Abstract
Seawater intrusion in coastal zone is a common gradually changed geological hazard. After seawater intrusion, the coastal groundwater is polluted into seawater, which directly causes great damage to local industrial and agricultural production and residents’ health. In order to better understand the geological hazard, the chlorinity of groundwater in six monitoring sites which had different distance with the coast was detected by silver nitrate titration method based on the groundwater model of Laizhou Bay coast to collect the chlorinity of groundwater in the six monitoring sites from April 2013 to April 2017. The local seawater intrusion was analyzed taking the monthly average precipitation data in February and March of the same year into account. The results showed that in terms of time distribution, groundwater chlorinity in the monitoring site far from the coast had no significant fluctuation as time went on, but had an upward trend. Groundwater chlorinity in the monitoring site near the coast fluctuated significantly with time and had an upward trend. Especially in 2015, drought caused more seawater intrusion and greater chlorine fluctuation; in spatial distribution, with the increase of distance with the coast, the chlorinity of groundwater decreased gradually except fluctuation in 2015 caused by drought, the seawater intrusion area was approaching inland as time went on, and the transverse distribution of seawater intrusion area was irregular.
1 INTRODUCTION
In the development of coastal cities, seawater intrusion is a unique and common environmental deterioration problem, which is a kind of gradually changed geological hazards [1]. The main factors causing seawater intrusion include natural factors such as the decline of groundwater level caused by long-time drought and human factors such as excessive exploitation of groundwater [2]. In addition to the water resources in surface runoff, groundwater is also an extremely important water resource. Residents’ lives and industrial and agricultural production depend heavily on freshwater resources. Especially in areas lacking surface water, groundwater is almost the only freshwater source. Therefore, the harm caused by seawater intrusion has many serious influences. The first one is groundwater pollution [3]. Seawater intrusion makes the content of various mineral ions in groundwater rise sharply, especially chloride ions, which eventually leads to the increase of chloride ion content in soil and the loss of fertility and results in the reverse evolution of plant communities on the surface. Next is land salinization [4]. The granular structure of soil leads to capillary phenomenon; as a result, groundwater can be transferred to the surface. But because of the invasion of seawater, the salinity of groundwater rises, and the salinity of soil also rises after groundwater is transferred to the surface through capillary phenomenon, which can lead to the loss of production capacity. Thirdly, the industrial production is also affected [5]. A large amount of fresh water is inevitably needed in industrial production. The salinity of groundwater rises after the invasion of seawater. The application of such groundwater in industrial production will accelerate the corrosion of machinery, which will not only reduce product quality, but also increase production costs. The last one is difficult living of residents [6]. Freshwater is essential for human survival. Long-term drinking of groundwater polluted by sea water will cause various diseases. Beaujean et al. [7] proposed a sequential method to evaluate the feasibility of identifying hydraulic conductivity and dispersivity in density-dependent flow and transport models from surface ERT-derived mass fraction and found that only the low salt mass fraction in the seawater-freshwater transition zone can be recovered at different times and that the mismatch between the mass fraction of the target salt and recovered salt occurred from a certain threshold. The mismatch between quantities and scores occurs at a threshold. Hussain et al. [8] studied the control effect of hydraulic barriers in different layout forms on sea water intrusion by combining the established evolutionary polynomial regression model with multi-objective genetic algorithm and found that the numerical simulation results were consistent with the results obtained by direct connection with optimization tools and the evolutionary polynomial regression model could effectively reduce the computational complexity and time. Zeng et al. [9] studied groundwater vulnerability to anthropogenic pollution and seawater intrusion in Kapas Island using DRASTIC and GALDIT models. Both models showed that the areas likely to be affected by human pollution and seawater intrusion were located in the alluvium in the western part of the island. Based on the groundwater model of Laizhou Bay coast, the chlorinity of groundwater in six monitoring points with different distance with the coast was measured by silver nitrate titration method. The chlorinity data of groundwater in six monitoring points were collected in April from 2013 to 2017 and combined with the monthly average precipitation data in February and March of the same year to analyze the phenomenon of seawater intrusion in the local area.
2 THE MODEL OF GROUNDWATER INTRUDED BY SEAWATER
The model of groundwater in coastal zone with seawater intrusion
3 INSTANCE ANALYSIS
3.1 Regional overview
Laizhou bay coastal zone which is located in the south of the Bohai sea starts from Yellow River Estuary and ends at Qimu cape, with a total length of 319.06 km. It has a temperate monsoon continental climate [12], with low annual rainfall and uneven time distribution of rainfall. The terrain descends from southeast to northwest. The geological and topographical condition is complex. Alluvial plain is the main landform, which forms under the alluviation of river and tide, and the alluvial deposit mainly includes yellow-river sediments and seabed mud. The water permeability of the bottom of Laizhou bay coast and surface original runoff is high; therefore seawater can easily penetrate the coast. Therefore Laizhou bay has rich brine resource and has become the first place with seawater intrusion.
3.2 Data source
As shown in Figure 2, the groundwater model of Laizhou bay coast [13] is divided into four layers, land, phreatic aquifer, pervious layer and confined aquifer. Water in the land layer is basically overground runoff. Water in the phreatic aquifer is mainly seawater. Water in the pervious layer is mainly the mixture of seawater and groundwater between the phreatic aquifer and confined aquifer. Water in the confined aquifer is mainly groundwater. The intrusion law of the seawater could be concluded from Figure 1, and it was found that the confined aquifer was the first layer to be invaded. Therefore the monitoring well was dig as deep as the confined aquifer to collect groundwater sample. The chlorinity of groundwater was detected using silver nitrate titration method [14] to determine the degree of seawater intrusion: chlorinity lower than 250 mg/l as no seawater intrusion, chlorinity between 250 mg/l and 1000 mg/l as seawater intrusion, and chlorinity higher than 1000 mg/l as severe seawater intrusion.
The monitoring of seawater intrusion based on the groundwater model of Laizhou bay coast
As shown in Table 1, six monitoring wells were set to monitor the chlorinity of groundwater. Monitoring well with larger number was farther away from the coast, and monitoring wells nearly covered the seawater intrusion region, transition region and non-intrusion region of Laizhou bay coast [15]. The sampling time was April from 2013 to 2017. In April, Laizhou bay is in dry season, and the ground precipitation has the smallest influence on seawater intrusion. Moreover the sample was collected at 11 am ~ 12 am, and there is no tide influencing seawater intrusion in that period.
The coordinate of monitoring wells and their vertical distance with the coast
| No. | Coordinate | Distance with the coast/m |
|---|---|---|
| 1 | N 37.25°, E 119.91° | 62.11 |
| 2 | N 37.21°, E119.86° | 261.33 |
| 3 | N 37.25°, E 119.93° | 1264.41 |
| 4 | N 37.25°, E 119.92° | 1968.15 |
| 5 | N 37.21°, E 119.89° | 2685.92 |
| 6 | N 37.20°, E 119.90° | 3620.88 |
| No. | Coordinate | Distance with the coast/m |
|---|---|---|
| 1 | N 37.25°, E 119.91° | 62.11 |
| 2 | N 37.21°, E119.86° | 261.33 |
| 3 | N 37.25°, E 119.93° | 1264.41 |
| 4 | N 37.25°, E 119.92° | 1968.15 |
| 5 | N 37.21°, E 119.89° | 2685.92 |
| 6 | N 37.20°, E 119.90° | 3620.88 |
The coordinate of monitoring wells and their vertical distance with the coast
| No. | Coordinate | Distance with the coast/m |
|---|---|---|
| 1 | N 37.25°, E 119.91° | 62.11 |
| 2 | N 37.21°, E119.86° | 261.33 |
| 3 | N 37.25°, E 119.93° | 1264.41 |
| 4 | N 37.25°, E 119.92° | 1968.15 |
| 5 | N 37.21°, E 119.89° | 2685.92 |
| 6 | N 37.20°, E 119.90° | 3620.88 |
| No. | Coordinate | Distance with the coast/m |
|---|---|---|
| 1 | N 37.25°, E 119.91° | 62.11 |
| 2 | N 37.21°, E119.86° | 261.33 |
| 3 | N 37.25°, E 119.93° | 1264.41 |
| 4 | N 37.25°, E 119.92° | 1968.15 |
| 5 | N 37.21°, E 119.89° | 2685.92 |
| 6 | N 37.20°, E 119.90° | 3620.88 |
3.3 Detection method
About 2 mL of groundwater samples collected from each monitoring point were put into test tubes. Two drops of potassium chromate indicator were added to each test tube, and 1 mol/l silver nitrate solution was added to the test tube using acid titrator until brick red precipitation appeared. The consumption of silver nitrate was recorded. The chloride ion content in the samples was calculated. The sample from each monitoring point was repeatedly operated for three times, and the average value was taken as the final result. Water with the content of salt lower than 0.5 g/l is called fresh water internationally.
3.4 Analysis results
3.4.1 Changes of seawater intrusion with time
As shown in Figure 3, the chlorinity of groundwater from monitoring points 3, 4, 5 and 6 which were far away from the coast fluctuated with time, but the fluctuation amplitude was not large. Only monitoring point 5 fluctuated slightly in 2015. On the whole, the chlorinity of groundwater from four monitoring points which were far away from the coast was relatively stable and had a small upward trend. The chlorinity of groundwater from monitoring points 1 and 2 which were near the coast had large fluctuation as time went on. Especially in 2015, the chlorinity of groundwater from monitoring point 2 even exceeded that from monitoring point 1 which was closer to the coast, reaching 11 000 mg/l, which was about 43 times higher than that of seawater intrusion index (250 mg/l). On the whole, the chlorinity of monitoring points 1 and 2 showed an upward trend.
Changes of chlorinity of groundwater at monitoring points with time.
In order to explore reasons for the large fluctuation of groundwater chlorinity in 2015, the average monthly precipitation data in February and March of 2013–2017 were collected. As shown in Figure 4, the monthly average precipitation in February and March of 2015 dropped sharply to 2 mm. According to the above groundwater model, the movement of the seawater–groundwater interface was related to the hydrodynamic balance, and the hydrodynamic balance was related to the groundwater level, salt concentration gradient and tidal action. Precipitation infiltration is the main factor ensuring the normal groundwater level, and its change will directly affect the groundwater level. The average monthly precipitation in February and March 2015 was only 2 mm. The long-term drought made the groundwater in Laizhou Bay coast insufficient to replenish, the groundwater level decreased and the hydrodynamic balance destroyed. Moreover, the long-term drought made the local people dependent more on the exploitation of groundwater, which further led to the decline of water level, the rise of groundwater salinity, and the aggravation of seawater intrusion. Figure 3 shows that the groundwater exploitation in monitoring sites 1, 2 and 5 was serious.
The monthly average precipitation in February and March from 2013 to 2017
3.4.2 Changes of seawater intrusion with space
It was found from the groundwater model and the schematic diagram of seawater intrusion monitoring based on the Laizhou Bay coastal groundwater model that the intrusive seawater and groundwater mixed and diffused at the seawater–groundwater interface which mainly located in the pervious layer of the ground floor, and soil particles in the pervious layer could absorb and exchange ions. Seawater contains high concentration of chloride ion. After being absorbed and exchanged by soil, the positive ions in seawater solution are diverse, but the negative ion only include chloride ion. Therefore, the evolution of seawater intrusion in space can be judged by detecting the concentration of chloride ion in groundwater.
As shown in Figure 5, the chlorinity of groundwater decreased with the increase of distance with the coast. Except the fluctuation of curve caused by drought mentioned above in 2015, the overall trend was that the larger the distance with the coast, the smaller the chlorinity. Except 2015 in which the curve change was relatively abnormal, the variation trend of curve in other years was the same: the chlorinity of groundwater decreased to the standard of freshwater when there was a distance of 2000 m with the coast. According to the chlorinity standard of seawater intrusion, monitoring points 1 and 2 were in the serious seawater intrusion area after 2014; monitoring point 3 was in the seawater intrusion area in 2013 and 2014 and in the serious intrusion area after 2015; monitoring points 4, 5 and 6 were basically in the non-intrusion area, except monitoring point 5 in the seawater intrusion area in 2015. Taking points of intersection of curves of different years and the dotted line of the judgment standard into account, it was found that the serious seawater intrusion area and seawater intrusion area were gradually approaching the land, and the approaching degree in 2015 was the largest.
Changes of chlorinity of groundwater with space.
Taking the coordinate of the monitoring points into account, it was found that the chlorinity of groundwater was different, i.e. degree of seawater intrusion was different, even when they were at the same latitude. Thus it was concluded that the transverse distribution was uneven when seawater intruded groundwater on land.
4 CONCLUSION
Based on the groundwater model of Laizhou Bay coast, the chlorinity of groundwater at six monitoring points which had different distances with the coast was measured using silver nitrate titration method, and the chlorinity data of groundwater at six monitoring points in April from 2013 to 2017 were collected. The phenomenon of local seawater intrusion was analyzed taking the monthly average precipitation data of February and March in the same year into account. The results were as follows. With the passage of time, the chlorinity of groundwater fluctuated little at the monitoring point away from the coast, but there was an upward trend. The chlorinity of groundwater fluctuated significantly at the monitoring point near the coast and showed an upward trend. The drought caused by the sudden drop of monthly average precipitation in February and March 2015 led to insufficient supplement of groundwater, damaged hydrodynamic balance and aggravated seawater intrusion; as a result, the chlorinity of groundwater rose rapidly. Groundwater chlorinity decreased with the increase of the distance with the coast. Except for irregular fluctuation caused by drought in 2015, the chlorinity of groundwater dropped to the level of freshwater when the distance with the coast was 2000 m. With the passage of time, the seawater intrusion area gradually approached the land, and the transverse distribution of the intrusion area was uneven. In this study, the degree of seawater intrusion was observed by monitoring the chlorinity of groundwater along the coast of Laizhou Bay, and the causes of seawater intrusion were analyzed, which provides a reference for the prevention and control of seawater intrusion disasters.
FUNDING
This study was supported by Key Project of Shandong Social Science Planning (2018): the impact mechanism of human activities on the coastal ecosystem of Laizhou Bay and its early warning, under grant number 18BJJJ05.
