Papers from Bolsena Conference (2002). Residence time in lakes:Science, Management, Education J. Limnol., 62(Suppl. 1): 42-48, 2003

Over twenty years trend of chloride ion concentration in

Yasuaki AOTA*, Michio KUMAGAI1) and Kanako ISHIKAWA2)

Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, 1)Lake Biwa Research Institute, Uchidehama, Otsu, Shiga 520-0806, Japan 2)Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan *e-mail corresponding author: [email protected]

ABSTRACT Recent increase of chloride ion concentration in Lake Biwa was considered. Over the past 20 years' data at the North Basin of Lake Biwa showed that chloride ion concentration has been continuously increasing from 7.4 to 9.9 mg l-1 at 0.5 m depth from lake surface and from 7.3 to 9.9 mg l-1 above the bottom (depth of over 80 m from lake surface). This low level salinity indicated, how- ever, about 35% increase through 20 years. In this paper, we reported the trend and the tendency of chloride ion concentration at some locations and the change of climatic data through 20 years in Lake Biwa. In a short period within one year, chloride ion con- centration clearly fluctuated in the upper water layer. This fluctuation was mostly influenced by precipitation. Similar trend of chlo- ride ion concentration could be seen in the South Basin of Lake Biwa with much higher concentration than that in the North Basin. We also discussed the long-term changes of chloride concentrations in 5 major rivers with large catchment area, water level and precipitation. The data of river discharge indicated that some rivers in the southern part of the North Basin in Lake Biwa contain a relatively high chloride concentration compared with others. Furthermore, we proposed a simple conceptual model with variable outflow and inflow fluxes. This conceptual model expressed an amount of chloride budget. We estimated the average chloride flux flowing from the watershed into Lake Biwa using the data of chloride ion concentration in the lake and the data of water discharge from the lake. Comparing the results of the model analysis with the data of 12 major rivers contributing large flux to Lake Biwa, we suggested that river inflow is one of the major sources to induce the increase of chloride ion concentration in the lake, and the inflow from non-point source can be also important sources.

Key words: Lake Biwa, chloride, salinity, precipitation, water level, river

Pyramid Lake, USA, which has 0.67 times of area and 1. INTRODUCTION almost same volume as Lake Biwa, salinity level -1 Lake Biwa is the largest (surface area of 670 km2, changed from 3.75 to 5.5 g l between 1933 and 1980 average volume of 27.5 km3) and the most ancient (Williams 2001). (nearly 5Ma) inland lake in Japan. The lake is located in In case of Lake Biwa, salinity level is quite lower (from 7.4 to 9.9 mg l-1 at 0.5m from lake surface and , Japan, which is near Kyoto and Osaka -1 Prefectures, and is divided into two basins by the from 7.3 to 9.9 mg l above the lake bottom) than those Biwako-Ohashi bridge (Fig. 1). The outflow from Lake of the salt lakes mentioned above. However, it increased Biwa is the only (Fig. 1), and the amount of about 35% linearly above the lake bottom during last 20 discharge is well controlled by the water gate called years (Fig. 2) even if it fluctuated from 1978 to 1980. In Arai-Zeki. There are 120 rivers flowing into Lake Biwa. this paper, we reported the monthly trend of chloride ion Because of the large freshwater body and the geo- concentrations in the North Basin of Lake Biwa. Also, graphical location near Kyoto – Osaka area, Lake Biwa we analyzed monthly change of water level, precipita- is one of the most important water resources for drink- tion, chloride ion concentration of the South Basin and ing, agricultural, pleasure, fishery and industrial use. the rivers flowing into Lake Biwa with top 5 catchment The increase of lake salinity is one of serious con- areas through 20 years. cerns, because a lake sustains safety water required for When we estimate or predict the present and/or the human being life and/or richness of biota. For example, future lake salinity, calculation of the water budget is Aral Sea in Central Asia, which is the closed inland lake essential. In Lake Biwa, water outflows to Seta River, and used to have the 4th area in the world (about 100 Uji hydroelectric power station and the canal of Kyoto- times of Lake Biwa), is now exposed to the serious Sosui, and all discharge rates are well monitored. How- threats of loosing water body and rising salinity level. ever, the effects of the inflows to Lake Biwa are very The increase of salinity was from 10 to >30 g l-1 be- complicated because different water flows such as rivers tween 1960 and 1991 in the lake (e.g., Williams 2001), flowing into the lake, groundwater, agricultural or in- and brought serious influences such as a destruction of dustrial drain water, or surface water due to heavy rain- the birds or wildlife ecosystem, collapse of fishery, and fall or flood containing various chemical materials, and damage of human health (Williams & Aladin 1991). In most of them were not measured well for analysis. To Over twenty years trend of Salinity in Lake Biwa 43

35 40 ΠN

35 30 ΠYogo River Ishida River Ane River

Ado River 90 80 60 35 20 Œ 70 40 20 Hikone Seri River Echi River 35 10 Œ

Biwako-Ohashi Bridge Otsu 35 00 Œ

Seta River 010km20

34 50 Œ 135 45 Œ 136 00 Œ 136 15 Œ 136 30 Œ

Fig. 1. Map of Lake Biwa.

10.5

] l -1 10.0 mg mg [ lake bottom on on

i 9.5 at r 9.0

8.5 concent

on 8.0 i 0.5m from lake surface de de i 7.5 or

chl 7.0 y

hl 6.5

Mont 6.0 80 84 85 90 94 95 00 78 79 81 82 83 86 87 88 89 91 92 93 96 97 98 99 01 Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Date

Fig. 2. Monthly concentration of chloride ion in the upper and bottom layers at the deepest point in Lake Biwa. understand the rising processes of the chloride ion con- chloride ion concentration of Lake Biwa, and show the centration, we considered some causes, and we pro- result of our conceptual model. All observed data were posed a conceptual model approach. collected and arranged from "Kankyo Hakusyo" which In the following section, we explain the procedure of are the environmental white papers published by Shiga our model approach. Then, we discuss the trends of Prefecture, and from "Shigaken Kisyo Geppo" which chloride ion concentration and the climate in order to are monthly weather report papers published by Japan clarify the processes that sustain the increase of the Weather Association. 44 Y. Aota et al.

2. CONCEPTUAL MODEL APPROACH The concentration of chloride ion contained in the -1 The water in Lake Biwa flows basically from the rain is almost less than 2 mg l in the region of Lake Biwa (Tanaka & Ichiki 1986). Therefore, the rain falling North to the South Basin. Water discharge from the lake is well controlled by the gate, but the estimation of the directly to the lake tends to dilute the chloride ion con- water budget in the lake is very difficult because of centration in the lake. Figure 5 shows monthly precipi- tation at Hikone (see Fig. 1). In 1994, due to relatively having complex sources of water inflows to the lake. Let us put the average chloride ion concentration of low rainfall compared with an average year (Fig. 5), one inflows to the lake as C , and the total amount of of the lowest water levels during the observation period R was recorded (Hori et al. 1996). From 1993 to 1995, inflows as QR. Assuming that the lake volume is constant, then the dynamics of the chloride flux is difference of chloride concentrations between bottom represented by: and upper layers changed dramatically (Fig. 3) with the amount of rainfall. In heavy rainy year of 1993 and dC (t) 1995, the chloride ion concentration at surface de- V N = −C (t)Q (t) + C (t)Q (t) , N dt N in R R creased largely, and it increased in dry year of 1994, re- spectively (Fig. 6a). Similar relationship between pre- dC (t) cipitation and chloride concentration can be seen in V S = C (t)Q (t) − C (t)Q (t) , S dt N in S out 1987-1989 (Fig. 6b). Therefore, the dilution due to the rain falling directly on the lake may be one of the key where VN and VS are the volume of the North Basin and factors causing great fluctuation of the chloride ion con- South Basin of Lake Biwa, and CN and CS are the con- centration in the upper layer. centrations of chloride ion in the North Basin and South Figure 7 shows annual average chloride ion con- Basin of the lake, respectively. Variables Qin and Qout centrations of the rivers with large catchment area: are the amount of flow from the North Basin to the Yasu River with 383.8 km2, Ane River with 369.8 South Basin of the lake and the discharge from the km2, Ado River with 306.9 km2, Hino River with South Basin of the lake, respectively. From the obser- 210.9 km2, Echi River with 202.8 km2 of the vation data, we assume that the concentrations of chlo- catchment area, respectively (Oonishi 1986). These ride ion in the North and the South Basins change as rivers are the five big rivers with large catchment exponential functions: area flowing into Lake Biwa, and the sum of the C ≈ α exp(β t) , catchment areas of the five rivers covers about 47% N N N of whole catchment area in Lake Biwa (Oonishi 1986). Each average chloride ion concentration is C ≈ α exp(β t) , S S S derived by: where α , β , α and β are constant, which are deter- N N S S C Q Q mined by the field data of the chloride ion concentra- ∑ i i ∑ i i i tions. With the assumption and the above equations, the chloride flux CRQR can be expressed by: where Ci is the chloride ion concentration, Qi is the C (t)Q (t) ≈V α β exp(β t) + discharge from each river, and subscript i denotes R R N N N N (1 month in a year. In figure 7, chloride concentrations + α β β +α β VS S S exp( S t) S exp( S t)Qout (t) of Yasu River and Hino River exceed the maximum level of Lake Biwa average data over most of the periods. This suggests that the high chloride might be 3. RESULTS AND DISCUSSION supplied from two rivers. However, in order to know Figure 2 shows the monthly change of chloride ion the contribution of each river to the chloride ion concentrations at 0.5 m depth from lake surface and 1m concentration level of the lake, the estimation of height above the lake bottom (over 80 m of depth from chloride flux is needed. lake surface) near the deepest point in the North Basin As one of the approaches to estimate chloride flux of Lake Biwa. As seen in figure 2, trends of chloride ion with complex input sources, we proposed conceptual concentrations near the surface and the bottom showed model approach. Figure 8 shows annual changes of similar change from February 1978 to June 1985 (r2 = chloride flux in the inflows estimated from equation (1 0.68, n = 2693, p <0.001). After June 1985, the differ- and of the total chloride flux from 12 rivers flowing into ence between both concentrations expanded, and the pe- Lake Biwa. In equation (1, we set the volume of the riod with big differences of the concentration became North Basin and the South Basin, VN and VS, as 27.3 longer than that of the previous period (Fig. 3). Similar km3 and 0.2 km3, respectively. The outflow Qout was tendency as the difference could be seen in the monthly taken as the sum of annual water discharges from Seta change of water level (Fig. 4). Water level has not River, Uji hydroelectric power station and Kyoto chan- fluctuated much until 1985. However, since 1985, the nel. Parameters αN, βN, αS and βS were taken as 7.6191, period of low water level has become prolonged. 0.0126, 8.2217 and 0.0131, respectively. Over twenty years trend of Salinity in Lake Biwa 45

0.8

e 0.6 ac f

Sur 0.4

-

om 0.2 t Bot ( 0 on on i

at -0.2 r

-0.4 concent -0.6 CL CL -0.8 78 81 86 89 92 95 79 80 82 83 84 85 87 88 90 91 93 94 96 97 98 99 00 01 Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Date Fig. 3. Monthly change of the differences between bottom and upper chloride ion concentration.

80 60 40 20 cm (

0 -20 Level

er -40

Wat -60 -80 -100 -120 78 81 89 93 97 00 79 80 82 83 84 85 86 87 88 90 91 92 94 95 96 98 99 01 Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Date Fig. 4. Monthly water level change.

450

400

350 mm ( 300 on on i at t 250 pi

eci 200 pr

y 150 hl

100 Mont

50

0 80 89 91 98 00 78 79 81 82 83 84 85 86 87 88 90 92 93 94 95 96 97 99 01 Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Jan- Date Fig. 5. Monthly precipitation change.

46 Y. Aota et al.

600 9.7 Precipitation [mm] 0.5m D epth 550 9.5 ] -1 500 l mg mg

450 [ 9.3

400 on

[m m350 ] 9.1 on on 300 ta ti

pi 8.9

250 concentrati on i P reci 200 8.7 150 de ori 100 8.5 Chl 50

0 8.3 -95 -94 -95 -93 -94 -93 Jul Jul Jul Sep-95 Sep-94 Jan-95 Mar Sep-93 Jan-94 Mar Jan-93 Mar N ov-95 N ov-94 N ov-93 M ay-95 M ay-94 M ay-93

600 9.2 Precipitation [mm] 0.5m Depth 550 9

500 ] -1 l 450

8.8 [m g 400 on on ati [m m350 ] 8.6 on 300 ta ti pi 250 8.4 concentr eci on i

Pr 200

8.2 de

150 ori

100 Chl 8 50

0 7.8 -87 -87 -88 -88 -89 -89 Jul Jul Jul Jan-87 Mar Sep-87 Jan-88 Mar Sep-88 Jan-89 Mar Sep-89 N ov-87 N ov-88 N ov-89 M ay-87 M ay-88 M ay-89

Fig. 6. Monthly concentration of chloride ion at 0.5 m of depth and precipitation in 1993-1995 (upper) and in 1987-1989 (lower).

40 ] 1

- Yasu Ane Ado Hino Echi l 35 mg mg [

ge 30

schar 25 di de

ori 20 chl

of

15

10 am ount

age 5

Aver 0 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 Date Fig. 7. Annual change in average amount of chloride discharge from rivers.

Over twenty years trend of Salinity in Lake Biwa 47

2200 Estimated chloride flux 2000 Chloride flux of the 12 rivers 1800 1600 ] -1

s 1400 g [ 1200 ux ux l f 1000 de

ori 800 Chl 600 400 200 0 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 Date

Fig. 8. Annual change in estimated chloride inflow and in total chloride flux from 12 rivers.

North Basin 11.5 South Basin ]

-1 11

10.5

10 y = 8.2217exp(0.0131x) 9.5 R2 = 0.8737 y = 7.6191exp(0.0126x) 9 R2 = 0.9131 8.5

8 Chloride ion concentration ion Chloride l [mg

7.5 12345678910111213141516171819202122 Year from 1978

Fig. 9. Annual change of chloride ion concentration in the North Basin and the South Basin of Lake Biwa.

These parameters were determined from a regression greater than chloride inflow from twelve rivers. Average analysis of annual changes of the chloride ion demand of the chloride inflow during calculation period concentration in the North and South Basin of Lake in our model was about three times of the inflow from Biwa (r2 = 0.9131 in the North Basin and 0.8737 in the the rivers. However, river's data reported here might be South Basin of the lake, see Fig. 9). Calculation was underestimated for the real water flux, because it is dif- started from 1978 (t = 0). Note that we assumed many ficult to detect the large flux in a short-term such as parameters as constant. Therefore, this simple model is flood due to heavy rain in the field. Therefore, the actual quite conceptual. In figure 8, the sum of the annual contribution of river flux may be greater than that of the water flux of these twelve rivers covers about 90% of river flux estimated from the measured data. Further- reported data. Unfortunately, the data before 1984 was more, average chloride inflow estimated by the model not sufficient to compare with chloride flux estimated analysis includes the content of whole chloride inflow from equation (1. from the lake watershed. For example, inflow from land In figure 8, the demand of chloride inflow to sustain surface due to heavy rain and/or flood and inflow the chloride ion concentration in the lake was always through drain water have not been evaluated. Therefore, 48 Y. Aota et al. this result may show underestimated flux of inflow. Lake Biwa. We suggested that the high concentrations Such unmeasured flux may take a large contribution to of chloride ion in river water might be one of the major the concentration of the chloride ion in the lake. contributions to induce the increase. Precipitation is one It is well known that the lake water sometimes flow of the dilution factors of the chloride in the surface lake from the South Basin to the North Basin in Lake Biwa, water, but it may also promote the increase of loading although it flows from the North Basin to the South Ba- from the non-point sources. We showed that the inflow sin on the average. Water flow from the South Basin to from the non-point sources could be also major sources the North Basin can be caused by an intrusion of density to induce the increase of chloride concentration. It is currents, and by an excess transport due to oscillation of very difficult to monitor the loading from non-point seiche and/or internal waves (Oonishi & Imasato 1974). sources continuously, but it is really important to meas- Therefore, as one of the possibilities for the increase of ure the flux from the non-point sources running into chloride ion concentration in the North Basin, we can Lake Biwa. suggest that the water exchanges between two basins may transport the high chloride ion concentration from REFERENCES the South Basin (Fig. 9) to the North Basin of the lake. Hori, T., Y. Sugiyama, M. Kanao, M. Nagai, N. Taniguchi, M. In case of 2001, average chloride concentration was Sugiyama, T. Fujinaga. 1996. Distribution of some chemi- somewhat high (about 10.2 mg l-1) in southern part of cal components in Lake Biwa under usual and unusual wet -1 and drought conditions, 1992-1995. Japanese Journal of the North Basin, and was low (about 10.0 mg l ) near Limnology, 57: 183-192. the center of the North Basin (Kankyo Hakusyo 2002). Kankyo Hakusyo. Environmental White Paper, Shiga Prefec- Although an estimation of the chloride transport be- ture. tween two basins was not treated in our model, chloride Shigaken Kisyo Geppo. Monthly Weather Report Paper. Japan Weather Association. transport from the South Basin to the North Basin may Oonishi, Y. 1986. Land Use and Water Budget by River ba- be one of the possible mechanisms to maintain continu- sins. In: Lake Biwa Research Institute (Ed.), Shiga Pre- ous increase of chloride ion concentration in the North fecture Regional Environment Atlas. Lake Biwa Research Basin of the Lake Biwa. Institute. Oonishi, Y. & N. Imasato. 1974. A note on the water ex- change between the South and the North Basins of Lake 4. CONCLUSIONS Biwa. Contributions, Geophysical Institute. Kyoto Univer- Over 20 years trend of chloride ion concentration in sity, 14: 11-19. Tanaka, Y. & S. Ichiki. 1986. On the chemical components in the North Basin of Lake Biwa was reported. Its concen- the rain precipitated in Shiga. Rep. Shiga Pref. Inst. Pub. tration level was lower than those in the so-called salt Hlth. & Environ. Sci., 21: 156-159. lakes, but the continuous increase of chloride ion con- Williams, W.D. 2001. Anthropogenic salinisation of inland centration was statistically significant. On the basis of waters. Hydrobiologia, 466: 329-337. Williams, W.D. & N.V. Aladin. 1991. The Aral Sea - recent the conceptual model analysis and the several environ- limnological changes and their conservation significance. mental data analysis, we discussed some possibilities Aquatic Conservation-Marine and Freshwater Ecosys- that explain the increase of chloride concentration in tems, 1: 3-23.