The Dawu water source area is a rare super-large underground water source area in Northern China, and it is the main source of urban water supply to Zibo city, which has important strategic significance for a water resource guarantee and water supply security for Zibo city [1
]. The specific scope of water source includes west of Zi river, east of Fengbei road, north of Liuzheng village and Xuwang village, and the closed area south of national highway 309. The total area of the water source is about 148 km2
. Figure 1
shows the location of Dawu water source area.
The Dawu water source area is a complete hydrogeological unit, in which the water-bearing rocks are mainly the pore water-bearing rocks of the upper quaternary loose rocks and the fissure karst water-bearing rocks of the lower Ordovician carbonate rocks. Pore water supply is mainly atmospheric precipitation infiltration, and water quality is relatively poor and generally used for agricultural irrigation. The recharge source of karst water is mainly the infiltration of atmospheric precipitation in the southern mountains and the leakage of the Zi river. The water quality is relatively good, and the water is very rich. It is mainly used for residential, urban, and industrial water, and it is the main mining layer of the Dawu water source. The water source has a strong capacity of water storage and regulation, among which there are about 300 deep wells. The water inflow of a single well is generally about 200 m3/h and the maximum is about 450 m3/h. Since the Yellow River diversion water supply project of Zibo city was put into operation in 2001, the exploitation volume remained at about 360,000 m3/d, and the annual and inter-annual variation of groundwater level was large, with the maximum annual variation range of 50–60 m. Since the exploitation of the Dawu water source in 1960, it has provided a strong water supply guarantee for Zibo urban residents’ living water supply and production water supply for large state-owned enterprises, such as Sinopec Qilu Petrochemical and so on in the Linzi area. Because of its strong capacity of water storage and regulation, the Dawu water source area is of great strategic significance to the water resource security and water supply safety of Zibo city.
In recent years, the Dawu water source area has adhered to the principles of ecological civilization, environmental coordination, and sustainable development. However, due to historical reasons, since the mid-1960s, Sinopec Qilu Petrochemical company’s Shengli oil refinery, rubber plant, first fertilizer plant, second fertilizer plant, and other large chemical enterprises have been built in the limestone area in the east of the water source area. In the 1980s, large ethylene plants, such as olefin plants, chlor-alkali plants, and plastic plants, were built successively in the limestone areas in the west of the water source area. The distribution of these chemical enterprise installations is basically the same as that of the rich underground water area in the water source area. The surface water environment of the water source area is extremely severe, and the ground pollution or quaternary soil pollution is widely distributed. Multiple pollution areas are formed in the water source area, resulting in different degrees of pollution of multiple wells in the water source area [2
]. Especially, the pollution in Hougao area with more chemical plants is the most serious. According to the monitoring data of the water quality monitoring points in the Dawu water source area in recent years which is shown in Table 1
and Table 2
, the pollutant concentration exceeding the limit is always the most serious in four wells in the Hougao area, among which the chloride reaches 1140 mg/L at the highest point, 4.56 times higher than the Ш standard. Ammonia nitrogen reached 151 mg/L at the highest point, 302 times higher than the Ш standard. The highest level for petroleum was 5.5 mg/L, 275 times higher than the Ш standard. Benzene reached 58.2 mg/L at the highest value, 5820 times higher than the Ш standard. It shows that the Hougao region is seriously polluted and it is the key pollution area of the Dawu water source. Even though the Hougao area has adopted strong mining measures to reduce the concentration of pollutants in groundwater since the 1990s, the concentration of pollutants in the Hougao area still exceed the standard limit of groundwater quality several times from the recent monitoring data from March 2016 to May 2017.
At present, analytical and numerical methods are the main methods used in the study of pollutant transport [1
]. The numerical method is effective for the simulation of pollutant transport in complex geological conditions. The most commonly used numerical simulation software is Modflow, developed by the U.S. Geological Survey in the 1980s. At present, there are a large number of successful cases of pollutant transport using Modflow at home and abroad. For example, Shen et al. [6
] used this software to simulate the movement of groundwater pollution in a chemical plant in Shanghai. Xu et al. [5
] used Modflow to simulate groundwater pollution in karst areas in southwest China. Li et al. [7
] used this software to simulate and predict groundwater pollution in typical areas of the Hunhe fan. Seyed et al. [8
] used this software to simulate the pollutant migration in confined water aquifers. Therefore, the four Hougao wells as the key pollution source are taken as the research background, and the temporal and spatial migration of ammonia nitrogen under the action of strong pumping and stop pumping is simulated using the groundwater simulation software Modflow. At the same time, the blocking effects of the other four hydraulic interception wells on the pollutant under the action of strong discharging are simulated. The research results can provide a reference for the treatment of similar polluted sites.
2. Establishment of Hydrogeological Model
2.1. Research Scope and Determination of Boundary Conditions
The Dawu water source area is located in the mid-east region of Zibo city, mainly in the Linzi district of Zibo city. The west includes the partial region of the Zhangdian district, Fengshui town. The south includes the partial villages and towns northeast of the Zichuan district. The east involves the western region of Weifang city and Qingzhou city. Geographical coordinates are as follows: northern latitude 36°30′31″–36°51′12″, and east longitude 118°02′16″–118°26′55″. The key simulated area is the Qilu Petrochemical sewage treatment plant, which is located in the west of the Dawu water source area and the east of the Jinling fault. For the convenience of research, the sewage treatment plant is taken as the core and a certain distance is extended to the periphery as the range of the model. The south and north sides are basically perpendicular to the groundwater flow line, as the zero flow boundary, and the east and west sides are parallel to the flow line, as the constant flow boundary. The boundary flow is calculated by the cross section method.
2.2. Generalization of Source and Sink Terms
Rainfall evaporation: The climate of the working area (Linzi district) has a warm temperate continental monsoon climate. The annual average temperature is 12.2 °C. The average precipitation is 648.4 mm. The maximum annual precipitation was 1118.7 mm in 1964, and the minimum annual precipitation was 319.5 mm in 2006. The annual distribution of precipitation is uneven, and the annual variation is great. The area is located in a hilly area. The distribution of precipitation in the region is unevenly affected by geographical location, terrain, and other factors. The trend of the precipitation contour line and the topographic contour line is roughly the same, basically showing an east-west trend. The distribution trend of annual average precipitation is decreasing from south to north. Due to the influence of local terrain, the precipitation center is in the area of the Heiwang iron mine and Miaozi district. The annual change process of precipitation presents the alternation of abundant, and flat and dry. Precipitation in dry years is only about 60% of the annual average. The rainfall infiltration coefficient is the ratio of precipitation infiltration supply in a certain period to the corresponding precipitation in the same period. Additionally, it is related to many factors, such as groundwater depth, early soil moisture content, lithology, and vegetation. The precipitation infiltration recharge coefficient correlates with the groundwater depth. According to the engineering exploration data and technical requirements of hydrogeological parameter acquisition methods, the rainfall infiltration recharge coefficient is set at 0.15, and it is assigned with a recharge module.
The annual average evaporation is 1500–1900 mm. The distribution law is basically the same with the precipitation. The evaporation from April to July is the largest, and accounts for more than 53% of the annual evaporation, and the monthly average evaporation exceeds 200 mm. The evaporation in winter is the smallest, and accounts for less than 10% of the annual evaporation, and the monthly average evaporation is about 50 mm. The groundwater depth of the water source is generally deep, and the average depth is 50–100 m. According to the technical requirements of hydrogeological parameter acquisition methods, the evaporation coefficient is set at 0.01, and it is assigned by the evaporation module.
Mining situation in the study area: The northeast area is mainly the water source of the Xindian power plant with 16 mining wells. The well depth is about 350–400 m, and the production capacity is about 20,000–30,000 m3/d. It is the domestic and production water of the power supply plant. In addition, there are four strong drainage wells with depths of 350–400 m in the Hougao area. The producing capacity is 15,000–20,000 m3/d. The groundwater enters the water treatment center and is reused after treatment. There are also a large number of company-owned wells in the region, and the producing capacity is about 50,000–100,000 m3/d. The pumping value is assigned with the well module.
2.3. Generalization of Stratigraphic Structure
The water source area is high in the south and low in the north, with low hills in the south and piedmont sloping plain in the north. The east of the Zi river is a low and middle mountain area composed of Maan mountain, Qingliang mountain, and Ma mountain. The altitude is about 300–780 m. The west of Zi river is the surface watershed composed of mountains. The area from the south to the north gradually lowers into the piedmont sloping plain. The altitude is about 50–150 m. The terrain elevation near the watershed is 200–700 m, which is a hilly area. The mountains are monoclinic, which are gentle in the northwest slope and steep in the southeast slope.
The working area is located in the northeast of Zibo basin, which belongs to the transition zone between the mountain area of Luzhong and the North China Plain. The south of the working area is the low hill to low hilly area, and intermontane basin and valley terrain are distributed in local areas. The north area is the north-sloping piedmont plain. According to the characteristics of aquifer medium in the Dawu water source area, the simulation is divided into two water-bearing rock groups. They are aquifer formation and artesian aquifer formation, respectively. The aquifer formation is mainly composed of quaternary unconsolidated rock pore water aquifer formation and carbonate rock fissure karst water-bearing formation. The pore aquifer formation of the quaternary unconsolidated rocks is mainly distributed in the northern part of the study area. Its thickness increases gradually from south to north, commonly 50–100 m, and the maximum thickness can get to 200 m. The groundwater quality of this aquifer formation is poor, which is mainly used for agricultural irrigation water and domestic water of a small number of rural areas. Additionally, this aquifer formation is also part of the main groundwater recharge sources of the underlying carbonate fissure karst aquifer formation.
The fractured karst aquifer formation of carbonate rocks is widespread in the study area. The lithology of its water-bearing section is argillaceous dolomite, brecciated argillaceous limestone, medium-thick layer gray limestone, and leopard skin limestone. Limestone fissure karst is relatively developed. Its depth is between 60 and 300 m with high water content. It is the largest water-rich area in the water source area and the main water-intake aquifer in the Dawu water source area.
(1) The three-dimensional geological model of the Hougao area of the Dawu water source area was established by analyzing and summarizing the hydrogeological data of the Hougao area of the Dawu water source area. The ammonia nitrogen was taken as an example, and the transport and diffusion of the pollutant in the Hougao area were simulated. It was found that the pollutant was mainly controlled by the convection of groundwater and moves with the flow direction of groundwater to the northeast. In the absence of any control measures, the pollutant will quickly spread to the eastern centralized water supply area, posing a threat to water supply security.
(2) From the concentration change curve of pollution monitoring wells, it can be seen that the closer to the pollution source, the faster the concentration rises and the larger the maximum peak value, and the pollutant concentration decreases from the pollution halo center to both sides.
(3) After taking strong pumping measures in the four Hougao wells, the transport of ammonia nitrogen was effectively controlled, and the peak concentration of monitoring well drops significantly, from 4–5 times of exceeding the standard to slightly exceeding the standard, but there was still a large range of exceeding the standard in Xixia area.
(4) Under the combined action of hydraulic interception wells and strong pumping wells, the scope of ammonia nitrogen was basically controlled in the vicinity of interception wells and does not continue to spread to the eastern water supply area. The maximum monitoring concentration value of pollution was 0.11 mg/L, which is within the groundwater standard limits of class Ⅲ, and the pollutant control effect is the best, which provides a certain reference for similar pollution control works.