Chengde City boasts a wealth of high-quality mineral water resources characterized by a high level of strontium (Sr), a low level of sodium, and low alkalinity. In order to study the mechanism of formation of Sr-bearing mineral water in Chengde and to scientifically guide future mineral water exploration, taking three typical mineral water exploration areas in Chengde as examples, this paper studies the sources of Sr in mineral water and the general rules of its dissolution via a laboratory static leaching experiment and impact experiments, and it provides an analysis of the characteristics of typical rock samples. The research results indicate that the content of Sr in surrounding rock and the characteristics of minerals existing in surrounding rock jointly control the dissolution of Sr in water; that CO₂ can promote the formation of mineral water containing Sr; and that temperature increases may boost the dissolution of Sr from carbonate minerals but also inhibit the dissolution of Sr from silicate minerals.

Blindly developing shallow geothermal energy (SGE) may lead to difficulties in reinjection, a low coefficient of performance (COP), and the waste of funds. In order to avoid these problems and improve development efficiency, it is necessary to scientifically conduct a development suitability evaluation and classify the suitability levels before development. This article takes Chengde City, Hebei Province, China as an example and constructs an evaluation index system based on the analysis of regional basic conditions. The Analytic Hierarchy Process (AHP) was used to calculate the constant weight. A Fuzzy C-means Clustering (FCM) analysis was used to determine the critical value of evaluation index classification and the interval threshold of variable weights. The parameters of the variable-weight model were calculated using the method of model backcalculation. Based on the constructed variable-weight model, the suitability of SGE development for groundwater sources in the study area was evaluated, and it was divided into five levels: most suitable area, suitable area, relatively suitable area, less suitable area, and unsuitable area. Through a verification analysis of engineering examples and a comparison with the evaluation results under traditional constant-weight models, it was found that the evaluation results based on variable weight theory have better data discreteness and a higher accuracy. Research has shown that variable-weight models can adjust the weight of each index based on its state value. Through this study, the accuracy of the suitability evaluation for regional SGE development can be improved. This can provide a certain reference for the suitability evaluation of SGE development in other regions.

The Double U-pipe ground heat exchanger, recognized for its straightforward process, cost-effectiveness, high heat exchange efficiency, and low thermal resistance, stands as the predominant ground heat exchanger type in today's shallow geothermal energy development and utilization. Recent years have seen significant research into the heat transfer influencing factors and heat exchange performance of Double U-pipe ground heat ex-changers via experimental testing methods. Yet, studies integrating numerical simulation with in-situ testing have been less frequent. Drawing on the cylindrical heat source model theory and the outcomes of regional in-situ thermal response tests, this paper develops a Double U-pipe ground heat transfer model by establishing physical, mathematical, and heat transfer geometric models. It assesses the impact of varying inlet temperatures, flow rates, and initial ground temperatures on heat exchange efficiency under heating conditions. The results validate the precision of the double U-pipe ground heat exchanger model based on in-situ testing. They show that enhancing the temperature differential between the inlet and initial temperatures, elevating the initial ground temperature, and moderately increasing the flow rate can boost the system's heat exchange efficiency.

Nitrate is one of the most prevalent inorganic pollutants in groundwater systems. Its concentration directly affects the safety assessment of groundwater quality. To scientifically identify nitrate sources in strontium-rich mineral water and facilitate the protection of mineral water resources, this study selects Chengde City, Hebei Province, as the study area. Nitrate source apportionment was quantified using the MixSIAR model, with uncertainties assessed via cumulative probability distributions. The results show that the nitrate concentration in strontium-rich mineral water of the study area ranges from <0.003 to 70.2 mg/L, with a coefficient of variation of 1.24, indicating high data dispersion. The nitrate sources present a mixed pollution characteristic dominated by natural processes and supplemented by human activities. Nitrification dominates biogeochemical processes of strontium-rich mineral water. Soil nitrogen is the primary contributor to nitrate in mineral water, with an average contribution rate of 70.4%, followed by manure; synthetic fertilizers and rainwater together account for less than 1% of the total. Uncertainty analysis shows that contributions of rainwater, synthetic fertilizer, manure, and soil nitrogen to nitrate in strontium-rich mineral water remain stable (UI₉₅ < 0.15), verifying reliable results. This study provides guidance for protecting mineral water quality and sustainable resource exploitation.