Origin, distribution and chemical mass balances of non-anthropogenic, brackish and (hyper)saline groundwaters in the Netherlands

2006 
An inventory was made of brackish to hypersaline groundwaters in the Netherlands down to 600 m below mean sea level, using all readily accessible hydrochemical data with Cl > 150 mg/L (13,500 samples in total). Five types of non anthropogenic, brackish to (hyper)saline groundwater are discerned in the upper 600 m of the dutch underground: (sub)recent, intruding North Sea water (NS); Holocene transgression water in a high energy environment like open marine, tidal gully or estuary (HOH); Holocene transgression water in a low energy environment like a lagoon (HOL); relict groundwater of Late Tertiary to Early Pleistocene age (LTEP); and Permian or Devonian halite leachate (HAL). Their recognition required the use of various available tracers like Cl/Br, 2H, 3H, 14C, 18O, SO4/Cl, SO4/HCO3, NH4/HCO3, I, Li, Sr and NO3. An algorithm is presented that uses data on Cl, Cl/Br and 18O to quantify the mixing ratio of ocean, rain and fluvial water, the concentration factor by evapo(transpi)ration losses of the recharge water, and the dissolution of halite. This algorithm is part of a mass balance calculation scheme, set in EXCEL spread sheet, to also estimate the contribution of the following hydrogeochemical processes to water quality: dissolution or precipitation of gypsum, carbonates (calcite, dolomite, siderite), silicates and iron(hydr)oxides, the oxidation of soil organic material, cation and anion exchange, and methane gas bubble migration from and to the aquifer sampled. The calculation scheme is briefly described. Two watertypes reveal, through their mass balance, signs of methane gas bubble migration: HOL water shows a strong methane depletion, and in LTEP water significant amounts of iron(hydr)oxides must have been reduced and transformed into siderite by ascending methane bubbles from deeper layers of the aquifer system. For desalination purposes we recommend the brackish exponents of relict HOH groundwater close to former river mouths (thus reducing clay and peat contact and salinity), and relict LTEP groundwater. These waters require less energy and reduce the risks on membrane scaling. Key words-Brackish and saline groundwater, tracers, mixing, chemical mass balance, hydrogeochemical reactions, gas migration
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