This paper was presented to the Congreso CARA "Hydrogeology and Water Resources Management: Working together for the future", carried out on March 9-11, 2005 in Managua, Nicaragua. CARA (www.caragua.org) is the Central American Water Resources Management Network.
Introduction
One of the main environmental issues in Nicaragua is the natural content of arsenic in groundwater in the northeast and south-eastern regions (Estrada, F., 2002), close to mineralized areas, along the most active tectonic structure, the Depression of Nicaragua.
The Sébaco Valley is located in the outer eastern margin of the Depression of Nicaragua, covering 52 km2.
Go to Fig. 1
The valley is home for 15 communities with a total population of 3,225 people, (INEC, 1995) the annual population growth rate is 2.6%. High concentrations of arsenic have been detected in the water supply in the subwatershed southeast of the Valley.
General Aspects
The main source of arsenic in the subwatershed located to the southeast of the Sébaco Valley aquifer is associated to the hydrothermal processes originated from tertiary volcanism. The precipitates of hydrothermal fluids are the main source of arsenic. In the area of study the basement is closer to the surface, in a structure of fault blocks. The surfacing blocks present an intense hydrothermal alteration, shadowing their own original composition and texture, where faults and fractures occur close the flowing groundwater, being these the true conducts and sources for the pollutant to gain access to the aquatic medium present in quaternary alluvials and in fractured tertiary volcanic bedrock. The problem arises when the drilling for a drinking water wells are done directly on top of the hydrothermal alterations, faults or fractures containing the pollutant.
Research methodology
Physicochemical analyses were performed for groundwater and total arsenic in rock, soil and water were measured for this study. A geological survey was carried out in order to correlate the surrounding environment with the hydrothermal zone. A geophysical survey was performed measuring the magnetometry as a way to identify buried structures which could be linked to the high concentrations of arsenic found in the groundwater.
Results & Discussion
From the physicochemical point of view, the samples taken from 25 boreholes in the subwatershed located to the southeast of the Sébaco Valley are considered to contain water of good quality. The ionic concentration of these waters are less than those chosen as standard concentrations for comparison (CAPRE,1994).
16 of the 25 (64%) boreholes drilled for the study are classified as calcic bicarbonated waters associated to recharge zone waters, having their origins in the infiltration of the Viejo River and the underling Estelí Plateau. The steep profile in the mountains of the plateau have a hydraulic conductivity of 0.1 m/ day, which is evidence of higher groundwater velocity, but then these waters reduce its speed in the flatter portion of the valley to 0.01 m/ day. Nevertheless, the average speed of 0.06 m/ day indicates a relatively high velocity.
Of the 57 water samples taken, 21 show total arsenic concentrations between 10 and 122 µg l-1, which are above the guidelines established for drinking water (WHO, 1987). The highest concentrations of arsenic where found in the community of El Zapote. The total arsenic concentration in rocks and soil were 14.98 µg g-1 and 57.19
µg g-1 respectively, water concentrations were found to be 122.15
µg l-1. Cases of hydroarsenicism were reported for this community in the year 1996 from people drinking water from a well for a period of 6 months with an arsenic concentration of 1,320 µg l-1. People from this community were affected by irreversible health problems, which forced the populations to change its water supply from a dug well, which, according to findings from this study, has an arsenic concentration of 122.15 µg l-1, which is still above the guideline for drinking water.
Two comparative soil samples, located outside our area of study, one at the entrance of La India mine and the other among the surroundings of the community of Agua Fría, showed high arsenic concentrations similar to those of El Zapote community, with values of 95.2 and 59.5 µg g-1 respectively, giving evidence that the extension of arsenic pollution goes far beyond the area of study, following a north-western route in a 12 km wide path. This means that the population, besides being exposed to arsenic by ingestion, it is also being exposed to contaminated soil, which makes this region an environmentally critical zone.
Fig. 2: Total arsenic concentration & Redox Potential in groundwater of the subwatershed southeast of the Sébaco Valley-Nicaragua
Arsenic was not detected in the groundwater of the Communities of Las Mangas and Tatazcame which makes this area suitable for drilling and digging future drinking water wells to supply the nearby communities due that:
a) The community of Las Mangas is located to the north, outside the 12 km wide NW path were the basement is relatively deep (~70 m), and the wells would not be able to reach the hydrothermally altered zones rich in arsenic.
b) The community of Tatazcame is inside the NW path, where a microstructure with an alluvial cover is deep enough for the safe supply of drinking water locally. The arsenic concentration in this microstructure is below the detection limit (2.0 µg l –1)
Conclusions
The arsenic content in rocks, soil and water are associated to singenetic (primary) and epigenetic (secondary) hydrothermal processes evidenced along the mainly NE and NW faults and fractures characteristic of the eastern Nicaraguan geodynamics dominated by the subduction of the Cocos plate under the Caribbean plate, which is the drive for the formation of the Nicaraguan Depression where the Sébaco Valley is located in its outer eastern margin.
- The highest concentrations of arsenic are associated to secondary fault systems E-W, which favour the formation of microstructures that give the aquifers its unique properties, specially regarding the thickness and depth of the basement.
- Of the 57 water samples 21 show arsenic levels between 10 and 122 µg l-1 which are beyond the guidelines established for drinking water (WHO, 1987).
Fig. 3: Map showing the spatial distribution of arsenic concentration in the groundwater of the subwatershed located southeast of the Sébaco – Nicaragua Valley
- 93% of water samples contained arsenic as arsenate with an oxidation state of V and the remaining 7% was present as arsenide III, which is the most mobile and toxic form of arsenic.
- The distribution of arsenic present in soil it evidence of its natural origins. The exposure of the population to this contaminant is permanent, not only through drinking water, but also through their daily routines, increasing the toxicological risk.
References
Alessandro Ajuppaa, Walter D’Alessandrob,
Cinzia Federicob, Barbara palumboc, Mariano valenzaa. The aquatic geochemistry of arsenic in volcanic groundwaters from southern Italy.
Custodio, E & Llamas, R, 2001, Hidrología subterránea. Segunda Edición, Tomo I y
II Edición Omega, S, A. Barcelona España.
P.L. Smedleya, H. B. Nicollib, D:M:J: Macdonalda, A.J. barrosb,J.O.Tullioc. Hidrogeochemistry of arsenic and other inorganic constituents in groundwaters from Las
Pampas, Argentina.
Organization name: Centro para la Investigación en Recursos Acuáticos de Nicaragua -CIRA/UNAN
City: Managua
Country: Nicaragua
Web: http://www.geocities.com/cira_unan/informacion.htm
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