The Characteristics of Karst Aquifers and the Variation of Karst Hydrological Processes in Northern China

Approximately 3.44 million km(2) of carbonate rock exists in China and occupies one-third of China's territory. In general, the karst in China can be classified as two types (i.e., the karst in north China and the karst in south China) based on the paleodepositing environment of carbonate rocks and subsequent karstification processes. On the one hand, the lithology of carbonate rocks deposited between the Mesoproterozoic Era and the Mid-Ordovician in north China are micritic, which is difficult to erode compared with the sparry limestones common to south China, and deposited between the Sinian Sub-Era and the Triassic period. On the other hand, the paleoclimate during karstification processes in north China was dry and cold, and karstification occurred mainly in the subsurface, whereas the paleoclimate during karstification in south China was wet and warm, and karstification took place mostly at or near the surface. Karst aquifers are therefore characterized by fractures and conduits in the north, and by caves, tunnels, and subsurface rivers in the south. One of the significant characteristics of a karst hydrological system in the north is that the groundwater in a karst aquifer converges in a topographical depression and creates a large karst spring. To better understand the effects of climate variation and anthropogenic activity on spring discharge, the authors focused on the karst hydrological processes in north China, examined the Niangziguan Springs as a case study of karst hydrology in north China, and analyzed the spring discharge from 1959 to 2015 using statistical methods, including piecewise analysis, Wilcoxon rank-sum test, and wavelet coherence. Results indicate that the data of the Niangziguan Springs discharge can be divided into two groups: a pre-development period from 1957 to 1980 and a post-development period from 1981 to 2015. The annual average precipitation was 531 mm in the pre-development period and 462 mm in the post-development, showing a 13% decrease in precipitation. The annual mean spring discharges in the pre- and the post-development periods were 12.54 and 7.67 m(3)/s, respectively, showing a 39% decline. Compared to the precipitation, the spring discharge has a 26% greater reduction on average between the periods. This fact indicates that the spring discharge was attenuated by almost 26% owing to the effects of human activity in the Niangziguan Springs basin. To see whether human activity alters the frequency (i.e., shortand long-term variations) of the discharge of Niangziguan Springs, the time series of spring discharge in both the preand the post-development periods were analyzed by wavelet coherence. Results indicate that although groundwater development has weakened the relation between spring discharge and precipitation, the resonant frequency between spring discharge and precipitation remain unchanged.