Industrial application of environmentally hazardous substances have led to contamination of important groundwater aquifers worldwide. Decreasing groundwater quality poses risks to human health, water and food supply, and biodiversity. In Europe, a total of 2.8 million contaminated sites was estimated in 2017 (Pérez & Eugenio, 2018), while in the US, the legal authorities had to manage up to 1.3 million contaminated sites in 2017 (U.S. Environmental Protection Agency, 2017). In Denmark, groundwater is the primary source for drinking water and >36,000 contaminated sites were registered in 2018 (Olsen et al., 2020). Source zone remediation by excavation is a typical, but expensive method that can lead to a significant carbon footprint (Søndergaard et al., 2018). In many cases, the source zone contamination depletes and creates a contamination plume in the groundwater aquifer (Fjordbøge et al., 2017;Murray et al., 2019;Steelman et al., 2020).The development of cost-effective in situ remediation technologies, where contaminant plumes are directly treated in the groundwater, is a high priority (
<p><strong>Background</strong></p><p>Evolving <em>In situ</em> methods are showing results for sustainable and efficient plume remediation of groundwater contaminations. By injecting reactive components such as oxidation agents, zero valent iron, substrate and/or bacteria, a treatment zone (TZ) is established. In the TZ, the contamination degrades into harmless components by chemical and/or biological processes. Successful <em>in situ</em> remediation depends on contact between injectants and contamination. Yet, monitoring the spreading of the injectant is difficult by point sampling. The cross-borehole geophysical method DCIP (Direct Current, Induced Polarisation) allows for detailed spatial information on subsurface electrical resistivity and induced polarisation properties. The information can be used to assess the success of the injection and the development over time. Furthermore, the IP properties can be used to infer spatial information on hydraulic conductivity, which can be used in planning of the <em>in situ </em>remediation and in quantification of contaminant mass discharge (CMD) at the site.<strong> </strong>The objective of this study is to develop a cost-efficient method for detailed spatial and temporal monitoring of <em>in situ</em> remediation and to develop better tools to retrieve spatial subsurface information, able to assist and improve CMD based monitoring.</p><p><strong>&#160;</strong></p><p><strong>Approach</strong></p><p>A TZ in a plume of chlorinated ethenes was established by injecting the micro zerovalent iron product and a bacterial culture into the groundwater. A network of 9 geophysical and 16 monitoring wells was established. Cross-borehole DCIP measurements and water samples were taken before and shortly after injection and during the following year. Soil cores were sampled for chemical analysis of iron shortly after injection, and slug tests and grain size analysis. Data from water samples, soil cores and hydraulic tests were compared to the geophysical measurements to assess correlation between water chemistry and electrical resistivity from cross-borehole DCIP. The hydraulic properties inferred from hydraulic tests and cross-borehole DCIP were compared. The hydraulic properties with uncertainties and the contamination data were used to estimate the CMD through the TZ.</p><p><strong>&#160;</strong></p><p><strong>Results </strong></p><p>The changes in electrical conductivity and specific water quality parameters caused by the injection, showed a strong correlation with the geophysical model. The observed correlation enabled a coherent, detailed understanding of both spatial and temporal spreading of the injected components, resulting in a re-injection. Hydraulic tests and hydraulic properties inferred from cross borehole DCIP showed a very good correlation, and applying the hydraulic properties inferred from cross borehole DCIP reduced the uncertainty of the CMD estimate before and after injection. In conclusion, cross borehole DCIP has the potential to improve planning and monitoring of <em>in situ</em> groundwater remediation and to reduce uncertainty of CMD estimation and thereby strengthen CMD as a metric in risk assessment.</p>
A new methodology was developed to support contaminant mass discharge (CMD)‐based risk assessment of groundwater contamination downgradient of point source zones. Geoelectrical cross‐borehole induced polarization (IP) data were collected at a site undergoing in situ remediation of chlorinated solvents for determining 2D hydraulic conductivity (K) distributions with an inversion model resolution of 0.15 m (vertically) x 0.50 m (horizontally) in three control planes from 10‐20 m depth. Additionally, 18 slug tests and 31 grain size distribution analyses (GSA) from the control planes, were used for K‐estimation. The geometric means and variance of the IP, slug test, and GSA derived K‐estimates were consistent with previously studied sandy aquifers. Furthermore, the vertical variation in K between two geological settings, a sandy till and a meltwater sand formation, was clearly identified by the IP K‐estimates. The vertical variation was backed up by hydraulic profiling tool (HPT) measurements. Random realizations of CMD were simulated based on the cross‐borehole IP derived K‐values. For comparison, the CMD was also estimated with a geostatistical conditional simulation approach, using the data from slug tests and GSAs. The high IP resolution captured the small scale variations in K across the transects and led to CMD predictions with a narrow uncertainty interval, whereas slug test and GSA either under‐ or overestimated the magnitude of the areas with the highest CMD. Applying the geophysical cross‐borehole method for estimating K‐distributions in addition to traditional methods would improve CMD‐based risk assessment and evaluation of remediation performance at contaminated sites.This article is protected by copyright. All rights reserved.
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