Research and application of groundwater level as a water replenishment control signal in the restoration of Lijiang Heilongtan spring group

The paper takes Heilongtan in Lijiang as an example to study the recharge control process of spring water restoration in the karst area. Despite much work on Heilongtan in the previous period, there still exist problems in terms of the spring water restoration of Heilongtan, such as difficulties of detecting recharge sources and of controlling the recharge time, and the high cost of constructing recharge channels. On the basis of the groundwater system theory, and the spatial geographic information and geologic information of Heilongtan spring system, this paper aims to study the restoration of Heilongtan spring group, which may provide a scientific basis for the restoration of karst springs and environmental protection.

The Lijiang area, where the study area is located, belongs to the low-latitude plateau monsoon climate zone, with unique mountain monsoon climate characteristics, distinct dry and wet seasons, and significant vertical climate changes, but slight seasonal variations in temperature. Because the Lijiang area is surrounded by the Jinsha River in the west, north and east, deep-cut canyons and a peninsula-like mountainous area protruding northward have been developed. With an elevation of about 2,400 m, a width of 4-5 km from east to west and a length of 32 km from north to south, the Lijiang Basin—a long and narrow mountainous basin—is located in the southern part of the Lijiang Peninsula.The study area is located in the northeast side of the mountainous area of Lijiang Basin, and the Heilongtan spring group is located in the southwest corner of the study area at the foot of Xiangshan hill in the north of the Lijiang ancient city at the east of Lijiang Basin. The study area generally belongs to the alpine mountainous terrain in the dissolution fault block, and the karst in this area is developed. Based on three-dimensional data on basic geological conditions, karst hydrogeological conditions, and characteristics of water balance, seepage field, temperature field, chemical field, etc., connection tests have been conducted to verify that there are two karst groundwater flow systems in the north and south of the study area: Jiuzihai-Lijiangba karst groundwater flow system (Ⅰ) and Hongshuitang-Bailanghua karst groundwater flow system (II). There is no hydraulic connection between these two systems. The Jiuzhihai-Lijiangba karst water flow system (Ⅰ) can be laterally divided into the karst water flow subsystem of Heilongtan spring basin (Ⅰ-1), the karst water flow subsystem of Qingxi spring basin (Ⅰ-2) , and the decentralized groundwater flow system of eastern spring group (Ⅰ-3). These three karst water flow subsystems are self-contained in terms of recharge-runoff-discharge conditions, with weak hydraulic connection, and there is no hydraulic connection between the Jiuzihai recharge area and the Qingxi spring basin and the eastern spring group.

Many discharge points in the Heilongtan spring group result to the difficulty in flow observation. At present, only monthly average flow values with low accuracy have been obtained but not in real time; consequently, it is difficult for us to realize the real-time and accurate control of the spring recharge process. Moreover, when the Heilongtan spring group dry up, it is even impossible for us to control the recharge process through the flow data. The observation values of water level per minute in a hydrological observation well near the Heilongtan spring group and the qualitative analysis of geological structure indicate that the hydrological observation well and the spring group are located in the same karst groundwater system. In addition, the high correlation coefficient of 0.8813 between the water level of the hydrological observation well and the flow of the Heilongtan spring group , indicating that it is possible to control the water level of the Heilongtan spring group through the regulation of water level of the hydrological observation well.

The Heilongtan spring group consists of four main springs, namely, Lieshimu spring, Wufenglou spring, Zhenzhu spring and Wanshousi spring, and the elevation of each spring is slightly different. The landscape of Heilongtan Park can meet the requirement only if the flow of these four springs is maintained. According to the monitoring data of the Heilongtan spring group, when all of these four springs flow, the overall flow of the Heilongtan spring group is 1.00 m³·s⁻¹, which is also the target flow rate for spring restoration. Based on the actual flow and its change trend of springs, two conditions can be determined: water maintenance and recharge when water is in shortage. Under the condition of water maintenance, the signal of replenishing water should be firstly determined, during which predictors needs to be set given the time of water replenishment. Then, an inversion can be conducted to obtain the water level of hydrological observation well and the flow of the Heilongtan spring group, based on the time when groundwater flows from Jiuzihai to the Heilongtan spring group. When the water level of hydrological observation well drops to 2,409.41 m or the spring flow reduces to 1.27 m³·s⁻¹, the spring water in Jiuzhihai recharge area should be recharged, with water flow of 2.08-2.35 m³·s⁻¹, and the recharge can be lasted to the next flood season in which the rainfall and water discharge achieve an equilibrium again and Jiuzihai is restored. Under the state condition of replenishment when water is in shortage, the spring water in Jiuzhihai recharge area needs to be recharged with the maximum flow rate (greater than the minimum flow rate of recharge), so that the spring flow rate can reach 1.00 m³·s⁻¹ as soon as possible. Then, the spring water can be recharged with a flow rate of 2.08 m³·s⁻¹, so that the groundwater recharge and discharge keep balanced and the spring flow rate maintains 1.00 m³·s⁻¹. The replenishment can be lasted to the next flood season, in which the rainfall and discharge achieve an equilibrium again and the Jiuzhihai is restored for the next recharge.

In summary, in order to determine the signal of control well given the time of water recharge and finally carry out the accurate and real-time water recharge based on the water level of observation well as the control signal, the following steps should be completed: formulating the target state of the spring group, corresponding to the water level of replenishment control well, considering the time predictor in the recharge process, and correcting the water level of control well.