Field data and numerical simulation of btex concentration trends under water table fluctuations: Example of a jet fuel-contaminated site in Brazil

Abstract

Mass transfer of light non-aqueous phase liquids (LNAPLs) trapped in porous media is a complex phenomenon. Water table fluctuations have been identified as responsible for generating significant variations in the concentration of dissolved hydrocarbons. Based on field evidence, this work presents a conceptual model and a numerical solution for mass transfer from entrapped LNAPL to groundwater controlled by both LNAPL saturation and seasonal water table fluctuations within the LNAPL smear zone. The numerical approach is capable of reproducing aqueous BTEX concentration trends under three different scenarios - water table fluctuating within smear zone, above the smear zone and partially within smear zone, resulting in in-phase, out-of-phase and alternating in-phase and out-of-phase BTEX concentration trend with respect to water table oscillation, respectively. The results demonstrate the model's applicability under observed field conditions and its ability to predict source zone depletion.

A Screening Model to Predict Entrapped LNAPL Depletion

“…In tropical and subtropical climates, the amplitude of the water table fluctuation is typically high [15,20]. Under these conditions, the LNAPL may be completely entrapped below the water table, as described by Isler et al [21].…”

“…The concentration decline is related to several factors, such as the LNAPL saturation, flow velocity, and biodegradation rates in the aqueous phase. Teramoto and Chang [15], Mackay et al [23], Huntley and Beckett [24], and Thornton et al [25], among others, demonstrated that the continuous loss of water-soluble compounds leads to continuous depletion of LNAPL in the source zone. Actually, there are a large number of analytical and numerical approaches, varying in scale and complexity, to simulate NAPL dissolution into the aqueous phase [18,[26][27][28][29][30][31][32][33][34][35][36][37][38][39].…”

“…When the LNAPL remains mostly as an immiscible fluid in the capillary fringe, the volatilization in the air/LNAPL interface represents the prevailing mechanism of LNAPL depletion with respect to benzene, ethylbenzene, toluene, and xylenes (BTEX) [1][2][3][4][5]; however, as the water level fluctuates in response to the cyclical alternation of the dry and rainy seasons, there is a continuous redistribution of the LNAPL because it migrates to lower aquifers as water levels decrease [6][7][8][9][10]. When the water level rises, LNAPL is retained by capillary force in the saturated zone, a phenomenon known as entrapment [4][5][6][7][8][9][10], which has been demonstrated in numerous previous studies (e.g., [11][12][13][14][15][16]). When entrapment occurs, LNAPL is entrapped by the capillarity in the saturated zone; in contrast, when the water level drops, LNAPL is released, and previously isolated ganglia clump together to gain mobility.…”

“…When entrapment occurs, LNAPL is entrapped by the capillarity in the saturated zone; in contrast, when the water level drops, LNAPL is released, and previously isolated ganglia clump together to gain mobility. Once entrapped, LNAPL releases soluble compounds into the groundwater, particularly BTEX, which represents the most soluble compounds [15,[17][18][19].…”

Transporte de solutos em diferentes cenários geológicos gerados por modelos estocásticos de cadeias de Markov

“…Como demonstrado por Teramoto & Chang (2017), a solubilização da fase líquida não-aquosa (NAPL) em área contígua à do presente estudo é resultado de inúmeros fatores, tais como velocidade de fluxo, taxa de biodegradação, saturação da fase não aquosa e da flutuação do nível d'água. Entretanto, para simplificação do modelo, admitiu-se a existência de equilíbrio termodinâmico entre a água e o NAPL no poro, baseando-se na Lei de Raoult (BANERJEE, 1984), com a fixação de uma concentração na porção compreendida pela área-fonte idealizada.…”