquifers are often thick karst limestones, so the volumes involved can be very large.

For a consultant, inrush is also an environmental issue. Every cubic metre that enters a mine has to be pumped out, and that pumping lowers groundwater levels around the mine (a cone of depression). The pumped water then has to go somewhere, usually a stream or river. So the same hydrogeology that controls mine safety controls the mine’s effect on local water.

For a wider look at how mines manage these hazards, see our guide to mine safety innovations.

The vulnerability index method explained

The vulnerability index (VI) method is the best-known part of Qiang Wu’s work. It was introduced by Wu and Zhou in 2008 and tested on several Chinese coal mines. The 2011 paper by Wu, Liu and Yang in Mine Water and the Environment shows how it works step by step at the Tashan Coal Mine, about 10 km southwest of Datong in Shanxi Province.

Chart

Where each risk class starts at Tashan

Lower bound of the vulnerability index for each class

Step 1: choose the controlling factors

At Tashan the team used six main factors: aquifer water pressure, aquifer water yield (how much water the aquifer can give), effective thickness of the protective rock barrier, fault density, the density of fault crossings and fault ends, and a fault scale index (fault throw times length, summed per unit area). Thicker barrier rock lowers the risk; all the other factors raise it.

Step 2: map each factor in GIS

Each factor is built into a map from drill-hole data: water pressure from groundwater levels, barrier thickness from drill logs, water yield from inflow, drilling-mud loss and core recovery, and the fault measures from mapped faults. The maps are gridded and normalised so that they can be added together.

Step 3: weight the factors

The weights come from the analytic hierarchy process (AHP), a method in which experts compare factors in pairs to decide how important each one is. At Tashan the comparisons passed the usual consistency check, with a consistency ratio of 0.0604 against a limit of 0.10. In other work Wu’s group trained the weights with an artificial neural network instead, and his student Yifan Zeng later developed a variable-weight version so that a factor’s weight can change with its value (Zeng, Springer thesis).

Step 4: add up the weighted maps and set thresholds

The weighted maps are summed into one vulnerability index for every grid cell. The thresholds between risk classes are then read from a frequency histogram of the index values. At Tashan the boundaries were 0.21, 0.31 and 0.58, giving four classes.

These thresholds belong to Tashan. Every mine gets its own, because they come from that mine’s own histogram. A consultant should never copy 0.58 to another site.

Water inrush coefficient vs vulnerability index

The water inrush coefficient is the older test and is still written into Chinese mine hydrogeology codes. It is simple: T = P / M, where P is the water pressure on the barrier (in MPa) and M is the effective barrier thickness (in metres). The result is in MPa per metre.

According to the Codes of Mine Hydrogeology as quoted by Wu, Liu and Yang (2011), a coefficient below 0.06 MPa/m is considered safe in a faulted area, and 0.06 or more is dangerous. In a normal, unfaulted area the limit is 0.15 MPa/m.

At Tashan the two methods disagreed in useful ways. The coefficient method rated the whole of one seam (C3-5) as safe and even rated some faulted parts of another seam as safe. The VI method instead split each seam into four classes and flagged the faulted ground around several named faults as dangerous. The lowest seam, C9, came out far more vulnerable than the others because it sits closest to the aquifer and has the thinnest barrier. Wu’s group recommends using both methods together, and notes that the results are only as reliable as the data, so they should be revisited as mining adds new information.

Wu’s IMWA 2014 paper lists the full index system as five main factors (the aquifers under the coal, the hydrologic barrier, geologic structure, damage from mining and the ineffective part of the barrier) with 16 sub-factors.

Three maps and two predictions for roof water

Floor water is half the problem. When coal is extracted, the roof above collapses and cracks upward, and the cracked zone can reach a sandstone or limestone aquifer overhead. To handle this, Wu’s group developed the “three maps and two predictions” method, first published in 2000 and summarised in his IMWA 2014 paper.

The three maps are:

1. a risk map of roof collapse based on safety factors;
2. a map of how much water the overlying aquifer can yield;
3. a combined risk map of water inrush from the overlying aquifer.

The two predictions are:

1. the groundwater inflow rate under natural conditions;
2. the inflow rate under artificial control, such as after drainage or grouting.

The method uses numerical models to predict how high the roof cracking will reach and how much water working faces will take in. For an environmental review this matters because the predicted inflow is also the predicted pumping volume, which sets how much groundwater the mine will remove and discharge.

Small faults as water paths

The same paper describes how small faults and collapse columns, which Wu calls size-limited structures, often carry water into coal seams. His group built prediction models for these structures from many data sources, weighted by a neural network, and accepted a model only when it matched known structures at a rate of about 90%; below that, the model was revised and run again.

Qiang Wu’s three steps for mine water

In his 2021 paper on China’s relationship with IMWA, Wu sets out the central goal of the China National Committee he chairs as a three-step approach. This is where his safety research becomes environmental policy.

Step 1: cut the water entering the mine

Engineering measures listed in the paper include sealing water channels by grouting, using room-and-pillar or backfill mining instead of longwall mining, limiting blasting so the cracked zone stays shorter below an aquifer, limiting full-height or top-coal caving mining where roof and floor damage must be kept small, intercepting groundwater at the surface before mining, draining water-rich zones in advance, building waterproof gates and leaving waterproof coal-rock pillars to national standards, and diverting surface water away from areas where it would soak into the mine.

Step 2: make the water harmless

Some water will still enter. The second step is to treat it underground or at the surface so that its quality suits whoever will use it next. Wu notes that discharge limits can then differ from mine to mine, depending on the intended use.

Step 3: use the water

The treated water is used for dust control, pre-grouting of the floor, surface reclamation and revegetation, and, after suitable treatment, irrigation of nearby fields or animal and human supply where local needs call for it. Wu argues this turns waste water into a resource, cuts the volume discharged and so reduces pollution of surface water.

The same committee writes China’s national technical standards in this area, including the “Detailed Rules for the Prevention and Control of Mine Water” and the water section of the “Coal Mine Safety Regulations”, according to the paper. By 2021 the committee had 315 members, and Wu reports that its members had obtained 35 international patents and more than 600 Chinese invention patents in mine water prevention and use.

How much water US mining uses

Wu’s work is mostly about Chinese coal, but the same questions apply to US mines. The latest national USGS water-use circular with a full sector breakdown, Circular 1441 for 2015, estimates total US mining withdrawals at 4,000 million gallons per day (4.00 billion gallons per day). That was about 1% of all US water withdrawals and 1% more than in 2010.

Chart

Largest US mining water users, 2015

Total mining withdrawals, million gallons per day

• Groundwater supplied 72% of mining withdrawals, and 65% of that groundwater was saline.
• 77% of the surface water withdrawn for mining was fresh.
• Saline groundwater and fresh surface water together made up 68% of the total.
• USGS counts dewatering as a mining withdrawal only when the water is put to use, such as dust control on roads. Water pumped out and discharged without use is not in the total.

Note that the USGS mining category also includes water injected for oil and gas recovery, so it is wider than metal and coal mining alone. That is one reason Texas leads: it withdrew 1,140 million gallons per day for mining in 2015, 28% of the national total, including 1,000 million gallons per day of saline groundwater.

The dewatering point links straight back to Wu’s work. Much of the water a mine pumps out never appears in national withdrawal tables, yet it lowers water tables and ends up in rivers. A site review should ask for the mine’s own pumping and discharge records rather than rely on national averages.

Acid mine drainage and abandoned mines

The second big effect of mining on water is quality. The US Environmental Protection Agency describes abandoned mine drainage as water polluted by contact with mining activity, most often linked to coal mining. EPA lists three types: acid mine drainage, the most common; alkaline mine drainage, where calcite or dolomite is present; and metal mine drainage, with high levels of lead or other metals.

Chart

Fresh vs saline mining water by state, 2015

Mining withdrawals, million gallons per day

Acid mine drainage forms when rainwater, snowmelt or shallow groundwater reacts with rocks containing sulphur-bearing minerals such as pyrite. The reaction makes sulphuric acid, which dissolves metals from the rock. The process carries on for as long as those minerals are exposed to air and water, which is why closed mines can pollute streams for decades.

A short glossary helps when reading monitoring reports. In the review by Wolkersdorfer on the effects of mining on surface water, “ochre” is the yellow to dark orange iron oxide that coats stream beds below many mines, and “orphaned” mines are abandoned mines with no owner. pH measures acidity; it has no units; the same review notes that the most acidic value ever measured in nature is pH -3.6.

For a wider view of land, forest and water effects at surface mines, see our article on how open pit mining affects the environment, and our companion page on mining impacts on water areas and the IMWA numbers.

▶ The seven environmental impacts of mining, explained in claymation with satellite maps. · 2:31

Catchment-scale management: the ERMITE guidelines

Wu’s three steps sit alongside a European framework that is often cited with them. The ERMITE guidelines, “Mining Impacts on the Fresh Water Environment”, were published in 2004 in a supplement of Mine Water and the Environment by the ERMITE Consortium with Paul Younger and Christian Wolkersdorfer. ERMITE (Environmental Regulation of Mine Waters in the European Union) was a European Commission research project.

The guidelines look at mine water at the scale of the whole catchment, the land area that drains to one river, and link it to the European Union’s Water Framework Directive. They cover the exploration and working phase, the closure phase and the problems that remain after closure, and argue that good practice during mining should reduce the need for costly treatment afterwards.

The two bodies of work fit together. Wu’s methods predict where and how much water will enter a mine. The ERMITE approach asks what that water does to the river basin once it leaves.

How to apply these findings on a mine site

A mining consultant can turn the research into a practical review. The steps below follow the logic of Wu’s papers, adapted for any coal or hard-rock mine.

1. Collect the hydrogeology. Gather drill logs, groundwater levels, pumping tests and water-quality samples. Identify every aquifer above and below the target zone and the barrier rock between them.
2. Run the two-factor test first. Work out the water inrush coefficient (pressure divided by barrier thickness) for each panel. It is quick and shows obvious problem areas. Use the calculator above for a first pass.
3. Map faults and structures. Faults are where Wu’s maps flag the highest risk. Use underground mapping, seismic data and surface lineaments; satellite imagery is a fast way to see surface faults, drainage lines and old workings before a field visit.
4. Build a multi-factor index. Where the data allow, grid each factor, weight it and sum it, as in the VI method. Set class thresholds from that mine’s own histogram.
5. Estimate inflow and pumping. Predict inflow under natural and controlled conditions, as in the two predictions. This is also the figure for groundwater drawdown and discharge.
6. Plan to cut, treat and reuse. Check grouting, mining method and barrier pillars to reduce inflow; size treatment to the discharge limit; look for uses on site.
7. Check the catchment. Map where the discharge goes, which streams and wells it affects, and whether old workings nearby already discharge acid or metal-rich water.
8. Update as mining proceeds. Wu’s group stresses that conclusions should be revisited as new data arrive.

Teams that also need to review mineral targets on the same ground can use satellite-based mineral detection, and send site questions through our mining query form or contact page.

▶ Remote sensing for mining explained in three minutes. · 2:01

Limits of the research

Wu’s methods are well tested in Chinese coal fields, but a reader should keep their limits in mind.

• Coal-focused. Most case studies are North China coal mines over karst aquifers. Hard-rock metal mines have different water paths, such as fractured granite and old stopes.
• Site-specific thresholds. VI class boundaries come from each mine’s own data, so values do not transfer between sites.
• Expert weighting. AHP weights depend on expert judgement; the 2011 paper reports passing a consistency check, but different experts can give different weights. Neural-network and variable-weight versions try to reduce this.
• Data quality. The authors state that the method is limited by the reliability of the data and how well unusual conditions are known.
• Safety first. The VI work predicts inrush risk. It does not on its own measure water quality, which needs sampling and catchment monitoring as in the ERMITE guidelines.

Frequently asked questions

Who is the leader of the research on mining impacts on water areas?

The name usually given is Qiang Wu (Wu Qiang), professor at China University of Mining and Technology (Beijing), academician of the Chinese Academy of Engineering since December 2015 and chair of the China National Committee of the International Mine Water Association.

What is Qiang Wu known for?

He is best known for the vulnerability index method for predicting floor water inrush in coal mines, the “three maps and two predictions” method for roof water, and a three-step approach to mine water: cut inflow, treat the water and reuse it.

What is the vulnerability index method?

It is a way of rating water inrush risk across a whole mine by mapping several controlling factors in GIS, weighting them with the analytic hierarchy process or a neural network, adding them into one index and dividing the result into risk classes from the mine’s own histogram.

What is the water inrush coefficient?

It is water pressure on the barrier rock divided by the barrier’s effective thickness, in MPa per metre. The Chinese code quoted by Wu treats less than 0.06 MPa/m as safe in faulted areas and less than 0.15 MPa/m as safe in normal areas.

How does mining affect groundwater?

Mines pump water out to stay dry. That lowers the water table around the mine, can dry up nearby wells and springs, and sends the pumped water into streams. Cracking above or below mined coal can also connect aquifers that were separate before.

How does mining affect surface water?

The main effects are changes in flow from discharge and diversion, sediment from disturbed ground, and pollution from acid, metal or saline drainage. EPA lists acid mine drainage as the most common form at abandoned mines.

How much water does mining use in the US?

USGS Circular 1441 estimates 4,000 million gallons per day for mining in 2015, about 1% of all US withdrawals. 72% came from groundwater, most of it saline.

What is acid mine drainage?

It is acidic, metal-rich water formed when water and air react with sulphur-bearing minerals such as pyrite in mine rock, producing sulphuric acid that dissolves metals. It can continue for decades after a mine closes.

What are the ERMITE guidelines?

“Mining Impacts on the Fresh Water Environment” is a 2004 set of technical and management guidelines from a European Commission research project. It covers mine water at the scale of whole river catchments, from exploration through closure.

Can treated mine water be reused?

Yes. Wu lists dust control, floor grouting, land reclamation and revegetation, and, after suitable treatment, irrigation and supply for animals or people, depending on local needs and discharge standards.

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