A study on the groundwater recharge range of Baotu Spring based on wavelet analysis

As a famous "spring city", karst groundwater used to be the main source of water supply in Jinan City. However, with the acceleration of urbanization and the rapid increase of population, the pressure on urban water supply has increased, and hence the contradiction between supply and demand has become increasingly prominent. This situation will aggravate the over-exploitation of karst groundwater, resulting in a substantial decline in the karst groundwater level and outflow of Baotu Spring. How to deal with the contradiction between the continuous gushing of spring water and the citizens' demand for drinking high-quality underground water has always been an important issue of general concern to the Jinan Municipal Government and the general public. In recent years, Jinan City has successively adopted measures such as groundwater recharge and reducing groundwater exploitation to maintain the gushing of Baotu Spring, but the situation of keeping the springs gushing in the dry season is still severe.

Scholars at home and abroad have conducted many studies on the formation mechanism, supply channels, dynamic characteristics, and influencing factors of Baotu Spring. However, there is still a lack of research on the groundwater recharge range of Baotu Spring, and on the proportion of urban and western suburbs to Baotu Spring's recharge, which has affected Jinan's scientific conservation of springs and rational development and utilization of groundwater resources. In order to find out the groundwater recharge range of Baotu Spring in Jinan City and the proportion of the recharge range of Baotu Spring in urban and western suburbs, this study selects the data on karst water levels from 20 long-term groundwater monitoring points in Baotu Spring area from 2010 to 2020 as well as the precipitation data of the same period, and analyzes the time lag of the groundwater level to the precipitation by using the method of cross wavelet transform. According to the hydrogeological conditions and the functional division of spring water, the groundwater recharge range of Baotu Spring is discussed, and the proportion of the groundwater recharge range of Baotu Spring in the urban and western suburbs is calculated.

The results show that, (1) From the direct recharge area to the exposed area of confluence in the Baotu Spring area, with the rise of the length of groundwater runoff, the time lag of groundwater level to precipitation shows an increasing trend from 78.58 days to 129.22 days. The time lag gradient in the western suburb of Jinan City is larger than that in the urban area, with the time lag gradient decreasing gradually, and then tending to be stable. (2) A large area of groundwater recharge is distributed in the lower reaches of the Yufu river, with the characteristics of relatively flat terrain, slow surface runoff, strong rainfall infiltration capacity, slow groundwater flow velocity, and long runoff path, so the time lag of groundwater level to precipitation along the river is greater than that on both sides. The area of Liuchang mountain-Langmao mountain-Wanling mountain is a local surface watershed, with fast surface runoff, weak rainfall infiltration capacity, and relatively fast groundwater flow. Therefore, the time lag of the groundwater level to the precipitation in this section is smaller than that on both sides, and there is a hydraulic connection between the urban area and the western suburb of Jinan City. (3) The time lag contour of Baotu Spring water level to precipitation (107.30 days) is taken as the northern boundary of the Baotu Spring recharge range; the eastern boundary is the Dongwu fault; the western boundary is the Mashan fault; the southern boundary is the surface watershed. The groundwater recharge range of Baotu Spring is 1,390.54 km². (4) The area of Liuchang mountain-Langmao mountain-Wanling mountain is taken as the dividing line between the urban area and the western suburb of Jinan City, and the indirect recharge area of the spring is divided according to the surface water catchment. It is calculated that the recharge range of Baotu Spring in the western suburb is 1,133.09 km², and the recharge range of Baotu Spring in the urban area is 257.45 km², with a ratio of 4.4∶1 between the two recharge ranges. The supply of groundwater in the western suburb of Jinan has played a strong role in supporting the continuous gushing of Baotu Spring. Since the karst water system is a complex dynamic system, the response time lag of groundwater level to precipitation is affected by factors such as precipitation seasonality and human activities. Later on, the recharge range of Baotu Spring can be refined and verified by a smaller time scale, or from the perspective of water chemistry.