Redai dili (Jun 2022)
Characteristics of Groundwater Flow in Tidal Wetland in Pearl River Estuary under Heterogeneous Conditions
Abstract
Groundwater flow in tidal wetlands is an important part of coastal estuarine hydrological cycle. The results of previous studies were based on the condition that the aquifer was homogeneous. Moreover, the understanding of groundwater circulation in tidal wetlands under heterogeneous aquifer medium is still insufficient. To explore the characteristics of groundwater flow in tidal wetlands under heterogeneous conditions and reveal the groundwater circulation process of wetlands, this study used a typical estuarine tidal wetland in Nansha District of Guangzhou as the research object. After analyzing the characteristics of the heterogeneous aquifer medium in the wetland, the groundwater flow characteristics under the heterogeneous aquifer were investigated using the radioactive radon isotope tracer technique. The results showed that the dry bulk density, porosity, radon release, and radon equilibrium capacity of the studied medium exhibited spatial heterogeneity. The dry bulk density of the medium varied from 0.39 to 1.44 g/cm3 and increased gradually from land to river and shallow to deep. The overall porosity of the medium varied from 0.42 to 0.78 without evident regularity. The main factor affecting the spatial distribution characteristics of the equilibrium capacity of the medium radon was its release. The former varied from 2,210.15 to 7,700.33 Bq/m3 and had a good positive correlation with the latter (R2=0.69). Under the condition of heterogeneous aquifer, the analysis of the total head of groundwater in the tidal wetland during ebb tide showed that the groundwater flowed from shallow to deep and land to river in the aquifer far from the tidal river. However, in the wetland aquifer close to the tidal river, the state of the groundwater flow changed and showed a trend from deep to shallow. According to the spatial distribution characteristics of radon specific activity, groundwater flow in tidal wetlands was divided into fast-flowing and retention zones. The radon-specific activity of the groundwater in the fast-flowing area was relatively low, with an average of 1,522.39 Bq/m3 during the observation period. The groundwater showed an approximately U-shaped flow mode from land to river. The radon specific activity of groundwater in the retention area increased substantially, with an average of 3,858.40 Bq/m3 during the observation period. In addition, the state of the groundwater flow in the retention area was affected by the difference in the tidal height. The larger the difference between the tidal heights, the greater the driving force of the tide on wetland groundwater; the seepage of groundwater in the retention area was relatively easy, and vice versa. Based on the actual heterogeneity of the aquifer, this study measured and analyzed the characteristics of groundwater flow in tidal wetlands, which is an important step relative to previous research on homogeneous conditions. It is of great importance to ascertain the groundwater flow in tidal wetlands under actual conditions. In the future, we can further clarify the influence mechanism of the heterogeneity of different media on the characteristics of groundwater flow in tidal wetlands, quantify the groundwater circulation process, and provide constructive suggestions for production activities and sustainable development.
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