AN EXTRACTION REGIME FOR MANAGING GROUNDWATER WITH HIGH

CONCENTRATIONS OF FLUORIDES, IN THE SAN LUIS POTOSÍ BASIN

FT3.28 Antonio Cardona, Ma. Deogracias Ortiz Pérez, Pedro Medellín MilánUNIVERSIDAD AUTÓNOMA DE SAN LUIS POTOSÍ

In arid and semi-arid regions of Mexico, surface water is scarce and shallow groundwater from urban centers is commonly contaminated with wastewater. The major source for drinking water supply is DEEP GROUNDWATER

INTRODUCTION

TERTIARY (FELSIC AND INTERMEDIATE) VOLCANIC ROCKS OUTCROPPING IN MEXICO

BACKGROUND TO THE PROBLEM

WATER-ROCK INTERACTION PRODUCES MINERAL SOLUTION. THE EXTENT OF REACTION IS

CONTROLLED BY LONG RESIDENCE TIMES AND TEMPERATURE. SILICATE ROCKS GIVE RISE TO SOFT

WATERS, BUT TRACE ELEMENTS ARE RELEASED PRODUCING EXCESS IN DRINKING WATER

WELLS TAPPING DEEP AQUIFERS (FRACTURED VOLCANIC ROCKS) EXCLUDE NEAR/SURFACE POLLUTION, BUT MAY

ENCOUNTER WATER ENRICHED WITH NATURAL RELEASED TOXIC ELEMENTS SUCH AS: FLUORIDE, ARSENIC, URANIUM, LEAD, CADMIUM, AMONG OTHERS.

PRECI PI TACI ON ANUAL (mm)

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San Luis Potosi City located in the semi-arid region of Mexico, is one of the conurbations of the country with the highest rate of growing (broadly 5-7% per year), at present with around 900,000 inhabitants.

14,57,0 8,3

18,8

38,7

68,4 64,956,7

65,9

30,0

9,3 12,4

0102030405060708090

100

Lám

ina

de ll

uvia

en

m

Ene Feb

Mar

Abr

May

Jun

Jul

Ago

Sep Oct

Nov Dic

Precipitacion Promedio Mensual

SAN LUIS POTOSI

SOLEDAD DE GRACIANO SANCHEZ

Significant amounts of FLUORIDE are found in groundwater of San Luis Potosíclosed drainage basin. Higher fluoride levels are between 3.5 and 4.0 mg/l. Health effects have been identified in people consuming this water.

Drinking water71%

Irrigation17%

Industrial 8%

Services4%

Total extraction from deep sources 120x106 m3/year

Q

Tp

ALUVIÓN

GRUPO IGNIMBRITICO

Ti Td DOMOS RIOLITICOS

K CALIZAS

MARGEN DE CALDERA

ZONA DE FUENTE

FALLA NORMAL

ANTICLINAL RECOSTADO

CONTACTO GEOLÓGICO

Deep aquifer (basin fill and fractured volcanics)

Shallow aquifer (basin fill material)

Hydrogeologic basement (limestones and clays)

AQUIFER SYSTEM

1 cm

Fractured volcanics rocks with fluoride rich minerals (topaz, Al2SiO4 (F,OH)2; apatite, Ca5(PO4, CO3)3(F, OH, Cl)

TOPAZ

TOPAZ

APATITE

Natural Fluoride content in volcanic rocks of the San Luis Potosi region

1005

358

592 608 595

2651

1032

116

649

0

500

1000

1500

2000

2500

3000

Fluo

ruro

tota

l en

roca

(ppm

)

AndesitaCasitaBlanca(Lava)

IgnimbritaSanta Maria

LatitaPortezuelo

(Lava)

IgnimbritaCantera(Toba)

IgnimbritaCantera

Riolita SanMiguelito(Domo)

Riolita SanMiguelito

(Lava)

RiolitaPanalillo

(Ignimbrita)

Materialgranular

ROCKS WITH TOPAZ AND APATITE CRYSTALS

2000

Rumbo N 75° W viendo al NE

1750

N 78° W viendo al NE

1500

N 65° W viendo al NEI

III-III'

I-70I-73 I-68I-72I-71

I-56I-60 I-64

I-2CNA-260

I-3

IV-IV'

D-D'II-II'

CNA-871I-5 I-13CNA-860

CNA-867

I-15 I-21I-11

I-22I-18

I-20I-79

B-B'

Santa Rita

N 26° W - NE

I

Medio fracturado

Conductividad electrica (mmohs/cm)

Temperatura (°C)

Fluoruro (mg/lt)

Sodio (mg/lt)

Litio (mg/lt)

Nitratos (mg/lt)

kilometros5 10 15 20

0.01

0.1

1.0

10

100

1250

1000

750

500

250

0

I-73I-72

I-70I-71

I-68

I-60I-68 I-64 I-2

CNA-260I-3

I-5 CNA-871

CNA-860

I-11CNA867 I-13 I-15

I-21I-22

I-18I-20

I-79 Santa Rita

SECCIÓN HIDROGEOQUÍMICA I-I´

Basamento

Basamento

Medio fracturadoMedio fracturado

Medio granular

Medio granular

EXPLICACIÓNMEDIO GRANULAR

MEDIO FRACTURADO

BASAMENTO

Drilling deep wells in fractured volcanic rocks guarantee high production but also high fluoride concentration

FLUORIDE CONCENTRATION IN DEEP AQUIFER

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Litio (mg l-1)

Fluo

uro

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Intermedio

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CASO (c)

CASO (e)

CASO (b)

CASO (d)

CASO (a)

FLUORIDE CONCENTRATIONS IN GROUNDWATER FROM DEEP AQUIFER

20.0

25.0

30.0

35.0

40.0

45.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Fluoruro (mg l-1)

Tem

pera

tura

(ºC

)

RegionalIntermedioMezcla

T= 3.5622F +25.005Corr.= 0.799

Concentración máxima de fluoruro permitida de acuerdo con la NOM-127-SSA1-

1994

FLUORIDE CONCENTRATIONS RELATED WITH WATER TEMPERATURE

Hidrogeological conditions favoring high (above 2.5 mg/l) fluoride concentration and water

temperature above 34oC

F >2.5 mgl

F <1.0 mgl

HIGH Vertical Hydraulic Conductivity Kvf

LOW Horizontal Hydraulic Conductivity (KhTGU)

0.0

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4.0

0.00 0.05 0.10 0.15 0.20 0.25

Litio (mg l-1)

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uro

(mg

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Regional

Intermedio

Mezcla

CASO (c)

CASO (e)

CASO (b)

CASO (d)

CASO (a)

Hidrogeological conditions favoring low fluoride (F<1.0

mg/l) concentration and water temperature below 28oC

HIGH Horizontal Hydraulic Conductivity (KhTGU)

HIGH Vertical Hydraulic Conductivity Kvf

0.0

1.0

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uro

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Mezcla

CASO (c)

CASO (e)

CASO (b)

CASO (d)

CASO (a)

Horizontal Hydraulic Conductivity (KhTGU)

Vertical Hydraulic Conductivity Kvf

Hidrogeological conditions favoring VARIABLE fluoride concentration and

water temperature between 28-35oC

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uro

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Mezcla

CASO (c)

CASO (e)

CASO (b)

CASO (d)

CASO (a)

Hidrogeological conditions favoring medium fluoride (F<2.0 mg/l)

concentration and water temperature below 30oC

Horizontal Hydraulic Conductivity (KhTGU)

Vertical Hydraulic Conductivity Kvf

0.0

1.0

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uro

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Mezcla

CASO (c)

CASO (e)

CASO (b)

CASO (d)

CASO (a)

Hidrogeological conditions favoring HIGHfluoride (F>2.0 mg/l) concentration and

water temperature above 34 oC

Vertical Hydraulic Conductivity Kvf

Horizontal Hydraulic Conductivity (KhTGU)

0.0

1.0

2.0

3.0

4.0

0.00 0.05 0.10 0.15 0.20 0.25

Litio (mg l-1)

Fluo

uro

(mg

l-1)

Regional

Intermedio

Mezcla

CASO (c)

CASO (e)

CASO (b)

CASO (d)

CASO (a)

1) Avoidance or limiting pumping in regions of the aquifer known to be contaminant-rich (fractured volcanic rocks).

2) Appropriate mixing between water with low fluoride (intermediate flow systems) and thermal fluoride rich water (regional flow systems) keeps fluoride within acceptable drinking water limits, in some cases the well lithology and its construction design may also control the resulting mixture.

LESSONS LEARNED

LESSONS LEARNED

3) Use solubility controls on the in siturelease of fluoride, for example a natural fluoride control may be postulated increasing calcium and lowering water temperature, this can be done with a proper design and construction of the well in a specifichydrogeological setting.

4) Well-head tests should be devised to determine particular operation conditions in each well, in order to keep low fluoride concentrations well-head discharge temperature should be lower than 28 and 30o C.