BIOREMEDIATION

Success in bioremediation

DEMANDS close monitoring

Using bioremediation to treat ground impacted by chlorinated hydrocarbons (CHCs) essentially requires building a bioreactor below the ground; therefore, direct observation of results is not possible. Rather, the process must be scientifically monitored through specialised tests to serve as the integral lines of evidence needed to assess the health and performance of the biobarrier. By Michelle Roux and Sathisha Barath

he monitoring of this biobarrier,

Twhich is a living system, is done using specific in situ and laboratory tests. These tests will give insight into the key prevailing conditions that are required to maintain the biobarrier’s health and achieve the expected results in terms of CHC degradation. The characterisation of the affected ground will determine which geochemical conditions are most relevant to be included in the ongoing project assessment.

In situ monitoring of redox and pH

For instance, certain microbes responsible for the degradation of the dense non-aqueous phase liquids (DNAPLs) will only thrive in an anaerobic environment – in which case a vital aspect of monitoring will be focused on checking for oxidation-reduction (redox) conditions.

Acidity in the groundwater must also be carefully checked, as the microbes require a pH level in a range from five to seven. As they break down CHCs, the microbes generate volatile fatty acids that reduce the pH level of the groundwater and could undermine their effectiveness. Where pH levels are found to be too low, a ‘buffer’ can be added to raise them to within the optimal range.

Inorganic indicators

It is also necessary to check other geochemical parameters such as the levels of nitrates and sulfates. This helps to identify and understand the complex geochemical dynamics in the aquifer and possible presence of other microbes in the aquifer that could influence remediation reactions; these other microbes can compete for the injected food source. Information on these factors enables the practitioner to adjust the food supply to ensure that the dehalorespirators are still well supplied.

Total organic carbon

Regularly sampling and measuring the total organic carbon (TOC) in the aquifer is crucial, as this is an indicator of the substrate (food) on which microbes survive and thrive. The TOC levels should be measured both spatially and temporally to determine whether more substrate needs to be injected and where. Aquifers – and even zones within a single aquifer – will differ in terms of their hydraulic properties and flow velocities.

During the characterisation phase, prior to full-scale implementation, specific measurements should be undertaken to provide insights into the aquifer dynamics that will later influence substrate migration. The focus needs to be on the zone being treated and on ensuring that the substrate is finding its way to the targeted area and is being retained in the treatment zone for long enough that the microbes can utilise it efficiently.

This highlights the importance of making allowance in the project conceptualisation and characterisation phase for identifying preferential groundwater flow paths in the aquifer, which leads to flow heterogeneity within the system. These flow paths could take the injected substrate in unintended directions and deprive the microbes of an adequate food source.

Changes in concentration

Groundwater samples also need to be analysed to measure the concentration changes of CHCs within and downgradient of the biobarrier over time. Samples are collected every few months from carefully located monitoring holes. Indeed, the frequency and timing of the sampling regime can be viewed as a science in itself.

Similarly, the sampling methodology is

Bioremediation of chlorinated hydrocarbons using emulsified vegetable oil substrate: Practical approaches – a four-part series

South Africa is seeing valuable innovations in the use of bioremediation to treat industrial sites impacted by chlorinated hydrocarbons. This groundbreaking work is particularly urgent as the country makes more use of groundwater resources – a receptor that is vulnerable to contamination from surface sources of pollution. In this four-part series, SRK Consulting discusses the current advances it is making locally and how these will benefit efforts to clean up legacy impacts in the subsurface environment. In this, the fourth article in the series, the authors examine how the success of an enhanced in situ bioremediation (EISB) system is best monitored and evaluated. In the ground covered to date, the first article provided an overview of why EISB is an effective option for degrading chlorinated hydrocarbons. In the second article, the focus was on the use of emulsified vegetable oil as substrate for EISB, while the third article highlighted some lessons learnt in the practical implementation of this technology.

Suite of bottles used for groundwater sample collection. Groundwater samples are analysed and provide insight into the performance of the biobarrier

pH/redox probe used for in situ monitoring of geochemical parameters

critical to the accurate measurement of aquifer conditions. The selection, layout and placement of the sampling pumps – whether at the bottom of the hole or across a fracture – become a terrain for experienced scientists and specialists. Incorrect choices could lead to drastically skewed results, providing a misleading picture of actual processes under way.

End products

Monitoring results must show that the relevant end products are being formed from bioremediation’s metabolic pathway. This includes monitoring for methane, ethene and ethane – measured as a dissolved gas in water samples.

Sampling challenges

The sampling of groundwater in these projects, however, comes with special challenges. Bringing a sample to the surface would expose it to the atmosphere and thus potentially alter its pH and redox condition. Therefore, specialised in situ geochemical probes are used to sample the groundwater at depth to deliver an accurate reflection of the pH and redox conditions under which the microbes are living. Despite their cost, these probes are vital to effective monitoring, acting as the practitioner’s ‘eyes and ears’.

Highly specialised groundwater sampling and analysis are also required to ensure that the correct microbial species are present in sufficient quantities in the biobarrier. This calls for DNA testing – usually conducted by commercial laboratories abroad – with samples needing to comply with stringent requirements to avoid any kind of contamination. The Covid-19 pandemic has complicated this process even further, as samples must reach the laboratory within a certain timeframe. Logistical delays due to Covid19 lockdowns can lead to samples expiring in transit – and costly resampling of the aquifer.

Accuracy key to success

Accurate data, it should be remembered, is the foundation of the complex endeavour that bioremediation represents. As a living entity, the biobarrier we create to degrade CHCs is constantly changing – demanding that practitioners make regular interventions to correct and maintain the specific conditions required. Without ongoing monitoring and technical input guiding appropriate and timely action, whole sections of the biobarrier can begin to die, potentially threatening the success of the project.

Key take-aways from this series

• Bioremediation has been used in the USA and Europe over the past 50 years or more for treating ground impacted by chlorinated hydrocarbons, and the technology holds great potential for

South Africa. • There is a growing level of scientific innovation and practical experience in this field locally, informed by a global knowledge base. • Local project experience – alongside extensive laboratory and on-site testing – has allowed South African experts to develop fit-for-purpose solutions to deal with both chlorinated ethanes and chlorinated ethenes. • Successful bioremediation depends on a thorough investigation and understanding of the chemistry and geological matrix of the source material.

About Michelle Roux

Michelle is a principal hydrogeologist and contaminated land scientist in SRK Consulting’s Durban office, with more than 14 years of experience in hydrogeology, microbiology, contaminated land characterisation and management. Her specialisation includes project management, groundwater assessment and remediation, and bioremediation in situ treatment design. Her work includes conducting contaminated site assessments, developing and implementing long-term groundwater monitoring projects, and developing site conceptual models for DNAPL and LNAPL sites. Michelle holds an MSc Geohydrology, and a BSc (Hons) Microbiology.

About Sathisha Barath

As a senior hydrogeologist at SRK Consulting’s Durban office, Sathisha has more than 11 years of experience in land contamination, remediation and groundwater projects. Her specialisations include enhanced in situ bioremediation of chlorinated hydrocarbons, site characterisation of LNAPL and DNAPL sites, and soil and groundwater remediation of contaminated sites. She holds an MSc Hydrogeology and a BSc (Hons) Geology.