CONTROLS ON GROUNDWATER CHEMISTRY AND RECHARGE ON THE KARSTIFIED RIM OF THE MEGHALAYA PLATEAU, INDIA

The Meghalaya Plateau (maximum elevation 1964 m asl) is a “pop-up” block marking the northeastern limit of the Indian craton. The plateau’s southern escarpment (~1200–1500 m high) is one of the rainiest places on Earth, with annual rainfall at Cherrapunji (Sohra) averaging 11706 mm/yr (1973–2017). Well-developed karst drainage networks occur where the Sylhet Limestone and Therria Sandstone overlie crystalline rocks. However, studies of groundwater chemistry and recharge in the area have been limited.

We monitored cave streams, springs, and surface streams from February through May 2023 (pre-monsoon season) for field parameters, solutes, and stable isotopes of H, C, and O. We logged electrical conductivity (EC) and water temperature (T) within Mawmluh Cave at 30-min intervals beginning 14 February 2023, and water level and T at 30-min intervals within Arwah Cave from 19 April to 15 December 2023. We also conducted qualitative dye traces (using eosin and fluorescein) in two streams within Mawmluh Cave.

Rainfall recorded by the Indian Meteorological Department at Sohra totaled 8753 mm for 2023. Initial rain fell 22 February; monthly totals were ≥ 300 mm from March through October, and 74.5% of rainfall occurred in June–August. Maximum monthly and 1-day rainfalls occurred in June. The dominant hydrochemical facies was Ca-SO₄ at eight of 14 sites, Ca-HCO₃ at five sites, and Ca-SO₄-HCO₃ at one site. The prevalence of SO₄ is consistent with oxidation of pyrite in sandstone and/or interbedded coal seams. δ¹³C of dissolved inorganic C ranged from 0.09 to -11.61‰ (median -6.90‰), which suggests HCO₃^- is derived both from limestone dissolution and oxidation of organic matter. Ranges of δ¹⁸O and δ²H (-4.1 to -7.37‰ and -30.21 to -51.96‰, respectively) were similar to previous studies from Mawmluh Cave. Values fall slightly subparallel to the local meteoric water line, indicative of limited evaporation. Dye tracing indicates at least two distinct flowpaths through Mawmluh Cave. EC in Mawmluh Cave and water level and T in Arwah Cave rise and fall seasonally with monsoonal rainfall, whereas water T in Mawmluh Cave appears to show more of an annual trend. All logged parameters show short-term responses to relatively intense rainfalls.