Abstract: In order to quantitatively evaluate the degree of groundwater hydraulic connection between aquifers, the concept of "hydraulic connection coefficient" C is first proposed. Based on the drawdown of groundwater level in full penetrating wells during the steady-state pumping test when the groundwater level is stable, the hydraulic connection coefficient C is defined as the ratio of the drawdown of groundwater level of the target aquifer to the drawdown of groundwater level of the pumping/dewatering aquifer in the observation borehole. The hydraulic connection coefficient can be used to quantitatively evaluate the degree of hydraulic connection between two points in different directions and different distances on the aquifer plan, and also between different aquifers in the vertical direction. According to the hydraulic connection coefficient C value of the aquifers in Luohe Formation in Ordos basin, the degree of groundwater hydraulic connection between the aquifers is divided into five grades:Among them, 0.000 0 ≤ C<0.062 5, the hydraulic connection is very weak; 0.062 5 ≤ C<0.125 0, the hydraulic connection is weak; 0.125 0 ≤ C<0.250 0, the hydraulic connection is medium; 0.250 0 ≤ C<0.500 0, the hydraulic connection is strong; C ≥ 0.500 0, the hydraulic connection is very strong. Taking the data of pumping test and dewatering test in Gaojiabu mine field as an example, the hydraulic connection coefficient and the response time of groundwater level in the observation borehole beginning to fall are used to quantitatively evaluate the internal groundwater hydraulic connection of the thick and layered Luohe Formation. Among them, when the hydraulic connection coefficient of the middle-upper layers in Luohe Formation is 0.373 0 and 0.413 8 respectively, and the response time of groundwater level in the observation borehole beginning to fall is very short (about 5 min), the groundwater hydraulic connection is strong; The hydraulic connection coefficient of the lower layers in Luohe Formation is 0.440 1 and 0.491 1, respectively, and the response time of groundwater level in observation borehole beginning to fall is short (9-20 min), the groundwater hydraulic connection is strong; When the hydraulic connection coefficient between the middle-upper and lower layers in Luohe Formation is 0.000 2, 0.007 2 and 0.089 7, respectively, and the response time of groundwater level in the observation borehole is longer (more than 60 min), the groundwater hydraulic connection is weak or very weak.

Key words: Luohe Formation aquifer, vertical layered aquifers, hydraulic connection coefficient, pumping (or dewatering) test, groundwater