 |
| Titel |
Dual isotopic approach for determining groundwater origin and water-rock interactions in over exploited watershed in India |
| VerfasserIn |
Philippe Négrel, Hélène Pauwels, Romain Millot, Stéphane Roy, Catherine Guerrot |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042054
|
|
|
|
|
|
| Zusammenfassung |
| Groundwater flow and storage in hard rock areas is becoming a matter of great interest and
importance to researchers and water managers either with regards to the quantity, quality of
water as well as delimitation of resources and aquifers. Degradation of groundwater resources
by abstraction, contamination, -¦ has been increasing in many areas and is of growing concern
for few decades. In terms of hydrogeology, hard rocks represent a quite heterogeneous and
anisotropic media with irregular distribution of pathways of groundwater flow, typically
consisting of three vertical zones, upper weathered, middle fractured and lower massive
bedrock.
Aim of this work is dual and the Maheshwaram watershed (53Â km2, Andhra Pradesh,
India) representative of watersheds in southern India in terms of geology, overpumping of its
hard-rock aquifer (more than 700 classical open end wells in use), its cropping pattern (rice
dominating), and its rural socio-economy mainly based on traditional agriculture is
investigated through stable isotopes of the water molecule and lead isotopes in groundwater.
The overall objective is to incorporate isotopic- and chemical-tracing data and constraints
into methods for evaluating groundwater circulation. It divides into fingerprinting the
groundwater recharge processes (e.g. the input by the monsoon) and the water use
in such agricultural watershed, which is of primary importance in such semi-arid
context and investigating the processes of water-rock interactions (e.g. granite-water
interaction).
In the frame of delimitation of resources and aquifers and long-term sustainability, we
monitored the input from monsoon-precipitation over 2 years, and measured spatial and
temporal variations in δ18O and δ2H in the groundwater and in precipitation. Individual
recharge from the two monsoon periods was identified. This led to identification of periods
during which evaporation affects groundwater quality through a higher concentration of salts
and stable isotopes in the return flow. In addition, such evaporation is further affected by land
use, rice paddies having the strongest evapotranspiration.
Lead concentrations span over one or two orders of magnitude up to approximately 20
μg. L-1. Pb-isotopes, measured in water by MC-ICPMS using an improved new procedure,
fluctuate largely as exemplified by the 206Pb/204Pb ratio, reaching values up to 25. Most of
the lead in the groundwaters is of geogenic origin, and through the lead isotopic signature in
groundwater we have traced and fingerprinted the processes of water-rock interactions
considering the granite matrix. Combining a weathering model and field observations, we
have defined a two step weathering process that includes a control on the Pb-isotopes ratios
by accessory phases and by the main mineral from the granite in a second step of
weathering.
For future studies, multi-isotope approach will be necessary for the identification of
possible flowpaths, in conjunction with the larger exploitation of the groundwater resources.
This is also challenging for generalising the use of isotope tools (such as Nd, Sr, Pb and
newly developed isotope systematics like Ca, Si-¦) in many other catchments that may face
structural problems of groundwater overdraft. |
|
|
|
|
|
|