| Thermal springs with low-to-moderate temperature are raised by heating of geothermal gradient during the cycle process of ground water. In order to utilize the geothermal resources sustainably, knowledge of geothermal fluid and water circulation should be known well. Analyzing the characteristic of the low-to-moderate thermal spring qualitatively and discussing the variation laws of spring discharge and temperature quantitatively may contribute to the study of the formation mechanism, distribution rules as well as development and utilization of water deep cycling. The author elaborates the geological background of the spring occurrence, analyzes and summarizes hydrogeochemical and isotopic characteristics of geothermal water, calculates the water resource, recharge elevation, age, depth and temperature of the geothermal water, builds effective mathematical models of spring discharge and temperature based on the study of 14 low-to-moderate thermal springs distributed in Hebei province and Beijing.Based on field investigation of the study area, the research results of these hot springs are collected, which are proved by the predecessor and a summary is also made. The study area is located in the intermountain basin southwest of the Yanshan Mountain the northwestern hills area. The springs emanate from piedmont, basin border, the lithology of the strata in the vicinity of the springs are unconsolidated sediments, limestone, granite and gneiss, etc, which were influenced by faults and bed rock. The temperature of hot springs ranges from 37 to 85℃, the spring discharge ranges from 0.11 to 13.9 L/s. The result of the hydrogeochemical study show that the major ions of the hot water are Na+, K+, Ca2+, Mg2+, the major anion are HCO3-, Cl-, SO42-, minor ions are NH4+, CO32-, NO3-, hydrochemical type is mainly of SO4·HCO3-Na type. The TDS range from 0.189 to 1.3 g/L, pH from 7.73 to 9.35,H2SiO3 from 26.1 to 96.3 mg/L,F- from 1.6 to 1.7 mg/L. The isotopic compositions indicate that the thermal water comes from the rainfall. The evaluated recharge elevation is about 140-1850 m or 200-1530 m, temperature of the recharge area is about 4.5-11.6℃or -1-8.7℃, ages of most of the springs range from 12 to 68 a. Temperature range of geothermal reservoir is from 62 to 121℃, circulation depth of the spring ranges from 1700 to 3700 m or from 2100 to 5300 m. Mixing ratio of the Huailai spring and Cuizhuang spring are 0.7 and 0.82 respectively. By analyzing the thermal balance with the (Q/K) approach, there is nearly no minerals (fluorite, quartz, anhydrite, chrysotile, chalcedony, amorphous silicon, talc) reaching the thermal balance under any certain temperature.Mathematical models of spring discharge and temperature are built based on the Darcy's law and heat transport theory. Single U-like pipes aquifer model and double U-like pipes aquifer model are established based on the study results of circular arc-like pipes aquifer model and four kinds of mathematical model are used to check the measured spring discharge and temperature data and discuss their affecting factors are discussed. The results indicate that temperature raises first and then descends. Temperature of most spring is fit well with the results of the mathematical models. Thus, spring discharge and temperature of low-to-moderate thermal springs with fault-depth cycling mode can be estimated by using these mathematical models.
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