However, the aquifers are found to be in redox disequilibria with multiple overlapping redox zones as indicated by the co-presence of multiple redox-sensitive solutes. The reducing nature of groundwater within the delta is also indicated by the prevalence of CH 4 and S 2− coupled with low dissolved oxygen content ( Mukherjee and Fryar, 2008). The groundwater is mostly found to be postoxic in nature, and Fe(II)/Fe(III) is found to be the most predominant redox couple within the aquifers. Fe–S–C redox cycles strongly controls the aquifer redox states across the delta. The aquifer redox states within the GBM river delta are found to be concurrently controlled by multiple geochemical processes that are hydrostratigraphy and depth dependent ( Mukherjee and Fryar, 2008). The solute concentrations are found to vary with depth, for example, Fe, Mn, and SO 4 2− concentrations are found to decrease with depth, while CH 4 shows an increasing trend ( Mukherjee and Fryar, 2008). On average, the delta groundwater is also characterized by the dominant presence of Fe and Mn as minor solutes along with a very low concentration of SO 4 2− and NO 3 − and undetectable levels of NO 2 − and H 2S ( Mukherjee and Fryar, 2008 Mukherjee et al., 2008). These highly brackish/saline waters represent entrapped seawater from the proto-BoB, which have undergone limited diagenetic evolution through interaction with aquifer sediments and fresher groundwater leakage ( Mukherjee and Fryar, 2008). The isolated aquifer lenses majorly host connate waters dominated by Na + and Cl − ions. The semiconfined and poorly connected aquifers show significantly higher proportions of brackish/saline water with a higher concentration of Na + and Cl − ions indicating contribution from marine source (i.e., BoB). The multilayered aquifer system to the south shows a somewhat varied hydrogeochemical signature, broadly falling within the Ca–Na–HCO 3 −–Cl − facies. The HCO 3 − ions are believed to be majorly derived from silicate weathering and oxidation of organic matter along with carbonate dissolution and root respiration ( Mukherjee and Fryar, 2008). The Ca 2+ ions are suggested to be introduced into the groundwater majorly through carbonate dissolution, while the Mg 2+ ions are derived from both carbonate dissolution and silicate weathering. The unconfined continuous main aquifer system to the north of the delta is mostly dominated by Ca–HCO 3 −-rich groundwater, with the predominance of Ca + and Mg + and HCO 3 − ions as major solutes. The multi-depth hierarchical groundwater flow system within the delta has resulted in highly heterogeneous hydrogeochemical signatures that vary both spatially and vertically across the GBM River delta.Ī total of seven distinct hydrogeochemical facies have been reported from the GBM river delta (namely, Ca 2+–HCO 3, Ca 2+–Na +–HCO 3, Na +–Ca 2+–HCO 3, Ca 2+–Na +–HCO 3–Cl, Na +–Cl–HCO 3, Na +–Cl, and Ca 2+–Cl), which were found to vary both spatially and vertically ( Mukherjee and Fryar, 2008). The groundwater in the main aquifer undergoes hydrogeochemical evolution along the groundwater flow paths by water–sediment interactions that are majorly dominated by carbonate dissolution and cation exchange along with a substantial influence of silicate weathering processes, all of which are guided by the aquifer redox states ( Mukherjee and Fryar, 2008). A significant fraction of the groundwater is also recharged as the irrigation return flow through the extensive stretches of agricultural fields that are cultivated using water-intensive irrigation practices. The shallow aquifers are dominated by relatively young groundwater that bears groundwater isotopic (δ 18O ) signature of meteoric recharge coupled with some meteoric diagenesis and evaporation loss ( Mukherjee, 2006 Mukherjee et al., 2007b). The groundwater chemistry of the GBM River delta aquifer system is controlled by the hydrostratigraphy and the groundwater flow paths and hence varies across the delta. Kazi Matin Ahmed, in Global Groundwater, 2021 10.5 Hydrogeochemistry
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |