arsenic origin determination by geochemical modeling: region lagunera aquifer system, northern...
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ARSENIC ORIGIN DETERMINATION BY GEOCHEMICAL MODELING:
REGION LAGUNERA AQUIFER SYSTEM, NORTHERN MEXICO
Carlos Gutiérrez-Ojeda
Introduction
• Groundwater with elevated As concentrations reported since 1962
• Originated health problems (people and animals)
• Unconfined alluvial aquifer is the main source of drinking water
• Hydrogeochemical and isotopic study (IMTA, 1990):
• Extensive areas have elevated As concentrations:
range of 0.003-0.443 mg/l
• Potential sources:
• Extinct intrusive hydrothermal activity
• Use of arsenical pesticides
• Mining activities
• Sedimentary origin
“Geochemical modeling was used in this work to show that surface water evaporation could have contributed to the elevated As conc.”
• Situated in a broad closed basin located in the central part of northern Mexico
• It covers a total area of about 12,000 km2 of Coah., Dgo and Zac., states
• 90,000 ha are irrigated every year with SW and GW
• Main cities are Torreón, Gomez Palacio and Lerdo (2 million inhabitants)
• Climate typical of the arid regions of Northern Mexico:
very dry
Temperature 25°C/year
Precipitation: 220 mm/year
Evaporation: 2,400 mm/year
Geographic location of the Region Lagunera
EDO. DE CHIHUAHUA
EDO. DE COAHUILA
GUANACEVI
EL PALMITO
NAZAS
RODEO
CUENCAME
EDO. DE DURANGO
EDO. DE ZACATECAS
PARRAS
DELICIAS
SAN PEDRO
VIESCA
TORREON
MAPIMI
GOMEZ
TLAHUALILO
Hydrologic Region # 36 "Nazas-Aguanaval"
Surface Water Nazas and Aguanaval rivers Nazas
Covers 63% of the total area Main source of surface water
Regulated by:
o Palmito or Fco Zarco (1968)
2,778 Mm3
o L. Cárdenas (1946) 235 Mm3 o About 600 – 830 Mm3 are
used for agriculture every year
Estimated water table elevation under natural conditions (masl)
Groundwater (aquifer):
• Unconfined type
• composed of alluvial and lacustrine deposits of sedimentary origin
• extends 50% of the total area
• contributes 50% of all water used for domestic, agricultural and industrial needs
• sources of recharge: Nazas river and infiltration of excess irrigation
• Natural flow directions: SW -> NE
Water table elevation in 1991 (masl)
Groundwater (aquifer):
• Aquifer overexploitation has caused:
• drawdowns of more than 100 m in less than 50 years.
• migration of GW with elevated As concentrations.
• 1986: abstraction was three times greater than recharge
• Abstraction 1000 Mm3/year by more than 4,800 wells
• Recharge 300 Mm3/year
• General flow directions: from the borders to the central portion of the region
Geologic map
The region contains rocks from Paleozoic to Quaternary:
• Cretaceous Limestones: broadly distributed in the area
• Instrusive Igneous rocks: “El Sarnoso”
• Extrusive igneous rocks: rhyolite to basalts
• Quaternary deposits
Quaternary sedimentologic model
Quaternary deposits:
• Alluvial: broadly distributed
• Lacustrine: clay and silt located in the lowest part of the basin
• Alluvial fan sediments: clastic material (gravel and sand)
Location of wells sampled
Parameter Min. Mean Max. SD Drinking Water Standard
T (C) 22 42
pH 6.91 7.54 8.50 0.37
Conductivity (μS/sec) 299 1,663 15,650 2,004
TDS (mg/l) 205 1,274 14,210 1,836 500
Hardness (mg/l) 14.46 485.63 2,281.55 545.92 120
Alkalinity as CaCO3 (mg/l) 38.00 129.20 316.00 46.48 600
Cl (mg/l) 4.30 72.00 709.74 106.15 250
SO4 (mg/l) 29.37 674.55 7,384.81 1,043.87 250
HCO3(mg/l) 46.40 157.02 385.60 55.48 50-350
NO3 (mg/l) 0.50 50.82 508.93 80.34 10
Na (mg/l) 19.90 219.61 3,972.00 455.01 100
K (mg/l) 1.15 5.77 16.54 3.56 100
Ca (mg/l) 3.73 142.85 633.26 157.01 100
Mg (mg/l) 0.28 31.25 196.87 42.93 30-40
F (mg/l) 0.07 2.48 6.97 1.41 1.5
B (mg/l) 0.11 0.67 6.01 0.81 5
Li (mg/l) 0.02 0.10 0.34 0.07 0.05
Fe (mg/l) 0.04 0.15 3.72 0.47 0.30
Pb (mg/l) < 0.20 0.20 0.20 0.00 0.05
Mo (mg/l) < 0.20 0.20 0.20 0.00 0.05
Hg (mg/l) 0.00 0.01 0.15 0.03 0.002
As (mg/l) 0.00243 0.07497 0.44300 0.10044 0.05
Note: Concentrations above the standards
Summary of GW samples with error<10%
Total arsenic levels in 1990 (mg/l)
• Reported concentrations:
0.003 to 0.443 mg/l
• Drinking Water Standard
As < 0.05 mg/l (1991)
As < 0.025 mg/l (2010)
Chloride levels in 1990 (mg/l)
• Reported concentrations:
4.3 to 709.7 mg/l
• Drinking Water Standard
Cl < 250 mg/l
Description As range
Agricultural soils (1992) 250-369 ppm
Abandoned Mines La Campana (1992)Tlahualilo (1992): SEl Mexicano (1992): Au & AgLa Bonanza (1992): Pb,Ag & SrLa Zorra (1992): Pb, Au & AgLa Zorrita (1992): FeDinamita (1992): Fe
(ppm)109-15091-184
224-2,400193-331
180-2,930115-5,680132-836
Santa Maria Mine (1992): Pb & Zn 1,840-97,000 ppm
Geologic formations Limestone "Fenix hill" (1992)Limestone (1983)Chloritized rock (1983)Igneous rocks (1983)Marble breccia (1993)
(ppm)70-12044-450
42033
2100
Talmessite mineral (1983) 22,000-87,000 ppm
Arsenic in different settings
of the Region Lagunera (1/2)
Description As range
Surface water from Reservoirs L Cárdenas (1990)F. Zarco (1990)F. Zarco (1992)Entrance of main canal (1992)Main canal at 11 + 420 (1992)
(mg/l)0.00657-0.017660.00739-0.01270
0.002410.002010.00665
Groundwater CINVESTAV (1986)IMTA (1990)
(mg/l)0.008-0.624
0.00243-0.4430
Sediments L. Cárdenas (1992)F. Zarco (1992)Entrance of main canal(1992)Nazas River (1992)Evaporation Zones (1992)Piedmonts (1992)
(ppm)100-210
22077-8334-218
129-34511-213
Arsenic in different settings
of the Region Lagunera (2/2)
As sediments > As solution especially:
- evaporation zones (lagoonal deposits)
- mineralized areas (of igneous origin)
1) Mining : Peñoles
+ process 430 ton/day of lead minerals which are associated with As—species.
+ produces 600—700 ton/month of “Speiss” mineral which consist of:
54 % Cu
18 % Pb
30 % As
2) Pesticides
+ herbicides —> arsenites
+ Insecticides —> arseniates
3) Alluvial sediments with As: Settled down in the basin as a result of the erosion - transport - deposition process
4) Hydrothermal aqueous system: extinct magmatic process that originated the intrusive and extrusive Igneous rocks. High concentrations of As, Li, Cl, S04, can be the result of such a kind of processes.
Hypothesis of the presence of As
Carbon-14 vs arsenic levels
River flowed from Mountains to Lagoons
Most water of the lagoons was mainly evaporated; the balance was infiltrated (very concentrated)
Lagoons were very broad > 200 km2 during flood times (2,000 Mm3)
High As-levels in the lagoonal Deposits
Volcanic origin of the As; transport by surface flow
Evaporation could have significantly increased the soluble As-levels
Evaporation effects can be observed from dam to dam
Under natural conditions
Mean arsenic and chloride concentrationsin the aquifer and reservoirs
Description Cl (mg/l) As (mg/l)
Fco Zarco Reservoir 4.80 0.00962
L Cárdenas Reservoir 3.80 0.00781
Aquifer 72.57 0.07410
Reservoir ratios (%) 126.32 123.81
1) select two samples
1 surface water sample No. 96 from L. árdenas
dam 1 groundwater lagoonal areas:
well 1387
2) artificially evaporate the SW by multiplying it by the Clgw/Clsw ratio.
3) Determine the SI of selected minerals -> WATEQ4F
4) Determine the possible geochemical mass-balance reactions between the two water samples by considering:
+ precipitation / dissolution of minerals + ion exchange of elements+ C02 outgassing
In order to evaluate this possibility two geochemical model were used: WATEQ4F and NETPATH
Surface water evaporation
Chloride levels in 1990 (mg/l)
GW SW Evap - SW
Element Well 1387 Sample No 96 Mod96
Ca 146.70 27.37 1,409.91
Mg 13.21 2.13 109.72
Na 542.26 10.83 557.88
K 7.12 3.84 197.81
Li 0.20 0.02 1.03
HCO3 168.40 112.20 5,779.75
Cl 199.87 3.88 199.87
SO4 1,179.38 10.54 542.95
F 3.22 1.13 58.21
NO3 43.02 3.38 174.11
CO3 7.20 --- ---
As 0.19250 0.00657 0.34
B 1.43 0.25 12.88
Fe 0.004 0.04 2.06
Pb 0.020 0.20 10.30
pH 8.10 8.10 8.10
FINALWATER
NETPATH INITIAL WATER
Cl ratio = ClGW / ClSW = 199.87 / 3.88 = 51.51
Analytic data of Well 1387, Sample No. 96
and Mod96 (mg/l)
51.51 lt of SW would produce 1 lt of GW
Description Well 1387 Mod96 Comments
Calcite 0.666 2.907 Precip. Only
Gypsum -0.634 -0.436 Dissol. Only
Fluorite 0.295 3.472 Precip. Only
pCO2 (atm) 0.00125 0.0314 CO2 Outgassing
I 0.04281 .011802 Debye-Huckel eqn.
CI (%) -5.58 -8.04
FINAL INITIAL
Calcite, Gypsum and Flourite Minerals that normally occur in lagoonal
deposits (Rankama & Sahama, 1949)
Other minerals
Aragonite Unstable at normal P & T (Drever, 1988)
Magnetite Metamorphic rock
Dolomite Unstable ( Calcite + Mg + CO2 )
Halite No geological evidence
Geochemical parameters of Well 1387
and Mod96 (from WATEQF)
_____ ______
Ca/Na EX: 2Na+ + Ca-X2 --> Ca2+ + 2Na-X
_____ _____
Ca/K EX: 2K+ + Ca-X2 --> Ca2+ + 2K-X
_____ _____
Ca/Mg EX: Mg2+ + Ca-X2 --> Ca2+ + Mg-X2
Ca Normally present in clays of the area.
It is expected to enter the aqueous solution
Excess of Na, K & Mg incorporated into the clays
Ion exchange reactions
Chemical constrains
Ca entering the aqueos solution
Precipitation
Dissolution
Initial Well : Mod96 Final Well : Well 1387
Final Inicial C 2.7290 89.7870 S 12.3060 5.7040 CA 3.6690 35.4990 F 0.1700 3.0920 MG 0.5450 4.5540 K 0.1830 5.1050 NA 23.6410 24.4880
Model 1 CALCITE ‑ ‑43.86450 FLUORITE ‑ ‑1.46100 GYPSUM + 6.60200 CO2 GAS ‑43.19350 Ca/Na EX 0.42350 Ca/K EX 2.46100 Ca/Mg EX 4.00900
1 models were tested. 1 models were found which satisfied the constraints.
Results from NETPATH: First run
Precipitation Precipitation + clay
Dissolution
Incorporated into the clays
Evaporation of 51.51 lt of SW would produce 1 lt of GW:
Precipitation of CaCO3 and CaF2 Dissolution of CaSO4 • 2H2O Outgasing of CO2 1 mmol of Ca entering the aqueous solution
per every x mmol of Na, K, Mg leaving the aqueous solution.
Description Sample No 96
Calcite 0.201
Gypsum -2.789
Fluorite -0.852
pCO2 (atm) 0.000888
I 0.00308
CI (%) -7.66
Additional run: Evaporation option included in NETPAH
Initial Water SW (sample No 96)
Final Water GW (same)
Change Fluorite from undersaturated to oversaturated conditions
Geochemical parameters of
Sample No. 96 (from WATEQF)
Most Plausible
Ca/Na missing
Initial Well : SAMPLE # 96 : ORIGINAL Final Well : WELL # 1387
CS
CAF
MGK
NA
Final2.729012.30603.66900.17000.54500.183023.6410
Initial1.84300.11000.68300.05900.08800.09800.4710
MODEL 1CALCITE ‑FLUORITE ‑GYPSUM +CO2 GASCa/Na EXCa/K EXEvaporation factor:
‑1.93795‑0.0977011.62475‑6.74708‑10.362010.211976.193
MODEL 2CALCITE ‑ FLUORITE ‑ GYPSUM + CO2 GAS Ca/K EX Ca/Mg EX Evaporation factor:
‑42.24198‑1.395706.78475
‑47.535102.367973.8720050.193
MODEL 3CALCITE ‑ FLUORITE ‑ CO2 GAS Ca/Na EX Ca/K EX Ca/Mg EX Evaporation factor:
‑98.74042‑3.21525
‑104.7120214.525535.390269.29980111.873
MODEL 4FLUORITE ‑ GYPSUM + CO2 GAS Ca/Na EX Ca/K EX Ca/Mg EX Evaporation factor:
‑.0352911.85747‑4.78585‑10.86025
.10830‑.186184.078
7 models were tested. 4 models were found which satisfied the constraints.
Results from NETPATH (considering the
evaporation option included in NETPATH).
1. It is termodynamically possible to get the high As-levels observed in the groundwater of the lagoonal deposits of the Región Lagunera from surface water evaporation.
2. The geochemical modeling (WATEQF and NETPATH) results show that evaporation of 51.51 liters of surface water from the Nazas River would produce one liter of well 1387; this would require the precipitation of calcite (43.86 mmol) and fluorite (1.46 mmol), dissolution of gypsum (6.60 mmol), outgassing of CO2 (43.19 mmol), and an exchange of 1 mmol of calcium (entering the aqueous solution) per every 0.42 mmol of sodium, 2.46 mmol of potassium and 4.01 mmol of magnesium (leaving the aqueous solution).
Conclusions (1/2)
3. The resulting As conc. (0.34 mg/l) altough not exactly the same as in well 1387 (0.1925 mg/l) could then be later subject to adsorption process.
4. Evaporation could be then considered an important factor of the presence of high As concentrations in the Region Lagunera.
5. Additional information (C-13. C-14, S-34) is required to corroborate the possibility of the suggested reactions paths along the flow line from the lagoonal deposits to the center of the aquifer.
Conclusions (2/2)
