Salt contamination of a coastal confined aquifer

Transcription

1 Relation of Groundwater Quantity and Quality (Proceedings of the Hamburg Symposium, August 1983). IAHS Publ. no Salt contamination of a coastal confined aquifer INTRODUCTION K. MORI Department of Geography, Mie University, Tsu, Mie 514, Japan ABSTRACT Quality of confined groundwater in the vicinity of the outlets of the Kiso, Nagara and Ibi Rivers in the central part of Japan was investigated to determine the origin of dissolved constituents. The concentration of chloride in groundwater showed a remarkable tongue distribution, having a maximum value of 2510 mg 1 _1 in the aquifer of m in depth. The area showing the highest concentration of chloride is located a short distance landward from a shoreline and coincides with an area of high intensity of withdrawal. Aquifer leakage by vertical downward movement of contaminated phreatic groundwater through non-pumping wells had an effect in lowering the quality of confined aquifers. The extrusion of fossil water from marine impermeable layers was a further source of dissolved material in confined groundwater. A mean residence time of years was deduced from measurements of environmental tritium for groundwater in the majority of the basin. Contamination par le sel marin d'une nappe aguifère confinée en bordure de la mer RESUME L'origine des matières dissoutes et la qualité de l'eau souterraine artésienne d'une nappe située près des embouchures des fleuves Kiso, Nagara et Ibi (Japon central) ont été analysées. La concentration en chlore montre une distribution en forme de langue ayant une valeur maximale de 2510 mg 1~ entre 40 et 70 m de profondeur. La zone de concentration maximale en chlore se situe dans les 1 terres, assez en arrière de la ligne côtière, où le soutirage de l'eau souterraine par unité de surface est relativement élevé. La nappe aquifère confinée se déplace en un mouvement descendant vertical que provoque l'infiltration d'eau libre contaminée, par les anciens puits de captage. La contrainte qu'exerce l'eau fossile des couches marines imperméables est une autre source pour les éléments dissous dans l'eau de la nappe. Les mesures de concentration en tritium montrent que le temps moyen de séjour de l'eau souterraine, qui occupe la majorité du bassin hydrogéologique, est de 30 à 40 ans. Groundwater is increasing in importance as a usable resource in 219

2 220 K.Mori meeting the rising demands imposed by human activity. Concurrently, the development of this resource has sometimes been counterproductive with environmental damage due to improper utilization. In this paper, salt contamination of confined groundwater was investigated in the vicinity of the outlets of the Kiso, Nagara and Ibi Rivers in the central part of Japan, where heavy withdrawal of groundwater has been practised. The main purpose of this paper is to clarify the origin of dissolved constituents in contaminated confined groundwater. The study area is underlain by younger sediments, being composed of alluvial fans, deltaic plains, reclaimed lands and in-filled areas (Iida et al., 1977). The lowest point in this area is 2.9 m below mean sea level due to land subsidence. There are three layers of sand and gravel, each perceived as forming a distinct confined aquifer. METHODS AND MATERIALS Routine observations were made over a period of six years from 1977 to 1982, generally in February and August, taking account of the annual variation of piezometric head (Mori, 1981). 117 observation wells were established for aquifer I, 52 wells for aquifer II and 73 wells for aquifer III. The quality of groundwater was assessed, using the following analytical methods: Both sodium and potassium were determined by the flame photometry. Calcium analysis was based on the EDTA titrimetric method. Magnesium was obtained as the difference between the total hardness measured by EDTA titration and the calcium hardness. Titration with mercuric nitrate yielded values of chloride concentration. The barium sulphate turbidimetric method was used for sulphate content and bicarbonate was determined by titration with sulphuric acid. In addition to these periodic observations, a 3 1 sample of groundwater was collected in August 1982 for determining tritium concentration at each of six observation wells (two replicates for each confined aquifer). The Department of Chemistry, Gakushuin University, undertook the isotopic analysis for tritium concentration of the samples. CHARACTERISTICS OF GROUNDWATER QUALITY Distributions of groundwater quality A generalized picture of the horizontal distribution of chloride concentration of groundwater for each confined aquifer in the Hokusei District is shown in Fig.l. In the case of both confined aquifers I and II, increasing chloride concentrations are noted with downstream progression in the vicinity of the respective outlets of the Kiso, Nagara and Ibi Rivers, located in the northern part of the district. A remarkable tongue pattern of chloride concentration in groundwater is formed along the lower reaches of a river. Furthermore, localized contamination can also be identified. From Fig.2, it is clear that the zone of maximum chloride concentration (2510 mg l -1 ) in aquifer I at m in depth, coincides with the

3 Salt contamination of a coastal confined aquifer 221 Aquifer I Aquifer II Aquifer III FIG.l Distribution of chloride concentration (mg I -1 ) of confined groundwater in the Hokusei District, Japan, for the years w "' F JEM < 10 m < S jgljj < S jjjllj < FIG.2 Distribution of groundwater withdrawal (m 3 day~ l km~ ) in the northern part of the study area (total amount for aquifers I, II and III).

4 222 K.Mori area of greatest intensity of withdrawal. Chemical composition of groundwater Figure 3 shows an east-west geological cross section to include the three confined aquifers. Superimposed on the cross section are resultant descriptions of chemical composition, using specialized pattern diagrams at the various sample points. These diagrams are formed by plotting the concentrations of cations to the left of a vertical zero axis and that of anions to the right. Three horizontal axes are used in depicting the proportions of specified ions as indicated in the legend of Fig.3. The resulting points form an irregular polygon with waters similar in quality defining a distinctive shape (Todd, 1980). The chemical composition of the upper two aquifers conforms to that of sea water whereas in the lowest aquifer an absence of sea water contamination is indicated. Aquifer I Aquifer II Aquifer III I I Sill. C.\,y Ca" Mg "' % ^ ^ F ScaW atcr % 100 _ cr sor HC0; + CO FIG.3 Chemical composition of groundwaters over a geological cross section. A triangular diagram for groundwater quality is shown in Fig.4. It is apparent that the chemical composition of groundwater of the deeper aquifer II is more consistent with that of sea water than that of the shallower aquifer I. In other words, the ratio of sulphate to the total anions for the groundwater of aquifer I is lower than that for sea water. This suggests that the dissolved constituents of groundwater in aquifer I originate from a comparatively old sea water in marine impermeable layers.

5 Salt contamination of a coastal confined aquifer FIG.4 Triangular and key diagrams for quality of groundwater including sea water. Dynamics in concentration of dissolved constituents Figure 5 shows the results of weekly sampling in aquifer III over a 7-month period. Temporary increases in chloride concentration appear to be coupled with a loss in piezometric head. During such periods of reduced pressure, access to the lowest aquifer is possible along non-pumping wells. Contaminated phreatic groundwater may flow vertically downwards, resulting in fluctuations in the quality of confined groundwater. JULY j "Âyo, TEPT. ~ÔCT. I NÔV7T DEC. "IXsTT (1978-'79) FIG.5 Changes in the chloride concentration of groundwater and piezometric head. RESIDENCE TIME OF GROUNDWATER AS DEDUCED FROM ENVIRONMENTAL TRITIUM The determination of a mean residence time of confined groundwater involved measurements of environmental tritium. The tritium concentrations of six samples of confined groundwater are presented in Table 1. Monthly variation in tritium concentration of precipitation at Tokyo and Tsukuba is shown in Fig.6 (Shimada, 1982) Except for one sample, the difference in tritium concentration between aquifers is marginal. It is obvious that those samples

6 224 K.Mori TABLE 1 Concentrations of tritium and chloride in groundwater for six samples collected in August 1982 Aquifer Depth of strainer (m) 3fJ (TU) Cl' (mg 1 i ) I II III FIG.6 Variations of tritium concentration in precipitation at Tokyo and Tsukuba. ranging between 0.85 and 1.97 TU have not been influenced by the thermonuclear testing since Thus, a mean residence time can be calculated as between 28.8 and 43.6 years. This determination assumes a piston flow mechanism (Kayane, 1972) and an initial value of tritium concentration in precipitation as 10 TU. The remaining sample of excessive tritium concentration at 18.7 TU was collected in the recharge area of the groundwater basin and has been contaminated with recent precipitation with elevated levels of tritium. CONCLUDING REMARKS Salt contamination of a coastal confined aquifer was investigated in the vicinity of the outlets of the Kiso, Nagara and Ibi Rivers in this study. Contamination was attributed to extrusion of fossil water from marine impermeable sediments. The majority of the basin

7 Salt contamination of a coastal confined aquifer 225 is occupied by groundwater having a mean residence time of the order of years. ACKNOWLEDGEMENTS The writer would like to express his gratitude to Dr Y.Araki, Professor Emeritus of Mie University, for his encouragement during the course of this work. The French abstract was prepared by Professor N.Takahashi of the University of Tsukuba, to whom the writer expresses his appreciation. REFERENCES Iida, K., Sazanami, K., Kuwahara, T. & Ueshita, K. (1977) Subsidence of the Nobi Plain. In: Land Subsidence (Proc. Anaheim Symp., December 1976), 1-8. IAHS Publ. no Kayane, I. (1972) Recharge mechanism of groundwater in Kanto groundwater basin as deduced from tritium data (in Japanese with English summary). Tokyo Geography Pap. 16, Mori, K. (1981) Considerations on the fluctuations of groundwater level in coastal areas (in Japanese with English summary). Rep. Environ. Sci., Mie Univ. 6, Shimada, J. (1982) Study of soil water movement in the Kanto Loam Formation using environmental tritium as a tracer. DSc thesis, Univ. of Tsukuba. Todd, D.K. (1980) Groundwater Hydrology (2nd Edition). John Wiley, New York.

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