Application of environmental tracers to mixing, evolution, and nitrate contamination of ground water in Jeju Island, Korea

Journal of Hydrology (2006) 327, 258– 275

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Application of environmental tracers to mixing, evolution, and nitrate contamination of ground water in Jeju Island, Korea Dong-Chan Koh a,*, L. Niel Plummer b, D. Kip Solomon c, Eurybiades Busenberg b, Yong-Je Kim a, Ho-Wan Chang d a

Korea Institute of Geoscience and Mineral Resources, Groundwater Resources Group, 30 Gajeong-dong, Yuseong-gu, Daejeon 305-350, South Korea b US Geological Survey, 432 National Center, Reston, VA 20192, USA c Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, USA d School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, South Korea Received 3 June 2005; received in revised form 7 November 2005; accepted 8 November 2005

Summary Tritium/helium-3 (3H/3He) and chlorofluorocarbons (CFCs) were investigated as environmental tracers in ground water from Jeju Island (Republic of Korea), a basaltic volcanic island. Ground-water mixing was evaluated by comparing 3H and CFC-12 concentrations with lumped-parameter dispersion models, which distinguished old water recharged before the 1950s with negligible 3H and CFC-12 from younger water. Low 3H levels in a considerable number of samples cannot be explained by the mixing models, and were interpreted as binary mixing of old and younger water; a process also identified in alkalinity and pH of ground water. The ground-water CFC-12 age is much older in water from wells completed in confined zones of the hydro-volcanic Seogwipo Formation in coastal areas than in water from the basaltic aquifer. Major cation concentrations are much higher in young water with high nitrate than those in uncontaminated old water. Chemical evolution of ground water resulting from silicate weathering in basaltic rocks reaches the zeolite–smectite phase boundary. The calcite saturation state of ground water increases with the CFC-12 apparent (piston flow) age. In agricultural areas, the temporal trend of nitrate concentration in ground water is consistent with the known history of chemical fertilizer use on the island, but increase of nitrate concentration in ground water is more abrupt after the late 1970s compared with the exponential growth of nitrogen inputs. c 2005 Elsevier B.V. All rights reserved.

KEYWORDS Environmental tracers; Residence time; Ground-water mixing; Nitrate contamination; Aquifer vulnerability; Volcanic island



* Corresponding author. Tel.: +82 42 868 3079; fax: +82 42 863 9404. E-mail address: [email protected] (D.-C. Koh).



0022-1694/$ - see front matter c 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.11.021

Application of environmental tracers to mixing, evolution, and nitrate contamination

Introduction Most of water supply for Jeju volcanic island, south of the Korean peninsula (Fig. 1), is derived from volcanic aquifers (Kim et al., 2003). Agricultural activities of vegetable garden and mandarine orange orchard are extensive in the coastal areas of the island, whereas mountainous areas are composed of natural forest and grass land. Human activities have resulted in substantial nitrate loadings to the aquifers and nitrate contamination of ground water is widespread in the coastal area of the island, where nitrate concentration exceeds the Korean drinking water standard of 10 mg NO3–N/L (Lee et al., 2002). The high permeability of the basaltic aquifer and high recharge rate can result in short residence times of ground water in the island. Using lumped-parameter models, ground-water residence times of 21 years for a coastal spring and 2.5 years for a mountain spring were estimated based on a time series of 3H data (Maloszewski and Zuber, 1982). However, a recent study showed variable vulnerability of the volcanic aquifers to nitrate contamination, and the occurrence of old ground water recharged before the 1950s, with negligible nitrate even in agricultural areas (Koh et al., 2005). Tritium/helium-3 (3H/3He) and chlorofluorocarbon (CFCs) analysis have been widely applied in ground-water dating. The 3H/3He method (Tolstikhin and Kamensky, 1969) has been extensively investigated and applied to ground-water age interpretation (Maloszewski and Zuber, 1983; Weise and Moser, 1987; Takaoka and Mizutani, 1987; Poreda et al., 1988; Schlosser et al., 1988, 1989; Solomon and Sudicky, 1991; Solomon et al., 1992; Solomon et al., 1993; Solomon et al., 1995; Ekwurzel et al., 1994; Szabo et al., 1996; Solomon and Cook, 2000). Since the early 1990s, CFCs also have been used to estimate ground-water

Figure 1

259

age up to about 50 years (Busenberg and Plummer, 1992; Dunkle et al., 1993; Plummer and Busenberg, 2000). Recently, multi-tracer approaches have been used to estimate residence time and characterize ground-water flow (Ekwurzel et al., 1994; Bauer et al., 2001; Plummer et al., 2001). Ground-water age was used to characterize nitrate contamination in sandy aquifers in the Atlantic coastal plain of Maryland, USA (Bo ¨hlke and Denver, 1995), spring waters in a karst area of Florida, USA (Katz et al., 2001), and an aquifer in Tertiary basalts (Zoellmann et al., 2001). Long-term measurements of environmental tracers over several years also have been used for study of hydrological systems. This method is much less expensive and less prone to error both in collection of samples and measurement of tracers if 3H and stable isotopes of water are used, compared to noble gases, 85Kr, CFCs and SF6. Kattan (1997) used 3H measurements over 20 years to evaluate residence time and reservoir size of karst springs, and Maloszewski et al. (2002) employed 3H and 18O monitoring data over 15–20 years to identify the flow system of a fissured-porous aquifer in a karst area. The advantage of long-term monitoring is that time-series data can provide numerous checkpoints to construct conceptual models and to constrain model parameters though it is not suitable for a study with short-term duration and involving the area which cannot be accessed frequently over a long period. In this study, we combine data from multiple environmental tracers (CFCs and 3H/3He) with hydrogeochemical parameters in ground water to obtain information on timescales and mixing properties of ground-water flow in the volcanic aquifers of Jeju Island. Nitrate contamination of the ground water also was interpreted in terms of recharge year and temporal variation of nitrate loading.

Location of ground water sampling sites. Air monitoring site for CFCs and Gosan station were also shown.

260

Hydrogeologic setting Jeju Island was formed by basaltic and trachytic volcanism from the Pliocene to Quaternary. A cross-sectional geologic map for Jeju Island is given in Fig. 2. The island has no volcanic activity at present, although there are historical records of volcanic activity. The basaltic rocks cover more than 90% of the island’s surface. These rocks consist of basaltic or trachytic lava flows and pyroclastic rocks (Won, 1976; Park and Kwon, 1993). The average soil layer thickness is 0.6 m and is less than 1 m over 85% of the total island area (Jejudo, 1997). The basaltic rocks have numerous interflow structures within a series of lava flows and unconsolidated scoriaceous formation. Because the voids in the permeable structures of the basaltic rocks are not filled, these rocks have high permeability and storage capacity. In most wells, ground-water yield can be as high as 1000 m3/d with only a few meters of drawdown, which is large compared to the ground-water yield from fractured crystalline rock aquifers in mainland Korea. Water levels in the basaltic aquifer are appreciably low and as much as 300 m below land surface in highaltitude (200 m above sea level) areas (Jejudo, 2000). A hydro-volcanic sedimentary formation (Seogwipo Formation) underlies the basaltic rocks. This Formation is composed of consolidated or semi-consolidated sedimentary rocks that consist of sand, tuffaceous materials, fragments of basaltic rocks, and molluscan shells (Sohn et al., 2002). The Seogwipo Formation is not found in the subsurface of the eastern coastal area. However, the formation is continuously distributed in other areas below the island with an average thickness of 100 m. The Seogwipo Formation is underlain by the U Formation that is composed of sand

D.-C. Koh et al. and silt. The U Formation is located from 15 to 205 m below sea level with a thickness from 70 to 250 m; the formation is present continuously below the island. The basement of the island consists of pyroclastic rocks, welded tuff, lapilli tuff and granite that formed in the late Cretaceous and early Tertiary. The basement is located from 250 to 300 m below sea level (Oh et al., 2000). The wells in this study were installed to the depth of the lower boundary of basaltic rocks (well type A) or upper parts of the Seogwipo Formation (well type B), which is described schematically in Fig. 2 (see Table 1 for lithology of each well).

Methods Ground-water samples were collected in April, 2002 from irrigation wells in Jeju Island (Table 1). There can be seasonal variatons in ground-water quality considering rapid infiltration and concentration of rainfall during the monsoon season in the island. The samples obtained in this study were collected during a dry period (Koh et al., 2005), and represent base flow conditions of ground water. The land use at each well was determined considering a circular area with 500 m radius around the well by the method described in Koh et al. (2005). This study focused on the western area (Fig. 2) because water use for agriculture is greatest in the area (Jejudo, 2000). The wells were purged by the dedicated pumps installed at the sites and samples were taken after field parameters (pH, EC, temperature, and DO) stabilized. Cations were analyzed by ICP-AES and anions by IC in the Department of Earth Systems Science, Seoul National University. The alkalinity was determined by Gran titration in the field. The analytical results are listed for the hydrogeochemistry in Table 2.

Figure 2 Lithologic cross-section along west-east direction in Jeju Island. The boundaries of lithologic units are inferred from geologic logs of wells whose depths are greater than 600 m (Oh et al., 2000). Well type A is that completed in basaltic rocks and type B is that completed to the upper parts of hydrovolcanic Seogwipo Formation.

Application of environmental tracers to mixing, evolution, and nitrate contamination Table 1

261

Selected ground-water sampling sites in Jeju island, April 2002

Sampling sitea

Elevationb (m a.s.l.)

Total depth (m)

Depth to water (m)

Lithologyc

Confining conditiond

Land usee

FW1 FW2 FW3 FW4 FW5 FW7 FW8 FW9 FW10 FW11 FW12 FW13 FW14 FW15 FW16 FW17 FW18 FW19 FW20 FW21 FW22 FW23 FW25 FW26 FW27 FW28 M1 M2 M3 M4 M5 M6 M8 M9 M10 M11 M12 M13 M14 M16 W54

44.8 45.0 73.0 152.8 131.0 310.0 32.1 71.5 85.0 19.4 18.0 88.0 143.5 145.5 283.0 24.0 43.5 61.0 23.0 71.4 78.0 85.9 186.4 322.0 118.0 173.9 26.0 25.0 81.6 144.0 25.0 83.6 46.0 338.0 338.1 430.0 338.0 338.1 417.0 434.0 61.6

70 100 100 170 184 327 100 160 105 100 100 160 170 220 310 101 119 100 84 100 140 110 214 350 159 186 70 97.5 95 170 90 117 146 380 325 420 500 240 410 460 80

39.0 38.0 62.0 140.0 124.0 242.0 29.0 41.5 75.0 9.5 18.0 74.0 129.0 90.0 245.0 22 37.0 48 23 50.0 39.0 56.0 145.0 211 65.0 70 25.7 15 81.0 120.5 24.0 36.5 32.5 328 198.2 266.5 307.5 190.0 204.0 285.0 57.0

B B S B B B B B B S S B B B B B S S B B B B B S S B B B B B S S S B B B B B B B B

U U C U U U U U U C C U U U U U C U U U U U U U C U C C U U U C U U U U U U U U U

A A A A M F M A A A A M A M F A A A A F A A A F A M A A A A A A M M F F F F F F A

a

See Fig. 2 for locations. Meters above sea level. c Surficial lithology for springs and lithology near screened zone for wells; B, basalts, S, Seogwipo formation. d Confining condition of aquifers is determined from drilling logs of the wells. Confined condition (C) is assigned when a abrupt rise in waer level is observed and unconfined condition (U) otherwise. e Land uses: A, agricultural fields; F, forest/grassland; M, mixed use of agricultural fields and forest/grassland. See text for classification method. b

Charge balance errors are less than 5% for most of samples with mean error of 4.5%. 3 H samples were collected in 1 L HDPE bottles. He and Ne samples were collected in 69 cm-long 3/800 copper tubes sealed at both ends by pinch-off clamps in duplicate with application of over pressure using a valve at the end of the discharge line to reduce formation of gas bubbles. He and Ne were analyzed by noble gas mass spectrometry and

3

H was analyzed by the He-ingrowth technique in the Noble Gas Laboratory, University of Utah. The analytical uncertainties (one standard deviation) in 3He/4He and 4He are less than 1% of the value and that in Ne is about 3%. For CFC measurements, ground water was collected in triplicate in 60 mL borosilicate glass ampoules by flamesealing at the ampoule neck using the apparatus similar to that of Busenberg and Plummer (1992). The CFC sampling

262

Field parameters and concentrations of major elements in ground-water samples

Sample

Collection date

T (C)

pH

EC (lS/cm)

DO (mg/L)

TDS (mg/L)

Ca (mg/L)

Mg (mg/L)

Na (mg/L)

K (mg/L)

HCO3a (mg/L)

Cl (mg/L)

SO4 (mg/L)

NO3–N (mg/L)

SiO2 (mg/L)

CBEb (%)

FW1 FW2 FW3 FW4 FW5 FW7 FW8 FW9 FW10 FW11 FW12 FW13 FW14 FW15 FW16 FW17 FW18 FW19 FW20 FW21 FW22 FW23 FW25 FW26 FW27 FW28 M1 M2 M3 M4 M5 M6 M8 M9 M10 M11 M12 M13 M14 M16 W54

Apr-1-2002 Apr-1-2002 Apr-2-2002 Apr-2-2002 Apr-1-2002 Apr-2-2002 Apr-4-2002 Apr-3-2002 Apr-3-2002 Apr-4-2002 Apr-4-2002 Apr-3-2002 Apr-3-2002 Apr-4-2002 Apr-3-2002 Apr-5-2002 Apr-5-2002 Apr-5-2002 Apr-6-2002 Apr-5-2002 Apr-8-2002 Apr-8-2002 Apr-5-2002 Apr-3-2002 Apr-8-2002 Apr-8-2002 Apr-10-2002 Apr-10-2002 Apr-10-2002 Apr-9-2002 Apr-8-2002 Apr-8-2002 Apr-4-2002 Apr-5-2002 Apr-9-2002 Apr-9-2002 Apr-10-2002 Apr-11-2002 Apr-11-2002 Apr-2-2002 Apr-10-2002

15.9 15.8 17.7 16.7 15.7 15.1 17.0 15.6 15.7 18.3 18.8 17.2 16.7 14.0 15.2 18.2 15.0 15.2 17.4 14.1 15.1 14.9 14.9 15.6 16.7 15.5 17.3 18.0 16.1 15.2 17.0 17.8 16.8 19.3 16.0 14.0 14.4 16.1 14.6 17.0 15.9

8.3 8.3 8.4 8.2 8.4 8.1 7.7 8.8 7.5 8.9 8.3 8.8 8.9 8.0 8.4 8.9 9.2 9.2 7.2 8.3 8.2 7.9 8.3 8.7 8.9 7.8 8.6 8.5 8.0 8.1 7.4 8.5 7.1 8.3 7.5 8.4 8.4 8.1 8.9 8.5 7.8

141 128 157 114 112 113 209 228 331 191 223 168 129 136 117 151 124 123 304 103 129 138 87 73 139 138 174 219 185 94 183 164 313 152 131 67 74 319 57 94 135

9.0 9.6 6.8 9.0 9.1 9.4 0.8 7.8 9.4 1.7 0.9 3.2 7.9 9.2 9.8 3.1 7.4 7.5 8.5 9.4 9.1 9.2 9.2 8.9 6.4 9.6 2.2 4.4 9.1 9.1 6.5 6.1 8.3 7.5 8.3 9.0 9.7 7.4 8.4 7.0 9.2

139 131 153 124 122 114 168 193 231 163 196 148 133 127 120 142 115 115 235 107 120 123 95 86 137 127 165 191 150 104 157 156 229 164 138 77 83 295 70 108 130

7.5 6.7 8.3 5.3 5.7 5.3 11.2 13.0 16.2 11.5 9.5 8.3 5.5 5.1 5.9 11.1 10.0 9.7 17.8 4.6 6.4 6.5 3.9 2.7 8.3 7.0 11.0 18.7 6.9 5.0 12.2 9.6 14.1 5.3 6.7 3.0 3.5 13.2 3.1 2.3 7.8

6.0 5.4 6.6 4.7 5.0 4.7 6.9 9.2 15.0 3.5 7.9 5.1 4.2 4.9 5.1 3.2 2.9 3.0 12.6 3.9 4.7 5.3 3.1 2.6 3.6 5.8 4.8 4.0 6.9 4.0 5.7 4.8 14.7 5.1 7.0 2.4 2.7 18.5 1.9 2.7 5.9

10.8 10.4 14.3 10.1 9.6 8.0 19.0 16.9 19.8 20.2 20.4 17.8 14.1 13.7 9.0 15.5 10.6 10.9 20.4 9.1 11.1 11.4 7.0 6.4 15.0 10.6 17.7 18.9 18.5 7.4 14.6 16.3 19.7 18.1 9.2 5.3 6.0 17.4 4.0 11.8 9.8

4.1 4.0 4.3 4.2 3.8 3.2 4.3 4.7 6.9 5.1 5.8 4.4 5.7 3.8 3.6 3.9 3.3 3.7 5.0 3.3 3.9 3.9 2.9 3.5 4.7 3.6 5.0 4.4 3.9 2.8 3.6 4.7 5.9 8.6 3.3 2.3 2.7 10.7 1.9 5.8 3.1

43.6 44.6 51.3 46.4 43.6 40.6 57.5 47.9 32.2 64.5 79.9 56.7 51.5 37.6 40.7 63.9 51.5 49.4 52.7 34.9 32.9 30.9 29.0 29.8 55.3 38.0 68.3 55.6 43.8 40.1 47.9 59.0 28.9 76.6 55.8 25.8 29.5 191.3 22.9 40.7 40.8

10.7 9.6 12.5 7.2 8.0 7.1 22.1 20.3 24.7 18.3 18.2 15.8 9.8 11.8 8.6 9.7 8.1 8.2 24.3 7.6 12.8 15.2 6.7 5.0 9.3 9.4 12.4 18.8 24.8 6.0 14.3 13.4 25.0 5.6 8.4 5.1 5.3 6.2 4.0 5.4 9.6

3.1 2.7 3.8 2.1 2.0 1.9 6.7 7.6 13.6 4.9 6.9 4.3 2.6 4.2 2.5 4.8 2.9 2.7 12.5 2.8 2.9 3.3 2.0 1.5 2.0 2.0 3.5 3.8 3.9 1.7 3.3 3.0 12.4 2.4 2.0 1.7 1.7 1.9 1.3 1.9 2.0

3.2 2.1 3.3 1.0 1.5 1.0 2.5 8.7 15.9 0.8 2.9 1.6 0.8 2.7 1.7 0.3 0.5 0.9 13.6 1.1 2.6 2.8 1.3 0.2 1.2 3.9 1.1 6.0 1.1 0.3 5.5 1.6 15.5 0.1 0.8 0.2 0.2 0.1 0.1 0.1 3.4

39.6 38.8 38.1 39.9 38.6 38.8 30.4 36.0 32.4 27.6 35.6 30.0 36.4 34.3 38.1 30.0 24.2 24.0 30.6 36.9 34.5 35.1 35.4 34.5 34.5 34.3 38.6 41.6 36.9 36.0 31.7 39.2 39.9 42.6 42.6 30.6 31.3 38.6 31.1 37.1 37.3

4.8 4.5 5.5 4.9 5.0 4.9 4.3 2.7 7.6 1.4 0.4 4.2 5.5 4.6 5.1 4.9 4.5 5.6 2.7 5.2 5.5 5.9 2.0 3.0 6.5 6.3 6.2 5.8 7.0 4.5 4.4 5.4 6.9 6.7 5.2 1.5 3.4 2.3 0.4 6.7 5.8

a b

Total alkalinity. P P P P Charge balance error = ( cations  anions)/( cations + anions) · 100.

D.-C. Koh et al.

Table 2

Application of environmental tracers to mixing, evolution, and nitrate contamination apparatus and ampoule neck were filled with ultra-pure N2 gas cleaned by an MS13X trap. The water sample was flame-sealed into the ampoule. CFCs were analyzed by closed-system purge and trap gas chromatography with electron capture detector at the Land and Water Division, CSIRO, Australia. For CFCs, triplicate analyses were performed and values reported only if at least two measurements agreed within 10% for concentrations above 100 pg/ kg. Below 100 pg/kg, values were reported only if at least two measurements agreed within 20 pg/kg (Johnston et al., 1998). The analytical uncertainty (one standard deviation) in CFCs is less than 5%. Atmospheric concentrations of CFCs were monitored from June 2002 to March 2003 near the center of the island on a monthly basis (Fig. 2). For evaluation of short-term fluctuations, the monitoring also was performed weekly from January to March 2003. There are no residential, agricultural, or industrial facilities near the monitoring site. The air samples were collected using a special air-sampling pump and SUMMA canisters (Biospherics Research) with a pump inlet located at 2 m above the ground. CFCs in air samples were analyzed in the US Geological Survey Chlorofluorocarbon Laboratory (Reston, VA, USA).

Results and discussion Estimation of recharge elevation and temperature To determine ground-water ages from CFCs and 3H/3He data, it is necessary to estimate recharge temperature and elevation. From the d18O values of ground water, it was estimated that recharge originated mainly from local rainfall for western and eastern areas of the island (Koh et al., 2005). Thus, recharge elevation was assumed to be the altitude of the sample site. The change in total atmospheric pressure (P) with altitude was estimated by the relation, ln P = H/8313 (H is the recharge elevation in meters above sea level) determined from the daily measurements of atmospheric pressure with altitude from 2000 to 2002 at Gosan station, Jeju Island (Korea Meteorological Agency, http://www.kma.go.kr). Recharge temperature was assumed to be the measured ground-water temperature at the time of sampling because there is no geothermal activity in the island and, thus, warming of ground water is likely to be negligible. We used the recharge conditions determined as above for calculation of apparent CFC and 3 H/3He ages. 3

H/3He dating

The measured 3H, He and Ne concentrations of ground water are listed in Table 3. 20Ne concentrations were converted to Ne concentrations using the atmospheric ratio of 0.905 (Ozima and Podosek, 1983). The measured ratios of 3 He/4He (Rtot) are well above the atmospheric 3He/4He (Ra) for most of samples; this result indicates that nonatmospheric 3He is considerable. The sources of He can be distinguished by a plot of the excess air-corrected isotopic ratio of 3He/4He versus solubility equilibrium 4He divided by the excess air-corrected 4He concentration (Fig. 3) (Aeschbach-Hertig et al., 1998). The sol-

263

ubility equilibria of He and Ne were calculated using data from Weiss (1971). Helium concentrations from excess air were calculated from the relations: 4 Heexc ¼ ðNetot  Neeq Þ 4 He ð Ne Þexc and 3Heexc = Ra Æ 4Heexc, with the assumption that 4 He/Ne of excess air is atmospheric (0.288), where tot = measured, eq = solubility equilibrium, and exc = excess air. In samples with small amounts of tritiogenic 3He (Fig. 3), the helium isotopic composition represents mixing of atmospheric and terrigenic He with a narrow range of the 3 He/4He ratio in the terrigenic source (Rterr). For samples with negligible 3H (<0.5 TU), the range of Rterr/Ra is 5.9 to 6.6. This Rterr/Ra range indicates mixing of radiogenic and mantle sources of helium (Plummer et al., 2000). A considerable number of samples plot to the right of air-saturated water (ASW) (Fig. 3), which indicates degassing of He. These samples have negative quantities of terrigenic 4He, indicating that the excess air quantities of Ne and He may be fractionated. The fractionation of excess air is due to either partial degassing of an initial amount of excess air at the water table or equilibration of a finite entrapped air volume with a finite water volume in the quasi-saturated zone (Holocher et al., 2001). Other samples plot to the left of ASW, although these samples also may have partially fractionated He. The apparent 3H/3He ages of ground water (Table 3) were calculated by the method of Schlosser et al. (1989). For the samples with negative terrigenic 4He, apparent 3 H/3He ages cannot be determined because the He/Ne ratio of fractionated excess air is not known. 3H/3He ages are sensitive to Rterr; these ages even reduced to zero, although the range of Rterr is narrow (Plummer et al., 2000). For samples with negative terrigenic He, the He/Ne ratio of excess air was varied to the level where terrigenic He becomes positive; these ratios vary from approximately 0.22 to 0.23 for most of samples, but below 0.1 for some samples with small excess air. This level of the He/Ne ratio is similar to the range constrained by the model of closed-system equilibration with entrapped air (Aeschbach-Hertig et al., 2000). Entrapment of excess air can occur during rapid infiltration of rainwater through permeable fractures of basaltic rocks, which enables fractionation of He. A sensitivity calculation was performed by adding He to the measured He concentrations maintaining a constant 3 He/4He ratio of the sample. Many samples showed high sensitivity to addition of He, and 3H/3He ages varied more than 50% toward younger ages when He was increased by even 5% of the measured He. For samples with negative terrigenic He, addition of 5–10% of the measured He changes calculated negative terrigenic He to positive values. This result indicates that at least 5–10% of fractionation of He relative to Ne occurred during incorporation of atmospheric gases in ground water. Considering the evidence of He fractionation and the presence of large amounts of mantle He, the results of 3H/3He dating have large uncertainties for most of the samples (Table 3).

Monitoring of atmospheric CFCs Atmospheric CFC concentrations at Jeju Island are similar to those in the Northern Hemisphere (Cunnold et al., 1994). Local sources of CFCs are not expected in the study area

Sample

264

Table 3

Analytical results of 3H, He, and Ne, and apparent 3H/3He ground-water ages 3

H (TU)

4

He (ccSTP/g · 108)

20

Ne (ccSTP/g · 108)

Rs/Raa

D4Heb %

DNeb %

3

Hetrit (TU)c and 3H/3He age (year)d

Max. 3

Hetrit

a

4.77 ± 0.24 4.55 ± 0.23 1.39 ± 0.11 3.21 ± 0.16 3.96 ± 0.20 5.08 ± 0.25 0.78 ± 0.08 1.85 ± 0.09 2.74 ± 0.14 0.10 ± 0.01 0.93 ± 0.13 0.77 ± 0.17 0.11 ± 0.07 3.74 ± 0.19 5.16 ± 0.26 0.08 ± 0.04 0.35 ± 0.11 0.21 ± 0.04 1.96 ± 0.10 3.27 ± 0.16 3.62 ± 0.18 4.30 ± 0.22 5.46 ± 0.27 2.24 ± 0.11 0.22 ± 0.05 4.02 ± 0.20 0.10 ± 0.01 2.26 ± 0.11 2.28 ± 0.11 3.93 ± 0.20 0.80 ± 0.04 0.85 ± 0.15 3.80 ± 0.19 0.35 ± 0.02 3.61 ± 0.18 4.55 ± 0.23 4.09 ± 0.20 1.16 ± 0.18 2.53 ± 0.13 0.48 ± 0.03 3.87 ± 0.19

15.6 12.0 13.3 – 36.7 5.24 6.11 6.22 4.50 10.8 13.1 31.7 17.6 86.5 6.30 20.6 7.69 7.30 111 9.28 – 5.52 5.47 9.52 9.60 8.35 5.72 5.82 5.37 5.33 13.7 9.28 5.19 78.6 4.67 7.33 5.28 112 4.42 68.8 4.75

20.0 20.4 20.3 – 24.7 19.4 23.9 22.7 16.9 21.5 19.8 30.4 23.4 35.0 20.5 27.4 24.2 24.7 353 24.4 – 21.1 19.8 19.1 21.2 28.3 22.0 21.9 21.0 20.6 21.3 25.5 20.8 25.5 18.5 21.1 16.4 13.3 17.6 24.9 18.7

4.60 4.11 4.03 – 5.60 1.56 0.98 1.07 1.05 3.53 3.87 5.07 4.31 5.73 1.94 4.26 1.57 1.41 1.60 2.53 – 1.19 1.11 3.50 2.96 1.01 1.06 1.25 1.12 1.28 3.80 2.44 1.03 5.90 1.24 2.33 1.99 6.34 1.55 5.40 1.36

246 167 198 – 722 19 36 38 0 140 192 609 296 1826 43 359 70 62 – 105 – 22 23 118 115 88 27 30 20 19 204 108 15 1724 7 69 20 2466 2 1504 5

16 18 20 – 45 15 40 32 1 27 17 79 39 102 22 62 40 43 – 40 – 22 16 14 25 66 29 29 22 21 24 51 22 57 11 26 2 20 6 53 9

32.7 37.0 30.3 36.5 11.1 39.4 – – 123.2 62.2 15.9 25.5 He fractionated He fractionated 1.6 8.1 19.8 94.9 0.0 0.0 89.4 85.4 21.2 94.4 184.7 70.3 13.3 22.8 23.4 102 0.0 0.0 5.2 58.3 – – 10.3 25.5 – – He fractionated 0.2 0.5 12.4 33.6 1.2 33.8 0.0 0.0 He fractionated He fractionated He fractionated He fractionated 0.0 0.0 17.9 55.5 He fractionated 188.4 113 He fractionated 10.7 21.7 4.8 14.0 375.9 104 He fractionated 0.0 0.0 He fractionated

Rterr · 106

3

Age

Rterr · 106

8.20 8.20 8.20 – 8.20 8.20

0.0 4.1 0.0 – 3.2 15.8

0.0 11.5 0.0 – 10.7 25.4

9.00 9.20 8.56 – 9.20 9.20

8.20 8.20 8.15 8.20 8.20 8.20 8.20 8.20 7.73 8.20 – 8.20 –

1.5 0.3 – 0.0 0.0 0.0 10.3 0.0 – 3.5 – 0.1 –

8.0 24.8 – 0.0 0.0 0.0 19.7 0.0 – 51.4 – 0.5 –

9.20 9.20 – 9.17 8.68 8.80 9.20 8.66 – 9.20 – 9.20 –

8.20 8.20 8.20 2.08

0.0 0.0 0.0 –

0.0 0.0 0.0 –

8.45 8.90 8.28 –

8.16 8.20

– 9.6

– 44.9

– 9.20

8.20

0.0

0.0

8.86

8.20 8.20 8.20

4.3 1.2 0.0

12.0 4.7 0.0

9.20 9.20 9.07

–

–

8.17

Hetrit

–

Rs = measured 3He/4He, Ra = atmospheric 3He/4He (1.384 · 106). D = excess of measured values relative to solubility equilibrium. c Tritiogenic 3He. d Range of terrigenic 3He/4He ratio (Rterr) from 8.2 · 106 to 9.2 · 106 was determined by the measured 3He/4He ratio of ground water with 3H less than 0.5 TU. For zero age, Rterr is the ratio that would give zero age. Determined 3H/3He ages have large uncertainties because of mantle He and fractionation of He. b

D.-C. Koh et al.

FW1 FW2 FW3 FW4 FW5 FW7 FW8 FW9 FW10 FW11 FW12 FW13 FW14 FW15 FW16 FW17 FW18 FW19 FW20 FW21 FW22 FW23 FW25 FW26 FW27 FW28 M1 M2 M3 M4 M5 M6 M8 M9 M10 M11 M12 M13 M14 M16 W54

Min. Age

Application of environmental tracers to mixing, evolution, and nitrate contamination

265

Figure 3 Excess air corrected ratio of 3He/4He versus solubility equilibrium 4He divided by excess air corrected 4He concentration. Initially air-saturated water (ASW) is evolved by addition of terrigenic He and tritiogenic 3He. The lower and upper limits of terrigenic 3He/4He (Rterr) were determined from the samples with negligible 3H (<0.5 TU). Diamonds are the end members of He sources described above and of radiogenic He.

because there is neither a major city nor industrial facilities in the area. Thus, it is unlikely that any local sources considerably bias the level of CFCs in the local atmosphere relative to that of the Northern Hemisphere. Monthly monitoring results of atmospheric CFCs in the island (Fig. 4a) are concordant with those of North American air (Plummer et al., 2000) and with the measurements of Kim et al. (2001). For identification of short-term fluctuations in atmospheric CFCs, weekly sampling also was carried out during a 3-month period. Results of this sampling indicate that there was no fluctuation in atmospheric CFCs (Fig. 4b). The North American Air curves for CFC-11 and CFC-12 (http://water. usgs.gov/lab/cfc) were used to determine apparent CFC ages.

Determination of apparent CFC ages The apparent CFC ages were determined using the procedures of Plummer and Busenberg (2000). The effect of excess air on CFC ages was corrected by an iterative procedure using the Ne excess above solubility equilibrium. However, most corrected apparent ages varied within 1 year of the uncorrected age (Table 4). There is no evidence for aerobic degradation of CFCs in ground water. Because the ground-water samples in this study have measurable concentrations of dissolved oxygen, it is unlikely that CFCs are affected by microbial degradation (Table 2). The apparent CFC-11 and CFC-12 ground-water ages are in good agreement, within 5 years, although CFC-11 ages have a small old bias relative to CFC-12 ages (Fig. 5). In fractured-rock systems, fluctuations in barometric pressure may induce appreciable movement of air to great depths. Plummer et al. (2000) measured CFCs of air in the

Figure 4 (a) Shows monthly atmospheric mixing ratios of CFC-11 and CFC-12 measured in Jeju Island (solid triangles) in this study and, those measured from 1994 to 1997 in the island by Kim et al. (2001) (open triangles). The solid lines are CFC mixing ratios for North American Air for comparison (Plummer et al., 2000). (b) Shows weekly atmospheric mixing ratios. They are nearly flat during measuring period.

unsaturated zone of a basaltic aquifer where the thickness of the unsaturated zone is up to 300 m and they suggested that barometric pumping is fairly efficient in mixing unsaturated zone air. Because of the thin soil layers (<1 m) and the common exposure of basaltic rocks at the land surface in Jeju Island (Jejudo, 1997), the transport of CFCs in the unsaturated zone is expected to be greatly enhanced by barometric pumping.

Comparison of CFC-12 and 3H/3He dating results The wells sampled during this study were developed for irrigation, and, consequently, have relatively long and multiple screened intervals to augment well yields compared to monitoring wells (Table 1). Therefore, the ground-water samples are likely to be mixtures of water from various depths including different flow paths and residence time. Lumped parameter mixing models can be applied to this type of sample, although the exact distribution of

266 Table 4 Sample

FW1 FW2 FW3 FW4 FW5 FW7 FW8 FW9 FW10 FW11 FW12 FW13 FW14 FW15 FW16 FW17 FW18 FW19 FW20 FW21 FW22 FW23 FW25 FW26 FW27 FW28 M1 M2 M3 M4 M5 M6 M8 M9 M10 M11 M12 M13 M14 M16 W54

D.-C. Koh et al. Analytical results of CFC-11 and CFC-12, and apparent ground-water ages Meaured concentration (pg/kg)

Atmospheric mixing ratio (pptv)a

Apparent ages (years)b

CFC-11

CFC-12

CFC-11

CFC-12

CFC-11

294.8 343.2 123.9 150.0 568.6 218.4 50.7 183.4 456.0 3.0 61.3 13.8 21.9 356.2 273.8 5.4 2.4 28.9 370.6 391.4 261.0 351.3 336.1 77.0 13.1 423.2 8.5 93.3 372.6 325.6 212.4 12.6 466.7 56.3 384.0 285.8 203.3 66.8 75.1 15.5 409.0

111.4 116.2 59.1 82.5 74.8 97.2 32.3 115.0 238.8 6.1 44.2 18.0 21.2 197.7 105.6 12.1 5.3 30.0 212.6 203.3 171.3 195.5 196.6 49.6 10.1 252.9 0.4 149.6 189.8 138.2 72.5 0.0 229.2 40.5 191.1 132.5 98.2 48.7 52.7 10.8 193.7

141 163 65 76 272 104 26 87 217 2 34 7 11 157 130 3 1 13 190 171 120 161 156 38 7 202 4 49 181 152 107 7 233 33 191 130 93 33 35 8 196

226 230 127 175 145 193 66 223 482 13 99 35 43 335 209 25 10 56 459 366 336 372 381 101 21 476 1 320 380 269 150 0 472 92 398 252 193 103 105 23 389

25.8 23.3 31.8 30.8 10.8 28.3 37.8 29.8 17.3 49.8 36.3 44.8 42.3 24.3 26.3 48.3 50.8 41.3 19.8 22.3 27.3 23.8 24.3 35.3 45.3 18.8 46.8 33.8 21.3 24.3 28.3 45.3 15.8 36.3 19.8 26.3 29.3 36.3 35.8 43.8 19.3

CFC-12 +1.0 +2.0 +1.0 +0.5 +4.0 +1.0 +0.5 +1.0 +2.0 +0.5 +0.5 +0.5 +0.5 +1.5 +1.0 +0.5 +0.5 +0.5 +2.5 +2.0 +1.0 +1.5 +1.5 +1.0 +0.5 +2.0 +0.5 +0.5 +2.0 +1.5 +0.5 +0.5 +2.0 +0.5 +2.5 +1.0 +0.5 +0.5 +0.5 +1.0 +2.5

1.5 2.0 0.5 0.5 9.5 0.5 0.5 0.5 2.0 0.0 0.5 0.5 0.5 2.0 1.0 0.5 0.0 0.5 2.0 2.5 1.0 2.5 2.0 0.5 0.5 2.0 0.0 0.5 2.0 1.5 1.0 0.5 2.0 0.5 2.0 1.0 1.0 0.5 0.5 0.5 2.0

27.3 27.3 32.8 29.8 31.3 28.8 37.8 27.3 12.8 49.3 34.8 42.3 40.8 20.8 28.3 44.8 50.8 38.8 14.3 19.3 20.8 18.8 17.8 34.3 46.3 12.8 59.8 21.3 17.8 24.8 31.3 62.3 13.3 35.3 16.8 25.8 28.8 34.3 34.3 45.8 17.3

+1.0 +1.0 +0.5 +0.5 +1.0 +1.0 +0.5 +1.0 +2.5 +0.5 +0.5 +0.5 +0.5 +2.0 +0.5 +0.5 +0.5 +0.5 +1.5 +1.5 +1.5 +1.5 +2.0 +1.0 +0.5 +2.5 +0.5 +2.0 +2.0 +1.5 +0.5 +0.0 +2.0 +0.5 +2.5 +1.5 +1.0 +0.5 +0.5 +0.5 +2.5

1.0 1.5 1.0 1.0 0.5 1.0 1.0 1.0 5.5 1.0 1.0 0.5 0.5 2.0 1.0 0.5 0.5 0.5 3.0 2.5 2.0 2.0 1.5 0.5 0.5 4.0 0.0 1.5 1.5 1.5 1.0 0.0 3.5 1.0 1.5 1.0 1.0 1.0 1.0 1.0 1.5

a

Corrected for excess air determined by Ne using an iterative technique. Determined using CFC history of North American Air (Plummer et al., 2000). Age uncertainties were calculated assuming 2 C uncertainty in recharge temperature. b

ground-water age is not known (Maloszewski and Zuber, 1982). In the piston-flow model (PFM), it is assumed that the tracer moves from the recharge area with the mean velocity of the water, as if it were in a parcel, as is the case for a confined aquifer with a narrow recharge area far from the sampling point. The PFM is the limiting case of the dispersion model (DM) where the travel time distribution, g(t), is symmetrical at mean transit time for low dispersion parameter and g(t) is asymmetrical for high values of the

dispersion parameter. When the sample includes flow lines with infinitesimally short transit time in an unconfined aquifer, the exponential model (EM) might be more appropriate (Zuber, 1986). Because the input functions of CFCs and 3H have different shapes, the comparison of ages obtained from the CFC and 3H/3He dating methods can provide a cross-check for behaviors of two transient tracers (Ekwurzel et al., 1994). Calculated concentrations of 3H and CFC-12 at the time of

Application of environmental tracers to mixing, evolution, and nitrate contamination

Figure 5 Comparison of apparent ages of ground water determined by CFC-11 and CFC-12.

measurement were based on various mixing models using a mathematical model, FLOWPC (Maloszewski and Zuber, 1996). The reconstructed 3H history in Koh et al. (2005) was used as an input function of 3H, and the atmospheric CFC-12 concentration history of North American Air (Plummer et al., 2000) was used for the input function of CFC12 in Jeju Island. This approach has been used in studies where multiple environmental tracers were applied for spring waters (Katz et al., 2001; Plummer et al., 2001). Measured concentrations of 3H and CFC-12 were compared with theoretical curves for the DM and EM (Fig. 6). Most of ground-water samples show highly mixed characteristics while a considerable number of samples have very low concentrations of both 3H and CFC-12. Low concentrations of 3H and CFC-12 are indicative of old ground water, though it is not clear which model fits the measured concentrations because measured 3H and CFC-12 are scattered over various model curves. For waters with 3H concentrations, the EM or DMs appear to correspond to the measured 3H and CFC-12 concentrations. However, the EM and DMs (even for a dispersion parameter of 2) do not appear to fit measured data for the samples with low 3H values. This feature is more pronounced for the wells in coastal areas (<100 m a.s.l.) than those located at higher altitude. The lower 3H level of those samples can be explained by binary mixing of old groundwater with negligible 3H and CFC-12 and young groundwater. This is interpreted in the following section in the context of the hydrogeologic setting of the island. The first step in the evaluation of the 3H/3He data is a self-consistency check to determine if the data are meaningful in terms of known 3H/3He geochemistry (Dunkle et al., 1993; Shapiro et al., 1998; Ekwurzel et al., 1994). 3 H in precipitation on Jeju Island was reconstructed by Koh et al. (2005). For the samples which are not significantly affected by terrigenic He, the sum of the measured 3H and tritiogenic 3He concentration is in a relatively good agreement with the reconstructed 3H input history at the island

267

Figure 6 3H versus CFC-12 in ground water. CFC-12 concentration is equivalent atmospheric concentration (pptv). Theoretical curves were also shown for dispersion model (DM) with D/vx of 0.5 (a) and 0.05 (b), and exponential model (EM). The numbers on the DM lines are ground-water ages in year. For binary mixing model (BMM), only two major mixing lines were shown although they were drawn for each sample respectively. Ground water was classified by the altitude (above sea level) of the well site as below 100 m (open circle), 100–200 m (open square) and above 200 m (open triangle).

Figure 7 Comparison of apparent 3H/3He ages to the 3H input history in Jeju Island. The samples were not shown in case 3 H/3He ages have high sensitivity to terrigenic 3He/4He. M14 and M2 were affected by ground-water mixing.

268

Figure 8 CFC-12 ground water age versus specific capacity of the well. Wells were divided into unconfined (open circles) and confined (solid circles) group according to the conditions in drilling. Vertical error bars represent uncertainties of CFC-12 ground-water age. See text for further explanation.

(Fig. 7). These samples are less affected by mixing with old ground water in the comparison with CFC-12 (see Fig. 6). Sample FW25 has a higher 3H level compared with the initial 3 H concentration (3H + 3Hetrit), which may indicate a time lag of 3H in unsaturated zone. Zoellmann et al. (2001) modeled the time lag of 3H using environmental tracers of 3H and SF6 in a basaltic aquifer and showed the time lag is more pronounced in the area with thick loess cover than in the area with fractured basaltic outcrops. However, much of the 3H values do not appear to be affected by residence time in the unsaturated zones compared to the 3H input history (Fig. 6), consistent with the observation that the basal-

D.-C. Koh et al.

Figure 10 Alkalinity versus pH according to ground-water ages and mixing pattern. Ground water was grouped into young (open circle), old (solid circle), and mixed water (crossed circle) considering 3H and CFC-12 concentrations (see Table 5). For alkalinity, M13 sample was not shown because its anomalously high value is far off the trend.

tic rocks on Jeju Island have no low permeability cover and frequently outcrop through thin soil layers (0.6 m thickness on the average).

Aquifer characteristics and environmental tracers Most of the basaltic aquifer is regarded as unconfined because high permeable structures, such as clinkers and vertical fractures, are commonly observed both in outcrops and rock cores. However, some wells have properties of a confined aquifer. For example, there is an abrupt rise in water

Figure 9 Aerial distribution of CFC-12 ground-water age. The size of sold circles is proportional to CFC-12 age. Ground water with an age older than 50 years was designated as old water.

Application of environmental tracers to mixing, evolution, and nitrate contamination level when a productive zone is penetrated during drilling. Most of the wells that respond as confined aquifers are completed in the Seogwipo Formation where sand and silt zones are intercalated between layers of low permeability. Waters from wells in the confined zones of the aquifer have much older CFC-12 age, and there is an inverse relation of CFC12 age and specific capacity of the well (Fig. 8). This relation indicates that ground water moves faster through permeable interflow zones in the basaltic aquifer, whereas

269

ground-water movement is greatly retarded by low-permeability zones in the hydro-volcanic Seogwipo Formation. This hydrogeologic setting explains the considerable presence of old groundwater. The CFC-12 apparent ground-water ages are much older in coastal areas that intercept confined zones in the hydrovolcanic Seogwipo Formation than in inner areas of the island, where altitude is lower than 200 m a.s.l. (Fig. 9). This feature is not evident in the eastern coastal area where the

Table 5

Binary mixing calculations for age, fraction and nitrate concentration of young ground water

Sample

Measured NO3 (mg/L)

FW01 FW02 FW03 FW04 FW05 FW07 FW08 FW09 FW10 FW11 FW12 FW13 FW14 FW15 FW16 FW17 FW18 FW19 FW20 FW21 FW22 FW23 FW25 FW26 FW27 FW28 M01 M02 M03 M04 M05 M06 M08 M09 M10 M11 M12 M13 M14 M16 W54

14.1 9.5 14.4 4.5 6.4 4.6 11.3 38.6 70.5 3.4 12.8 6.9 3.7 12.0 7.4 1.2 2.3 4.2 60.4 5.0 11.6 12.3 5.6 0.7 5.5 17.4 4.8 26.5 5.0 1.4 24.4 7.2 68.8 0.6 3.8 0.8 0.8 0.6 0.6 0.6 15.0

DM (D/vx = 0.5)a

DM (D/vx = 0.01)a

Groupb

Age (year)

fraction

NO3 (mg/L)

Age (year)

fraction

NO3 (mg/L)

34 32 16 30 44 43 18 11 3 Old 13 Old Old 17 40 Old Old Old 1 13 16 18 20 36 Old 10 Old 8 4 23 2 Old 11 Old 13 29 34 17 39 Old 35

0.82 0.77 0.26 0.54 0.62 0.86 0.10 0.44 0.89 – 0.17 – – 0.80 0.85 – – – 0.16 0.80 0.79 0.92 1.00 0.31 – 0.99 – 0.62 0.70 0.75 0.23 – 1.00 – 0.88 0.79 0.68 0.19 0.36 – 1.00

16.3 11.2 44.7 4.9 7.9 4.7 79.3 82.9 79.1 – 56.5 – – 14.0 8.0 – – – 71.1 5.3 13.7 13.0 5.6 – – 17.5 – 40.5 5.4 – 90.8 – 68.5 – – – – – – – 15.0

26 25 19 25 27 27 20 16 12 Old 17 Old Old 19 26 Old Old Old 9 17 19 19 19 26 Old 13 Old 15 13 23 12 Old 14 Old 17 24 26 19 26 Old 18

0.85 0.83 0.28 0.58 0.55 0.78 0.11 0.48 0.98 – 0.18 – – 0.90 0.81 – – – 0.14 0.86 0.87 1.00 1.00 0.31 – 0.99 – 0.69 0.78 0.85 0.26 – 1.00 – 0.94 0.89 0.71 0.22 0.34 – 0.97

15.8 10.6 41.5 4.9 8.5 4.8 71.1 75.7 71.7 – 52.7 – – 12.9 8.2 – – – 69.7 5.2 12.7 12.3 5.6 – – 17.5 – 36.6 5.3 – 82.3 – 68.5 – – – – – – – 15.3

Young Young Mixed Mixed Mixed Mixed Mixed Mixed Young Old Mixed Old Old Young Young Old Old Old Mixed Young Young Young Young Mixed Old Young Old Mixed Mixed Young Mixed Old Young Old Young Young Mixed Mixed Mixed Old Young

a Dispersion model which were used to estimate young ground-water component from binary mixture. Old ground water is assumed to have CFC-12 concentrations less than 30 pptv (atmospheric mixing ratio) and nitrate concentration of 1 NO3–N mg/L (an average concentration of old ground-water samples). b Based on dispersion model with D/vx = 0.01. Young water was assigned for the samples with young-water fraction greater than 0.80.

270 Seogwipo Formation is not found in the subsurface, although the number of irrigation wells is much smaller in the eastern than western parts of the island. The relatively old CFC-12 apparent ground-water ages in mountainous areas (>200 m a.s.l.) appear to result from much deeper well depths than those depths in the coastal area. The relation between the transient tracers 3H and CFC12 (Fig. 6) has implications for the age structure of ground water sampled from irrigation wells. For many of the sampled wells, ground water shows highly mixed characteristics, which indicates that travel time of groundwater is distributed over a wide range. This appears to be attributed to the long screened interval with multiple productive zones in the irrigation wells. The length of the screened interval seems to have little effect on the extent of ground water mixing. This result indicates heterogeneity in frequency and location of permeable zones in the basaltic aquifer. Pumped groundwater from wells in the coastal area, especially for the western area, where the hydrovolcanic formation is frequently reported in driller’s logs, is composed of two components, young water from basaltic

D.-C. Koh et al. aquifer and old water from hydrovolcanic formation. In mountainous area (>200 m a.s.l.) most wells do not intercept the hydrovolcanic formation, which accounts for the lack of binary mixing patterns in waters from the wells.

Geochemical evolution and anthropogenic input in ground water The interaction between ground water and aquifer materials involves kinetic processes such as the dissolution and/or precipitation of minerals (Lasaga, 1984). Therefore, concentrations of conservative species derived from the geochemical processes can be functions of residence time in ground-water systems. The basaltic aquifer is composed of Ca-rich plagioclase and mafic minerals such as olivine and pyroxenes, and a matrix with a chemical composition similar to phenocrysts (Park and Kwon, 1993). The ground water gradually acquires major cations with increase of pH and alkalinity during ground-water/basaltic rock interactions (Gislason and Eugster, 1987). However, nitrate sources can also contribute dissolved solutes to ground water as well

Figure 11 Cations versus nitrate according to ground-water ages and mixing pattern. Ground water was grouped into young (open circle), old (solid circle), and mixed water (crossed circle) considering 3H and CFC-12 concentrations (see Table 5).

Application of environmental tracers to mixing, evolution, and nitrate contamination as nitrate in Jeju Island, where nitrate contamination is considerable in the coastal area (Koh et al., 2005). To evaluate the degree of chemical evolution and relative contribution of nitrate sources to ground water solutes, the concentrations of some major ions and pH were plotted (Fig. 10) according to ground-water classification, ages and mixing patterns (Table 5). pH and alkalinity clearly separate ground-water samples into young, old and mixed waters though a few samples overlap (Fig. 10). This indicates ground-water mixing patterns observed in the comparison of 3H and CFC-12 are consistent with the hydrogeochemical evolution of ground water over a relatively large geographic area. Cations poorly distinguish ground-water samples according to ages and mixing (Fig. 11), which may suggest that concentration of each cation is more sensitive to chemical and mineralogical composition of basaltic rocks along the flow paths than ground-water age. For ground water contaminated with nitrate, major cation concentrations are higher than in more evolved, that is, old water. This result indicates that dissolved cations, especially Ca and Mg, are supplied more by nitrate contamination processes than interaction between CO2-charged water and basaltic rocks. As a whole, nitrate contamination of ground water appears to proceed to the point where it affects dissolved solutes more than natural mineralization does. The saturation index (SI) of calcite was calculated by PHREEQC (Parkhurst and Appelo, 1999) and compared to the CFC-12 apparent age with groups according to ages and mixing (Fig. 12a). The SI can be regarded as an indicator for the progress of basalt–water interaction because calculation of the indices involves the major variables in the geochemical processes, such as Ca, pH and alkalinity, simultaneously. SI of calcite increases with the increase of CFC-12 apparent age for young water, whereas this trend is not clear for the samples regarded as mixtures. Although the concentration of each dissolved species is poorly correlated with the CFC-12 apparent age, pH and SI calcite co-vary, and, thus, indicate a similar relation with CFC-12 age (Fig. 12b). These results indicate that the apparent CFC-12 ground-water age is correlated with the geochemical evolution of ground water along the flow paths. Most of ground-water samples from the wells in the confined parts of the Seogwipo Formation (Table 1) are oversaturated with calcite (Fig. 12a) and have high pH (Fig. 12b). The high pH values appear to result from dissolution of calcareous materials such as molluscan shells in the formation. Calcite cements are common in Seogwipo Formation, whereas they are not found in outcrops or drilled cores of basaltic rocks (Park, K.H., personal commun.). Thus, dissolved calcium and carbonate species appear to re-precipitate as calcite within the Seogwipo Formation where they are originally derived. Evolution of ground-water chemistry is also identified in silicate mineral equilibria with the increase of groundwater age, and compared to waters from Springs (Fig. 13) located over a wide range of altitude between the coastal area and central mountain peak of the island (Koh et al., 2005). Most young ground-water composition is located near the kaolinite–smectite phase boundary or in the smectite stability field. For the K2O–Al2O3–SiO2–

271

Figure 12 CFC-12 ages versus saturation index (SI) of calcite (a) and pH (b) in ground water. Ground water was grouped into young (open circle), old (solid circle), and mixed water (crossed circle) considering 3H and CFC-12 concentrations (see Table 5). Labeled points are the wells in confining condition (Table 1).

H2O system, ground-water samples plot along the K–feldspar–muscovite phase boundary. This pattern of chemical evolution is similar to those of Nesbitt and Wilson (1992) and Aiuppa et al. (2000). Unlike ground water from wells, most spring water plots in the kaolinite field where the degree of chemical evolution is controlled by altitude of spring location. Old water has more evolved characteristics than young water and plots toward the zeolite and chlorite fields. The composition of old water appears to be controlled by the smectite–zeolite boundary for Ca, and the smectite–chlorite boundary for Mg. Mixtures of old and young water are located between the two components, although they are widely scattered. The poor separation of young water and mixed water suggests that hydrogeochemical evolution is more controlled by other factors such as reactive mineral proportions in the aquifer materials than ground-water age. The occurrence of smectites in chemical weathering of basaltic rocks was reported by Banfield et al. (1991) in Oregon, and Prudencio et al. (2002) in Portugal. Jercinovic et al. (1990) identified zeolites in closed-system environments with low ground-water flow rates during weathering of basalts.

272

Timescales for nitrate contamination of ground water The effect of nitrate contamination on ground-water quality is clearly identified in this study as in previous studies (Woo et al., 2001; Lee et al., 2002) where nitrate concentrations considerably exceed the background level (about 1 mg NO3– N/L) in many of the samples (Table 2). The source of nitrate contamination of ground water is mainly chemical fertilizers used extensively in coastal farm land (Oh and Hyun, 1997; Song et al., 1999). In Jeju Island, the use of chemical fertilizers increased rapidly from the early 1960s until the mid1990s (Fig. 14). Therefore, the utilization history of fertilizers is expected to be related to the recharge date of ground water in terms of nitrate contamination, as shown by Bo ¨hlke and Denver (1995). The nitrate concentration was compared to the fertilizer use with the timescales of ground-water recharge. Nitrate concentration dramatically increases in ground water recharged after the1980s for the wells with agricultural land use, consistent with the rapid growth of chemical fertilizer use in the island (Fig. 14). The fertilizer-derived nitrate was calculated as concentrations of total chemical fertilizer

D.-C. Koh et al. amount dissolved in average recharge water (Jejudo, 2000) assuming that fertilizers are applied evenly over the entire agricultural area in the island, which was scaled to compare with measured nitrate concentration in ground water. Organic fertilizers like manures and plant residues also can be used as fertilizers. But their use is unlikely to be favorable for profitable crop production during the period from 1970s to 1990s because they are slow and inefficient in supplying nitrogen to crops. Thus, the strong temporal correlation of nitrate in ground water and fertilizer use indicates that the major source of nitrate is chemical fertilizers used in agricultural fields. This result is consistent with the previous works by nitrogen isotopic study (Song et al., 1999; Koh et al., 2005). When plotted with apparent CFC-12 ground-water age, the temporal trend in the upper limit of the measured nitrate conforms well to the annual use of chemical fertilizer in the island. This appears to be attributed to the very similar growth curves of chemical fertilizer use and atmospheric CFC-12. As previously discussed, ground-water samples in this study were affected by mixing of old water with negligible 3 H and CFC-12 (Fig. 6). Fractions of young and old water

Figure 13 Activity–activity diagrams showing silicate mineral-water equilibria at 25 C (solid line) and 10 C (dotted line) with P = 1 bar. Thermodynamic data were taken from Helgeson (1969) and Helgeson et al. (1978). Mineral abbreviations are: Gibb, gibbsite; Kaol, kaolinite; Laum, laumontite; Sm, smectite; Chl, chlorite; Ab, albite; and Musc, muscovite. Ground water was grouped as young (open circle), old (solid circle) and mixed (crossed circle) ground-water considering 3H and CFC-12 (see Table 5). Spring waters (Koh et al., 2005) was also plotted as open triangles for comparison.

Application of environmental tracers to mixing, evolution, and nitrate contamination

Figure 14 Nitrate concentration versus CFC-12 age of ground water. Fertilizer-derived nitrate and atmospheric mixing ratio of CFC-12 were also shown for comparison where ages mean years from sampling time of ground water (2002). Fertilizerderived nitrate was scaled as 9 wt% of total chemical fertilizer amount dissolved in average recharge water.

were determined by CFC-12 concentrations. The CFC-12 concentration of the old water was estimated to be that in equilibrium with air containing 30 pptv CFC-12, which is the mean concentration for samples with 3H less than 0.3 TU (Koh et al., 2005). The CFC-12 concentration of young water was estimated as that of the DMs at the intersection point with the line extended from old water through the sample concentration. Using the measured CFC-12 concentrations in binary mixtures, the fractions of young and old components were calculated (Table 5). Dilution of nitrate concentration by an old-water component was corrected using the fractions calculated by CFC-12 concentrations with the

273

assumption that the nitrate concentration of the old-water component is 1 mg NO3–N/L, an average of old ground-water samples. The results of the calculation are listed in Table 5. Comparison of DMs and nitrate concentration of ground water was made for a young-water component, not for the mixture. Of two cases of DMs with dispersion parameters of 0.5 (a) and 0.01 (b), the DM with the smaller dispersion parameter estimates ground-water nitrate concentrations more closely than those with a large dispersion parameter (Fig. 15). This indicates that travel time distribution is relatively narrow in ground-water pumped from irrigation wells for the young water component. An increase in nitrate concentration of ground water is abrupt after the age of about 25 years whereas that in the history of chemical fertilizer use is exponential (Fig. 15). This indicates that before the late 1970s, the mass of nitrates that leached out of root zones and reached the water table is not sufficient to appreciably affect the ground-water quality. Thus, if nitrate sources could be controlled to the levels used before the late-1970s, additional nitrate contamination of ground water would be greatly reduced. Bo ¨hlke (2002) discusses various scenarios in controlling nitrate levels in ground water. The relatively high vulnerability of the basaltic aquifer to nitrate contamination can be attributed to the thin soil cover, high permeability of the basaltic rocks, and high recharge rate. This finding indicates that the fertilizer use is excessive and inefficient, and that fertilizer application rates need to be controlled to protect ground water resources from worsening nitrate contamination.

Summary and conclusions The environmental tracers, 3H, He, Ne, and CFCs (CFC-11 and CFC-12) were investigated along with hydrogeochemical characteristics for ground water from irrigation wells completed in basaltic and hydro-volcanic aquifers of Jeju Island. The apparent 3H/3He age was considerably affected by terrigenic helium, but could be estimated, in some cases, by narrowing the ratio, Rterr/Ra, to the range of 5.9–6.6.

Figure 15 Nitrate concentration versus age of young ground-water fraction in a binary mixture. Dispersion models (DM) were shown as solid lines for dispersion parameters of 0.5 (a) and 0.01 (b) where ages are mean ground-water age. DM results were calculated using nitrate input history of 10 and 13 wt% of total chemical fertilizer amount dissolved in average recharge water for (a) and (b), respectively.

274 Atmospheric concentrations of CFCs in the island showed little weekly fluctuations and were in good agreement with that of North American Air, which was used to determine ground-water age from CFCs. Lumped-parameter mixing models (dispersion models and exponential model) were compared to the measured concentrations of 3H and CFC12. Ground water was distinguished as old water with negligible 3H and CFC-12 and younger water. However, low 3H levels in a considerable number of samples cannot be explained by the mixing model, which was interpreted by binary mixing between old water and younger water. The binary mixing is also identified in alkalinity and pH whereas it is not clear in major cation concentrations. The groundwater CFC-12 age is much older in water from wells completed in confined zones of the hydrovolcanic Seogwipo Formation in coastal areas. Old water shows more evolved characteristics in water–silicate mineral equilibria. The saturation states of ground water with calcite are in relatively high correlation with the apparent CFC-12 ages for younger water whereas there is no apparent correlation for water mixed with old water. Nitrate contamination of ground water was analyzed in terms of recharge date and compared to the fertilizer use history on the island. The temporal correspondence of nitrate concentration in ground water to the fertilizer use indicates that nitrate sources are mainly from chemical fertilizers used in agricultural fields. Nitrate concentration in ground water increased abruptly compared with exponential growth of nitrate loadings, which indicates that the levels of fertilizer use before the late-1970s did not appreciably affect ground water quality. This result indicates that application of chemical fertilizers is inefficient at least after the late-1970s and that the fertilizer application rates need to be controlled to prevent further contamination of the volcanic aquifers.

Acknowledgement This research was supported by a Grant (Code 3-2-2) from Sustainable Water Resources Research Center of 21st Century Frontier Research Program. We thank H.-J. Seong and C.-H. Kang for their assistance in sampling ground-water, and D.-C. Moon for periodic air sampling, and Dr. W.-B. Park and Dr. G.-W. Koh for providing helpful information on the ground water systems in Jeju Island. Comments of Dr. K.-H. Park greatly helped to construct a conceptual geologic model. The local managers of irrigation wells are also thanked for allowing access to the wells for sampling. This study was inspired and facilitated in the early stage by the late Dr. Dae-Ha Lee and it is much benefited from his profound thrust for upgrading ground-water study in Korea. The use of brand names in this report is for identification purposes only and does not represent endorsement by the U. S. Geological Survey.

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