Nicola Balbarini

Contaminant mass discharge to streams: Comparing direct groundwater velocity measurements and multi-level groundwater sampling with an in-stream approach

Abstract Quantitative knowledge of contaminant mass discharge is useful when assessing the risk posed to streams by point sources. However, due to a multi-directional flow field at the groundwater-surface water interface, reliable estimates close to streams are particularly challenging to obtain. Moreover, since the “true” value of the total contaminant mass discharge across a defined control plane is typically unknown at field sites, it is difficult to assess the accuracy of estimates at the field scale. We estimated the mass discharge of a chlorinated ethene plume entering a low-land Danish stream across a control plane on the stream bank. This was done using multi-level groundwater sampling combined with 1) specific discharge obtained from Darcy’s law, and 2) two different specific discharge fields (a constant equal to the mean, and a varying) obtained from direct groundwater velocity measurements using point velocity probes (PVPs). The methods yielded contaminant mass discharges ranging from 204 to 372 kg/y (PCE equivalents). To help account for the entire contaminant mass, we also quantified the contaminant mass discharge of chlorinated ethenes from measured contaminant concentrations in the stream water and the stream discharge to obtain an in-stream contaminant mass discharge. By subtracting contaminant input upstream from the plume discharge zone, as well as input from two culverts discharging contaminated water to the stream, this estimate was used to calculate a groundwater-borne in-stream contaminant mass discharge of 558 kg/y (PCE equivalents). This method has not previously been explored to access the accuracy of stream bank contaminant mass discharge estimates. Differences between the groundwater-borne in-stream value and those on the stream bank may be due to high concentration zones located in fast flow conduits or missed high concentration zones in the control plane. It is also possible that the groundwater-borne in-stream mass discharge was overestimated due to uncertainty in the input from two discharging culverts. The agreement of these results is sufficient to be of practical usefulness for risk assessment. It is concluded that the combination of direct groundwater velocity measurements and multi-level groundwater sampling can provide a useful extension of Darcy-based methods when quantifying the contaminant mass discharge to streams. Moreover, this study demonstrates the efficacy and usefulness of the in-stream contaminant mass discharge in risk assessment if fully mixed stream water concentration and corresponding stream discharge are available.

Dilution and volatilization of groundwater contaminant discharges in streams

An analytical solution to describe dilution and volatilization of a continuous groundwater contaminant plume into streams is developed for risk assessment. The location of groundwater plume discharge into the stream (discharge through the side versus bottom of the stream) and different distributions of the contaminant plume concentration (Gaussian, homogeneous or heterogeneous distribution) are considered. The model considering the plume discharged through the bank of the river, with a uniform concentration distribution was the most appropriate for risk assessment due to its simplicity and limited data requirements. The dilution and volatilization model is able to predict the entire concentration field, and thus the mixing zone, maximum concentration and fully mixed concentration in the stream. It can also be used to identify groundwater discharge zones from in-stream concentration measurement. The solution was successfully applied to published field data obtained in a large and a small Danish stream and provided valuable information on the risk posed by the groundwater contaminant plumes. The results provided by the dilution and volatilization model are very different to those obtained with existing point source models, with a distributed source leading to a larger mixing length and different concentration field. The dilution model can also provide recommendations for sampling locations and the size of impact zones in streams. This is of interest for regulators, for example when developing guidelines for the implementation of the European Water Framework Directive.

Permeability Estimation Directly From Logging‐While‐Drilling Induced Polarization Data

In this study, we present the prediction of permeability from time domain spectral induced polarization (IP) data, measured in boreholes on undisturbed formations using the El-log logging-while-drilling technique. We collected El-log data and hydraulic properties on unconsolidated Quaternary and Miocene deposits in boreholes at three locations at a field site in Denmark, characterized by different electrical water conductivity and chemistry. The high vertical resolution of the El-log technique matches the lithological variability at the site, minimizing ambiguity in the interpretation originating from resolution issues. The permeability values were computed from IP data using a laboratory-derived empirical relationship presented in a recent study for saturated unconsolidated sediments, without any further calibration. A very good correlation, within 1 order of magnitude, was found between the IP-derived permeability estimates and those derived using grain size analyses and slug tests, with similar depth trends and permeability contrasts. Furthermore, the effect of water conductivity on the IP-derived permeability estimations was found negligible in comparison to the permeability uncertainties estimated from the inversion and the laboratoryderived empirical relationship.

Efficient water table evolution discretization using domain transformation

Domain transformation methods are useful techniques for solving problems on non-stationary domains. In this work, we consider the evolution of the water table in an unconfined aquifer. This nonlinear, time-dependent problem is greatly simplified by using a mapping from the physical domain to a reference domain and is then further reduced to a single, (nonlinear) partial differential equation. We show well-posedness of the approach and propose a stable and convergent discretization scheme. Numerical results are presented supporting the theory.

Geophysics‐Based Contaminant Mass Discharge Quantification Downgradient of a Landfill and a Former Pharmaceutical Factory

Contaminant mass discharge is a commonly applied tool to evaluate the environmental impact of contaminated sites on water resources. At large contaminated sites with heterogeneous sources, such as landfills, the number of wells available is often not sufficient, leading to a high uncertainty of mass discharge estimates. In this study, we tackle the uncertainty of the contaminant mass discharge due to low sampling densities by interpolating limited water-sample data with the support of surface direct current resistivity and induced polarization geophysical data. The method relies on finding a conceptual link between the bulk conductivity imaged from geophysics and the contaminant concentrations. We investigate the link between (1) imaged bulk and electrical water conductivity, (2) water conductivity and conservative ionic species, (3) water conductivity and redox-sensitive species, (4) water conductivity and semipersistent organic species, and (5) water conductivity and biodegradable organic compounds. The method successfully identify similarities between the distribution of the bulk conductivity and chloride and pharmaceutical compounds in a landfill leachate plume and between the bulk conductivity data and benzene and chlorinated ethenes for a contaminant plume from a former pharmaceutical factory. Contaminant concentrations were interpolated through regression kriging, using geophysical data as the dependent variable. The distribution of concentration determined with the novel method showed a lower mean relative estimation error than the traditional method of kriging only contaminant concentration data. At large sites, the method can improve contaminant mass discharge estimates, especially if surface geophysical measurements are integrated in the site investigation at an early stage.