Influence of Shallow Geology on Volatile Organic Chemical Attenuation from Groundwater to Deep Soil Gas

Abstract

Vapor intrusion pathway evaluations commonly begin with a comparison of volatile organic chemical (VOC) concentrations in groundwater to generic, or Tier 1, screening levels. These screening levels are typically quite low reflecting both a desired level of conservatism in a generic risk screening process as well as limitations in understanding of physical and chemical processes that impact vapor migration in the subsurface. To study the latter issue, we have collected detailed soil gas and groundwater vertical concentration profiles and evaluated soil characteristics at seven different sites overlying chlorinated solvent contaminant plumes. The goal of the study was to evaluate soil characteristics and their impacts on VOC attenuation from groundwater to deep soil gas (i.e., soil gas in the unsaturated zone within 2 feet of the water table). The study results suggest that generic screening levels can be adjusted by a factor of 100× at sites with fine‐grained soils above the water table, as identified by visual observations or soil air permeability measurements. For these fine‐grained soil sites, the upward‐adjusted screening levels maintain a level of conservatism while potentially eliminating the need for vapor intrusion investigations at sites that may not meet generic screening criteria.

Vapor Intrusion Investigations and Decision-Making: A Critical Review

“…A number of studies have documented the effect of high soil moisture on reducing VOC diffusion rates in the vadose zone and reducing VI potential (Figure C). ,− Diffusion rates are significantly lower through water-connected soils versus air-connected soils. , Field studies demonstrated that the presence of fine-grained silt or clay layers with high enough water content in the vadose zone to break the air-connected diffusion routes resulted in high VOC attenuation from groundwater to deep soil gas. This study found that, at these sites, generic groundwater to soil gas attenuation factor could be adjusted by 100-fold to account for the increased attenuation attributable to these fine-grained layers . However, because VOC diffusion is more strongly influenced by soil moisture than soil types, measuring the moisture content of fine-grained layers is an important part of evaluating their effectiveness as a VI barrier …”

“…This study found that, at these sites, generic groundwater to soil gas attenuation factor could be adjusted by 100-fold to account for the increased attenuation attributable to these fine-grained layers . However, because VOC diffusion is more strongly influenced by soil moisture than soil types, measuring the moisture content of fine-grained layers is an important part of evaluating their effectiveness as a VI barrier …”

“…In other words, building-specific variations in VOC attenuation may overwhelm the effect of soil type. A more focused field study demonstrated a clear relationship between soil type and VOC attenuation from groundwater to soil gas with higher attenuation in fine-grained soils . However, currently, most VI guidance documents do not account for soil type during initial pathway screening.…”

Oxidation of trichloroethylene, toluene, and ethanol vapors by a partially saturated permeable reactive barrier

“…During the past few decades, the migration of volatile organic compounds (VOCs) by diffusion from contaminated soil or groundwater into overlying buildings has received considerable attention (McHugh et al, 2013;Provoost et al, 2009). Human health risks of VOCs are typically dominated by the extent of exposure through inhalation of indoor air.…”

Intrusão de vapores do solo: breve histórico sobre desenvolvimento da tecnologia, cenário brasileiro e avanços recentes

“…Alguns guias sugerem que camadas de alta permeabilidade, de ocorrência natural (por exemplo, areia, cascalho ou rochas verticalmente fraturadas), também podem atuar como caminhos preferenciais para migração de vapores(API, 2005;ITRC, 2007;ITRC, 2014;USEPA, 2015).Características inerentes ao meio físico, podem atuar como fator limitador para a intrusão de vapores. Exemplos de barreiras físicas incluem: lentes de água limpa na porção superior do aquífero, camadas de silte ou argila saturadas, posicionadas acima da fonte de COVs, processos de biodegradação e/ou outras transformações físicoquímicas que podem ocorrer na zona de vadosa, bases de edificações que possuam algum tipo de impermeabilização, ventilação ou indução de pressão positiva(MCHUGH et al, 2013). A Figura 03 a seguir ilustra exemplos de barreiras potenciais para a intrusão de vapores.…”