Eco-Environmental Geological Features of Mangrove in Dongchong, Shenzhen City

Mangroves are halophytic forest plant communities located on saline marshes in estuaries of tropical and subtropical bays. They are one of the most vulnerable ecosystems in the world and are severely threatened by urban development, environmental pollution, aquaculture, and other problems. The Dongchong mangrove forest is a relatively well-preserved mangrove forest in China, with a large area and a typical Excoecaria agallocha landscape, which has significant ornamental and ecological conservation value. The aim of this study is to provide basic support not only for mangrove ecological conservation and restoration, but also to construct and manage nature's reserves. The eco-environmental geological characteristics of mangroves were investigated using geology, geomorphology, pedology, ecology, and other methods, and a typical eco-environmental geological profile of the mangrove in Dongchong was drawn. The results show that the strata in the study area are mainly Quaternary sediments and rhyolites of the Nanshancun Formation of the Early Cretaceous. Faults are developed in the west of Dongchong Mangrove Wetland Park. The main environmental geological problems are uneven ground settlement, ground subsidence, and ground cracks. The concentration of F- in the surface water are high, and the water quality is slightly lower than that of the Class III water standard; however, the groundwater is freshwater with low salinity and hardness. Cd and Tl are locally significant in the surface soil of the study area. N, P, CaCO3, Org, and B are deficient in the soil, whereas the K content is mainly medium. A part of the soil is polluted by heavy metals, particularly As, followed by Cd. However, the mangrove leaves are rich in nutrients, indicating that the lack of soil nutrient elements and heavy-metal pollution are not the major factors limiting the growth of mangroves in this region. Improving the growth environment of mangroves should include expanding the landscape area and reinforcing reserve management. In the rock-soil-plant ecosystem, As, Pb, Cd, and B are significantly rich in the soil, indicating that their contents have a slight correlation with their parent rocks. Ni, Cu, Zn, and K are limited by the parent rock contents, and some elements including P, Mo, and Cr show enrichment capacity in the soil. The BCF >2 of mangroves of the mangrove forests in Dongchong are P and B, indicating that the mangrove trees have a higher absorption capacity for P and B. In contrast, the BCF values of As, Pb, and Cd are relatively low, which, in addition to the weak absorption capacity of the mangrove trees for these elements and combined with the geochemical characteristics of the soils in the study area, are also affected by the high Cd and As contents of the soil. All the data presented in this paper are from the project, "Ecological and Environmental Geological Survey of Shenzhen Nature Reserves," for which we express our sincere gratitude