Nghin cu s phn b khng ng nht v

hm lng Asen trong nc ngm trn mt

phm vi hp minh ha ti x Vn Phc, Thanh

Tr, H Ni

Nguyn Ngc Mai

Trng i hc Khoa hc T nhin

Khoa Mi trng

Lun vn Thc s ngnh: Khoa hc mi trng; M s: 60 85 02

Ngi hng dn: TS. Phm Th Kim Trang

Nm bo v: 2011

Abstract. Tng quan v s hnh thnh v mt s tnh cht a ha ca nc ngm;

nhim asen trong nc ngm trn th gii; vn nhim asen trong nc ngm

Vit Nam; nhng gi thit v s xut hin ca asen trong nc ngm. Nghin cu s

phn b khng ng nht v hm lng asen trong nc ngm trn mt phm vi hp

ti khu vc Van phc, huyn Thanh Tr, H Ni. Tm hiu v phng php nghin

cu: phng php khoan ging v ly mu trm tch; phng php ly mu nc;

phng php phn tch mu. a ra kt qu v tho lun: c im trm tch, hm

lng asen v mt s thnh phn ha hc khc trong nc ngm ti vng nghin

cu; phn tch mt s nguyn nhn dn n s phn b khng ng nht v hm

lng asen trong nc ngm khu vc nghin cu (tnh kh th hin trm tch;

nc ngm; bng chng v s khc nhau tnh kh trong trm tch v nc ngm

cc ging c su khc nhau cng mt v tr nghin cu)

Keywords. Khoa hc mi trng; nhim nc; Asen; Nc ngm

Content

M U

S nhim As trong nc ngm c bit n t lu, vi cc nh hng xu n

sc khe con ngi khi s dng ngun nc c nhim As n ung v sinh hot. Tuy

nhin, nhng nguyn nhn dn n s xut hin As trong cc tng ngm nc vn ang c

cc nh nghin cu bn lun. Ngoi ra, s phn b As khng ng u trong cc vng b nh

hng cng ang l mt vn kh hiu. S khc nhau ln v hm lng As trong nc

ngm c tm thy Bangladet trong khong cch 100 m. Cc nghin cu gn y mt

s vng thuc ng bng sng Hng cng pht hin s khc nhau ln thm ch trong mt

khong cch ngn t 10 20 m. c mt vi s gii thch c a ra v s phn b As

phc tp theo khng gian, bao gm cc s khc nhau v , khong, a ha, thy hc v s

phong ph cc vt cht hu c lp di t. Tuy nhin, cn nhiu vn cha c r

rng, cc bng chng vn cn kh hn ch cha th gii thch mt cch ton din v y

cho s phn b phc tp ca As trong cc tng ngm nc c tm thy.

Vi nhng l do nu trn chng ti la chn ti: Nghin cu s phn b

khng ng nht v hm lng As trong nc ngm trn mt phm vi hp. Minh ha ti

x Vn Phc, Thanh Tr, H Ni vi cc ni dung sau:

Phn tch v so snh c im ha hc trm tch v ha hc nc ngm gia cc

ging c hm lng As thp v cao khu vc nghin cu

Phn tch mt s nguyn nhn dn n s gii phng As t trm tch vo tng

ngm nc

Lun vn uc thc hin trong khun kh ti hp tc gia trung tm CETASD,

trng H KHTN vi trng i hc Tng hp Columbia, M v nghin cu xu hng

nhim asen cc tng nc sch. Cc kt qu ca lun vn c trnh by poster ti Hi

ngh Quc t v Asen trong nc ngm khu vc Nam , thng 11/2011 t chc ti H Ni.

Chng 1. TNG QUAN

1.1. nhim asen trong nc ngm trn th gii

Vn nhim As trong nc ngm khng cn l mt vn mi, n c pht

hin nhiu ni trn th gii nh Achentina, Mhic, Chil, M, Canada, Trung Quc, i

Loan, Thi Lan, n , Bangladet v Vit Nam, v.vvi nng ln hn 50 g/L [20].

Mc d As c pht hin nhiu khu vc, nhng n khng phi l ph bin trong tng

ngm nc. Nhng tng ngm nc b nh hng bi As l c gii hn, mi trng no

v thng xut hin ngoi l hn l theo quy lut. Mt nt c trng ca cc vng nc ngm

c As cao l s thay i ln v nng As trong nc ngm theo khng gian. iu ny c

ngha l rt kh hoc khng th d on c chnh xc nng As ph hp mt ging c

th t kt qu ca nhng ging bn cnh [20].

Hnh 1. Bn nhim asen trn th gii [20]

Mt v d in hnh nh Bangladet. Mc d y l khu vc b nh hng nng n

nht, nhng s phn b As trong tng ngm nc ca khu vc l khng ng nht (hnh 1.3).

Khu vc b nh hng ti t nht l ng nam ca Bangladet ni m mt s huyn c hn

90% ging b nh hng [20]. Nguyn nhn dn n s phn b khng ng nht y l

cha c hiu r, cha tm thy ti liu no nghin cu v vn ny.

Hnh 2. Bn phng th hin s phn b As trong nc ngm t cc ging khoan (< 150

m) Bangladesh [20]

1.3. Vn nhim asen trong nc ngm Vit Nam

Vn nhim asen trong nc ngm Vit nam nhng nm gn y c nhiu

t chc v cc nh nghin cu trong nc, nc ngoi quan tm. Nhiu nghin cu cho

thy nc ngm c s dng nhiu ng bng sng Hng v sng Cu Long cho cc mc

ch sinh hot v sn xut ang ng trc nguy c nhim asen t nhin.

Mt nghin cu trn din rng ng bng sng Hng do Trung tm Nghin cu

Cng ngh Mi trng v Pht trin Bn vng (CETASD), Trng i hc Khoa hc T

nhin, i hc Quc gia H Ni kt hp vi cc chuyn gia thuc Vin Khoa hc v Cng

ngh Nc (Eawag), Lin bang Thu S tin hnh (t nm 2005 n 2007) kho st ngu

nhin trn ton khu vc ng bng sng Hng. Kt qu phn tch cho thy hm lng asen c

s thay i trong mt khong rt rng, t mc nh hn < 0,1 g/l cho ti hn 810 g/l.

Trong , 73% s mu c hm lng nh hn 10g/l, t tiu chun nc ung v asen ca

B Y t Vit Nam, 16% s mu c lng asen nm trong khong 1050 g/l, 8% s mu c

lng asen nm trong khong 50200 g/l, v 3% s mu c lng asen rt cao, ln hn 200

g/l [10].

Hnh 3. nhim asen trong nc ging khoan ti ng bng sng Hng [10]

Tuy nhin s nhim li tp trung ch yu ti vng b tri sng Hng, xuyn qua

mt s tnh ng dn nh H Ty, H Nam, H Ni, Nam nh, Hng Yn v Thi Bnh

(hnh 1.4). Nhiu ging khoan vng ny c hm lng asen cao khong 200300 g/l (gp

2030 ln tiu chun nc ung). Pha lu vc sng ung cng xut hin gii nhim vi

mc nh hn. l a phn tnh Bc Ninh v Hi Dng. Cc khu vc ven vng ni

trung du, ven bin c chiu hng t b nhim asen trong nc ging khoan. Phn trm s

mu c hm lng asen vt tiu chun ca nc ngm l 45% ti H Nam, 28% ti H Ty,

17% ti Hng Yn, 10% ti Nam nh v Bc Ninh. Cc tnh cn li c t l ny nh hn

10%. Tuy nhin, cc t l nhim tng khu vc s thay i nhiu khi c mu kho st tng

ln. Nhiu mu nc ti cc khu vc lin k nhau c hm lng thch tn rt cao (> 200

g/L). V d, ti cc huyn an Phng, Hoi c, Thanh Oai, Thng Tn, Ph Xuyn (H

Ty), Duy Tin, L Nhn (H Nam). C nhng x t l ging nhim thch tn cao gn

100%.

S khc nhau ln v nng As trong nc ngm theo khng gian ng bng sng

Hng cng c quan st thy (hnh 1.4), thm ch khong cch rt gn khong 20 m [10].

Tuy nhin, nguyn nhn ca vn ny l cha c hiu mt cch r rng.

1.4. Nhng gi thit v s xut hin ca asen trong nc ngm

Nh trnh by trn, c th thy nc ngm nhim As cao (cao hn tiu chun cho

php trong nc ung) khng phi l him thy, nhng chng khng c ngha l ph bin

trong hu ht cc tng ngm nc v n ch tn ti di nhng iu kin c bit. iu ny

lin quan n c mi trng a ha v a cht thy vn trc v c hin nay. Nghch l

l nc ngm c As cao th khng phi lun lin quan n cc vng gc c As cao. Vn

As trong nc ngm xy ra di c iu kin oxy ha v iu kin kh; cng nh c

iu kin kh hu m t v kh hn. iu cho chng ta thy rng As khng c sn trong

cc tng ngm nc m n b gii phng ra t cc trm tch trong cc khu vc c iu kin

thch hp.

Vit Nam, nc ngm thng c khai thc c tng Holocene v Pleistocene.

Hai tng ny c cch bit nhau bi mt lp st dy hng mt. khu vc ng bng sng

Hng, trm tch Holocene hnh thnh nn cc tng nc nng, vi su khong 30 60 m

[8, 29]. Nc ngm thng c c tnh kh, vi nng Fe v amoni (NH4+) rt cao [8, 10].

Nhiu tc gi nghin cu cho rng nhim asen trong nc ngm ti y c th do s kh

ho tan ca cc oxit st cha asen, tn ti t nhin trong tng cha nc [9, 10, 18, 27]. S

liu phn tch trm tch v cc thng s ho hc trong nc ngm chng t mi tng

quan gia nng asen vi cc iu kin thiu oxy trong tng cha nc v dn n s ho

tan kh ca cc khong cha asen vo nc ngm.

Nghin cu ca tc gi Charles F. Harvey v cc cng s (2002) v tnh hnh nhim

asen ti tnh Munshiganj (cch sng Dhaka 30 km v pha nam v sng Ganges 7 km v pha

bc, Bangladet) cng cho thy c ch gii phng asen t trm tch vo nc ngm lin quan

ti iu kin mi trng kh. Tc gi chng minh qu trnh gii phng asen c nguyn

nhn t dng cacbon tr i vo tng cha nc su do s bm nc ti tiu t vo ma

kh Bangladet. S kh hp cht hu c thc y s linh ng ca asen t trm tch vo

nc ngm [7].

Cc tc gi C. A. J. Appelo v D. Postma cho rng c hai c ch chnh chi phi qu

trnh gii phng asen t trm tch vo nc ngm l c ch oxy ho v kh. Bn cht ca c

ch oxy ho l qu trnh oxy ho cc qung pyrit c cha asen nh asenopyrit (FeAsS),

loellingit (FeAs2) bi cc tc nhn oxy ho c trong t (nh NO3) v sn phm ca qu

trnh l s gii phng asen vo nc ngm cng vi sulphat. Theo c ch kh, trm tch cha

cc xit st m trn b mt ca chng hp ph asen s c lng ng cng vi cc hp

cht hu c t nhin, cc vi sinh vt trong t tiu th cht hu c v ly oxy khin mi

trng tr nn c tnh kh theo thi gian, chnh mi trng ny chuyn cc pha st trong

trm tch v dng Fe2+ ng thi gii hp asen ra khi trm tch v tan vo nc ngm [8].

Trong cc nghin cu nhm tm hiu c ch gii phng As c tm thy cc vng

c kh hu kh cn Argentina. Cc tc gi nghin cu cho rng di cc iu kin kh cn,

cc phn ng phn hy ca silicat v cabonat rt d xy ra v nc ngm thng c pH cao

(6,3 9,2). Chnh iu kin pH cao ny l nguyn nhn chnh dn n s gii hp As t cc

oxit kim loi c mt trong trm tch (c bit l oxit, hidroxit ca Fe, Mn) vo mi trng

nc ngm, mc d s ha tan trc tip ca ni la cng c th l mt ngun gc gy ra s

nhim As trong nc ngm y [21].

Trong mt nghin cu ng bng sng Mekong, tc gi Polizzotto li cho rng As

c gii phng trong cc tng t b mt bi cc qu trnh oxy ha kh v sau c th vn

chuyn i xung tng ngm nc c ct [15].

Nh vy, cc nguyn nhn lm gii phng As t trm tch vo tng ngm nc vn

ang c tranh lun, vn cn nhiu quan im bt ng, mt c ch r rng cho tt c cc

tng ngm nc b nhim As l cha c.

Chng 2. I TNG V PHNG PHP NGHIN CU

2.1. a im nghin cu

a im nghin cu nm x Vn Phc cch th H Ni khong 20 km v pha

ng nam. X ny nm gia con sng Hng v mt con bo v vng ty nam ca thnh

ph H Ni trnh khi l lt hng nm (hnh 2.1). y thnh thong chu b ngp lt vi

ngy trong ma ma. Tng ngm nc gm c lp trm tch mng Holocene v Pleistocene

su > 40 m. Mt s nghin cu trc y cho thy hmlng asen trong tng Holocen

x Vn Phc l khng ng nht, pha Ty Bc khng b nhim, cn pha Nam v ng

nam b nhim cao [10].

2.2. i tng nghin cu

Vi mc ch tm hiu cc nguyn nhn dn n s phn b khng ng nht v hm

lng As trong nc ngm ca khu vc. Chng ti tp trung vo phn tch v nghin cu 3

i tng chnh l :

Trm tch

71 Mu nc ging khoan (29 mu ging khoan ca d n, 42 ging nh dn)

1 Mu nc sng

Chng 3. KT QU V THO LUN

3.1. c im trm tch, hm lng asen v mt s thnh phn ha hc khc trong nc

ngm ti vng nghin cu

Cc kt qu t phn tch bng phng php quang ph hp th nguyn t (i vi 29

ging ca d n) cng nh cc kt qu t b th hin trng (t 42 ging ca h gia nh)

cho thy phn ln cc ging trong khu vc c nng As ln hn 10 mg/L (chim 64% ton

khu vc, trong c ti 57% > 50 gAs/L). c bit c nhiu ging c nng As ln hn

300 mg/l. iu cho thy tnh cht nghim trng v vn nhim As trong nc ngm

ca khu vc nghin cu.

Hnh 4. S phn b As trong nc ging khoan khu vc x Vn Phc

Mt iu rt c bit y l s khc bit rt ln v nng As trong cc ging

khu vc nghin cu, thm ch vi cc ging cch nhau cha y 100 m. Hu ht cc ging

pha bc ca x c nng As thp (100% ging c nng As thp < 10 g/L), trong khi

phn ln cc ging nm pha ng nam ca lng (hu cc ging c nng As cao t 10

n > 400 g/L chim 77%, n=60). Mc d cng xut hin mt s ging c nng As thp

pha nam nhng chim t l rt nh (~11% ging c nng As < 10 g/L, n=60). S phn

b khng ng nht theo khng gian c th hin r hnh 3.1.

Ngoi ra, cc kt qu cho thy c im ha hc nc ngm rt khc nhau gia cc

ging c hm lng As cao v cc ging c hm lng As thp ca khu vc nghin cu. Cc

thng s ha hc cc ging c As cao thng cao hn rt nhiu so vi cc ging t As. y

l kt qu ca cc phn ng oxy ha kh xy ra mnh m trong tng ngm nc mang tnh

kh.

3.2. Phn tch mt s nguyn nhn dn n s phn b khng ng nht v hm lng

As trong nc ngm khu vc nghin cu

3.2.1. Nguyn nhn lin quan n tnh kh th hin trm tch

Khng c s khc bit ln c tm thy trong cc thnh phn ch yu ca cc

khong v a ha ca trm tch khu vc nghin cu, ngoi tr trng thi oxy ha kh ca

Fe oxyhidroxit suy ra t mu ca trm tch v quang ph pht x khuch tn. S khc bit v

mu sc ca ct trm tch cc ging l d dng nhn thy khi chng ti khoan cc ging

a im nghin cu. Mu sc trm tch khng ch khc nhau gia cc ging, m s khc

nhau theo su cng c ghi nhn. Cc ging c nng As cao th ct trm tch trong

tng ngm nc thng c mu xm hn. Nhng ngc li, ct trm tch trong tng ngm

nc cc ging c nng As trong nc ngm thp thng c mu vng hay mu cam

(hnh 3.4; 3.5). C th ly ging AMS1 v AMSNS4 nh mt v d in hnh. Cng

su khong 25 m, ct trm tch ging AMS1 c mu xm tng ng vi nng As > 250

g/L trong nc ngm, trong khi ct trm tch c mu vng ging AMSNS4 ng vi

nng As ~10 g/L (hnh 3.4).

C th suy on rng ct trm tch cc ging c As thp mang tnh oxy ha (Fe(III)

cha b kh), cn ct trm tch cc ging c As cao th hin tnh kh mnh hn. Trong iu

kin kh, ngoi qu trnh kh ha tan cc oxit Fe dn n gii phng As b hp ph ln ,

th qu trnh kh As(V) v As(III) ngay trn b mt cc oxit Fe cng c th din ra. As(III) c

kh nng hp ph km hn As(V), v vy qu trnh ny cng gp phn tng tnh linh ng

ca As trong mi trng. S thay i trng thi oxy ha ca As cn c ngha quan trng

bi s cnh tranh v v tr hp ph ca cc anion khc nh PO3, SiO3

, HCO3

-,cng nh

ca chnh As(V). Nh vy khi b kh v As(III) th kh nng gii phng ca As t trm tch

ra nc ngm s ln hn [10, 14].

Hnh 5. S khc nhau v mu sc c

quan st thy t trm tch 2 ging c

nng As khc nhau, ct trm tch

mu xm ng (nh trn) vi nng As

cao trong nc ngm (> 200 g/L) ct

trm tch mu vng (nh di) ng vi

nng As thp trong nc ngm (< 10

g/L).

Hnh 6. S bin i mu sc ca trm tch ly ln t mt s ging khoan khu vc

nghin cu v v tr cc ging tng ng

3.2.2. Nguyn nhn lin quan n tnh kh th hin nc ngm

Hnh 7. Tng quan gia As v Fe, Mn, SO42

, PO43

, NH4+, HCO3

, DOC, CH4, trong

nc ngm khu vc nghin cu

Kt qu nghin cu hnh 3.6 cho thy tng quan rt tt gia As v Fe, y l minh

chng cho vai tr quan trng ca s kh ha tan cc oxit Fe i vi qu trnh gii phng As

t trm tch vo nc ngm khu vc nghin cu. Fe trong trm tch tn ti trong cc

khong nh Fe oxit v nh hnh (ferrihydrite hay HFO), Fe oxit tinh th (geothite (

FeOOH), hematite (Fe2O3)), hay Fe oxit a ha tr (magnetite Fe3O4). Trong iu kin

kh mnh cc oxit ny c th b kh v Fe2+ vi s xc tc ca cc vi sinh vt. Hot ng ca

vi sinh vt ng vai tr rt quan trng trong s kh gii phng st v asen khi trm tch. Mt

s loi vi khun n st oxit trong mi trng kh, As theo thot ly khi b mt lp trm

tch b kh ny. St oxit b kh hoc do tng tc trc tip vi hp cht hu c, hoc ph

bin hn l do tc dng xc tc ca vi sinh vt s dng cht hu c lm cht nn. S tng

quan cht ch gia As v Fe cho thy y c th l qu trnh chnh lm gii phng As t trm

tch ra nc ngm khu vc nghin cu.

CH2O + 8H+ + 4Fe(OH)3 4Fe

2+ + 8HCO3

+ 3H2O

2Fe2O3.xH3AsO3+CH2O+7H+4Fe2++HCO3

+4H2O+2xH3AsO3

Tng quan nghch gia As v SO42

trong nc ngm l rt r rng. iu kin kh

mnh trong tng ngm nc dn n s kh SO42

theo phng trnh sau: 2CH2O + SO42

2HCO3 + H2S. Trong mi trng nhiu Fe

2+ phn ng xy ra nh sau: 9Fe2+ + SO4

2 +

20H2O 8Fe(OH)3 + FeS + 16H+. Pyrite c th c hnh thnh sau theo phng trnh

phn ng: FeS + H2S FeS2 + H2. Hu ht cc mu c nng As cao (> 150 g/L) th c

nng S l thp hn 1 mg/L. Mi tng quan ngc ny c th gi rng As c gii

phng t trm tch vo trong tng ngm nc di iu kin kh mnh, m khng phi l kt

qu ca qu trnh oxy ha sulphide.

S phn hy ca hp cht hu c l mt qu trnh quan trng to ra iu kin kh

trong nc ngm. ng thi qu trnh ny s lm xut hin amoni (NH4+) v HCO3

, cc hp

cht hu c ha tan (DOC) v CH4 trong nc ngm. iu ny c ngha nng cc cht

ny l cc ch th cho mc phn hy cht hy c v mc kh ca mi trng. S tng

quan thun ca As v DOC, NH4+

, HCO3 cng nh CH4 chng t s gii phng asen gn

lin vi qu trnh phn hy cht hu c. S phn hy cc cht hu c to ra mi trng kh

trong tng ngm nc, tip qu trnh kh ha tan cc Fe oxit lm asen b gii phng t cc

trm tch vo nc ngm.

3.2.3. Bng chng v s khc nhau tnh kh trong trm tch v nc ngm cc ging

c su khc nhau ti cng mt v tr nghin cu

Ngoi s phn b khng ng nht v hm lng As trong nc ngm theo khng

gian th nghin cu cn tm thy s phn b khng ng nht v hm lng As theo su.

3 ging khoan cng mt v tr nm trn mt ct giao gia hai khu vc c As v khu vc

khng c As c su khc nhau (25 m, 32 m, v 47 m) c s khc nhau ln v hm lng

As cng nh cc thng s ha hc trong nc ngm. Hm lng cc thng s ha hc nh

As, Fe, PO43

, CH4, NH4+, HCO3

trong nc ca 3 ging ny cng nh ca nc sng c

th hin hnh 3.8 Hu ht cc thng s ha hc ging nng 25 m u cao hn so vi 2

ging cn li, trong khi mu nc sng cc thng s ny l rt thp. Kt qu ny ging 25 m

th hin tnh kh mnh nht, trong khi mu nc sng c tnh oxy ha. S khc bit v mu

sc ca ct trm tch cng c quan st thy (hnh 3.9). Trm tch ging 25 m c mu xm

th hin tnh kh, trong khi cc trm tch ca hai ging cn li c mu vng. Hm lng As

cao trong nc ngm cc ging c trm tch mu xm ging 25 m l tng t vi cc

ging khc khu vc nghin cu.

Hnh 8. Hm lng As, Fe, PO43

, CH4, NH4+, HCO3

trong nc sng v trong nc ngm

ca 3 ging c cng v tr nhng c su khc nhau

Hnh 9. S khc nhau v trm tch v nng As ca 3 ging khoan cng mt v tr trn

mt ct giao gia hai vng c As v vng khng c As

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