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Introduction

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)

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Introduction

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  1. ARSENIC ORIGIN DETERMINATION BY GEOCHEMICAL MODELING: REGION LAGUNERA AQUIFER SYSTEM, NORTHERN MEXICOCarlos Gutiérrez-Ojeda

  2. 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.”

  3. EDO. DE CHIHUAHUA EDO. DE COAHUILA DELICIAS TLAHUALILO GUANACEVI MAPIMI SAN PEDRO EL PALMITO GOMEZ TORREON PARRAS NAZAS VIESCA RODEO CUENCAME EDO. DE DURANGO EDO. DE ZACATECAS Geographic location of the Region Lagunera • 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

  4. Hydrologic Region # 36 "Nazas-Aguanaval"

  5. 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

  6. 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

  7. 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

  8. 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)

  9. Location of wells sampled

  10. Summary of GW samples with error<10% Note: Concentrations above the standards

  11. 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)

  12. Chloride levels in 1990 (mg/l) • Reported concentrations: • 4.3 to 709.7 mg/l • Drinking Water Standard • Cl < 250 mg/l

  13. Arsenic in different settings of the Region Lagunera (1/2)

  14. Arsenic in different settings of the Region Lagunera (2/2) • As sediments > As solution especially: • - evaporation zones (lagoonal deposits) • - mineralized areas (of igneous origin)

  15. Hypothesis of the presence of As • 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.

  16. Carbon-14 vs arsenic levels

  17. Under natural conditions

  18. Mean arsenic and chloride concentrations in the aquifer and reservoirs

  19. Surface water evaporation In order to evaluate this possibility two geochemical model were used: WATEQ4F and NETPATH • 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

  20. Chloride levels in 1990 (mg/l)

  21. Analytic data of Well 1387, Sample No. 96 and Mod96 (mg/l) Cl ratio = ClGW / ClSW = 199.87 / 3.88 = 51.51 51.51 lt of SW would produce 1 lt of GW

  22. Geochemical parameters of Well 1387 and Mod96 (from WATEQF) 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

  23. Ion exchange reactions _____ ______ Ca/Na EX: 2Na+ + Ca-X2 --> Ca2+ + 2Na-X _____ _____ Ca/K EX: 2K+ + Ca-X2 --> Ca2+ + 2K-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

  24. Results from NETPATH: First run Dissolution Precipitation + clay Chemical constrains Precipitation Incorporated into the clays Precipitation Dissolution Ca entering the aqueos solution

  25. Geochemical parameters of Sample No. 96 (from WATEQF) • Additional run: Evaporation option included in NETPAH • Initial Water SW (sample No 96) • Final Water GW (same) • Change Fluorite from undersaturated to oversaturated conditions

  26. Results from NETPATH (considering the evaporation option included in NETPATH). Most Plausible Ca/Na missing

  27. Conclusions (1/2) • 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. • 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).

  28. Conclusions (2/2) • 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. • Evaporation could be then considered an important factor of the presence of high As concentrations in the Region Lagunera. • 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.

  29. www.imta.gob.mx cgutierr@tlaloc.imta.mx

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