Influence of Ambient Temperature, Precipitation, and Groundwater Level on Natural Source Zone Depletion Rates at a Large Semiarid LNAPL Site

Abstract

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 CO2 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 CO2 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.

Multimethod Analysis of NSZD and Enhanced SZD by Solar‐Powered Bioventing at the Guadalupe Restoration Project

“…2016; Eichert et al. 2017; McAlexander and Sihota 2019) and aligns with steady rates observed in southern California (Mackay et al. 2018a).…”

“…The stability of NSZD rates 6 months apart contrasts with seasonal variation observed at sites in the middle portion of the U.S. (Sihota et al 2016;Eichert et al 2017;McAlexander and Sihota 2019) and aligns with steady rates observed in southern California (Mackay et al 2018a). The primary weather component that varies at the site is precipitation, with rainfall (30 cm, on average; 45 cm in 2019) typically in the winter months and dry or foggy conditions the remainder of the year.…”

“…Time lags in the range of 1 month to multiple months for hydrocarbon‐related CO 2 vadose zone transport have been observed elsewhere (Sihota et al. 2016; McAlexander and Sihota 2019). For these relatively permeable sandy soils, the elevated April CO 2 effluxes are consistent with higher precipitation in the months preceding the spring and higher natural soil respiration.…”

“…Background CO 2 effluxes at the H-8 and ITA-5 locations were higher in April 2019 (average = 0.8 to 0.9 μmol CO 2 m −2 s −1 , SE = 0.05) than October 2019 (average = 0.4 μmol CO 2 m −2 s −1 , SE = 0.02 to 0.06). Time lags in the range of 1 month to multiple months for hydrocarbon-related CO 2 vadose zone transport have been observed elsewhere (Sihota et al 2016;McAlexander and Sihota 2019). For these relatively permeable sandy soils, the elevated April CO 2 effluxes are consistent with higher precipitation in the months preceding the spring and higher natural soil respiration.…”

“…CO 2 efflux measurements were taken when shallow soils were dry to provide data representative of prevailing conditions, avoiding short term gas transport limited conditions. NSZD rates have been correlated with precipitation rates at a semi‐arid site (McAlexander and Sihota 2019), but the depth to groundwater was less than 5 m. At the current site, the water table was deeper than 15 m at all studied locations (Figure 4). The large transport distance for percolating water to reach the microbially active zone may have dampened a detectable seasonal effect.…”