Assessing hydrochemical evolution of groundwater in limestone terrain via principal component analysis |
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| Authors: | Zargham Mohammadi |
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| Institution: | (1) Department of Earth Science, College of Science, Shiraz University, 71454 Shiraz, Iran |
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| Abstract: | This paper describes the use of multivariate statistical analysis to trace hydrochemical evolution in a limestone terrain
at Zagros region, Iran. The study area includes a deep confined aquifer, overlaid by an unconfined aquifer. The method involves
the use of principal component analysis (PCA) to assess and evaluate the hydrochemical evolution based on chemical and isotope
variables of 12 piezometers drilled in both the unconfined and confined aquifers. First PCA on all variables shows that water–rock
interaction under different conditions with respect to the atmospheric CO2 is the main process responsible for chemical constituents. As a result, combinations of several ratios such as Ca/TDS, SO4/TDS and Mg/TDS with physico-chemical and isotope variables reveal different hydrochemical evolution trend in the aquifers.
Second PCA on the selective samples and variables reveals that displacement of the unconfined samples from dry to wet season
follows a refreshing trend towards river samples that is characterized by reducing electrical conductivity and increasing
sulphate and tritium contents. However, the refreshing trend cannot be traced in the confined aquifer samples suggesting no
recharge from river to the confined aquifer. Third PCA reveals that, chemical composition of water samples in the unconfined
aquifer tends to have considerable difference from each other in the end of recharge period. In contrast, the confined aquifer
samples have a tendency to show similar chemical composition during recharge period in comparison to end of dry period. This
difference is caused by different mechanism of recharge in the unconfined aquifer (through the whole aquifer surface) and
the confined aquifer (through the limited recharge area). |
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