(Cfcs) and Tritium to Improve Conceptual Model of Groundwater flow in the South Coast Aquifers of Laizhou Bay, China
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HYDROLOGICAL PROCESSES Hydrol. Process. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.8450 Using chlorofluorocarbons (CFCs) and tritium to improve conceptual model of groundwater flow in the South Coast Aquifers of Laizhou Bay, China D. M. Han,1* X. F. Song,1 Matthew J. Currell2 and Maki Tsujimura3 1 Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China 2 School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, Australia 3 Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan Abstract: The southern coastal plain of Laizhou Bay, which is the area most seriously affected by salt water intrusion in north China, is a large alluvial depression, which represents one of the most important hydrogeological units in the coastal region of northern China. Chlorofluorocarbons (CFCs, including CFC-11, CFC-12 and CFC-113) and tritium were used together for dating groundwater up to 50 years old in the study area. There are two cones of depression, caused by intensive over-exploitation of fresh groundwater in the south and brine water in the north. The assigned CFC apparent ages for shallow groundwater range from 8ato>50 a. A binary mixing model based on CFC-113 and CFC-12 concentrations in groundwater was used to estimate fractions of young and pre-modern water in shallow aquifers and to identify groundwater mixing processes during saltwater intrusion. Discordance between concentrations of different CFC compounds indicate that shallow groundwater around the Changyi cone of depression is vulnerable to contamination. Pumping activities, CFC contamination, mixing and/or a large unsaturated zone thickness (e.g. >20 m) may be reasons for some groundwater containing CFCs without tritium. Saline intrusion mainly occurs because of large head gradients between fresh groundwater in the south and saline water bodies in the north, forming a wedge of saline water below/within fresh aquifer layers. Both CFC and tritium dates indicate that the majority of the saline water is from >50 a, with little or no modern seawater component. Based on the distribution of CFC apparent ages, tritium contents plus chemical and physical data, a conceptual model of groundwater flow along the investigated Changyi-Xiaying transect has been developed to describe the hydrogeological processes. Three regimes are identified from south to north: (i) fresh groundwater zone, with a mixing fraction of 0.80–0.65 ‘young’ water calculated with the CFC binary mixing model (groundwater ages <34 a) and 1.9–7.8TU of tritium; (ii) mixing zone characterized by a mixing fraction of 0.05–0.65 young groundwater (ages of 23–44 a), accompanied by local vertical recharge and upward leakage of older groundwater; and (iii) salt water zone, mostly comprising waters with ages beyond the dating range of both CFCs and tritium. Some shallow groundwater in the north of the Changyi groundwater depression belongs to the >50a water group (iii), indicating slow velocity of groundwater circulation and possible drawing in of saline or deep groundwater that is tracer-free. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS Laizhou Bay; coastal aquifers; mixing processes; saltwater intrusion; groundwater age dating Received 15 June 2011; Accepted 9 December 2011 INTRODUCTION recharged waters (e.g. since the 1950s). These ‘young’ groundwater age tracers have been successfully used to Following improvements in analytical techniques and determine groundwater recharge ages and flow paths of understanding of hydrological processes, environmental water in many studies (e.g. Busenberg and Plummer, 1992; tracer methods have become increasingly useful for Ekwurzel et al., 1994; Cook et al., 1995; Szabo et al., extracting information on the hydrological cycle. Evalu- 1996; Oster et al., 1996; Cook et al., 1996; Plummer ation of groundwater age is important in understanding et al., 1998; Cook et al., 2003; Cook et al., 2006; groundwater flow system dynamics and timescales of Hinsby et al., 2007; Newman et al, 2010 and references water circulation. The combination of the information therein), contributing enormously to our understanding from groundwater tritium and chlorofluorocarbons of the complex hydraulic dynamics of shallow aquifers, (CFCs; CFC-11, CFC-12 and CFC-113) concentrations, including those affected by anthropogenic influences. which have continuous known variations in concentration Coastal aquifers often are complex and heterogeneous in the atmosphere over the last 60 years, can provide systems, where multiple sources of water and salinity are valuable insight into movement and mixing of recently present. Hence, the use of multiple age indicators, along with physical and chemical data, has the potential to aid *Correspondence to: D. M. Han, Jia11, Datun Road, Chaoyang District, understanding of complex processes occurring in these 3 100101, Beijing, China. aquifers. Age/residence-time obtained from H and CFCs E-mail: [email protected] has been used to aid in the estimation of the timing of Copyright © 2012 John Wiley & Sons, Ltd. D. M HAN ET AL. seawater intrusion and time required for re-equilibration intrusion into the south coast aquifer of Laizhou Bay, of the saline/fresh groundwater interface (Zakhem and based on analysis of groundwater major ion and multi- Hafez, 2007). In addition to direct dating methods, the isotope composition (2H/18O, 3H, 14 C and 34S), which age of saltwater intrusion can be indirectly evaluated by a determined sources of groundwater salinity and esti- comparison of the chemical composition of the investi- mated ages of the deep, regional groundwater. However, gated saline water with that of the host aquifer fluid there is still a great deal of uncertainty in relation to the (Jones et al., 1999). However, in general, there have been circulation of shallow groundwater in the system, which relatively few studies, which have examined multiple has not been previously addressed. This study provides young groundwater age dating indicators in heavily information on the time scales and rates of recharge of exploited coastal aquifers subject to the combined effects this water, and the degree of mixing between modern of saline intrusion, inter-aquifer mixing and leakage of and pre-modern waters, which is difficult to determine irrigation water. Apparently, contradictory age informa- without the multi-tracer approach. Although great care is tion given by different tracer methods has been observed needed during water sampling, and many parameters in many past cases (e.g. Cook and Solomon, 1995; such as excess air, recharge temperature and degradation Plummer and Busenberg, 2005) and may indicate of CFCs, can influence the accuracy of the ages, the complex recharge processes (e.g. incorporation of excess CFCs and tritium are considered to be robust age air, diffusion through thick unsaturated zones and local indicators; the presence of existing hydrochemical and contamination) and/or mixing process of groundwater isotope data from previous studies in the aquifer allows components of different age. the findings to be interrogated and checked using This research focuses on groundwater and solute multiple lines of evidence (e.g. Kazemi et al., 2006). movement in the south coastal Quaternary aquifer of The objective of this paper is thus to improve the Laizhou Bay (Figure 1). This is an important aquifer conceptual model for groundwater movement in the south from a water resources point of view and one where a coast aquifers of Laizhou Bay and show how such a number of previous hydrochemical-isotope and salinity model can be developed and improved through the monitoring studies have helped to understand the combined use of CFCs, tritium and other groundwater salinization and hydrochemical processes. Zhang tracers. The controls on the natural baseline water quality (1993) and Xue et al. (2000) discussed the origin and and subsequent impacts of the significant water resource evolution of underground brines in the coastal plain of development of the past few decades on the groundwater Laizhou Bay using stable isotopes (d18O and d2H) and quality are considered against knowledge of the recharge tritium, showing that the brine is not simply formed by history and the conceptual model. Only by understanding the evaporation of sea water but that brine also mixed flow processes will it be possible to advance the with meteoric water, fresh groundwater and/or river numerical tools available (e.g. calibrate flow models, cf. water during formation. Han et al. (2011) provided Szabo et al, 1996) and so improve local groundwater geochemical and isotopic evidence for palaeo-seawater management. Figure 1. Location map of study area. P-P’ line is the location of the cross section in Figure 2 Copyright © 2012 John Wiley & Sons, Ltd. Hydrol. Process. (2012) DOI: 10.1002/hyp CONCEPTUAL MODEL OF GROUNDWATER FLOW IN THE COAST AQUIFERS STUDY SITE (with depth intervals of 8–20, 20–44 and 35–88 m, respectively, Xue et al., 2000) and two deep saline The study area is located in northern Shandong province aquifers (with depth intervals of 105–165 and 160–290 m, (Figure 1) in a warm temperate zone with subhumid- respectively, Han et al., 2011). This paper mainly monsoon climate. The mean annual temperature and discusses the aquifers within 85 m of the surface. The precipitation are 11.9 C and 660 mm, respectively.