| Seawater intrusion is the phenomenon in which seawater or other saltwater travels into the continent along the aquifer. It can be affected by climate change and human activity, both of which alter the hydrodynamic conditions of aquifers in coastal areas. This breaks the equilibrium between seawater and freshwater, causing the salt-fresh interface to move landward. Laizhou Bay suffers the most serious seawater intrusion in China, with an intrusion area of over4000km2. This is has been concretely attributed to climate change and human activity. Seawater intrusion decreases the quality of drinking water by raising salinity to levels exceeding acceptable drinking water standards. This causes environmental contamination and soil secondary salinization. Local economies and social development can be considerably constrained by this increase in seawater intrusion, which affects industry and agriculture. Using data describing groundwater levels, groundwater quality, and the geological environment from1976-2010, this paper discusses the development and variation of the seawater intrusion in the Laizhou Bay over the past40years, quantifies the mechanisms that influence and underlie of seawater intrusion, discusses the involvement of climate change and human activity, and establishes a comprehensive early warning assessment system, which includes a status appraisal model, a risk-assessment model, and a early warning model. The results of the present work may be used to develop an ideal response process and describe the ways in which human activity and climate change affect the mechanism by which seawater intrudes into the Laizhou Bay area.The intrusion of seawater into the Laizhou Bay area has shown five stages of development over the past40years:i) initiation, ii) development, iii) deterioration, iv) release, v) differentiation. According to the investigation in2010, the intrusion area in the Laizhou Bay has exceeded4300km2. Based on the characteristics and hydrogeological environment of the seawater intrusion, the seawater intrusion region can be divided into types:1) original seawater intrusion area,2) mixed area,3) human-disrupted area, and4) intrusion-retreated area. The area (areal fraction) of the five sections are approximately1000km2(12%),2500km2(30%),800km2(10%), and70km2(less than1%), respectively.Seawater intrusion may occur due to human activity or because of natural events. Over-abstraction is considered the main cause of seawater intrusion. Climate change can speed up seawater intrusion. The effect of climate change is mainly reflected in annual scale and inter-annual scale, which involves seasonal changes in seawater intrusion (seasonal salty groundwater) and changes in seawater intrusion over the course of the year (movement of the interface seaward or landward in a wet and dry seasons). The impact of human activity is shown as seawater intrusion in the linear growth trend. Rapid economic development has increased in abstraction from aquifers, causing a serious imbalance between the seawater and freshwater, and the risk of seawater intrusion has increased. A model incorporating climate change, human activity, and factors that can be used to evaluate seawater intrusion, such as the intensity of Cl-, groundwater height, area of sea water intrusion, and negative equivalent area, was established to assess the correlations among climate, human activity, and the variability in seawater intrusion variability. The model is as follows: F(s)=F(w, t)=F(r, e, p, t). In the present formula, F(s) stands for seawater intrusion, w stands for groundwater level, which is one of the characteristics of seawater intrusion; r stands for rainfall; and e stands for evaporation, both of which can reflect the impact of climate change on seawater intrusion; p stands for groundwater exploitation variations represented by intensity and coefficient of groundwater exploitation, which refers to the effects of human activity on the seawater intrusion; and t is time.To determine the mechanism by which human activity and variations in climate influence seawater intrusion, a multi-parameter regression analysis was conducted based on a monthly long time series data of groundwater level,rainfall,evaporation and human activity under the various time scales for nearly30years and on the response of seawater intrusion to these two outside influences. The results show that human activity can account for59%of seawater intrusion, and variations in climate variation can account for41%.An automatic monitoring network of seawater intrusion was established to facilitate assessment of seawater intrusion. The relationship between the conductivity of groundwater and Cl" concentration can be quantitatively analyzed:Cl-(mg/L)=157.26+261.20Ec (ms/cm), and take1.56ms/cm as the standard amount of seawater intrusion into the Laizhou Bay area. The current situation with respect to evaluation of seawater intrusion in Laizhou Bay area involves monitoring networks and geostatistical analysis. Seawater intrusion presents a banded distribution in the study area, and the seriousness of the intrusion increases from north to south. The areas with the most serious invasion are mainly distributed in Dajiawa, Yangzi, and Xiaying, covering a total of2073km2. These areas account for43.7%of the total. Invasion areas in Taitou and Changyi cover547km2, accounting for11.5%. The longest single invasion was30km in Shouguang. Areas with no intrusion were mainly distributed around the city of Shouguang in Weifang and the southern parts of Changyi, accounting for44.8%of the total.The risk evaluation index system for seawater intrusion is established. This system incorporates5factors and27variables. The risk evaluation of seawater intrusion in the Weifang coastal area is performed according to the information model, and the results divide the area in northern Weifang area into four levels:a high danger zone, a moderate danger zone, a low danger zone, and danger-free zone. The high danger zone was calculated to cover an area of2138km2, accounting for45.4%of the total evaluation area; the moderate danger zone covered an area of521km2,11.0%of the total; the low danger zone covered479km2,10.0%of the total; and the danger-free zone covered1583km2,33.6%of the total. Two-thirds of the Weifang coastal area are in a potential disaster zone with respect to seawater intrusion.According to the probability of disaster and level of risk assessment, the Laizhou Bay area early warning level can be divided into4using a sequential indicator simulation that was performed in2013. The early warning result shows that the early warning area of seawater intrusion in Laizhou Bay has a zonal distribution and the danger level gradually decreases from north to south. Based on the early warning result and the characteristic of seawater intrusion, three prevention and controlling measure of seawater intrusion from engineering technology, ecological restoration and administrative management are put forward. |
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