Estimating recharge in fractured aquifers of a temperate humid to semiarid volcanic island (Jeju, Korea) from water table fluctuations, and Cl, CFC-12 and 3H chemistry
Highlights
► Comparison of different methods for predicting groundwater recharge. ► Estimates from water table fluctuations, and Cl, CFC-12 and 3H chemistry are lower than those from a soil water budget. ► All methods yield highest recharge rates in areas of highest rainfall. ► Complex climate and fractured aquifers inhibit regional extrapolations of point estimates of recharge.
Introduction
On Jeju Island (Korea), surface water resources are unreliable because aquifers with high hydraulic conductivity limit permanent surface runoff to only a few select locations near the coast (Hamm et al., 2005, Kim et al., 2003, Won et al., 2005, Won et al., 2006). Given that groundwater provides ∼92% of the potable water for the island’s ∼600,000 residents (Kim et al., 2003), a detailed assessment of the temporal and spatial variations of recharge is important to evaluate and manage the island’s water resources sustainably.
Recharge on islands is generally estimated as the residual in a soil water budget (SWB) (Engott and Vana, 2007, Giambelluca, 1983, Giambelluca et al., 1996b, Izuka et al., 2007, Shade, 1995). The method is popular, considering that it is not hindered by limiting assumptions regarding the mechanisms that control the individual budget components (i.e., rainfall, evapotranspiration, direct runoff, baseflow/interflow in and outflow, vegetation interception and soil water storage) (Dripps and Bradbury, 2007, Scanlon et al., 2002, Tindall and Kunkel, 1999). In addition, the method does not require the drainage-basin boundary and groundwater divides to coincide; a condition that is hard to satisfy (Izuka et al., 2010). The major limitation of the approach is that the accuracy of the recharge estimate depends on the accuracy of the estimates of the other components in the SWB. This limitation is critical when the magnitude of the recharge rate is small relative to that of the other variables. As an illustration, on Jeju Island, direct runoff only occurs when a rainfall event exceeds a threshold value of 40–50 mm over several days (KOWACO, 2003). In this case, averaging over longer time periods may dampen out extreme precipitation and runoff events, which mainly control recharge. The usefulness and reliability of the SWB has therefore been questioned in regions with low runoff (Gee and Hillel, 1988, Hendrickx and Walker, 1997, Lerner et al., 1990). Recharge estimates using SWB models can also differ substantially depending on whether irrigation is carried out (Giambelluca et al., 1996a), which adds another source of uncertainty if averaging values over time periods longer than days. Oki (2008) concluded that the accounting order of recharge and evapotranspiration may result in a large uncertainty of the estimate if soil moisture storage capacity is small and when water budgets are computed using monthly time intervals as compared to daily time steps. Furthermore, as the SWB yields recharge values only for below the root zone, it is not clear where and when exactly the infiltrating rainwater will cause the water table to rise if the unsaturated zone is thick and heterogeneous (Dripps and Bradbury, 2007, Westenbroek et al., 2010). It becomes clear that recharge estimates from the SWB method should be validated and refined by incorporating other independent estimates.
Many techniques exist for quantifying recharge rates. These include numerical modeling of groundwater flow, lysimeters installed in the soil zone, remote sensing, water table fluctuations, radioactive (e.g., 3H, 14C) or other (e.g., chlorofluorocarbons) tracers, and chemical mass balance (see, e.g., the reviews by Walker et al., 2002, Scanlon et al., 2002, Cartwright et al., (2007) and Brunner et al. (2007)). However, choosing appropriate techniques for a specific site is not straightforward, considering that a particular method can be more suitable for certain time and spatial scales. For example, water table fluctuations are more suitable for estimating local recharge over a few days to a few years, while 3H, chlorofluorocarbons (CFCs), 14C and Cl in groundwater provide average local recharge estimates over years to millennia (Herczeg and Edmunds, 2000, Scanlon et al., 2002, Walker et al., 2002). On Jeju Island, the residence time of groundwater is generally less than 50 years (Koh et al., 2007b, Koh et al., 2006a), there is low direct runoff (Won et al., 2006) and water table fluctuations show direct responses to episodic rainfall events (Koh et al., 2006c, Won et al., 2006). Considering these characteristics, recharge is estimated at multiple point locations from water table fluctuations (WTF), chloride mass balance (CMB), apparent CFC-12 ages and modeled 3H mean residence times. The main objectives of this study are to: (1) assess the applicability of these recharge estimation techniques to Jeju and similar settings, and (2) validate and refine previous recharge estimates based on the SWB method published by the Korean Water Resources Corporation (KOWACO, 2003). Special attention is given to Jeju’s volcanic island setting where water levels can be as low as 300 m below the land surface (Koh et al., 2006c), fractured high and low conductivity aquifers may alternate over several meters (Hahn et al., 1997), and rainfall minus potential evaporation is high, but direct runoff is low (KOWACO, 2003).
Section snippets
Topography, climate and land use
Jeju Island is comprised of a dormant shield volcano with one central mountain peak, Mt. Halla, rising to an elevation of 1950 m (Fig. 1a). The island is elliptical in shape with a semi-major axis width of 74 km, a semi-minor axis width of 32 km, and a total area of 1830 km2. The smooth flanks of Mt. Halla are disrupted by about 360 secondary cinder and tuff cones that are mainly located along the eastern and western coastal regions (Hahn et al., 1997). The 16 watersheds of the island are
Data
In this study, recharge (R in mm/yr) was calculated from available climate, physical hydrogeology and geochemistry data from the literature. Porosity and specific yield values were taken from Kwon et al. (1993) and Hahn et al. (1997), respectively. Hourly groundwater levels for the time period 01/01/2008 till 12/31/2009 used in the WTF method were provided by the Institute of Environmental Resource Research, Jeju Special Self-Governing Province (IERR-JSSGP, 2010). For the CMB approach, rainfall
Water table fluctuations
Bore hydrographs recorded at regular intervals from multiple wells across the island show that recharge is episodic (Koh et al., 2006c, Won et al., 2006) and linked to seasonal rainfall cycles (Fig. 4). The variations in the water tables allow calculating recharge using the WTF method (Healy and Cook, 2002):where dh = change in water level (mm), dt = change in time (years), and Sy = specific yield (dimensionless), defined as the volume of water released from an unconfined aquifer per unit
Conclusions and Implications
This study clearly demonstrates that the magnitude and spatial distribution of recharge may vary on Jeju depending on which estimation technique is chosen (Fig. 5). Relying solely on one method to quantify recharge can therefore be very misleading.
The main advantage of the WTF method in comparison to the CMB, CFC-12 and 3H techniques is that, alike the SWB, it can be used to detect both short-term interseasonal and long-term average recharge rates making it a useful calibration tool for local
Acknowledgements
The authors would like to thank D.-C. Koh, G.-W. Koh and Y.-C. Kim for providing helpful information on the hydrogeology of Jeju Island. We also thank G.-P. Kim for providing hourly groundwater level data. L. Charlet, N. Goldscheider, P. Cook and an anonymous reviewer are acknowledged for their insightful suggestions that improved the manuscript. This work was supported by the Korea Institute of Geoscience and Mineral Resources for the Leading Industry Development of Jeju Economic Region.
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