Concentration trends and water-level fluctuations at underground storage tank sites

Abstract

Concentration trends of monitor wells utilized in monitored natural attenuation at petroleum underground storage tank sites can be used to predict achievement of regulatory standards if the data approximate a first-order decline trend. However, declining concentration trends often display seasonal and other fluctuations that complicate trend interpretation. Seasonal correlations between concentration and water-level elevation, including in-phase and inverse relationships, constitute one of the most common types of variation. The in-phase fluctuations are most common for monitor wells located in or near the source area of the release. This relationship may be the result of increased contact with the smear zone in the source area during periods of high water table. Conversely, inverse trends of water-level elevation and concentration are most common in downgradient wells beyond the limit of the source area. In a year long study of short-term fluctuations in BTEX and other parameters in a downgradient monitor well, the data suggest that the winter/spring recharge event significantly controls the concentration trends of BTEX as well as inorganic compounds in the well. Recharge and associated water table rise began in late fall and were soon followed by a slug of inorganic ions strongly influenced by road salt application. This slug of recharge diluted the concentrations of petroleum compounds and alkalinity (bicarbonate). Electron acceptors including oxygen, nitrate, and sulfate, which is a component of road salt, are also contributed to the water table during recharge. Oxygen and nitrate were not detected in the monitor well samples and were most likely consumed quickly in biodegradation reactions at the top of the contaminant plume. Sulfate peaked during winter/spring recharge and then slowly declined during the summer and fall, along with redox potential. Alkalinity (bicarbonate) increased during this period, which may represent the coupled oxidation of organic carbon to CO 2 with sulfate as the electron acceptor. BTEX concentrations peaked in the fall probably due to the lack of diluting recharge. The slow changes in concentration over the summer and fall months, interpreted to be caused by biodegradation, contrast with the rapid changes associated with dilution during the recharge event.

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

“…Similar patterns of the BTEX concentration changing over time have been described elsewhere (Davis et al, 1993;Kehew andLynch, 2011 andZhou et al, 2015). Kehew and Lynch (2011) presented three similar BTEX trends from a contaminated site located in Michigan (USA), suggesting that these patterns are widespread and can be explained by the conceptual model proposed in the present work.…”

“…One important process that can affect the aqueous BTEX concentration is biodegradation, which was not considered in the simulation. Biodegradation kinetic changes in response to water table fluctuations was demonstrated in several works, e.g., Rainwater et al (1993), Sinke et al (1998), Rainwater et al (1993), Dobson et al (2007), Kehew and Lynch (2011) and Rezanezhad et al (2014). These works suggest that biodegradation enhancement during episodes of a rising water table are attributed to aquifer oxygenation.…”

“…Although the role of a large variety of influential factors on the mass transfer phenomena has been well documented, the impact of the water table is still underestimated. Only a few works using either numerical simulation (Reddi et al, 1998), laboratory experiments (Dobson et al, 2007) or field evidence (Davis et al, 1993;Kehew and Lynch, 2011;Zhou et al, 2015) have shown that dissolution is strongly controlled by water level fluctuations. These authors verified that the concentration of soluble hydrocarbons in water is correlated with the natural fluctuation of the water level, with an increase during the rise of the water level and a decrease during its fall.…”

A Critical Review of Groundwater Table Fluctuation: Formation, Effects on Multifields, and Contaminant Behaviors in a Soil and Aquifer System

“…17 In addition, the migration of contaminants is closely related to the physiochemical properties of the groundwater environment. Recently, the migration and transformation of heavy metals, 18 non-aqueous phase liquids, 19 and colloids have become hot topics. 20 The distribution of contaminants tended to be associated with the effect of GTF.…”

Feasibility of greenhouse gas emissions offsets for natural source zone depletion of petroleum hydrocarbons

“…Petroleum releases to the environment occur across the extraction (Fisher & Sublette, ; Nigerian Federal Ministry of Environment [FME], ), refining (Pasadakis, Gidarakos, Kanellopoulou, & Spanoudakis, ; Schneider et al., ; Zargar, Sarrafzadeh, Taheri, & Tavakoli, ), and storage (Arnold, Parfitt, & Kaltreider, ; Biesterveld, Scheiter, & Marsh, ; Kehew & Lynch, ) phases of resource use. Released hydrocarbons in the terrestrial subsurface tend to be spread across multiple phases, including as sorbed chemicals to soil organic matter, as dissolved components within groundwater, as gases within vadose zone pore spaces, and as light nonaqueous phase liquid (LNAPL) across the soil column and particularly near the water table (Newell, Acree, Ross, & Huling, ).…”