A Generic Method for Predicting Environmental Concentrations of Hydraulic Fracturing Chemicals in Soil and Shallow Groundwater

Abstract

Source-pathway-receptor analyses involving solute migration pathways through soil and shallow groundwater are typically undertaken to assess how people and the environment could come into contact with chemicals associated with coal seam gas operations. For the potential short-term and long-term release of coal seam gas fluids from storage ponds, solute concentration and dilution factors have been calculated using a water flow and solute transport modelling framework for an unsaturated zone-shallow groundwater system. Uncertainty about dilution factors was quantified for a range of system parameters: (i) leakage rates from storage ponds combined with recharge rates, (ii) a broad combination of soil and groundwater properties, and (iii) a series of increasing travel distances through soil and groundwater. Calculated dilution factors in the soil increased from sand to loam soil and increased with an increasing recharge rate, while dilution decreased for a decreasing leak rate and leak duration. In groundwater, dilution factors increase with increasing aquifer hydraulic conductivity and riverbed conductance. For a hypothetical leak duration of three years, the combined soil and groundwater dilution factors are larger than 6980 for more than 99.97% of bores that are likely to be farther than 100 m from the source. Dilution factors were more sensitive to uncertainty in leak rates than recharge rates. Based on this dilution factor, a comparison of groundwater predicted environmental concentrations and predicted no-effect concentrations for a subset of hydraulic fracturing chemicals used in Australia revealed that for all but two of the evaluated chemicals the estimated groundwater concentration (for a hypothetical water bore at 100 m from the solute source) is smaller than the no-effect concentration for the protection of aquatic ecosystems.

Probabilistic Groundwater Flow, Particle Tracking and Uncertainty Analysis for Environmental Receptor Vulnerability Assessment of a Coal Seam Gas Project

“…At a regional scale this method was appropriate as a screening assessment to identify and prioritise potential source-receptor combinations prior to employing dedicated local contaminant transport models for contamination risk assessment. This vulnerability assessment approach can be applied as a useful predecessor or first step of a generic preliminary contaminant risk assessment workflow as reported in Mallants et al [26,27]. Median estimates of groundwater flow in the region were less than 1.5 m per year meaning particles generally did not travel far from their origin.…”

Water yield modelling, sensitivity analysis and validation: a study for Portugal

“…The minimum requirement when validating assessment models is to establish an adequate scientific basis for credibility [18]. The model is said to be validated if its accuracy and predictive capability in the validation period have been proven to lie within acceptable limits [53].…”

“…Sensitivity analysis is an effective tool for identifying important uncertainty sources and improving model calibration and predictions [18]. It is an integral part of model development and involves an analytical examination of input parameters to aid in model validation and provide guidance for future research [19].…”

“…It is an integral part of model development and involves an analytical examination of input parameters to aid in model validation and provide guidance for future research [19]. This technique consists of changing values of inputs and internal parameters of a model to determine the effect on the behavior and output of the model [20], increasing our understanding of which parameters must be determined with the greatest accuracy, and thus helping to prioritize data collection requirements [18].…”

How pore structure non‐stationarity compromises flow properties representativity (REV) for soil samples: Pore‐scale modelling and stationarity analysis

“…This traditional view suggests that if one increases the volume of the sample, REV should eventually be achieved, whereas non‐representativity is due to the heterogeneities within samples not being large enough. The general idea of REV and its application is shown schematically in Figure 1, which depicts two important stages of REV application: homogenization (the replacement of the complex pore structure and flow field within it with a single continuum‐scale property; e.g., Torquato (2002)) and upscaling (the substitution of the homogenized property for a larger volume of porous media, which is a common practice in hydrological models; e.g., Mallants et al (2020)).…”