## 2 Methodology  
### Study Area Overview  
The study area was a chemical park in China. The study area was 5 million m\({}^{2}\). The maximum and minimum elevation of the area were 480 and 520 m. It has a typical continental arid climate with hot summers and cold winters, a multi-year average rainfall of 15.8 mm, and an average annual evaporation of 2293.3 mm. The recharge of rainfall to the groundwater in this area is not obvious. The groundwater is not significantly affected by evaporation because of its general depth of more than 10 m in the study area. The study area has no rivers and lakes and groundwater is not significantly exploited. The groundwater in the study area only receives recharge and discharge from lateral seepage. The interannual variation in the groundwater level is not significant, and it can be assumed that the groundwater level remains stable all year round.  
Figure 1 shows the 19 water level observation points of the study area unconfined aquifer (which is the focus of this study). The groundwater flows from south to north, while the unconfined aquifer thickness also gradually increases from south to north. The lithology is a sand and pebble gravel layer, with the presence of a powder layer in some areas. The study area, guided by the different cementation degree of gravel and sand layer, is divided into 4 parameter zones. The eastern and western boundaries of the study area are perpendicular to the water table and there are zero flow boundaries. The southern and northern boundaries are known as hydraulic boundaries.  
The chemical company in the study area, which produces methanol, was established in February 2013, In order to ensure the production and living water quality of downstream residential areas, it is necessary to identify the current status of pollution of regional groundwater by the chemical industry park, and to sample the water level monitoring points. The monitoring indexes were pH, salinity, metasilic acid, total hardness, carbonate, bicarbonate, sulfate, chloride, fluoride, cyanide, nitrate, nitrite, ammonia nitrogen, permanganate index, COD, volatile phenol, iodide, arsenic, mercury, hexulent chromium, lead, cadmium, manganese, aluminum, selenium, zinc, etc. During the August 2021 groundwater sampling run, the pollutants exceeding the water quality standard were Mn\({}^{2+}\), ammonia nitrogen and volatile phenol. This paper considers only the Mn\({}^{2+}\) contamination. The groundwater contamination in the chemical park was divided into three main areas (Figure 2). The leakage points in the accident pond and pretreatment area were determined via a site survey, and the contamination range of two sources was determined via groundwater monitoring at points C1-C19. The observed chemical concentrations at monitoring points Obs1 and Obs2 were significantly higher than those of points C3 and C11, indicating that there was at least one more source of pollution between these four points. However, the site investigation did not reveal any possible contamination leakage from any ground structures. It is therefore likely that the contamination occurred during the historical operation of the company, and possibly there is only one source of contamination. During the site investigation, no suspected source of pollution was found. The possibility of long-term anthropogenic discharge of pollutants was ruled out; therefore the length of the pollution leak should not be too long. The longest conservatively estimated length for pollution leakage should not exceed 1000 days. If taking the day the company began operations as day 1, it was estimated that the leak would have occurred no later than day 3150. Table 1 shows the aquifer and the contamination source parameters.  
Figure 1: Overview of the study area.