Hydrogeology and geochemical characterization of groundwater in a typical small-scale agricultural area of Japan

Journal of Asian Earth Sciences 29 (2007) 18–28 www.elsevier.com/locate/jaes

Hydrogeology and geochemical characterization of groundwater in a typical small-scale agricultural area of Japan Adrian H. Gallardo b

a,*

, Norio Tase

b

a AIST Geological Survey of Japan, Higashi 1-1-1, Center 7, Tsukuba 305-8567, Japan School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Ibaraki 305-8572, Japan

Received 14 October 2005; accepted 9 December 2005

Abstract Tsukuba city is located in the center of the Kanto Plain, the largest basin in Japan. Its dramatic increase in population and agricultural production make it imperative to quantify the groundwater system to ensure the long-term sustainability of the resource. In response to this need, the hydrogeology and chemistry of groundwater in a typical agricultural catchment of the region were characterized on a detailed scale in order to serve as a scientific tool for the local management authorities. The uplands include two aquifers separated by confining clays. In the lowlands, these aquifers behave as a single unit. Infiltration from precipitation recharges the groundwater, which moves laterally through the sands and discharges mainly into an artificial drainage. The age of groundwater increases with distance from the drain, with a maximum of 63 years for particles that originated at the catchment divide. The chemistry of shallow groundwater reflects a fertilizer’s source, with approximately 75% of the samples collected within the croplands exceeding the Japan’s drinking water standards for NO 3 . Biochemical processes and the absence of agricultural practices reduce the salts concentrations throughout the rest of the area. Better water quality was found at depth, where groundwater of Ca–HCO3 type prevails.  2006 Elsevier Ltd. All rights reserved. Keywords: Hydrogeology; Groundwater; Water quality; Agriculture; Tsukuba city; Japan

1. Introduction Groundwater contamination has been recognized as one of the most serious environmental problems in Japan since the environment agency of Japan (EAJ) carried out a nationwide investigation in 1982 (Tase, 1992). In spite of the early warning, neither the government nor the public have noticed the real dimension of the problem until recently. Therefore, pollution of groundwater continued growing in conjunction with the expansion of urban and industrial areas, and the intensification of agricultural practices to cope with the ever increasing demand for food supply in the region. An inappropriate management of groundwater promotes the rapid deterioration of the resource, putting the human community under a severe risk. Among the sources of contamination, agriculture *

Corresponding author. Tel./fax: + 81 29 861 3240. E-mail address: [email protected] (A.H. Gallardo).

1367-9120/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2005.12.005

exerts both direct and indirect effects on the rates and compositions of groundwater recharge and aquifer biogeochemistry (Bo¨hlke, 2002). Agriculture constitutes the largest user of the resource, averaging about 70% of all freshwater supplies on a global scale (Ongley, 1996). In Japan, many surficial aquifers throughout the country have been severely degraded by overuse of fertilizer and pesticides, and are now unsuitable for human consumption  especially due to the high content of NO 3 and NO2 (Tase, 2004). Significant amounts of nutrients can be exported from agricultural areas into streams and lakes through various hydrological pathways and from all of them, it can be expected an especially large contribution from groundwater, as it remains in contact with the geologic media during longer periods, and usually presents higher concentrations of dissolved solids than rainfall or surface waters. Thus, understanding the patterns and rates of groundwater flow is the first and most important step before investigating the migration of contaminants at any specific site. Quanti-

A.H. Gallardo, N. Tase / Journal of Asian Earth Sciences 29 (2007) 18–28

tative estimates of the flow paths enable a better definition of the extent and movement of contaminants within the system (Buxton et al., 1991), while the chemistry of groundwater is an essential parameter to assess the environmental characteristics of an area. On a national scale, groundwater contamination has been reported to concentrate essentially in municipalities within the Kanto Plain (Tase, 1992, 2004), the largest and the most altered basin of Japan. Even though the geology of the Plain has been intensively studied (e.g. Kimura, et al., 1991; Suzuki, 1996; Marui and Seki, 2003), most of the research is restricted to the regional scale, sometimes overlooking local heterogeneities and variations according to the location. Furthermore, other studies only compare the concentrations of major ions in groundwater against drinking water standards, without any concern of their relation with other parameters. In the core of the Kanto Plain, the area of Tsukuba Science City deserves a special consideration. The city was designed to decongest the Tokyo Metropolitan Area, and as a result it is experiencing a steady development and dramatic intensification of agricultural production, with consequent pressure over the water system. While the city had a population of 188,000 in the year 2000, it is expected to reach 350,000 by 2025 (Municipality of Tsukuba, 2005).

19

In view of this, there is an urgent need to adequately manage the use of water resources in order to ensure a reliable and sustainable supply in the near future. The purpose of this study is to define a conceptual framework of the shallow deposits underlying a representative agricultural catchment within Tsukuba, and describe the patterns and rates of groundwater movement within it. In addition, major ions are analyzed to get a better understanding of the groundwater chemistry of the site, and as a basis for the construction of a numerical model of contaminant transport in the district. Finally, the present investigation aims at assisting the planners in the development of the city. 2. Area of study A typical agricultural catchment of about 400·300 m was selected for investigation in the northern part of Tsukuba City, Ibaraki Prefecture, approximately 60 km northeast of Tokyo, Japan. The site is located at the boundary between the uplands of the Tsukuba plateau, and the floodplain of the Sakura River (Sakuragawa Teichi), with heights ranging from nearly 28 m above sea level at the catchment divide, to 16 m in the lowlands (Fig. 1). A number of steep slopes, formed as a consequence of landslides, conforms the tran-

Fig. 1. Land use and location of the area of study.

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sition between both zones. A few valley corridors have been dissected in a semi-parallel pattern along the lowlands, although they remain dry for most of the year, indicating that overland flow is limited to heavy storms. On the other hand, a permanent stream originates in the southwestern corner of the area of study, and an open drain constitutes the final collector of the system, removing water from the site towards the Sakura River about 1 km to the north. The canal width averages 1 m and is only partially filled. The flow is slow, sometimes even stagnant, as a result of the low gradient and the presence of plant roots and clogging materials on the bottom. Agriculture is the dominant land use, mainly developed in small farms distributed throughout the relatively flat uplands. The principal crops grown are Chinese cabbage, wheat, and pastures. According to the farmers’ information, there is an approximate input of 400 kg ha1 of fertilizers N:P:K 14% at each growing season of wheat, 1000 kg ha1 of N:P:K 15:10:10 and 400 kg ha1 of urea 45% for the cabbage, and 380 kg ha1 per month for the pastures. A dense forest covers part of the uplands, extending along the banks of the stream and the slope’s escarpments. A riparian zone and wetland are located downgradient in the lowlands, just up the drain channel.

during 14 months. Prior to groundwater sampling, the depth of the water table was determined using a measuring tape and fox whistle. Chemical analyses of major cations were performed by an inductively coupled argon plasma atomic emission spectrophotometer (ICP-AES; Nippon   2 Jarrel-Ash ICAP 757V), while NO 3 ; NO2 ; Cl ; and SO4 were measured by a Shimadzu CDD-10A ion chromatograph both at the University of Tsukuba. Ammonium ðNHþ 4 Þ concentrations were determined by a DKK Corp IOL-40 multi channel ion meter. Bicarbonate ðHCO 3Þ was calculated by the titration method using an E645 Multi Dosimat device. A solution of 0.005 M sulfuric acid was utilized as reagent. Electrical conductivity, temperature, pH, redox potential (ORP) and dissolved oxygen (DO) were measured in the field by a Quantas sensor, Hydrolab Corp. As field observations are restricted by the density of the measurement points, the flow of groundwater was determined through a three-dimensional model developed by the finite-difference program MODFLOW (McDonald and Harbaugh, 1988). PEST (Doherty et al., 1994) was utilized to calibrate the model, and MODPATH (Pollock, 1989) calculated the advective pathlines of the water particles.

3. Materials and methods

4. Results and discussion

The subsurface was studied through a network of 43 wells, 23 of which followed a transect along the expected groundwater direction. Although, some irregularities in its design were inevitable, the multilevel transect was placed at the center of the studied site providing the maximum coverage of land use and geological units. The installation process involved the use of hand augers (DIK-100A-B1, U70: Daiki Rika Kogyo Co.) and a drilling machine (Geoprobe Systems Model 4220), resulting in a set of boreholes with depths from 1 to 10 m. All wells were stabilized by a sand pack, precluded with bentonite, and surveyed to establish their absolute elevation. The stratigraphy was determined by well logging, and undisturbed sediment cores were taken for measurements of vertical conductivity and porosity. Horizontal hydraulic conductivity was estimated by slug tests at several of the monitoring wells, and its value calculated by the solution of Bouwer and Rice (1976). Rainfall (1272 mm) was derived by averaging the values measured from 1991 at one of the facilities of automatic observation located in Tsukuba, automated meteorological data acquisition system (AMEDAS), whose data is of public domain. Actual evapotranspiration was interpolated from the literature according to the land use. It ranges from 537 mm yr1 at the croplands (Nakagawa, 1984) to a maximum of 1043 mm yr1 within the wetland (Takamura, 2001). Samples of groundwater, the main stream, and the drainage were collected approximately on a monthly basis

4.1. Geologic setting More than 3000 m of sediments accumulated in the center of the Kanto Plain (Suzuki, 1996). The regional stratigraphy is depicted in Table 1 (Unozawa et al., 1988). At a refined scale, four units define the geologic setting of the site in descending order: the Kanto Loam, J oso clay, Ryugasaki Fm, and Narita Fm (Fig. 2), the last one is attributable to the upper part of the middle Pleistocene Shimousa Group (Masuda and Shindou, 1986). The Kanto Loam is a brownish layer of volcanic ash that extends throughout the area to a depth between 0.7 and 1.6 m. Even though its upper zone is highly affected by weathering, it is unaltered below 0.75 m, as a well consolidated and massive structure. Clays of the J oso Fm occur beneath the Loam in the uplands. With a thickness of about 1 m, they disappear abruptly by erosion at the slopes. The strata contains important amounts of sesquioxides in veinlets, and micas are largely the primary mineral (Tang, 1989). These deposits are attributed to the floodplain of the ancient Kinu River, which used to run near the actual plain of the Sakura River. The predominant white coloration of the sediments is caused by pumice transported by rivers coming from the volcanoes of Nikko, about 90 km to the north (Matsumoto, personal communication). The 3–4 m thick sands of the Ryugasaki Fm constitute the top of the saturated zone, and although in the strict sense they do not transmit significant quantities of water to install production wells, they were informally considered

A.H. Gallardo, N. Tase / Journal of Asian Earth Sciences 29 (2007) 18–28

21

Table 1 Summary of the regional geology (after Unozawa et al., 1998)

the upper aquifer of the catchment. This unit is generally made up of bimodal sands dominated by fines, interpreted as deposits of point bars in a low energy stream as indicated by the dominance of angular grains, the presence of feldspars, and tabular cross-bedding observed in the Hanamuro outcrops a few kilometer south of the studied site. The occasional presence of thin and discontinuous lenses of gravel intercalated with sands beneath the uplands can be explained as lag deposits transported by the stream at maximum velocities during floods (Boggs, 1995). In the lowlands, there is a substantial decrease in the grain size of the sands and the disappearance of the gravel beds as well. Here, the deposits would have accumulated by mass movement that resulted from hillslope failure. Landslides occur every few years mainly triggered by earthquakes of large magnitude or heavy storms, and constitute the majority of hillslope units in Japan (Oguchi, 1996). In the uplands, the Ryugasaki Fm is underlain by an approximately 3 m thick layer, consisting of grayish clays with subordinate sand lenses less than 0.5 m thick. The unit

can be considered as the top of the Narita Fm and originated in an environment characterized by alternating high and low energy currents, likely a small delta at the mouth of the Kinu River. The bulk of the clay would have been deposited by suspension in the prodelta, while the sands might be attributed to distributary channels in the delta plain. The so-called deep aquifer of green-olive, sub-rounded, and well-sorted sands underlies the sequence described above. This unit is dominated by fine and very fine sediments, and constitutes the main member of the Narita Fm. Its base could not be reached by drilling beneath the uplands, but its thickness is estimated to be at least 4– 5 m. In contrast, this thickness decreases from 3 to less than 1 m at some locations within the lowlands. The aquifer is effectively confined beneath the clayey sequence in the uplands, but as this layer is not laterally continuous, it comes in direct contact with the Ryugasaki sands throughout the lowlands. It is widely accepted that the Narita sands correspond to a shallow marine environment, likely an estuary (Masuda and Shindou, 1986). Furthermore,

Fig. 2. Geology along the main transect.

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A.H. Gallardo, N. Tase / Journal of Asian Earth Sciences 29 (2007) 18–28

Narita sands can be interpreted as a marine-dominated estuary experiencing prevalent wave action, as these systems are commonly characterized by sand-size sediments (Davis, 1992). In addition, hematite is the characteristic cement in the uppermost 1–1.5 m of sands below the uplands. The origin of the red bed might relate to biogenic processes: estuary environments support large populations of suspension feeder organisms, which would incorporate dissolved iron for their metabolic activities. The organisms would contribute to the sedimentation of iron either by the excretion of sand-size pellets, or by accumulation of skeletal debris on the floor (Ikeda, personal communication). As low energy environments are more favorable for organisms, it is expected that the red bed might have developed in calmer water facies with negligible tidal currents and waves. The basement of the sequence was observed in the lowlands where a greenish-gray silt of dome morphology, likely deposited under stagnant marine conditions, was found. The unit acts as an aquiclude in the hydrological system. Finally, a layer of organic soil formed in the vicinity of the wetland by the accumulation of plant remains in a waterlogged and anaerobic environment. In spite of its relative small extent, this horizon plays a major role in the removal of groundwater pollution (Gallardo and Tase, 2005) 4.2. Sediment properties Slug tests in the upper aquifer show that the hydraulic conductivity varies from 1.3·103 at the northern boundaries of the field to 3.5·107 cm s1 at the foot of the slopes, although values on the order of 105 cm s1 are predominant, especially in the uplands. These results are somewhat lower than the measurements of Tang (1989) and Bae (1986) in the neighboring Dejima area. They reported conductivities of 2.8·102 and 4.5·102 cm s1, respectively, supporting the idea of high heterogeneities at relatively short distances. In addition, there seems to be a direct relation between hydraulic conductivity and grain size, as larger conductivities were usually associated with a higher content in gravels, while the lowest permeability occurs in the fine-grained facies downgradient of the slopes. More data on grain size must be obtained through sieve analyses to corroborate this observation. Taking advantage of the number of measurements carried out, several conductivity values were defined by interpolation. A distribution array created by the natural neighbor method shows that conductivity of the upper unit follows a Gaussian distribution, with most of the points clustered around a mean of 5.5·105 cm s1 (Fig. 3), in coincidence with the expected lognormal distribution usually reported in the literature (Sudicky, 1986). As for the lower aquifer, hydraulic conductivity was derived from the drawdown data at one well in the lowlands and averages 1.9·105 cm s1. In comparison with the surficial aquifer, permeability slightly decreases at the Narita formation probably as a result of a more rigid and homogeneous

Fig. 3. Distribution of hydraulic conductivity within the upper aquifer.

structure, and also to the significant increase in the proportion of fine-grained materials. Finally, measurements confirmed a mean conductivity of 1.3·106cm s1 for the deep clay beneath the uplands, and 3.3·108cm s1 for the silty basement, confirming its role as a confining unit. Boreholes were also used to determine the saturated thickness of the major units and to get an idea of the transmissivity of each layer. The maximum saturated thickness of the upper unit in the uplands is normally constant between 3.6 and 3.7 m, although it can be greatly reduced during the dry season when the water table can drop by 1 m. On the other hand, the saturated thickness for the deep sands could not be calculated with precision as most wells did not fully penetrate the unit. However, the available information indicates a thickness over 3.5 m beneath the highplains, reducing to no more than 1 m for most of the lowlands. With these values in mind, it is clear that transmissivity is not uniform neither spatially nor in time, although some inferences can still be made. In the uplands, maximum transmissivity for the upper unit was estimated to range between 0.094 and 0.12 m2 d1. Moreover, its value at the top of the main slope was approximated at one nearly fully penetrating well by the Cooper et al. (1967) solution. Although, the solution is valid for confined aquifers, and the reliability of the estimate could be somewhat questionable during periods of depressed water table, the calculated value of 0.14 m2 d1 seems to be reasonable. For the Narita unit, the estimated transmissivity is over 0.033 m2 d1. These calculations clearly show that both sandy units cannot be considered aquifers in the strict sense, as they are far from being suitable for development. Transmisivities greater than 0.015 m2 d1 are required for water well exploitation (Freeze and Cherry, 1979). The heterogeneity of the upper sands results in a complex porosity distribution from 16 to almost 70%. Accord-

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23

Table 2 Properties of the main sedimentary units Formation

Prevailing sediments

Thickness (m)

Transmissivity (m2 d1)

Hydraulic conductivity (cm s1)

Total porosity (%)

Effective porosity (%)

Velocity (m d1)

Kanto loam J oso Ryugasaki Narita

Silty-clay

0.75–1.5

–

2·104 to 8·105a

71–79

–

–

Clay Sand. gravel Sand. silt. clay

< 1.3 2.8–4.2 1 to  8

– 0.094–0.14 0.033

8·105 (?)a 1·103 to 3·107 1.9·105 to 3.3·108

27–31 16–68 25–58

– 15 9.8

– 1·104 1·105 to 1·1010

Vertical hydraulic conductivity.

ing to McWorter and Sunada (1977), effective porosity for sands of similar characteristics is expected to vary between 18 and 43%. An estimation of only 15% indicates that pore spaces are not well connected, and water tends to have limited mobility through the aquifer. In contrast, the overall mean total porosity for the deep sands is 38.5%, with an effective porosity of 9.8%. Based upon field evidence, it is reasonable to think that the reduction in porosity as a function of depth is essentially because of compaction, and to a minor extent to the presence of iron and carbonate cement in the pores spaces of the Narita Fm. A summary of the sediment properties is displayed in Table 2. 4.3. Patterns of groundwater movement A numerical simulation under steady-state provided an understanding of the groundwater behavior in the area. Prior to any interpretation, the model was calibrated against the observed water-level data until a reasonable value of the objective function (U) was obtained. Initially, calibration was done manually by adjusting a set of hydrologic parameters, and then values of hydraulic conductivity were optimized by automated procedures to achieve an even better match between observed and calculated heads. Details on the calibration procedures and their results are presented in Gallardo et al. (2005). Shallow groundwater recharges uniformly throughout the area, while deep groundwater essentially originated near the divide, where it flows downward and moves

beneath the uplands while bypassing the effects of the topsoil (Fig. 4). Both fluxes finally mix at the riparian zone, where they move as a unique flow before discharging into the drainage. In contrast, the water flow is predominantly vertical in the confining clays. As recharge derives from precipitation entering at the ground surface, some particles were randomly introduced at the top layer and their paths tracked in the direction of the flow. Particles entering near the divide rapidly diverge from it in a sort of loop influenced by a lateral component of the flux, which moves to the stream. Finally, these particles undergo redirection and flow towards the drainage, especially when reaching the lowlands (Fig. 5). In contrast, most of the particles recharged downgradient of the center of the site follow straight pathways all the way to their discharge point. Flowlines indicate that the drainage is a major sink capturing both the flow that originated as far as the divide, as well as the water that originated nearby. On the other hand, the stream is fed by a lateral component of flow essentially restricted to particles recharged relatively close to it, within the upgradient forest and part of the croplands. The particle-tracking scheme also facilitated the calculation of travel times along the flowpaths. As can be seen in Fig. 5, short flowpaths usually correspond to short residence times, whereas groundwater age increases several times for particles entering far from the drainage. Travel time is no more than 180 days for water that originated in the lowlands and the northernmost edge of the uplands.

South

North

Upper Aquifer

25 Clay

Silt

20

Deep Aquifer Vertical Exaggeration x 5

60

120

180

240

300

Equipotential Line

20

Groundwater Head (m) Equidistance: 0.5 m Flow Vector

Fig. 4. Steady-state groundwater flow along the main transect.

15

a

360 m

24

A.H. Gallardo, N. Tase / Journal of Asian Earth Sciences 29 (2007) 18–28

Fig. 5. Pathways and travel time of particles entering the water table.

However, substantially older groundwater is found throughout the rest of the area and especially near the cathment divide, where the greatest residence time is 63 years. The increase of groundwater age with distance from the drainage is easily explained by an increase in the length of the pathways and therefore in the time required to terminate at the sink, as well as the reduction in the head gradient and groundwater velocity within the uplands. Moreover, the travel times also increase with depth. Water moves faster through the upper aquifer because of its proximity to the recharge source and because of the higher velocities promoted by a more elevated conductivity, but it is delayed within the low-permeability clays beneath the uplands. This also helps explain the correlation of ages with distance to the sinks: with increased distance from the discharge area, groundwater moves deeper and its residence time is increased. Particles that originated near the divide travel the longest flow paths and take the longest time to move through the system. Given that a great part of these pathways are across the low permeability units, particles travel at very slow rates and ages of several years are therefore reached. Calculated velocities generally increase from South to North along the flow lines, in direct response to variations in the land gradient. Movement in the uplands is greatest in

the upper sands with maximums of about 1·104 m d1, while the lowest velocities take place in the confining units, on the order of 1·107 in the clays to 1·1010 m d1 at the silty basement. Water entering the clay/sandy layer moves faster in the vertical direction at rates of about 1 to 3·105 m d1. On the other hand, velocities in the Narita sands range between 1·105 and 106 m d1. These values are lower than in the overlying aquifer, probably because comparatively less water flows within the deep unit (Buxton and Modica, 1992). 4.4. Water balance A water balance was used to determine the mean flows entering and leaving the system through the various sources and sinks I  O ¼ DV

ð1Þ

where I are the inflows into the system, O the outflows, and DV the change in water storage within it. Since, the system tends to reach equilibrium over the long term, DV vanishes and the inputs and outputs become equal. Water movement through both the drain and the stream are gathered together under a unique term. The data confirms that precipitation is nearly the only source of recharge to ground-

A.H. Gallardo, N. Tase / Journal of Asian Earth Sciences 29 (2007) 18–28 Table 3 Simulated water mass balance Unit

Source/sink

Inflows (m3 d1)

Outflows (m3 d1)

Upper aquifer

Swamp Precipitation Stream + drainage From/to aquitard From/to lower aquifer Evapotranspiration Total From/to upper aquifer From/to lower aquifer Total From/to upper aquifer From/to aquitard From/to basement Stream + drainage Total From/to lower aquifer Total

2 191 8 0 94 0 295 54 2 56 51 56 0 0 107 1·103 1·103

0 0 187 54 51 2 294 0 56 56 94 2 0 12 108 1·103 1·103

Aquitard

Lower aquifer

Basement

Residual (m3 d1)

1

0

1

25

201 m3 d1, from which 105 m3 d1 (52%) migrates further downward into the deeper layers (54 m3 d1 into the clays, 51 m3 d1 directly to the Narita sands). However, these water losses can not be considered definitive, as there is a permanent exchange of waters between the different units in an upward–downward movement, which is negative (downward migration) at about 12 m3 d1 beneath the croplands, but becomes positive (upward migration) within the lowlands, as a result of the extra contribution made by those fluxes coming from the deep aquifer near the discharge zone. Finally, it is interesting to note that despite its low conductivity, an important amount of the percolated water is still able to move through the aquitard towards the deep sands. 4.5. Hydrochemistry

0

water (95%), with a minor contribution by infiltration through the bottom of the canals (4%), and seepage from the swamp downward from the stream (1%). In contrast, water outputs occur almost exclusively through the channels (99%). Although these inferences are reasonable when considering the system as a whole, they must be taken with caution when focusing on a particular area of the catchment, as some components may be greater than the calculations. The clearest example is seen in the wetland, where evapotranspiration alone represents 82% of the water losses. A more detailed water budget through each of the most significant hydrogeological units of the catchment was additionally estimated: upper aquifer, confining unit, lower aquifer, and basement (Table 3). The new figures show that the basement acts as an impermeable barrier to the groundwater flow, so it can be neglected in the analysis. Fluxes through the upper aquifer dominate the overall balance, representing approximately 64% of the total flows in the system. Infiltration of water to the upper sand is

Two hydrochemical facies were distinguished based on the vertical distribution of ions in groundwater (Fig. 6). The first group comprises groundwater enriched in 2 NO 3 and SO4 within most of the upper aquifer and part of the clay aquitard, while the second group corresponds  to waters with Caþ 2 and HCO3 in the deep sands. Ions in shallow groundwater are typical of agricultural settings, and correlate well with the land use. While more than 75% of the samples taken on the croplands had NO 3 concentrations exceeding the maximum of 45 mg L1 recommended for drinking purposes (Water Environmental Department, 2001), there was a relative depletion of dissolved elements near the water table beneath the forest, where only 23% of the samples presented values above the maximum standard (Gallardo and Tase, 2005). Nitrate is thermodynamically stable under the oxidizing conditions prevailing near the ground surface, and rapidly moves with flowing water as a result of its high solubility. In the absence of reducing substances in the aquifer, oxygen saturated groundwater may travel a long way with no major changes in its chemical content. Moreover, the residence

Altitude masl

28

20

Na + K Ca Mg

10 meq/L 3

Cl HCO3 SO4 (NO3)

0

3 meq/L

0

50

100 m

Loam

Upper Sand

Organic Soil

Clay

Lower Sand

Clay/Sand Lenses

Fig. 6. Groundwater chemical characteristics in cross section.

Silt

26

A.H. Gallardo, N. Tase / Journal of Asian Earth Sciences 29 (2007) 18–28

time of water through most of the upper sands is on the order of weeks to a few months, a relatively rapid transit which reduces the possibilities of chemical interactions with the surrounding environments. In general, NO 3 concentrations presented a similar distribution as SO2 4 and furthermore, there was a strong positive correlation between them throughout most of the shallow waters in the uplands (r: 0.75), suggesting a common source. Both ions would be introduced with the urea (CO(NH2)2) and ammonium-sulfate ((NH4)2SO4) applied during fertilization of the croplands, and since their excess is not absorbed to the negatively charged sites on soil colloids, they would rapidly migrate downward to the water table. Despite the fact that applied fertilizers are of the type N:P:K, no correlation was found between either 2 NO and K + , indicating that the cation would 3 or SO4 be effectively held in the soils by organic matter and clays, without conversion into any mobile form that is able to migrate down to the groundwater. As stated by Bo¨hlke (2002), the fate of K + from KCl applications is complicated in part by ion exchange, and evidence for agricultural K + enrichment over background concentrations is sporadic. Although its leaching does not represent a major threat to water quality, K + concentrations increased several-fold during March and April in coincidence with the fertilization of Chinese cabbage, suggesting the fixing capacity of the soils might become insufficient during periods of heavy loads. Other cations that deserve consideration are Ca2 + and Mg2 + , which present a strong correlation within the uplands (r: 0.84) interpreted as the result of losses from the dolomite (CaMg(CO3)2) added to the soils to neutralize the acidic water produced after nitrification of the fertilizers (Ii et al., 1997): 2Hþ þ ðCa MgÞCO3 ¼ CO2 þ ðCa2þ ; Mg2þ Þ þ H2 O

ð2Þ

The distribution of salts shows a decrease in concentrations towards the lowlands, in direct response to the absence of agricultural activities. Enhanced by substantial amounts of organic matter and the anoxic conditions resulting from the waterlogged soils, microbial-catalyzed 2 processes promote the depletion of NO 3 and SO4 within the riparian zone, and especially at the wetland. Supplied on a permanent basis through decomposition of plants and root exudates, organic carbon (OC) is a fundamental energy source to sustain the bacteria population and the natural attenuation of contamination in the saturated zone. Groundwater at a few wells near the wetland showed some seasonal variations in OC, with an increase in concentrations during in autumn and winter, and a reduction after spring, having an inverse relationship between concentrations and precipitation. The decline of the water table during the dry season in winter would promote better oxygenation of the soils, which in turn would restrict the bacterial activity, resulting therefore in a net decline of the attenuation rates. In addition, bacterial metabolism is

affected by temperature, generally diminishing in conjunction with its decrease. Therefore, greater reaction can be expected under hot conditions and lower rates in winter (Littke and Hallberg, 1991). In addition to the lateral variability, changes in water chemistry were observed with depth. Along with an overall reduction in the total salinity of groundwater, there was a notorious increase in the concentrations of Ca2 + and  HCO 3 across the lower aquifer. Concentrations of HCO3 at depth normally duplicate the values registered in shallow groundwater. Even though each well presented its own characteristics, the most drastic rise in the ion concentration coincided with the upper boundary of the deep aquifer. The inverse trend between both pH and HCO 3 2 against DO, NO 3 and SO4 suggests that part of the existing HCO 3 could be the product of biochemical reactions. However, a great part of HCO 3 would have originated from dissolution of cement in the lower aquifer through the action of percolating waters enriched in CO2 after being in contact with the atmosphere (Apello and Postma, 1993). As expected, the dissolution of CaCO3 releases Ca2 + into solution, yielding water of type Ca–HCO3 as a final product. Aside from the major ions, some minor elements have also been investigated in order to further clarify the characteristics and chemical composition of groundwater. The dissolution of silicate minerals is generally a very slow process that does not produce significant variations in the water chemistry (Condesso de Melo et al., 1999). Concentrations of H4SiO4 ranged between 0.8 and 29 mg L1 independent of the land use. However, they showed a tendency to increase with depth that was interpreted as the result of longer residence times and thereby more possibilities for the breakdown and release of minerals present in the sediment’s matrix. Muscovite and K-feldspar were present in large amounts and could have served as a source of silica in groundwater. Furthermore, Tang (1989) suggested that silica concentrations in the Narita unit are higher than in other formations because of the presence of clay within the deposits. It is widely known that Fe2 + in concentrations greater than 0.3 mg L1 may form encrustations on pipes and wells. Measured values are usually low, with an average of 0.01 mg L1, except for two wells in the vicinities of the wetland where concentrations exceeded 2 mg L1. Although Fe2 + is a common constituent of anoxic groundwater, it is still difficult to recognize the causes of its selective delivery. Pyrite is not commonly found in sediments in the region (Tase, personal communication), so it can not be considered an important source of Fe2 + . Since, concentrations of Fe2 + in solution increase under anoxic conditions (Apello and Postma, 1993), it is reasonable to expect that the anomalous values are related to small zones of highly depleted oxygen beneath the peaty soils. This hypothesis is supported by the negative correlation (r: 0.72) observed between Fe2 + and DO in the riparian zone. The Fe2 + present in limonitic minerals would be

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picked up much faster by groundwater traveling through the peaty sediments and remain in solution while highly anoxic conditions prevail. It is also interesting to note that even when ammonium ðNHþ 4 Þ is the main constituent of the applied fertilizers, its concentrations in groundwater were undetectable for most samples due to sorption into the cation exchange capacity of the soils and effective uptake by plants. In addition, þ the high NO 3 =NH4 ratio found throughout the area indicates nitrification is the dominant process in aerated soils. The occasional high concentrations of NH4 + in groundwater of the croplands can be attributed to remnants of applied fertilizers, while anomalous occurrences at the riparian zone would be associated with anaerobic degradation of organic matter within the saturated zone (Berner, 1971). The redox potential (ORP) fluctuated from 115 to 521 mV, although most measurements concentrated between 50 and 300 mV. It was correlated to DO within the lowlands (r: 0.75), and even though both of them decreased with depth, reductions in ORP were more gradual. As expected, the highest values in ORP were registered in shallow groundwater due to exposure to atmospheric conditions. In contrast, the oxidation of organic matter removed most of the oxygen in the organic-rich layers of the lowlands, and caused the ORP to decline. The lowest values were measured during summer, reflecting greater biological activity during that period (Panno et al., 2001). An unusual increase in ORP and DO with depth was observed at one well near the groundwater divide. As was already explained, the vertical flows prevailing in the area of the divide would facilitate the influx of more oxygenated waters during recharge. This observed pattern would reflect a more open conduit system, suggesting rapid migration of chemical compounds along cracks, root channels or macropores. As stated by Casey et al. (2004), macropores are sites of pipe flow that allow for the transport of nutrientrich water through the subsurface, and would bring chemical constituents further into areas that would otherwise not be exposed to its migration. 5. Summary and conclusions The present study provided a conceptual framework of groundwater characteristics in a typical agricultural area of Japan, which will assist municipal and governmental authorities in the design of an integrated plan for the future management of Tsukuba’s water resources. The investigation attempts to provide detailed information on the movement and quality of groundwater in a system potentially sensitive to anthropogenic impact. The main hydrological units correspond to sands of the Ryugasaki Fm and deeper Narita Fm, which are separated by a clayey layer extending throughout the uplands. Groundwater occurs under both confined and unconfined conditions, with a general flow towards the north, turning to the northeast at the lowlands. Downward fluxes prevail

27

adjacent to the water divide, but the geological setting determines that groundwater beneath the rest of the uplands is essentially restricted to two flow systems that follow predominantly horizontal paths. Waters mix downgradient at the break of slopes and valley corridors, before being discharged through the stream channel. Although the low permeability of the clay limits the migration of groundwater downward, about 19% of the water entering the water table is still able to infiltrate into deeper units. As expected, the residence time of groundwater was shortest for particles recharged closest to the sinks, but longest where particles entered the aquifer near the divide. The time required for water to flow to a sink ranged from 180 days when recharged within the lowlands, with a maximum of 63 years for particles entering at the groundwater divide. Groundwater salinity is directly dependent on land use and geology, decreasing both with depth and along the flowpaths. Even though shallow groundwater exhibits considerable heterogeneity in its chemistry, the highest salinity occurred throughout the uplands, strongly influenced by agricultural practices. The substantial amounts of 2 NO detected in the groundwater beneath the 3 and SO4 croplands are a consequence of fertilizer application and its mobilization into the saturated zone. The decrease in solute concentrations near the water table within the forest and the lowlands is mainly attributed to the lack of fertilization. Biochemical reduction is the most important process of ion depletion in the wetland and lower reaches of the valley corridors. This study demonstrates that the groundwater resource is already deteriorating. Therefore, some measures should be taken in advance to prevent spreading of the pollution front. Local authorities could carry out a permanent advisory campaign to promote good agricultural practices by collaborating with the farmers. For example, farmers might be encouraged to plant crops that can accumulate sizeable amounts of nutrients, while replacing or minimizing the use of highly leachable fertilizers. On the other hand, natural wetlands of the region could be designated as protected zones and any activity that alters their characteristics (like desiccation) should be restricted. Periodic monitoring of water quality is also necessary in order to assess the effectiveness of the programs.

Acknowledgements The authors want to express their sincere gratitude to Dr Eiji Matsumoto, former professor of the Institute of Geosciences, University of Tsukuba, and Dr Hiroshi Ikeda, School of Life and Environmental Sciences, University of Tsukuba, for their invaluable comments and suggestions to effectively reconstruct the geological sequence of the area. Besides, many thanks to the Ministry of Education and Culture of Japan (Mombusho), for the financial support to carry out the research.

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References Apello, C.A.J., Postma, D., 1993. Geochemistry, Groundwater and Pollution. Balkema, Rotterdam, The Netherlands. Bae, S.K., 1986. Study on the Three-Dimensional Groundwater Flow System in an Upland Area. PhD Thesis, University of Tsukuba, Ibaraki, Japan. Berner, R.A., 1971. Priciples of Chemical Sedimentology. Mc Graw-Hill, New York, USA. Boggs Jr, Sam, 1995. Principles of Sedimentology and Stratigraphy, second ed. Prentice Hall, Englewood Cliffs, NJ. Bo¨hlke, J.K., 2002. Groundwater recharge and agricultural contamination. Hydrogeol. J. 10, 153–179. Bouwer, H., Rice, R.C., 1976. A slug test method for determining hydraulic conductivity of unconfined aquifers with completely or partially penetrating wells. Water Resour. Res. 12 (3), 423–428. Buxton, H.T., Modica, E., 1992. Patterns and rates of ground-water flow on long Island, New York. Ground Water 30 (6), 857–866. Buxton, H.T., Reilly, T.E., Pollock, D.W., Smolensky, D.A., 1991. Particle tracking analysis of recharge areas on long Island, New York. Ground Water 29 (1), 63–71. Casey, R.E., Taylor, M.D., Klaine, S.J., 2004. Localization of denitrification activity in macropores of a riparian wetland. Soil Biol. Biochem. 36 (4), 563–569. Condesso de Melo, M.T., Marques da Silva, M.A., Edmunds, W.M., 1999. Hydrochemistry and flow modelling of the aveiro multilayer Cretaceous aquifer. Phys. Chem. Earth (B) 24 (4), 331–336. Cooper, H.H., Bredehoeft, J.D., Papadopulos, S.S., 1967. Response of a finite-diameter well to an instantaneous charge of water. Water Resour. Res. 3 (1), 263–269. Davis Jr, R.A., 1992. Depositional Systems: An Introduction to Sedimentology and Stratigraphy, second ed. Prentice Hall, Englewood Cliffs, NJ, USA. Doherty, J., Brebber, L., Whyte, P., 1994. PEST—Model Independent Parameter Estimation. User’s Manual. Watermark Computing. Australia. Freeze, R.A., Cherry, J.A., 1979. Groundwater. Prentice-Hall, Englewood Cliffs, NJ, USA. Gallardo, A.H., Tase, N., 2005. Role of small valleys and wetlands in attenuation of a rural-area groundwater contamination. In: Heathwaite, L. (Ed.), Dynamics and Biogeochemistry of River Corridors and Wetlands. IAHS Publ 294, pp. 86–92. Gallardo, A.H., Reyes-Borja, W., Tase, N., 2005. Flow and patterns of nitrate pollution in groundwater: a case study of an agricultural area in Tsukuba city, Japan. Environ. Geol. 48, 908–919. Ii, H., Hirata, T., Matsuo, H., Nishikawa, M., Tase, N., 1997. Surface water chemistry, particularly concentration of NO3 and DO and d15N values, near a tea plantation in Kyushu, Japan. J. Hydrol. 202, 341–352. Kimura, T., Hayami, I., Yoshida, S., 1991. Geology of Japan. University of Tokyo Press, Japan. Littke, J.P., Hallberg, G.R., 1991. Big Spring Basin Water-Quality Monitoring Program: Design and Implementation. Iowa Department

of Natural Resources, Geological Survey Bureau, Iowa City, OpenFile Report 91-1. Marui, A., Seki, H., 2003. Deep groundwater in the Kanto Plain. J. Jpn. Assoc. Hydrol. Sci. 33 (3), 149–160. Masuda, F., Shindou, S., 1986. Geology in the Kasumigaura region. In: Kankyo-kagaku Kenkyu-hokokusho (Environmental Science Research papers), B262-R12-2 (3), pp. 128–154. (in Japanese). McDonald, M.G., Harbaugh, A.W., 1988. A Modular Three-Dimensional Finite-Difference Groundwater Flow Model. Techniques of Water Resources Investigations of the US Geological Survey, Book 6. McWorter, D.B., Sunada, D.K., 1977. Ground-Water Hydrology and Hydraulics. Water Resources Publications, Ft Collins, CO, USA. Municipality of Tsukuba, 2005. Outline of Tsukuba Science City. Available at www.info-tsukuba.org Nakagawa, S., 1984. Study on Evapotranspiration from Pasture. University of Tsukuba, Environmental Research Center Publications No. 4. Oguchi, T., 1996. Factors affecting the magnitude of post-glacial hillslope incision in Japanese mountains. Catena 26, 171–186. Ongley, E.D., 1996. Control of Water Pollution from Agriculture. FAO, Rome, Italy. Panno, S.V., Hackley, K.C., Hwang, H.H., Kelly, W.R., 2001. Determination of the sources of nitrate contamination in karst springs using isotopic and chemical indicators. Chem. Geol. 179, 113–128. Pollock, D.W., 1989. Documentation of Computer Programs to Compute and Display Pathlines Using Results from the US Geological Survey Modular Three-Dimensional Finite-Difference Ground-Water Model, USGS Open File Report 89-381. Sudicky, E., 1986. A natural gradient experiment on solute transport in a sand aquifer: spatial variability of hydraulic conductivity and its role in the dispersion process. Water Resour. Res. 22 (13), 2069–2082. Suzuki, H., 1996Geology of Koto Deep Borehole Observatory and Geological Structure Beneath the Metropolitan Area, Japan, vol. 56. National Research Institute for Earth and Disaster Prevention, Japan, pp. 77–123. Takamura, Ch., 2001. Water Balance, Heat Balance, Special Quality of CO2 Flux in a Paddy Field in the Tsukuba Region. MSc Thesis, University of Tsukuba, Ibaraki, Japan. (in Japanese). Tang, C., 1989. Evolution process of groundwater quality in dejima upland. Sci. Rept. Inst. Geosci., Univ. Tsukuba—Sect. A 10, 59–86. Tase, N., 1992. Groundwater contamination in Japan. Environ. Geol. Water Sci. 20 (1), 15–20. Tase, N., 2004. Groundwater contamination by nitrate and nitrite. Environ. Manage. 40 (3), 47–55 (in Japanese). Unozawa, A., Isobe, I., Endo, H., Tagutsuchi, Y., Nagai, S., Ishii, T., Aihara, T., Oka, S., 1988. Environmental Geologic Map of the Tsukuba Science City and its Surroundings. Geological Survey of Japan, Miscellaneous Map Series No. 23-2. Water Environmental Department, 2001. Water Environment Management in Japan. Environmental Management Bureau, Ministry of the Environment, Tokyo, Japan.