Barren, dusty, polluted. When it comes to abandoned mining subsidence sites, it is difficult to associate them with clear water and green trees. The serious problem is: Can we restore the beautiful and healthy environment after people recklessly exploited natural resources and destroyed the habitats of wildlife?

In the process of urbanization and industrialization, many cities in China sought economic development at the expense of the ecological environment. Xuzhou was one of the most important coal-resource-based cities, with a total area of about 23,000 km2 degraded lands from long-term coal mining, and Pan’an Lake was in the most extensive existing coal mining subsidence area in the northeast of Xuzhou (Zhou and Wang, 2016).

Figure 1. Location of Pan’an Lake Wetland Park in Jiawang District, Xuzhou City, Jiangsu Province, China(Credit: Xu et al., 2020, https://doi.org/10.1371/journal.pone.0238243)

Figure 2. Satellite image of Pan’an Lake Wetland Park (Credit: Google Earth, 2020)

When I visited my relatives living in Jiawang District ten years ago, I saw this area covered with muddy pits and ponds, and overgrown weeds. Moreover, due to the collapse of the land surface, local farmers were not able to cultivate and support their livelihoods. Most areas became uninhabited regions. In 2010, the local government implemented several ecological restoration projects for mining subsidence areas of several urban lakes, Pan’an Lake was one of them.

Now, Pan’an Lake has been restored to a national wetland park and has become habitat for various species of animals and plants. The wetlands laying around the reservoir forms a very attractive landscape. During my field trip, I was impressed by the huge change of the local environment, natural landscape and the developed ecotourism. The fantastic restoration project in Pan’an lake is a great example for us to learn about ecological restoration in urban areas and sustainable development in mine subsidence areas with damaged ecosystems.

Figure 3. Pan’an Lake subsidence area before resotration and during enhancement of wetlands (Credit: Jiangsu Natrual Resources Department, 2010)

Figure 4. Pan’an Lake wetland park after resotration (Credit: Jiangsu Natrual Resources Department)

The background of Pan’an Lake wetland restoration

Before the wetland restoration, the coal mining activities caused detrimental impacts on the community ecosystem, especially on the water quality, soil composition, and wetland plants adjacent to the area. The parameters of water quality in abandoned subsidence areas such as suspended matter, mineral composition, dissolved oxygen, toxic heavy metal, and pH value deviated from the standard range because of the coal mining. Mining water which contained a large number of suspended impurities such as coal powder and rock powder flowed into the lake. The water pH value was over 7 and presented as alkaline due to the oxidation of sulfur compounds such as pyrite.

After the mine collapsed and subsided, only part of the ground was flooded all year-round, and part of the gentle slope around the flooded area was prone to be seasonal flooding, resulting in soil erosion and salinization. The mine leaching water penetration caused the soil to turn very alkaline, have a high content of heavy metals, with decreased soil aeration and weakened microbial activity. Compared with non-subsidence areas, the unnatural coarser grain-sized soil reduced the water holding capacity, making it easily lose water due to evaporation, and was more susceptible to drought.

Soil and water are essential elements for plant growth, thus pollution from both affected plants significantly. The barren soil’s water holding capacity was inadequate, leading to a lack of water and nutrients needed by plants during the growing period. At the same time, mine wasted water containing heavy metal ions had toxic and harmful effects on most plants, causing plants to wither and die (Wang, 2016).

The wetland restoration of Pan’an Lake was designed to recover the damaged local ecosystems through multiple approaches, from water quality improvement to plant recovery. In order to enhance biodiversity and ecological restoration capacity of the wetlands, the restoration project was conducted over several years and included three phases of construction.

The Process of Pan’an Lake wetland restoration

The main restoration process of Pan’an Lake wetland ecosystem included building revetments and planting vegetation around the lake (Shen, 2018).

Figure 5. Workflow of Pan’an Lake wetland restoration project (Credit: Shen, 2018)

Restoring plants

The wetland vegetation restoration strategy adopted the comprehensive method of planting a combination of emergent plants, floating leaf plants, submerged plants, and ecological floating islands. From the shore to the center of the water body, the community types are vegetation buffer zone, emergent plant community, floating leaf plant community, submerged plant community plant floating island.

I learned when I visited the wetland park that vegetation buffer zones were constructed between the aquatic and terrestrial ecosystems to filter, retain, and absorb the surface runoff and seepage to remove the sediments, nutrients, organic matter, and other substances. Accordingly, it reduces the concentration and toxicity of pollutants entering the water. The vegetation buffer zone is the first protective barrier for lake water. The terrestrial vegetation buffer zone in Pan’an Lake is mainly divided into three zones, and the width of the three zones is arranged according to 1:3:1. Planting native trees in the first zone functions in retaining soil and protecting slope and aquatic habitat. The second zone planted artificial forests and shrubs to ensure the diversity of plants and provide living space for all kinds of small animals. Herbaceous plants were planted in the third zone, which mainly retained and filtered various pollutants.

Figure 6. Three vegetation buffer zones designed in Pan’an Lake wetland (Credit: Shen, 2018)

Figure 7. Landscape of vegetation buffer zones in Pan’an Lake Wetland (Taken by Wang Jifei)

Building revetments

Another vital part of the plant restoration system is the building of revetments. Revetments not only function in protecting the stability of bank slopes and preventing soil erosion, but also can connect the vegetation of the waterfront with that of the embankment to form a fully functioning wetland ecosystem. The steps of revetment bioremediation are as follows: First, establishing floating islands of plants and intercepting sewage net (eliminating solid waste). Second, building ecological revetment. Third, planting vegetation for purification.

In the first step, plants with a strong ability to block pollution such as canna, calamus, and other plants are planted to form a floating plant island. Then, sand and gravel are used to build a filtration zone to further remove the suspended particles in the water. Meanwhile, plants with well-developed roots, strong sand fixation ability, and flood tolerance are chosen to stable filtration layers along the shoreside.

In Pan’an Lake, natural and planted revetments are built along the river bank and lakeshore with slow or gradual slopes (Figure 8). For steep slopes, engineers would use gabions or rock revetments. Water-loving plants, such as willow, salicylates, and reeds, are planted with well-developed roots to stabilize embankments. These plants have flexible branches and leaves to adjust the water flow and increase the flood resistance of the embankments. When walking along the shores of the wetlands, we can discover many natural revetments were built and strengthened by tree stumps, branch cuttings, bamboo hedges, and straw bags. Then the plants’ roots are fixed to form the bank when they grow.

Figure 8. Ecological planted revetments built in Pan’an Lake Wetland (Taken by Wang Jifei)

The landscape and biodiversity after restoration

The construction and restoration of Pan’an lake wetland have significantly improved the ecological environment. The water quality of the lake meets the level III standard (Environmental quality standard for surface water, GB 3838-2002), which is considered to be protected surface water resources area.

The species of wildlife increased in the wetland habitats. According to an investigation conducted in 2016, there were 12 mammalian species found in wetlands around the Pan’an Lake, such as hedgehog, hare, and mole. 209 species of birds such as Pond heron, Bean goose, Swan goose, and Shelduck were found. Thirteen reptiles including  Chinese turtles, geckos, and red chain snakes and six species of amphibians including Chinese frog. Forty-four species of fish like Silver carp, Herring, Grass carp were identified.

When walking around the bird islands in the middle of the lake, I spotted many species of birds. Most of the birds I encountered belonged to Anser genus. A large part of wetlands was covered with dense plants, such as tall trees and thick bushes, thus providing ideal shelters for birds and small animals.

Figure 9. Some species of birds and fish in Pan’an Lake Wetland Park that I saw on my trip

Figure 10. Species of birds in Pan’an Lake Wetland Park (Credit: Du Junxi, 2018)

239 species (including varieties) in 97 families, 234 genera of plants were identified by scientists in Pan’an Lake Wetland Park. There are 117 species of trees in the park, accounting for 33.1% of all plant species. The number of dawn redwood (Metasequoia glyptostroboides), pond cypress (Taxodium ascendens), eodar cedar (Cedrus deodara), bull bay (Magnolia grandiflora), and ginkgo trees is over 2,000 individuals. Shrubs can be found around the park such as Christmas berry (Photinia × fraseri), Nerium (Nerium indicum), and crape myrtle (Lagerstroemia indica), which is the city flower of Xuzhou. The shrubs consist of about 27% of all plant species. Grasses like monkey grass (Ophiopogon japonicus) and Mexican aster (Cosmos bipinnatus) are planted in the park. In addition, 41 species of aquatic plants grow in the wetland park, including reeds, lotus water chestnuts, and allium, which account for more than 80% of all the aquatic plants (Ding and Lu, 2018).

Figure 11. Some species of plants in Pan’an Lake Wetland Park

The wetland park provides habitats for endangered species of birds such as Milvus, Oriental Scops Owl (Otus sunia), Common Kestrel (Falco tinnunculus), Sparrowhawk (Accipiter nisus), and Black Kite (Milvus migrans). The Dawn redwood (Metasequoia glyptostroboides) and Ginkgo (Ginkgo biloba) are national first-level protected plants growing and thriving in this wetland ecosystem.

Summary and Reflections

The restoration and recovery of the wetland ecosystem in Pan’an Lake wetland park still continues. The monitoring and maintenance of the aquatic environment and wildlife species are necessary for supporting the development of this artificial wetland ecosystem. After ten years, the landscape is totally changed from the barren coal mine subsidence area. The government devoted huge amounts of money and labour to the restoration project, which reminds people of the cost of exploiting natural resources by devastating the ecological and other aspects of the environment.

Nowadays, the beautiful Pan’an Lake wetland park provides local residents with places for entertainment. The abundant species of wildlife in this park attracts tourists, especially teenagers, to go and explore. The developed ecotourism gets more people to be involved in the protection and restoration of the wetland ecosystem. This area has also become a typical case of an urban ecological reconstruction project in China. I believe it has played a positive role in the regional ecological enhancement and environmental protection.  I thoroughly enjoyed my visit and felt restored and refreshed after connecting deeply with these wetlands and the species that inhabit them.

If you have a chance to come to Xuzhou, the Pan’an Lake Wetland Park is a wonderful place for you to explore nature and enjoy an eco-friendly trip.

Author: Wang Jifei