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In Situ Bioremediation Overview


​The basic premise of bioremediation is to accelerate microbial activity using nutrients (carbon, phosphorus, nitrogen, etc.) to create conditions conducive to biodegradation of target contaminants. In general, in situ bioremediation is designed to modify the environmental conditions, typically through oxidation/reduction manipulation, to create an environment that stimulates the growth of indigenous and added microbes and the subsequent degradation of the target contaminants via microbial catalyzed reactions. To accomplish these events, the following primary factors must be considered:
  • Types of contaminants present and the target contaminant(s)
  • Types and numbers of microorganisms present or added
  • Environmental /Geochemistry conditions within the treatment zones
  • Types and amounts of nutrients needed
  • Site hydrology
 
There are no regulatory barriers preventing the use of in situ bioremediation. The EPA’s policy on reinjection of treated groundwater to promote in situ treatment is allowed as long as the groundwater is treated to eliminate contaminants of concern prior to reinjection; the treatment is intended to substantially reduce the hazardous constituents in the groundwater before or after reinjection; and the cleanup is protective. However, when in situ treatment is used in an uncontained environment, EPA approval is usually required.

Site and water assessments are combined with bench-scale studies to determine if in situ bioremediation can be effective in a given situation. In situ treatments can require longer time periods in sensitive environments where conservative application rates are appropriate, there is less certainty about the uniformity of treatment because of environment variability, and process efficacy is sometimes more difficult to verify without extensive monitoring. Biotreatments augment and stimulate the naturally-occurring microorganisms to grow and transform environmental contaminants and are influenced by specific contaminants, metals and metal forms present, their concentrations, oxygen supply, moisture, temperature, pH, and nutrients present.
 
In situ bioremediation is designed to reduce or stabilize source contaminant mobility and/or establish treatment zones. It has the potential to significantly reduce and, in some cases, eliminate down gradient water treatment requirements. In situ bioremediation has been successfully demonstrated to achieve site closure results in various hard rock mining influenced wastewater (MIW) environments at significantly reduced treatment costs. 
Inotec offers two approaches to in-situ bioremediation.

1. Conventional In Situ Bioremediation
Conventional in situ bioremediation includes application of the appropriate microbes and/or rebalancing C:N:P ratios to foster desired redox conditions and/or establish subsurface treatment zones. To a large extent, redox chemistry governs the large- and micro-scale microbial and chemical changes occurring in mine site environments. Properly applied, in situ bioremediation can be used to adjust the microbial/chemical environment within mine waste sites and associated contaminant flow paths to foster conditions conducive to contaminant transformation and/or immobilization. Such methods can produce almost immediate and potentially long-term results that can be achieved in a wide range of climates with relative ease of construction and maintenance to treat large volumes of mining impacted waters and associated source materials.
2. Hybrid In Situ Bioremediation
Given the high costs of implementing and operating large active water treatment systems, especially where higher nitrate concentrations are also present, Inotec has proposed a hybrid water management strategy for mine sites. The holistic approach proposed by the Inotec team includes integration of source control measures, in-situ nitrate reduction and metals stabilization, and combinations of semi-passive and active treatment processes for more economical and comprehensive water management. 
Please see the case studies below for examples of Inotec's in situ projects.

Denitrification with Arsenic Stabilization

Mining activities introduced nitrate and mobilized arsenic in approximately 125 million tons of waste rock contained in various spent ore depositories, and associated down-gradient alluvial materials. Nitrate-N levels in down gradient compliance wells had exceeded groundwater standards of 10 mg/L for about two years. Mitigation plans, including a series of pump back wells, failed to halt a rise in groundwater nitrate and arsenic levels.  Following site assessments and successful bench-and pilot-scale testing, in situ treatment methods were developed, EPA and State approvals were obtained, and Contingency Plans were put in place for in situ treatments. Due to the potential to impact sensitive down-gradient trout fishery waters, the treatments were approached in a very conservative manner. Treatments initiated in 2002 had cut nitrate levels by about 50% by 2003 to below treatment target levels; additional treatment applications were required to permanently lower nitrate and arsenic levels to below treatment targets. Treatment goals were met in 2008 and contaminants have remained below target treatment levels. Treatment costs were approximately $0.12/1,000 gallons treated.

In Situ Selenium Stabilization

A high concentration selenium (selenate) plume (measured at levels 10 mg/L to 16 mg/L), was located beneath an old refinery site and was moving toward a sensitive lake environment. A number of wells, including monitoring wells in the heart of the plume and at various intervals around the plume, had be drilled to more fully characterize the extent, concentrations and movement of the plume.
Samples of both contaminated and uncontaminated plume associated alluvial materials were available for testing. Scaled laboratory testing demonstrated that stable selenium reduction of selenate to elemental selenium could be achieved in the alluvial materials through a series of injections of a microbial/nutrient inoculum into various simulated plume alluvial material samples.  Furthermore, selenate concentrations in the heart of the plume, the preferred target area, could be reduced to the desired concentrations.
A full-scale in situ injection design was developed to target the highest selenium concentrations in the plume.  Injections of a microbial/nutrient mix, developed in laboratory testing were scaled and injected into several areas considered to be the heart of the plume. In full-scale application, the initial 2001 test injections into the plume alluvial materials were successful at significantly lowering plume selenium concentrations; selenium concentrations dropped from over 10,000 ug/L to below 200 ug/L, and the elemental selenium formed appeared to be stable in the aquifer materials. Selenium levels dropped to below the target treatment value (1,000 ug/L) in the bulk of the plume center by early 2002. A third and final microbe/nutrient application was completed in 2003. The in situ selenium reduction treatment was considered fully successful.

Hybrid Treatment for Coal Mines

The weathering of coal mine waste rock releases selenium and other co-contaminants, such as sulfate and iron, into the seepage waters. Additionally, various levels of nitrate are released into these waters from residual blasting compounds. Since nitrate is the preferred electron acceptor in nitrate and selenium containing waters, effective management approaches for water treatment should include in situ denitrification treatments to reduce nitrate loads. In situ denitrification treatment also holds promise for significant selenium reduction and stabilization within the source materials, both of which could significantly reduce active treatment CAPEX and OPEX costs.
To reduce treatment costs, especially where higher nitrate concentrations are present, Inotec has proposed a combined nitrate/selenium management strategy for water treatment at coal mining sites. The holistic approach proposed by the Inotec team includes integration of source control measures (in-situ nitrate reduction and selenium stabilization) and active treatment processes for a comprehensive water flow management. In situ nitrate reduction has been successfully implemented at full-scale at several US gold mining sites. For example, amended Electro-Biochemical Reactor (EBR) water treatment system effluents have been used to inoculate waste rock in place and waste rock as it is mined and placed in depositories. This approach has a potential for both immediate and long-term contaminant reductions to active treatment systems, along with reduced active treatment costs.
​A column testing program for coal mining waste rock was designed based on screening results obtained. Four bench-scale columns were constructed: 1) EBR column, treating mine waters, was used to produce an amended microbial inoculum for the in-situ columns; 2) a control, down-flow column filled with coal waste rock source materials was used to determine baseline selenium and nitrate elution rates; and 3-4) two duplicate down-flow columns filled with coal waste rock source materials were inoculated periodically with EBR amended effluents to evaluate in-situ denitrification and selenium reduction/stabilization.
Average nitrate concentration in the control column effluent was 20 mg/L, while it was below the detection limit in the EBR inoculated columns effluents. Average selenium concentration in the control column effluent was 25 ug/L, with initial concentrations as high as 33 ug/L and stabilizing with time at around 20 ug/L. The average Se concentration in the EBR effluent inoculated columns was 2.7 ug/L.