ISOTOPE HYDROGRAPH SEPARATION FOR MODELING OF RUNOFF MECHANISMS OF ATMOSPHERICALLY DERIVED CHEMICAL AND RADIOACTIVE POLLUTANTS

In a tributary watershed of the Kuji River of Japan, the hydrological components of runoff associated with a precipitation event were investigated using isotope tracers of hydrogen (2H) and oxygen (18O) in precipitations and stream water. The runoff was separated into "old water" (pre-existing in the ground before the precipitation event) and "new water"(from the precipitation). It was found that the discharge of several hazardous trace elements (Sb, Cu, Cr) was largely (24-54%) attributable to that of the new water in spite of its small contribution to the total water discharge. These investigations suggest that the new water may play an important role in the migration of atmospherically derived, hazardous trace elements to streams during precipitation events. The present findings will contribute to current necessity of assessment of a risk of long-term exposure to pollutants at low concentrations by providing information on their transport among different environmental media

ISOTOPE HYDROGRAPH SEPARATION FOR MODELING OF RUNOFF MECHANISMS OF ATMOSPHERICALLY DERIVED

CHEMICAL AND RADIOACTIVE POLLUTANTS

T Matsunaga*, N Yanase*, Y Hanzawa*, K Tsuduki* and H Naganawa*

*Department of Environmental Sciences, Tokai Research Establishment, Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken, 319-1195, Japan

ABSTRACT

In a tributary watershed of the Kuji River of Japan, the hydrological components of runoff associated with a precipitation event were investigated using isotope tracers of hydrogen ( 2 H) and oxygen ( 18 O) in precipitations and stream water The runoff was separated into "old water" (pre-existing in the ground before the precipitation event) and "new water"(from the precipitation) It was found that the discharge of several hazardous trace elements (Sb, Cu, Cr) was largely (24-54%) attributable to that of the new water in spite of its small contribution to the total water discharge These investigations suggest that the new water may play an important role in the migration of atmospherically derived, hazardous trace elements to streams during precipitation events The present findings will contribute to current necessity of assessment of a risk of long-term exposure to pollutants at low concentrations by providing information on their transport among different environmental media

KEYWORDS

deuterium; isotope hydrograph; oxygen-18; pollutants; runoff

INTRODUCTION

Since the 1980s, non-point source contamination has been one of the central issues in water quality management, particularly with regard to the contamination of fresh water bodies with sources distributed over a catchment Some of these sources can be considered to originate from the atmosphere The widespread use of metals and their strict controls at point-sources (relevant firms of smelting, fabrication, etc.) have now raised concerns about spreading of atmospheric depositions of anthropogenic heavy metals at trace levels (NIES, 1991) While a number

of related studies have been carried out, the process of inflow of contaminants to water bodies such as streams or impoundments has not been studied enough to allow quantification of its impact An understanding of the discharge of trace elements will serve to predict migration of hazardous elements in the environment Even radionuclides that may be released to the atmosphere in a nuclear accident can contribute to the concern of atmospherically derived, broadly spread contamination (Cooper et al., 2002) There are such instances, the Windscale and the Chernobyl accidents Other than these instances, it is hardly possible to track those radionuclides in the actual environment, because an accidental release seldom occurs Instead, mathematical modeling has been considered as a useful tool as for tracking those radionuclides (e.g Monte et al., 2000) Thus, findings about the migration of stable trace elements will be also suggestive in providing a scientific basis for a migration model of those radionuclides by a similarity in their pathways including the atmosphere and the hydrosphere

We have discussed fluvial elemental loads in particulate and dissolved forms in relation with the discharge rate (Nagano et al., 2003) The present study focuses on the hydrological components of runoff after a precipitation event and on their relation to the load of dissolved elements The Oda River and its watershed, located in a central Japan, was investigated in the present study Hydrogen (2H) and oxygen (18O) isotopes of precipitations and stream water samples are used as tracers of water to separate the runoff into i) that originating from the ground water and the sub-surface water existing in the ground before the precipitation event ("old water"), and ii) that from the storm precipitation ("new water") This separation allows us to identify the component responsible for carrying trace

elements to a stream The analytical results of trace elements including hazardous metals in those water samples

are then coupled with the isotope analysis Finally, the importance of the component originating from the

precipitation (new water) in a carrier of hazardous metals is considered Our present findings will be included in

our hydrological model to describe migration of accidentally released radionuclide and hazardous trace elements

(Tsuduki et al., 2004)

EXPERIMENT

Site description

The Oda River is a tributary of the Kuji River located in Fukushima Prefecture of Japan (Fig 1) It is 10 km long

and has a 40 km2 watershed, 92 % of which is categorized as hilly forests (National Land Agency, 1980) Its plain

area is limited to 2.8 km2 (7 %) and the open surface of the stream is only 1.2 km2 (3 %)

Sampling and on-site monitoring

River water River water was collected at a site downstream of the Oda River (36o 31' N, 140o 26' E), 6 river-km to

the confluence of the Oda tributary and the Kuji main stream The collection was performed with the aid of an

automated sampler at 2 h intervals At a late stage of recession, this was adjusted to 4 h intervals River water

samples were collected in a 1 L polypropylene bottle and were transported in a cooler to our laboratory (60 km away

from the sampling location) for filtration and analyses

Precipitation Precipitations were collected at two locations at 600 - 650 m in altitude in a central part of the

watershed A supplemental sampling was done at a foot of the watershed (220 m) Each 1 mm of the first 8 mm

after the start of the raining was collected separately The remaining precipitations were collected as a whole

On-site monitoring The water level was monitored continuously at the river water sampling location using a level

gauge This was calibrated to the water flow using manually observed data regarding the water level and flow rate

at sectioned streams The value of pH was also monitored

Analysis

Isotopic composition of 2 H and 18 O The compositions of the isotopes 2 H (D) and 18 O were analyzed to obtain δD

(‰) and δ18O (‰), respectively, which are defined as the relative deviation of the parts per thousand ratios from that

of a standard according to the following equation (Kendall and Caldwell, 1998):

where R refers to the ratio of 2H/1H, or 18O/16O in atomic concentrations

Oda River watershed

an

Kuji River watershed

N

■

Fig.1 Location of the Kuji River and the Oda River watersheds The closed square denotes the location of our

laboratory, where analytical works in the present study were conducted and the sampling of atmospheric

depositions was conducted by Ueno and Amano (2003), see text

The standard employed in the present study was the standard mean ocean water (SMOW) distributed by the International Atomic Energy Agency (IAEA) The hydrogen isotopic composition was measured by reducing the water by passage over chromium at 840 oC, chiefly following Coleman et al (1982), with a mass spectrometer (MAT252, Finnigan MAT GmbH) The composition of oxygen was analyzed according to a principle of

CO2-H2O equilibration (Epstein and Mayeda, 1953; Horita and Kendall, 2004) Briefly, an aliquot of water sample (200 µl) was equilibrated with 3% CO2 in hyperpure grade helium in a glass vessel (5 ml) at 40 oC for 12 hrs Then, the oxygen composition of CO2 was analyzed with a mass spectrometer (Isoprime-Multiflow System, VG Instruments) equipped with a gas chromatography column for separation of CO2 The standard deviation of our measurements was in the range of ±0.1 to ±0.3 ‰ and ±0.2 to ±0.4 ‰ for hydrogen and oxygen, respectively

Inorganic elements After filtration, water samples were acidified with nitric acid for ultra-trace analysis (Wako

Pure Chemical Industries Ltd.) to 0.3 N HNO3 Inorganic elements other than Si were analyzed with an inductively coupled plasma mass spectrometer (ICP-MS) (HP-4500, Yokogawa Analytical Systems) The elements were classified into several groups by concentration level as appropriate for the analysis Mixed standards (XSTC-1, -7 , -8 and -13, SPEX) were used for calibration The concentration of Si was determined by spectrophotometry using ammonium molybdate

RESULTS AND DISCUSSION

Runoff and isotopic composition

This paper deals with an observation for a precipitation event during Oct 22-23 of 2003 The amount of precipitation was 30 mm in total Figure 2 shows the precipitation record of the studied event (Fig 2a) and the record of the river flow rate (solid line, Fig 2b) of the Oda River at the water sampling location

Fig 3 Variation in the δD of precipitations (a) , the river water (c), and the accumulated rainfall (b)

-40 -35 -30 -25 -20 -15

0 5 10 15 20 25 30

accumulated precipitation (mm)

0 10 20

-52 -50 -48 -46 -44 -42 -40 -38

(c)

Elapsed time (hrs) since 0000 Oct 21

0

2

4

6

8

10 Oct 21 Oct 22 Oct 23 Oct 24

(a)

0

1

2

3

old water new water

0.0

0.1

0.2

0.3

0.4

0.5

f1

(c)

Elapsed time (hrs) since 0000 Oct 21

Fig 2 Record of precipitation (a) and river water flow rate (b)

(total, solid line) with evaluated fraction of new water (c) and

separated flow rates (b) (dashed and dotted lines).

In response to the major part of the raining (0600 to 1400 of Oct 22, 22 mm in total), the flow rate increased to its peak value of 2.2 m3/s (1400 of Oct 22) It then decreased gradually with two small peaks that corresponded to light rains Four days after the major raining ceased, the flow rate maintained a certain value (0.8 m3/s, 1400 of Oct 26) that was higher than the lowest value (0.5 m3/s) before the precipitation event

Variations in the δD of precipitations and the river water are depicted in Fig 3a and Fig 3c, respectively The isotopic records in Fig 3 are of the highest location (650 m) The records at this location were considered to be representative in the central part of the watershed and are discussed hereafter This is because records of the δD of precipitations at another location in the same part differed only by 0.5 ‰ from the former in terms of amount-weighed mean The δD of precipitations decreased from -17.2 ‰ (the first 1 mm) to -38.3 ‰ (6-7 mm), and then returned to -27.6 ‰ in the last 14 mm (1000 - 1400 of Oct 22, Figs 3a and 3b) Its amount-weighed mean was -28.1 ‰ On the other hand, the δD of river water was -47.9 ‰ at the start (0600, Oct 22) and reached

a maximum value of -44.1 ‰ at 1600 of Oct 22 It then returned to -48.2 ‰ 12 hrs later, at which time it most resembled the earlier low value (-47.9 ‰) before the precipitation event Interestingly, it continued to decrease with time, reaching -50.1 ‰ at 1200 of Oct 24

The oxygen isotopic composition δ18O of the precipitations varied from -4.9 to -8.7 ‰ in separate samples Its amount-weighed mean was -6.0 ‰ (Fig 4) Machida and Kondoh (2001) showed that the relationship between

δD and δ18O in shallow ground water and river water in Japan statistically is given by the equation:

-60 -50 -40 -30 -20 -10 0

δ18O (‰) river water (during the main part of the flooding) river water (after the main part of the flooding)

□

■

△

▲

river water (pre-event) precipitations

precipitations (weighted mean) river water (dry season, mean)

relation to those in the δD The dashed lines correspond to probable relationships reported for Japanese shallow ground waters and river waters (see text)

δD = 6.72 δ18O + 3.94 ± 11.88 (3σ) (2)

The relationship of the studied precipitations was reasonably found between the two expected lines (dashed lines in

Fig 4) The δ18O of river water exhibited less variations than the δD The δ18O was -7.6 ‰ before the start of

raining (0600 of Oct 22) It then decreased slightly to its lowest value -8.0 ‰, at 0200 Oct 23, and then increased

to -7.4 ‰ (1200 of Oct 24) This ambiguity may come from the intrinsic few variation in the δ18O relative to that

in the δD as indicated by Eq.(2), and also from relatively large uncertainty (±0.2 to ±0.4 ‰) in the analysis of the

δ18O with our instrument The isotopic composition of river water samples collected in a dry season (Feb 2004)

indicated that the composition of the river waters in the studied precipitation event (Oct 2003) were affected by

seasonal variations to some extent

Hydrograph separation

A hydrograph separation was carried out based on data of the flow rate and the isotopic composition The isotopic

composition of hydrogen (δD) was used in the separation because its value was more significant than that of oxygen

in our measurement, considering of their respective analytical errors A concept of two-component mixing was

employed, in which it was assumed that runoff is composed of a mix of two components: one originating from the

ground water and the sub-surface water existing before the precipitation event ("old water" or pre-event water), and

one from the precipitation event ("new water" or event water) (Buttle, 1994; Kendall et al., 1995) The δD of river

water is expressed by the following equation:

Cr (t) = f1 (t) C1 (t) + (1- f1 (t)) C2 (t) (3)

where Cr (t), C1 (t), and C2 (t) are the isotopic compositions of hydrogen (δD) of the river water, new water, and old

water, respectively, and f1 (t) represents the fraction of new water in the runoff at time t Further, C1 (t) and C2 (t)

are assumed to be constant In the present case, C1 was set to the amount-weighed mean δD of the precipitation

samples (-28.1 ‰), and C2was set to the δD of the river water just before the precipitation event (-47.9 ‰) For

the main part of the runoff that is described later, it was considered that these two (precipitations and pre-event river

water) could be end members of mixing (see Fig 4) Although there are many arguments for the general

applicability of this two component concept (e.g Harris and McDonnell, 1995), it was employed in the present study

only in order to grasp a first insight into the relationship between the discharge of new water and the migration of

hazardous elements, which has not been sufficiently analyzed or reported previously

As a result of the calculation, and by setting C1 and C2 as mentioned above, the fraction of new water evaluated for

the main part of the runoff (0000 of Oct 22 to 1600 of Oct 23, 40 h in total) was found to be as shown in Fig 2c

The fraction reached its maximum, 0.21 (21 %) at 1600 of Oct 22, while the peak intensity of the precipitation event

occurred 6 hrs earlier and the peak of the flow rate occurred 2 hrs earlier Figure 2b shows the hydrograph of the two

components This separation suggests that the increase of water discharge in the precipitation event was mainly

caused by old water, and that the temporal contribution of new water was limited to about 20 % at most This

feature has been commonly found in rural areas, but not in urban areas It has been reported that the statistical

mean of the fraction of new water is 0.23 (i.e 0.77 is the fraction of peak discharge consisting of old water) for 32

forested areas in precipitation events (Buttle, 1994) It seems that the Oda River watershed also is one of these

much-studied forested areas, at least from this viewpoint

Discharge of major and trace inorganic elements

In the present study, 68 major and trace inorganic elements in river water were analyzed This large number made

it possible to consider the discharge of inorganic elements as a function of their origins, including anthropogenic,

detrital, and other natural sources Figure 5 shows the temporal variations in concentrations of several selected

elements in river water Sodium, which must have come from the weathering of base rocks (detrital origin) and

also from sea salt fallout, showed a slight decrease at the time of peak flow rate, possibly due to dilution by the

precipitations The variation in concentrations of potassium has not been explained Chlorine, which is often

ubiquitously distributed in the sub-surface and ground water, exhibited no change in response to this increased runoff,

as well as magnesium In contrast, concentrations of copper and antimony exhibited clear increase in response to

increase in the flow rate

To determine the relative importance of the separated water components as carriers of trace elements to a stream, we

evaluated the mass load of those elements associated with each component This load was evaluated as a

cumulative value during the main part of the runoff of interest, for which f1 (t) was evaluated (0000 of Oct 22 to

1600 of Oct 23, 40 hrs in total) First, the total cumulative load (ML total) in gram was calculated for each element

as follows:

, where C i (g/m3) represents concentration of the element in river water at time i, Q i (m3/s) is river water flow rate at

time i, and T is time interval of observation (s)

The cumulative load attributed to the old water, ML old was calculated in a similar manner by assuming that C i was

constant at its pre-storm value Then, ML new, the cumulative load attributed to the new water, was determined as the

difference between ML total and ML old:

The resultant load is listed in Table 1 for selected elements The fraction of ML new in ML total, which is the new

water contribution, is summarized in Table 2 as well as other elements listed in the order of the contribution

In Table 2, the new water contribution is limited for the major detrital elements such as Ca and Mg This indicates

reasonability of this method of load separation in this case It was found that Al, Ti, Y, U, and Th have similarly

Fig.5 Temporal variations in concentration of several elements in river water during the freshet

period Elapsed time is since 0000 of Oct 21, 2004.

0

400

800

1200

0 1 2 3

Oct 21 Oct 22 Oct 23 Oct 24

Elapsed time (hrs)

(c) Cu

0 20 40 60

0 1 2 3

(d) Sb

Oct 21 Oct 22 Oct 23 Oct 24

Elapsed time (hrs)

0 4 8 12

0 1 2 3

- (mg/l)

Oct 21 Oct 22 Oct 23 Oct 24

Elapsed time (hrs)

(b) Cl

0

2

4

6

0 1 2 3

Na

K

Mg

(a) Na, K, Mg Flow rate (m

Oct 21 Oct 22 Oct 23 Oct 24

Elapsed time (hrs)

high contributions (30-60 %) Generally, aluminum hydroxide forms colloids in neutral river water, as in the present circumstances (pH 7.1-7.3) It also tends to be associated with other types of colloids including Ti, Y, U, and Th in freshwater (e.g Pokrovsky and Schott, 2002) Indeed, in this case their concentrations in river water are strongly correlated each other Taking into account these reported colloidal size distributions and the observed correlation, the present result implies that a discharge of these elements and a formation of colloids including these elements are closely related to the discharge of new water

Table 1 Cumulative mass load of selected elements in fluvial discharge of the studied precipitation event

Element Cumulative mass load ( g/km2 ) * MLtotal MLold MLnew

* The load is normalized for the area of the watershed ( 40 km2 )

Table 2 Evaluated new water contribution for selected elements in stream water

in the studied precipitation event

New water contribution* (%) Trace elements Major elements

30-60 Al, Ti, Cu, Y, Sb, U, Th

* A fraction of MLnew in MLtotal

Table 3 Ratio of relative abundance of inorganic elements to aluminum in atmospheric depositions

in the Tokai village, Japan

5 < Cr(24)#, Br(16), Sb(54) Na(27)

# Figure in parenthesis denotes the ratio of that element

The most notable feature in Table 2 is that some trace, hazardous elements were increased in the new water contribution, namely Sb (36 %) and Cu (52 %) Ueno and Amano (2003) reported on recent atmospheric depositions of a number of major and trace elements in the Tokai village, which is located at the foot of the Kuji River watershed (Fig 1) Note that the village faces the Pacific Ocean, and the deposition is influenced by the

ocean to some extent Based on their data, we estimated the anthropogenic influence on the depositions by comparing their content with that of aluminum Aluminum was chosen as a reference because of its naturally high content in soil In principle, the ratio of their content of the depositions to that of the surface soil can reflect the enrichment of the element through the atmosphere In the case of no increase, the ratio will be equal to 1 Table 3 shows the obtained ratios in the present study Copper was not analyzed by Ueno and Amano due to methodological limitations While the elements Na, Cl, and Br have high ratios because they are mostly contained

in incident sea salt spray, the elements Sb (ratio 54) and Cr (24) also have high ratios and may be of anthropogenic origin The industrial use of Sb and Cr is constantly increasing in Japan, along with the use of Cu Especially, the domestic use of Sb in Japan markedly increased from 1800 tons (1980) to 11000 tons (2000) (Metal Mining Agency

of Japan, 2001) Thus, atmospheric depositions of Sb, Cu, and Cr probably do have major anthropogenic origins

Implication in studies of hazardous trace elements

Other than several serious episodes caused by localized sources, concentrations of heavy metals in Japanese aquatic environments have been generally far lower than legal standards and guideline values (e.g Ministry of Environment, 2005) However, Nriagu (1988) posed the question that the risk caused by "trace levels" has been underestimated This might be especially true for substances that have not been used until now For such substances, few studies have been carried on their environmental behavior and associated toxicity Antimony, which is known as a carcinogen (Eisler, 2000), is one of these substances Although its toxic properties have not been well understood (Urano, 2001; Fiella et al., 2002), it is used as a flame retardant additive in large amounts (Fiella et al., 2002)

The frequency of the detection of antimony over 0.002 mg l-1 in Japanese water bodies has recently reached 20 to

30 %, while the guideline value for drinking water is 0.015 mg l-1 Increasing is the risk of long-term, low-dose exposure to toxicants which are broadly distributed in the environment and whose toxicities are not well known

As Arizono (1999) pointed out, assessing such type of risk has become very necessary

The present investigation will contribute to knowledge of transport of heavy metals of non-point sources in different environmental media The above assessment requires such knowledge

CONCLUSIONS

In the present study, runoff from a precipitation event was successfully separated into two components which were designated old water and new water, based on their hydrogen isotopic compositions The sources of these components correspond respectively to the ground and sub-surface water existing before the precipitation event, and the precipitation This separation approach made it possible to attribute the fluvial discharge of major and trace elements associated with the precipitattion event to the respective components It was found that higher levels of

Sb, Cu and Cr occurred in new water, along with Al and its related elements An analysis of the reported atmospheric depositions near the studied watershed and the statistics regarding metal use suggests that there has been an anthropogenic enrichment of Sb and Cr, at least, in their atmospheric depositions These investigations suggest that new water may play an important role in the migration of atmospherically-derived, hazardous trace elements to streams during rain storms The present findings will contribute to current necessity of assessment of a risk of long-term exposure to heavy metals at low concentrations

Acknowledgements

This research has been supported by the River Environment Fund of the Foundation of River & Watershed Environment Management Necessary allowances related to the field work by the authorities of the Fukushima Prefecture and the Ministry of Land, Infrastructure and Transport are appreciated The authors thank Dr Mariko Atarashi-Andoh for her aid in mass spectrometry We also thank Mr Takashi Ueno and Mr Morio Takada for their support in laboratory analysis and field sampling Critical comments for this work from Dr Masahiro Kumata are acknowledged

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