From last year, the water gate construction started at the mouth of Nagara River. There are two purposes for the construction of the gate; one is to control the flooding, another is to obtain a water resource. Regarding the former problem, the Water Resources Development Public Corporation, planned to deepen the river bottom in order to expand the area of the river section. But the deepening of the river bottom would lead the flow of much more seawater into the upper stream than at present. The authorities of the above corporation have emphasized the necessity of gate to eliminate the influence of seawater by possible rice field damage due to saline water which might penetrate through the bank. I will discuss the water pollution problems after the construction of gate at the mouth of river.Article信州大学理学部付属諏訪臨湖実験所報告 7: 121-128(1991)departmental bulletin pape

Saijo,  Yatsuka

English

47032

Shinshu University Institutional Repository

   Rep. Sii,ya H.y`frobiot. Z' ?2J- 128"99i) l2!PROBLEMS OF WATER POLLUTION AFTER THE WATER GATE CONSTRUCTION AT THE MOUTHOF THE NAGARA RIVERYatsuka SaijoFuculty of Liberal Arts, Aichi University, Aichi-ken, 470-02, JapanAbstract From last year, the water gate construction started at the mouth of NagaraRiver. There ate two purposes for the constru¢tion of the gate; one is tocontrol the flooding, another is to obtain a water resource. Regarding theformer problem, the Water Resources Devetopment Pubiie Corporation, plannedto deepen the river bottom in order to expafid the area of the river section. But the deepening of the river bottora wou}d lead the f!ow of mu¢1'i moreseawater into the upper stream than at present. The authorities of theabove corporation have emphasized the necessity of gate to eliminate theinfluence of $eawater by possible rice fieid damage due to saline waterwhich might penetrate through the bank. I will discuss･the water pollutionproblerns after the construction of gate at the rnouth of river.KEYWORDS: BOD, Nagara River, nitrogen and phosphorus, phytoplankton growth,          water gateINTRODUCTION Three big streams, the Kiso, Nagara and Ibi Rivers (Fig. I), flow into theinner part of Ise Bay, which is a rather big bay iocating in the raiddle ofJapan along the Pacific coast. The region of these three rivers is wellknown for its heavy floods frora the ancient times. The idea for water gateconstruction at the mouth oi Nagara River {Fig. 2) took shape in 1959.There have been two purposes for the construction of the gate, one was tocelltrol the £loods, and another was to obtain a water resource. Regardingthe former problem, the MiBistry of Construction, later the ,Water ResourcesDeveiopment Public Corporation, planed to deepen the river bottom in orderto expand the area of the river'section, instead of buUding elevatedembankraents. But the deepening o£ the river bottora would lead to the flowof much more seawater into the upper stream than at present. Theauthorities o£ above corporation, have emphasized the necessity to constructthe gate to eliminate the influence of $ea water by possibile rice fielddamage due to the saiine water which rnight penetrate through the bank. TheFig･ 2 Location of water gate in Nagara RiverLakeItNss7{5""' v"- -ttN1'-- N t"'-t  N tttN} lBiwauft} 2. Nt}l !te2,lfl,, t̀kttNWater gateNitNxNTsu Otstvt  s`ilrg"tn- pt ""Ytt N .Ns1` eN. 'ofof           ""o   4tge             " 'toGifu .'"' N     - t N   tNt N..- tt     N- xt'NNIse BayO Nagoya  I t N-J"tt' -Ne}tt  '  t  `  t  '  t  x  }  1  l  t  t 1t1NO 24 kmtew-L...-t.....s...-n,Fig･ } Locatlon of the Kiso, Nagara and Ibi Rivers;N                                                                    l23 construction of the gate started last year. After the cornpletien of thegate, the downstreara area of the river wiil change to a £reshwaterstagnatlng area (sometking like a reservoir) exteBding for about 25 km. Thefreshwater of 22.5 m3/sec wili be takefi frora this reservoir as a new waterresource for the prefectures of Aichi, Mie and Nagoya City.･ Concerning the changes in water characteristics above the gate, theauthorities (4) insist that there will be no change in water quality evenafter the construction of gate, because tke water will be changed frequentlyand maintain a condition sirnilar to that of a river haviBg a steady flowthough a part of the water is taken £or the water supply. They recogAizethe possibility of increased sedimentation of mud upstream of the gate,caused by the s!ower flow rate; ltowever, they claim that the mud would befrushed out at the period of the highest water level (about 60 days everyyear). I would like to exarnine this projection of the water characteristicsafter the gate construction from the standpoint of the organic matterproduction and rnatter cycle in the aquatic region. The first probiefn is the di£ference in algal pvoduction between the riverand the iake. In the river, geRerally, the periphyton (attached algae> growoR the stone or sorae other place, whiie in the lake where there is almost noflow, the phytoplanktoll (free floating algae) grow in the water. When theriver is dairuned up, during the period of high flow, the water will !naintaina siinilar condition to that of river. However, how about during the periodof water shortage ?DATA ON WATER FLOW VOLME To examine this problem, from the data of £lowing water volurne at Sunematastation of Nagara River frora 1979 to l988 <2), the period when the arnount ofdaily flow did not exceed about 50 m3/sec was iisted in Table l consideringthat 22.S m3/sec of water will be taken for the water supp!y. If the volumeof reservoir <stagnaring water) 37,60e,OOe m3 is divided by 58 m31sec, itbecornes 8.7 days. Here, the winta" season was eliminated because in wiRter,geRerally £rom Deceraber to February, the ilow rate was below 50 m31sec, andoften below 30 rn31sec, during SO to IOO days every year.Table 1 The period when the amount of daily fiow did not exceed about50 m31sec from l979 to 1988. The winter season was eliminated.Frorn the data o£ Sunemata Statiofi, Nagara River.124¥ear1979l980i981l982l983l9841985l986l987l988Period Days Year!yMay 19-June     no     noJune 2fi-julyMay 27-JuneMay l9-JuneSep. I6-Nov.Aug. 24-Sep.Aug. 6-Sep.Sep. 24-Dec.Aug. 28-Sep.Sep. 29-ect.Oct. I6-Nov.218l28 Il 6l7 l8 24 B 232822172157l li4386282839mean £!g.w ratel23l56155H9l4S 85l53 95 87l14..'.....･.l554894694m3/sec As seen in this table, thete were ll times when the period of watershortage exceeded 14 days evef IO years, arnong them 7 times it exceeded 4weeks. It was longer ill the dry year. ''CHEMICAL FEATURES OF RIVER WA[{rER (Table 2)Table 2 Data of BOD, total nitrogen and total phosphorus yearly meanvaiues of Nagara River at T6kai-6hashi frorn l978 to l988 <I>. The valueswere shown in mglL. '1978 1979 l98e 1981 1982 1983 l984 l985 1986 l987 1988BODTotal-NTotal-P1.8l.3O.16l.9 l.71.3 l.3O.12 O.l2  ttllo'..56lll1o...#309l1o...34gsl1o...34ll1lo...241}1lo...45IS}lo...44lgolo'.･9209 Data of BOD vaiues at T6kai-6hashi, which locates near the top of areservoir (water stagnating area}, changed from about 2 to 1 mglL frorn l978to 1988. It raeans a decrease o£ organie matter pollution iG the riverwater. Similar tendencies of BOD decrease were fouBd also at Nann6-6hashiand Ise-6hashi. However, there were no appreciabie changes in the values oftotai nitpogen and total phosphorus which are important elements as thesource of artificial eutrophication in lakes, from 1978 to 1988. Thus, the                                                                    125total nitrogen ranged froiTt 1.2 to l.6 ittg/L and total phosphorus irorn O.08 tos.l6 mg/L. It is noteworthy that these values are comparable with those inthe water in Lake Suwa, which is one of the typical lakes having heavypoHution by eutrophication. Which raealls that if tke water stagnates as inthe iakes, in sumvrter the phytoplankton will grow to the level of IOO mg/rn3o£ clt!orophyH-a, which is almost equivalent to the organic raatter of 25mg/L as BOD. But the species of phytoplankton would not be lirnited toMcrocystis, which is the major algae in Lake Suwa in sunurier.DIFFERENCE IN PHYSICAL ENVIRONMENIr BETWEEN RIVER AND LAKE Cempared with rivers, the characteristic$ of physicai environment of lakesare the low exchange rate of water and tlte stratification of watertemperature in summer. There are many factors inEluencing the relationbetween the exchange rate of lake water aBd the productiofi of phytoplankton.But, generaRy, frorn one week to IO days would be acceptable as enough timefor the growth of phytoplankton. Assufriing that the volurae of the river where the water wili stagnate afterthe construction of the gate is 37,600,OOO rn3, the days'necessary for riverwater to pass through that region were calcuiated as showB in Table 3..Table 3 The days necessary for river water to pass through the waterstagnated region after the gate construction. 'Flow rate <m31sec> 200 leo 80 60 50 46 3eDays to pass (days) 2.l 4.4 5.4 7.3 8.7 IO.9 lq.5 And by assuming that the length of water stagRate region is 2S kra, the flowvelocity was also caleulated for the days to pass as seen in Table 4.Table 4 The ilow ve!ocity calculated from Table 3.     Days to pass <days) 3 4 5 6 7 ,8 9 IO ll    -iil-U.'"''i'tr'r.6"l'£MY"M'('6'.1,.,> g.6 7.2 'T'i"i"L"5'1-s--'mua:-'i'--''-i.U''S'.i''3.o 2.6              ttt      ... ..t." . .tt.t.t--t-.tt.+.. +.t t.t. . t .t ... .t.tttt.t-t.t.- .. .t .... When the flow ve!ocity of river water decreased to the leve! o£ severalcentimeters per second, it is said that the river cannot easily maintain asteady flow because the influeRce oi wind and other factors becornes muchstronger (Okuda, personal cerrwnunicatioB>. Another probiera is that the vertical mixing of water is generally much faorestronger in the river than in the lake. But the turbidity of water in the  126Nagara River is generally low except in the period of high water level.Therefore, even if the mixing of water takes p}ace during the period of !owwater levei, there is a enough light for the algai photosynthesis down tothe bottom of the'river. Using high amounts of nitrogen and phosphorus, thephytoplanktoR will grow extensively. This rneans an increase of pollutioll byorganic matter in that region. Furtherraore, the stratification o£ watertemperature was sornetimes observed in surTt;rter when the velocity of the streamis very slow. This wHl give a more favorabie cQadition £or the growth ofphytoplankton. It was oEten said that even if the phytoplackton grow duringthe period oi !ow water levei, it will sQon be flushed out by the increaseof water flowing, however, as I showed above, the period of low water leveloften continues a rather !ong time, especially in the year of watershortage. Though above discussions were mainly directed to the period from spring toauturnn, there is also a rather high possibility of algal growth during thepeviod oE low water level in winter, based, Qn enouglt uutrients, aconsiderable amount of light and the temperature of 5 ℃ or so. Such analgai bloorn was observed in sorae reservoirs or lakes in winter. Forinstance, high values o£ chlorophyH-a were determined in Lake Fukami-ike,in Nagano Prefecture almost every year (5).SOME OTHER PROBLEMS l) The possibilty of sedimentation of bottorn mud containing a large arnountof organic rnatter in the region above the gate is very high. The organicmatter will be supplied not only as phytoplankton during the period of lowwater level, but as the periphytoR, the detritus of aguatic raacrophyte andthe allochthonous substances (i.e., the matters supplied from outside of theriver>. Narnbu (3> suggested the possibility oi dissolved oxygen decrease to2.3 mg/L by the dlssolved substanee in the region above the gate. Thesedimentation of mud containing a high amouBt of otganic matter willaccelerate the consuraption of dissolved oxygen and the deterioration inwater quality. Though the Corperatien insist the e£fe¢t of rnud flushing atthe period of highest water level, the effects should be examined careiuiiyfrom the hydrological viewpoint, especially relatillg to the gate operatioB. 2) Another fact of･ the probiem is the changes in the water features belowthe gate. In the dowllstream, the seawater wi!1 be covered with a freshwaterlayer and stagnate send perrnanently even in winter. There will be almost noflow, though the tidal changes in water levei will take place. The effeeto£ ireshwater fiowing down through the gate as the cause o£ vertical raixing                                                                   127wouid be very !ow except at the highest water level because of a largedifference of density between seawater and freshwater. In that regien, ananoxic condition will prevail and a large amount of hydrogensuifide will beproduced. The in£luence oi this water mass on the benthic organisms as wellas on the fish passing through this region should be examined carefully. 3) It is clear that the macrophyte zone along the river wil! decreaseextensively with the construction work, naraely by deepeBing the river bottornand the constrllction of a so ca!led biaBket <the ilat plane on the innerbanks to elininate the penetration of water through the bank). Theirnportance o£ the macrophte zone for the protection of natural environmentin iakes and rivers i$ well known. Furthermore, the decrease of macrophytewiH increase the solub!e nutrients, such as nitrogen and phosorous in riverwater, aBd wiil have sorne influence so as to increase the phytoplanktongrowth.CONCLUSION To anticipate the changes in aquatic environment by the construction of thewater gate in the Nagara River, a series oi extensive researeh was carriedout by the group of late Professor Koizumi frorn l964 to l968 with theparticipatioll of about 70 scientists. The major part of this research was,however, directed to the changes in fisheries. Furthevmore, it is probablethat during the more than 2e years since the time of research, there werevast changes in enveironrnental conditions in the river and in the adjacentsea area. On the other hand, significant progress has been made in thetechniques of environrnental assessrnent. As pointed out above, there aremany environfnental probiems requiring more extensive research reiating tothe gate eofistructioll. Consequently, I would iike to suggest extensiveenviroBraeRtal researclt ifcgrtediately using new techaiques while interruptingthe construction work.ACKNOWLEDGEMENTS l would like to thank Dr. Setsuo Okuda, Okayama University of Science, Dr.Tokio Okino, Shinshll University, Dr. ¥asunori Watallabe, Tokyo Metropolitangniversity and Mr. Akihiko Yagi, Nagoya Wornen's University for theirsignificant suggestions.REFERANCES1. Environraental Agency, ed. (l980-1990) Water quaiity yearbook of public   region in Japan. Fuy6 Information Center, Tokyo. <In Japanese) l282. Ministry of Construction, ed. (1981-1990) Yearbook of river diseharge in. (In   Japanese)3. Nanbu, Y. "971> Legal eonsu!tation report on the ease to suspend the   gate construction at the mouth of Nagara River. (In Japanese)4. Water Resources Developrnent Public Corporation, Nagara River Water Gate   Coilstruction Office {1990) Water Gate at the meuth of Nagara River,   l2pp. (In Japanese)S. Yagi, A., Shimodaira, I. Terai H. and Saijo Y. (l983} Seasonal change of chlorophyll-a and bacteriochlorophyll in Lake Fukarni-ike, Jap. J. Lirnnoi. 44, 283-292.

PROBLEMS OF WATER POLLUTION AFTER THE WATER GATE CONSTRUCTION AT THE MOUTH OF THE NAGARA RIVER

From last year, the water gate construction started at the mouth of Nagara River. There are two purposes for the construction of the gate; one is to control the flooding, another is to obtain a water resource. Regarding the former problem, the Water Resources Development Public Corporation, planned to deepen the river bottom in order to expand the area of the river section. But the deepening of the river bottom would lead the flow of much more seawater into the upper stream than at present. The authorities of the above corporation have emphasized the necessity of gate to eliminate the influence of seawater by possible rice field damage due to saline water which might penetrate through the bank. I will discuss the water pollution problems after the construction of gate at the mouth of river

Saijo  Yatsuka

https://soar-ir.repo.nii.ac.jp/record/16448/files/Suwa07-15.pdf

PROBLEMS OF WATER POLLUTION AFTER THE WATER GATE CONSTRUCTION AT THE MOUTH OF THE NAGARA RIVER

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