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Influence of Ambient Temperature, Precipitation, and Groundwater Level on Natural Source Zone Depletion Rates at a Large Semiarid LNAPL Site
Author(s) -
McAlexander Benjamin,
Sihota Natasha
Publication year - 2019
Publication title -
groundwater monitoring and remediation
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.677
H-Index - 47
eISSN - 1745-6592
pISSN - 1069-3629
DOI - 10.1111/gwmr.12309
Subject(s) - vadose zone , groundwater , water table , precipitation , environmental science , hydrology (agriculture) , volatilisation , soil gas , elevation (ballistics) , soil science , environmental chemistry , soil water , geology , chemistry , physics , geotechnical engineering , geometry , organic chemistry , mathematics , meteorology
Natural source zone depletion (NSZD) is increasingly being integrated into management strategies for petroleum release sites. Measurements of NSZD rates can be used to evaluate naturally occurring hydrocarbon (HC) mass losses, and provide a baseline for evaluating engineered recovery systems. Here, nominal saturated and unsaturated zone HC losses were quantified by groundwater sampling and ground surface CO 2 effluxes approximately monthly over a 1‐year period. In addition, subsurface gas profiles and temperature, precipitation, and groundwater elevation were evaluated to elucidate dominant environmental factors controlling NSZD rates. Results showed that NSZD rates estimated by surface CO 2 effluxes were, on average, more than a factor of 3 greater than those estimated by uptake of electron acceptors (primarily sulfate) in extracted groundwater. This may indicate that vadose zone mass loss mechanisms (e.g., volatilization and subsequent biodegradation) were dominant in this system, but possible transfer of gases from the saturated zone to the vadose zone confounds this interpretation. Results for this semiarid site revealed that increasing NSZD rates tended to occur with increasing ambient monthly precipitation and temperature when accounting for time lags associated with subsurface transport. However, groundwater elevation was not found to be significantly related to NSZD rates. This result is counter to an expectation that increased smear zone exposure increases HC mass losses, and suggests that the pump‐and‐treat system did not greatly influence total NSZD rates directly through smear zone flushing or indirectly by lowering the regional water table.