Monitoring chemical processes and transport phenomena in the water table region of phreatic aquifers - review of a study conducted under agricultural land irrigated with sewage effluents
1998
Ronen, D. (Weizmann Institute of Science, Rehovot (Israel). Dept. of Environmental Sciences and Energy Research)
Results of field studies conducted at the water table region of the phreatic Coastal Plain aquifer of Israel, under agricultural land irrigated with sewage effluents since the 1960's, revealed: (i) intensive biochemical activity as reflected by the consumption of dissolved O2 with the concomitant oxidation of organic matter and the development of an anoxic layer, and the production of N2O (up to 400 mg per L) and CO2 (pCO2 equal to 10-1.8 to 10-1.3 atm); (ii) the presence of an almost stagnant water layer (q equal to 0.5 m per year) down to a depth of 60 cm below the water table, and (iii) the presence of microscale isothermal water parcels (characteristic vertical and horizontal length dimensions of the order of less than 1 m) which differ from each other in their chemical composition and density an are characterized by very sharp boundaries between them. It is postulated that the gases produced during the biodegradation of organic matter accumulate as a distinct gas phase (bubbles) down to a depth of 1 m below the water table, diminishing groundwater flow. The replenishment of the aquifer by water to different chemical composition and the almost stagnant conditions prevailing at the water table region (where mechanical dispersion by advection is negligible) lead to the development of microscale parcels of water of different chemical composition. It is suggested that haline convection is a major transport and mixing mechanism at the water table region.
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