B. S. Sukhija

Differentiation of palaeomarine and modern seawater intruded salinities in coastal groundwaters (of Karaikal and Tanjavur, India) based on inorganic chemistry, organic biomarker fingerprints and radiocarbon dating

Abstract Hydrosalinity evolution is examined in the Karaikal and Tanjavur coastal aquifers situated about 300 km south of Madras, Tamil Nadu, India. Ancient and modern sources of solutes in groundwaters have been differentiated using hydro-geochemical data, environmental radioisotope dates and organic biomarkers. Results from different techniques show consistently that the Pliocene aquifer possesses characteristic palmitoleic and oleic acid biomarkers, apparent 14 C ages of more than 20 000 years and high I/Cl ratio; the modern marine-estuarine intruded groundwaters in the alluvial aquifer display relatively younger apparent 14 C ages (modern to 3000 years), enriched potassium ion, low I/Cl ratio and contain vaccenic and hopanoic acid biomarkers, and they also tend to display transient changes in chemical characteristics. These results have practical importance with regard to identifying and controlling seawater intrusion in coastal areas.

Validity of the environmental chloride method for recharge evaluation of coastal aquifers, India

The work presented in the paper was carried out with the objective of ascertaining the validity of environmental chloride method for evaluation of groundwater recharge to the coastal aquifers situated in semi-arid tropical area under the influence of monsoonic type of hydrologic regime. Towards this end a comparative study, using environmental chloride in conjunction with injected tritium method (the more commonly used method), was undertaken in the coastal aquifers of Pondicherry situated, about 160 km south of Madras, along the coast of Bay of Bengal. Five each of environmental chloride (repeated twice) and injected tritium profiles were studied in the alluvium and Cuddalore aquifers of Pondicherry. Recharge from the chloride study was estimated using (1) concentration approach, which involves estimating the relative enrichment of soil chloride concentration in relation to the input chloride, and (2) flux approach, determining the turnover time of soil moisture by estimating the total chloride in the profile and yearly input. Recharge from the injected tritium profiles was estimated by studying the displacement of soil moisture during the time interval between sampling and injection of the tracer. Repeated chloride profiles show fairly good reproducibility (within 15–30%) of recharge rates determined by the two approaches for two different sampling times at the same site. Also, the results compare well among the two approaches used for the chloride method. Mean recharge rates obtained by the injected tritium method for the Cuddalores is about 26 cm yr−1 and by the chloride method about 22 cm yr−1; and for the alluvium the respective rates are 10 and 14 cm yr−1. This shows fairly good agreement between the two methods. The study, thus, demonstrates conformity of recharge rates determined by the environmental chloride method and injected tritium technique and proves the validity of the chloride method for recharge measurements for tropical coastal aquifers situated in semi-arid regions.

Paleoliquefaction evidence and periodicity of large prehistoric earthquakes in Shillong Plateau, India

Abstract The tectonic setting and the occurrence of the great Assam earthquake (M=8.7) of 1897 in the Shillong Plateau succeeded by three great earthquakes (1905, 1934 and 1950) in the adjoining Himalayan frontal arc, indicates the vulnerability of the Shillong Plateau to large earthquakes. The lack of seismicity records of the region earlier than 100 years and data on the recurrence of damaging earthquakes led us to investigate the paleoseismicity of the Shillong Plateau. Our paleoseismic investigations in the meizoseismal area of the 1897 earthquake revealed well-preserved liquefaction and deformed syndepositional features at 10 selected sites in the alluvial deposits along two north flowing tributaries of the Brahmaputra river. These features are 14C dated using associated organic samples. As the liquefaction of sediments was an important feature of the 1897 earthquake, we identified this seismic event, and other large/major prehistoric earthquakes through paleoliquefaction and other coseismic structural deformation at the investigated sites. In addition to the 1897 event, we provide geological evidence for at least three large seismic events. Two of them occurred during 1450–1650 and 700–1050 AD, the third predates 600 AD. The analysis of the 14C data suggests a return period of about 400–600 yr for the large earthquakes in the Shillong Plateau. This finding is the first of its kind from the Himalaya and adjoining region.

Timing and return period of major palaeoseismic events in the Shillong Plateau, India

Abstract The close temporal occurrence of four great earthquakes in the past century, including the great Assam earthquake of 1897 in the Shillong Plateau, necessitated examination of the palaeoseismicity of the region. The results from such investigation would definitely aid in addressing the problem of the earthquake hazard evaluation more realistically. Our recent palaeoseismological study in the Shillong Plateau has led us to identify and provide geological evidence for large/major earthquakes and estimate the probable recurrence period of such violent earthquakes in parts of the Shillong Plateau and the adjoining Brahmaputra valley. Trenching along the Krishnai River, a tributary of the River Brahmaputra, has unravelled very conspicuous and significant earthquake-induced signatures in the alluvial deposits of the valley. The geological evidence includes: (1) palaeoliquefaction features, like sand dykes and sand blows; (2) deformational features, like tilted beds; (3) fractures and syndepositional deformational features, like flame structures caused by coeval seismic events. Chronological constraints of the past large/major earthquakes are provided from upper and lower radiocarbon age bounds in the case of the palaeoliquefaction features, and the coeval timing of the palaeoseismic events is obtained from the radiocarbon dating of the organic material associated with the deformed horizon as well as buried tree trunks observed wide distances apart. Our palaeoseismic measurements, which are the first from the area, indicate that the Shillong Plateau has been struck by large/major earthquakes around 500±150, 1100±150 and >1500±150 yr BP, in addition to the well-known great seismic event of 1897, thereby the 14 C dates indicate a recurrence period of the order of 500 yr for large earthquakes in the Shillong Plateau.

ENVIRONMENTAL AND INJECTED TRACERS METHODOLOGY TO ESTIMATE DIRECT PRECIPITATION RECHARGE TO A CONFINED AQUIFER

Abstract Direct precipitation recharge to a confined aquifer is a vital parameter for groundwater budgeting, management, and modelling. We describe the conjunctive use of environmental isotopes and geochemical tracer technique, and injected tracer method to estimate direct precipitation recharge to an important coastal aquifer undergoing large withdrawal stress. The methodology evolved delineates the intake area of the confined aquifer based on environmental isotope and geochemical data and then utilises the recharge rates of the intake area only, as determined using injected tracer and geochemical data, to estimate the amount of recharge by direct precipitation to the confined aquifer.