The Hydrochemical Response of Heilongtan Springs to the 2010
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THE HYDROCHEMICAL RESPONSE OF HEILONGTAN SPRING TO THE 2010–2012 DROUGHTS OF YUNNAN PROVINCE, KUNMING, CHINA Hong Liu International Joint research Center for Karstology, Yunnan University, No. 5 Xueyun Road, Wuhua District, Kun- ming, Yunnan, 650223, China, [email protected]; School of Resource Environment and Earth Science, Yunnan University, Yunnan University Chenggong Campus, East Outer Ring Road, Chenggong District, Kunming 650500, China Ruiyong Chen School of Resource Environment and Earth Science, Yunnan University, Yunnan University Chenggong Campus, East Outer Ring Road, Chenggong District, Kunming 650500, China Huacheng Huang School of Resource Environment and Earth Science, Yunnan University, Yunnan University Chenggong Campus, East Outer Ring Road, Chenggong District, Kunming 650500 Yinghua Zhang School of Resource Environment and Earth Science, Yunnan University, Yunnan University Chenggong Campus, East Outer Ring Road, Chenggong District, Kunming 650500, China Yongli Gao Department of Geological Sciences, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas, 78249, USA, [email protected] Abstract 2010–December 2012 covering two complete hydro- Karst waters from a mountainous recharge area drains logic years were used to investigate the response of hy- toward basin and emerges at the edge of the basin af- drochemical changes to prolonged and severe droughts ter encountering quaternary sediments. The flow paths in Yunnan from 2010 to 2012. During the drought, in are partly covered by Quaternary sediments or other addition to the decline of water table, the EC of spring sedimentary rocks, which makes the spring acts as an decreased progressively from 319.5 μS/cm (yearly av- artesian spring. The spring is more vulnerable to hu- erage, ranging from 294.0 to 339.1 μS/cm) in 2010 to man activities and climate change than the classic con- 299.2 μS/cm (ranging from 248.9 to 323.3 μS/cm) in fined karst spring. Weakly confined spring is typical in 2011 and 277.6 μS/cm (ranging from 237.4–311.4 μS/ faulted karst basin. Heilongtan Springs are located at the cm) in 2012. At the same time, instead of the tendency of foothills of Wulao Mountains in the northern suburb of temperature to decrease, pH values increased. In 2010, Kunming City. They consist of Qingshuitan (QST), Hun- the ratio of Ca/Mg was constantly increasing from May, shuitan (HST) and Xiaoshuitan (XST) springs. Spring but in 2011 and 2012, Ca/Mg ratios fluctuated greatly, water appears clear and muddy in QST and HST Spring, which highlights the rapid responses of spring to pre- respectively, even though they share a common outlet cipitation. All the evidence has shown that as the drought at the southern end of QST Spring pool. From 2010 to went on, the water from the karstic aquifer kept draining 2012, a comprehensive hydrochemical study has been out and the residence time of water got shorter. Mean- performed in Heilongtan Springs. In coincidence, the while the water quality of the spring was more sensi- study area experienced continuously severe droughts in tive to the rain events. With a small to medium amount – – Yunnan Province, China, which offered an opportunity of precipitation, the concentrations of Cl and NO3 of to better understand the impact of global climate change the spring water started to increase a few hours after the on such weakly confined karst springs. Both concentra- rain, which would last 7 to 14 days. In summary, this tions of hydrochemical parameters at high-resolution study has shown the high sensitivity and variability of a (every 15 min) and weekly measured data during March weakly confined karst spring to environmental changes. 16TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 8 200 Introduction from the 2 springs normally do not mix. Xiaoshuitan A faulted karst basin is a special type of hydrogeologi- spring is an epikarstic spring with very small discharge cal unit. It originated from neotectonic movement and Figure 1. formed a basin-like topography surrounded by moun- tains, which is at least partly karstified with several hun- The discharge of Heilongtan Spring, based on monitor- dred meters of Quaternary sediments in the basin (Wang ing results of 1960, ranged from 82.78 l/s to 365.5 l/s et al., 2003). The karst water from mountainous recharge with an average flow rate of 208.3 l/s. Water tempera- areas drains toward the basin and emerges at the edge of ture of Qingshuitan Spring is 18 to 20℃, which is 2–4℃ the basin in contact with Quaternary sediments. The flow higher than shallow karst springs in surrounding areas. paths to the spring are partly covered by Quaternary or Since the exploitation of groundwater in the 1960s, Tertiary sediments or other types of rocks, which makes water levels began to decline. In the early 1980s, due the spring act like an artesian spring, but it is more sensi- to the substantial increase in exploitation, water levels tive to climate change than the classic spring of a con- dropped by 3–4 m within 5 years. The decline of water fined aquifer. Such karst springs are named as weakly levels caused Heilongtan Spring to be completely dry in confined spring, which are typical of springs in faulted the mid-1980s to the end of the 20th century during dry karst basins. These karst springs play essential roles in seasons. Consequently, instead of being a local indus- local water supplies. Heilongtan Spring, one of the most trial and municipal water supply source, the spring only important karst springs in Kunming, Yunnan, had been serves as an important landscape feature of Heilongtan supplying water to Kunming City until the late 20th cen- Park now. tury. The spring dried out many times because of exces- sive water demand and urbanization of Kunming in the The geological setting of the study area is very complex 1980s to 1990s. (Figure 1). Lithology of the Heilongtan Spring karst sys- tem corresponds to massive to thin carbonate rocks of the Heilongtan Spring karst system is a complex and het- Paleozoic Era, which is highly folded. The lithology con- erogeneous system in terms of structure and hydrologic sists of fine-grained limestone and dolomitic limestone, characteristics (Huang & Liu, 2015; Liu et al., 2011). interbedded with 10–30 m thick mudstone or sandstone However, the dynamics and origins of groundwater bands. The N-S oriented Heilongtan fault is the most and the mechanism of contaminant transport are poorly important fault in the study area. In the recharge area, understood. From 2010 to 2012, a comprehensive hy- Fengcong-depression and Fengcong-valley morphologi- drochemical study has been performed. In coincidence, cal features are well developed. The karst water is thought the study area experienced the prolonged and severe to flow under a Permian basalt along the east branch of droughts of Yunnan Province, China, which offered an Heilongtan fault,and emerges to the surface which is in opportunity to better understand the impact of global contact with Quaternary sediments in the basin. climate change on such a weakly confined karst spring. The geographical setting belongs to a north subtropi- Site Description cal plateau monsoon climate. For the 1961–2016 peri- Heilongtan Spring, one of the main karst springs in Kun- od, the average annual rainfall is 996.6 mm, and 85% ming, China, is located in the foothills of Wulao Moun- of precipitation occurs between May to October. From tains in the northern suburb of Kunming City. The spring 2010 to 2012, the entire Yunnan Province suffered a pro- emerges along the east branch of Heilongtan Fault after longed and severe drought. The annual precipitation of karst water meets the Quaternary sediments at the edge Kunming was 982.2 mm in 2009, 565.8 mm in 2010, of Kunming Basin. Heilongtan Spring is a typical weak- 869.1 mm in 2011, 659.0 mm in 2012 and 802.1 mm in ly confined karst spring, with a main conduit developed 2013 respectively. below spring level bounded by a major regional fault. This spring is composed of Qingshuitan, Hunshuitan, Methods and Xiaoshuitan springs. Qingshuitan means clear water A CTDP300 multi-parameter water quality meter (made spring and Hunshuitan means muddy water spring. They by Greenspan Corporation in Australia) was placed at are connected by a bridge. Though all of them share an Qingshuitan Spring pool in the end of March 2010 to ob- outlet at the end of Qingshuitan spring pool, the water tain water temperature, electrical conductivity, pH, and 201 NCKRI SYMPOSIUM 8 16TH SINKHOLE CONFERENCE Figure 1. Hydrogeological map of the Heilongtan karst system and simplified N-S geological cross section. water level parameters with an interval of 15 min. Pre- trical conductivity, and pH parameters of water samples cipitation data was recorded by a RG2-M rain gauge in- were measured in situ by WTW350i. stalled on the roof of the Heilongtan Park headquarters. Results and Discussion Water samples from two springs (QST and HST) were Results of hydrochemical data are presented in Table collected weekly from March 2010 to November 2012, 1. Figure 2 illustrates daily record of seasonal (wet and which were analyzed for concentrations of Ca2+, Mg2+, dry seasons) variability with fine details of storm-scale – – – + 3– HCO3 , Cl , NO3 , NH4 , and PO4 . The hydrochemical fluctuations. Figure 3 shows detailed hydrochemical samples were stored in 1000 ml high density polyeth- variations of the spring during the drought. Though the ylene bottles. Ca2+ and Mg2+ values were determined data of water levels in Figure 2 demonstrates seasonal by ethylenediaminetetraacetic acid (EDTA) titration. variability of spring water level, we did not take it in – – HCO3 and Cl values were measured by hydrochloric account, because a dam was built 100 m downstream to acid and argentum nitricum liquor titration, respectively, maintain the water level of the spring pool for the sake in the Karst Laboratory of the International Joint Re- of tourism and some spring water was used for watering search Center for Karstology, Yunnan University on the the garden daily.