Contaminated land dealing with hydrocarbon contamination Conceptual models

Contaminated land: dealing with hydrocarbon contamination Conceptual models for petroleum hydrocarbon sites

Contents of presentation n Key contaminants – Compounds of particular interest – The challenge of TPH analysis n Hydrocarbon-contaminated sites – – n n Important source considerations NAPL sources Important pathways Common receptors Developing and refining the conceptual model Contaminant interactions Environmental Simulations International

Key components n Will depend on the case but most common hydrocarbon risk drivers are: – – n n n BTEX PAH’s Weathering may change priorities with time When relevant, consider MTBE (etc. ), lead, other metals You will need to consider component interactions – Discussed later Environmental Simulations International

Key components – more on BTEX n Main pathways are often migration as dissolved plume and/or as vapour – Relatively soluble in water n Benzene ~1790 mg/l – Relatively volatile n Toxicology – Benzene is a carcinogen – TEX relatively toxic but not carcinogenic Environmental Simulations International

Key components – more on PAH’s n Main pathways are often ingestion, dust inhalation, direct contact – Low to very low solubility in water n e. g, naphthalene ~30 mg/l; benzo[a]pyrene ~0. 003 mg/l – Adsorb onto solids – Relatively low volatility (although naphthalene can be significant) n Variable toxicity – – Some are carcinogen’s (e. g. , benzo[a]pyrene) Effects can be additive Environmental Simulations International

Key components n Don’t forget NAPL migration Environmental Simulations International

A brief digression – TPH analysis TPH = Total petroleum hydrocarbons n A quick and easy screening assay n – Everyone uses them – Can help identify the type of oil and the extent of weathering n n Don’t always believe the lab. ’s interpretation But what do the concentrations tell us? Environmental Simulations International

TPH analysis n Various methods – Gravimetric n Not recommended! – IR – GC/MS Environmental Simulations International

What’s wrong with TPH measurement? n Nothing, provided the data are used correctly – Do you know which standards were used? – Different methods give very different answers – No direct relationship to hazards posed by the components n Work in progress to provide a standard TPH analysis methodology that can link directly into standard risk assessment techniques – – Environment Agency/Institute of Petroleum US methodology

The US approach 1. Define appropriate hydrocarbon fractions for assessment 2. Define key properties of each fraction 4. Integrate fate/toxicity data and determine risk 3. Determine toxicity based on data derived for the fraction or key components within it http: //www. aehs. com/ Environmental Simulations International

How does the process work? GC/FID screen to derive TPH fractions n Screening concentrations per fraction for main exposure routes n More detailed GC/FID or GC/MS analysis of fractions n Procedure for deriving site-specific values n Environmental Simulations International

Conceptual models Hardstanding River intercepts plume Deep borehole Environmental Simulations International

Why do we need conceptual models? n A ‘pollutant linkage‘ is the relationship between – – – n n a contaminant (source) a pathway a receptor The presence of all three elements is required for land to be defined as ‘contaminated’ So, we need a conceptual understanding of our site! Environmental Simulations International

A conceptual model … n Allows targeting of subsequent investigation – Use the model iteratively – And is a requirement for SI (BS 10175: 2001) n n Provides the context for quantitative risk assessment Demonstrates an appreciation of the problem – …beyond the purely factual data gathering n n Aids communication Is a good QA tool Environmental Simulations International

Some considerations for hydrocarbon sources - 1 n What? – What was released? n n n Source material Additives (MTBE, lead? ) Co-contaminants? – e. g. , degreasing solvents – When? n Weathering? – How? n Distribution and weathering? Environmental Simulations International

Some considerations for hydrocarbon sources - 2 n How much? – Estimate of source mass – NAPL likely? – Residual? n Where? – Weathering? – Spatial variability – Fracture versus matrix flow n And so on … Environmental Simulations International

NAPL sources n Flowing or immobile? – If flowing, where to? Smearing? n How much? n How reliable are thickness measurements? n Do it’s properties match theory? n Environmental Simulations International

Some possible source behaviours Metals (e. g. , lead) Ethanol Antifreeze Oil and petroleum products Chlorinated solvents Environmental Simulations International

NAPL sources Environmental Simulations International

NAPL smearing – spreading the source? Idealized spreading on water table. (actually, capillary fringe) Raising water table may not displace free product lens which remain in initial position. Soil heterogeneities result in “fingering” Water table moving below initial level results in further descent of free flowing NAPL. Environmental Simulations International

Some pathway considerations - 1 n n Geological setting Hydrology/hydraulics – Recharge processes and unsaturated zone flow – Groundwater flow n Essentially horizontal or vertical? – Discharge points n Natural or anthropogenic? – Fast pathways ( early arrival of contaminants) – Slow pathways ( persistence of contaminants) – Barriers to flow Environmental Simulations International

Some pathway considerations - 2 n Groundwater pathways – Advection and Dispersion – Sorption/retardation – Biodegradation n NAPL flow Gradient of water table n Penetration of preferential flow paths n Environmental Simulations International

Some pathway considerations - 3 n Vapour pathways – Advection and Dispersion – Sorption/retardation – Biodegradation n Solid phase contact – Dust transport – Direct contact n n Inter-phase partitioning Plant uptake? Environmental Simulations International

Some possible important receptors n Who/what? – Humans – Controlled waters n n Aquifers Surface waters – Water supply n Fit for purpose ? Plastic pipelines – Utilities structures n n – – Cabling Sewers Ecosystems Buildings Environmental Simulations International

Component interactions n Hydrocarbon components will behave very differently from each other – MTBE can behave very differently in groundwater – high mobility, often slow biodegradation n Components can affect the behaviour of others – – Biodegradation Sorption Solubility Vapour composition… Environmental Simulations International

Component interactions An example - benzene Parameter Pure benzene Typical petrol Solubility in groundwater (mg/l) 1790 36. 5 Equilibrium vapour concentration (mg/l) 400 6. 7 Environmental Simulations International

Conclusions n n We need conceptual models Hydrocarbons have some specific properties that are important to consider – And many that are common to all contaminants n n Consider the effect of the component mixtures when testing the model Ask the questions Environmental Simulations International