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					Principles & Applications
of BTEX Bioremediation



 Pedro J.J. Alvarez, Ph.D., P.E., DEE




           University of Iowa
Prospectus

 What are BTEX and why care about them?
 What is needed to biodegrade them?
 How to exploit biodegradation for site cleanup?
 What are the more serious technical and political
  challenges related to BTEX bioremediation?
 What is epistemology and how can it help us
  address some of these challenges?
“Water, water everywhere, nor any drop to drink”
 The Rime of the Ancient Mariner, Samuel Taylor Coleridge
          Contaminants of Concern: BTEX

                                  CH3                CH2CH3



          Benzene             Toluene         Ethylbenzene
              CH3                 CH3                CH3
                    CH3

                                        CH3
                                                     CH3
          o-Xylene           m-Xylene            p-Xylene

Importance:
• Relatively high solubility = High migration potential
• Toxicity: Benzene can cause leukemia at 5 µg/l
• Volatile, hydrophobic, biodegradable
      Requirements for Biodegradation

  1. Existence of organism(s) with required
       catabolic potential.

Xenobiotic will be degraded to an appreciable extent only if the
organism has enzymes that catalyze its conversion to a product that is
an intermediate or a substrate for common metabolic pathways.

The greater the differences in structure between the xenobiotic and the
constituents of living organisms (or the less common the xenobiotic
building blocks are in living matter), the less likelihood of extensive
transformation or the slower the transformation.
Requirements for Biodegradation (contd)

  2. Presence of organism(s) in the environment.

BTEX degraders are commonly found, but differences in relative
abundance of dissimilar phenotypes may lead to apparent discrepancies
in the biodegradability of a given BTEX compound at different sites.

Depending on the relative abundance of different strains,
B could degrade earlier than T at one site, but the opposite may be
observed at other sites.
                                               Frequency Analysis of Biodegradation
                                              Capabilities of 55 Hydrocarbon Degraders
                                              100
           % Strains that degraded compound


                                               90
                                               80
                                               70
                                               60
                                               50
                                               40
                                               30
                                               20
                                               10
                                               0
                                                    B   T    E    p-X   m-X   o-X   N

Gülensoy and Alvarez (1999). Biodegradation. 10:331-340
Requirements for Biodegradation (contd)

3. Compound must be accessible to organism:

a)   Physicochemical aspects (bioavailability).
     Desorption, dissolution, diffusion, and mass transport

b)   Biochemical aspects.
     Membrane permeability (important for intracellular enzymes),
     uptake regulation.
Requirements for Biodegradation (contd)

4. If catabolic enzymes involved are not constitutive,
   they must be induced
Inducer(s) must be present above specific treshold (e.g., [T] > 50 mg/L)
     Benzene Degradation by Pseudomonas CFS-215:
          Toluene enhanced enzyme induction


                             Benzene Concentration (mg/L)
                                                            50
                                                                                                  Control

                                                            40


                                                                                                  T=0
                                                            30

                                                                                   T = 0.1 mg/L
                                                            20

                                                                     T = 50 mg/L
                                                            10


                                                            0
                                                                 0         5             10             15
                                                                               Time (days)


Alvarez and Vogel (1991) Appl. Environ. Microbiol., 57: 2981-2985
                 Cometabolic Degradation of o-Xylene
                 by Denitrifying Toluene Degraders
                                                    10

                                                     8

                                                     6                   TOLUENE


                                             mg/l
                                                     4
                                                                                 Active
                                                     2                           Controls

                                                     0
                                                          0   10   20       30   40       50
                                                                        days

                                                    2.0


                                                    1.5

                                                                         o-XYLENE
                                            mg/l




                                                    1.0


                                                    0.5
                                                                                 Active
                                                                                 Controls
                                                    0.0
                                                          0   10   20       30   40       50
                                                                        days

Alvarez and Vogel (1995) Wat. Sci. Technol., 31: 15-28
Requirements for Biodegradation (contd)

5.   Environment conducive to growth of desirable
     phenotypes and functioning of their enzymes:

a)   Presence of “recognizable” substrate(s) that can serve as energy
     and carbon source(s) (e.g., the BTEX) and limiting nutrients
     (N and P, trace metals, etc.).

b)   Moisture (80% of soil field capacity, or 15% H2O on a weight basis,
     is optimum for vadose zone remediation. Need at least 40% of
     field capacity).

c)   Availability of e- acceptors (e.g., O2 for oxidative reactions) or e-
     donors (e.g., H2 for reductive transformations). The e- acceptor
     establishes metabolism mode and specific reactions.
                                                         The electron tower concept


                                          Half Reaction
       EH°´                         Reduction Potential Hierarchy
       volts                        Reduced         Oxidized

-0.50                                                          +
                                    H2                      H
                                                                    Benzene degradation to CO2 and CH4 under methanogenic conditions
                                benzene                     CO2     C6H6 + 4.5 H2O  2.25 CO2 + 3.75 CH4
                                                                    DGo’ = -(30 e-/mol) (96.63 kJ/volt) (-0.24 -(-0.29) volts)
-0.25
                                   CH4                      CO2     DGo’ = -133 kJ/mol of benzene, or -4.5 kJ/e- equiv transferred
                                                                    (barely feasible)
                                   HS-                      SO42-
0                                                                   Benzene degradation to CO2 under aerobic conditions
                                                                    C6H6 + 7.5 O2  6 CO2 + 3 H2O
               Electron Tower




                                                                    DGo’ = -(30 e-/mol) (96.63 kJ/volt) (+0.82 -(-0.29) volts)
                                                                    DGo’ = -3,200 kJ/mol of benzene, or -107 kJ/e- equiv transferred
0.25                                                                (24 x more feasible)



0.50



                                    N2                      NO3-
0.75
                                   H2O                      O2
 Aerobic BTEX Degradation
 BTEX are hydrocarbons (highly reduced) so their Oxidation to CO2 is
  highly feasible thermodynamically (fuel)
 Aerobic BTEX biodegradation is fast (O2 diffusion is often rate-limiting)
 Aerobic BTEX degraders are ubiquitous (e.g., Pseudomonas)
 Need oxygenase enzymes (i.e., enzymes that “activate” O2 and add it to
  carbon atoms in the BTEX molecule)
 The ring must be dihydroxylated before ring fission. Once the ring is
  opened, the resulting fatty acids are readily metabolized further to CO2.
 Anaerobic BTEX Degradation
 Rates are much slower because anaerobic electron acceptors
  (e.g., NO3-, Fe+3, SO4-2, and CO2) are not as strong oxidants as O2.

 Benzene, the most toxic of the BTEX, is recalcitrant under anaerobic
  conditions (i.e., it degrades very slowly – after TEX, or not at all)

 Anaerobic degradation mechanisms are not fully understood.
  Benzoyl-CoA is a common intermediate, and it is reduced prior to ring
  fission by hydrolysis. The oxygen in the evolved CO2 is from water.

                             O       S-CoA
                                 C




 Anaerobic BTEX degradation processes (e.g., denitrifying, iron-
  reducing, sulfidogenic, and methanogenic) are important natural
  attenuation mechanisms.
In aquifers, electron acceptors are used in sequence.
Those of higher oxidation potential are used preferentially:

           O2 > NO3- > Mn+4 > Fe+3 > SO4-2 > CO2
                                    Source: Wiedemeier et al., 1999
Requirements for Biodegradation (contd)

5.   Favorable environment (continued):

d)   Adequate temperature (rates double for ∆T = +10°C).

e)   Adequate pH (6-9).

f)   Absence/control of toxic substances (e.g., precipitation of heavy
     metals, dilution of toxic conc.).

g)   Absence of easily degradable, non-target substrates that could be
     preferentially metabolized (ethanol?).


6.   Time.
     Without engineered enhancement, benzene half-lives on the order
     of 100 days are common in aquifers.
     Want degradation rate > migration rate
 What is Bioremediation?
 It is a managed or spontaneous process in which biological,
  especially microbiological, catalysis acts on pollutants,
  thereby remedying or eliminating environmental
  contamination present in water, wastewater, sludge, soil,
  aquifer material, or gas streams. (a.k.a. biorestoration).
    Ex Situ (Above ground)
    In Situ (In its original place, below ground)
       Engineered Systems
           (biostimulation vs. bioaugmentation)
       Natural Attenuation (intrinsic/passive)
Why Use Bioremediation?
 Can be faster and cheaper (at least 10x less expensive
  than removal & incineration, or pump and treat)
 Minimum land and environmental disturbance (e.g.,
  generation of lesser volume of remediation wastes)
 Can attack hard-to-withdraw hydrophobic pollutants
 Done on site, eliminates transportation cost & liability
 Environmentally sound (natural pathways)
 Does not dewater the aquifer
                   When is engineered bioremediation feasible?


                                                                 Feasibility depends on:
                   2
                             Feasible
                                                                 1) Kh  distribution of nutrients
                   1            with
                                                  Feasible          and e- acceptors (Kh > 10-5 m/s)
                            Enhancement
log k; (per day)




                   0
                                                                 2) Adsorption  bioavailability
                                                                    (depends on Kow and foc,
                   -1
                                                                    problem for PAHs)
                            Not feasible
                   -2
                                                                 3) Potential degradation rate
                                                                    (half life < 10 days)
                   -3
                        6       5         4       3      2   1
                                       - log Kh (cm/s)
                                               Bioventing
        Used to bioremediate BTEX trapped above water table
        Vacuum pumps pull air through unsaturated soil
        Need to infiltrate water (with nutrients) to prevent desiccation




Source: MacDonald and Rittmanm (1993) ES&T, 27(10) 1974-1979
                    Water Circulation Systems
   Used to bioremediate BTEX in saturated zone (Raymond)
   Contaminated water is extracted, treated (air-stripping, activated carbon,
    or biodegradation), and recycled.
   Some is amended with nutrients and reinjected (pulsing is better).
   Clogging near injection well screens and infiltration galleries can be a
    problem (bacterial growth, mineral precipitation) but pulsing reduces
    clogging (may need occasional Cl2, H2O2)
                               Air Sparging
   Injection of compressed air directly into contaminated zone stimulates
    aerobic degradation, strips BTEX into unsaturated zone to be removed
    by vapor-capture system
   Not effective when low-permeability soil traps or diverts airflow
                              Biobarriers
   Containment method that prevents further transport (hydraulic or
    physical controls on groundwater movement may be required to
    ensure that BTEX pass through barrier
   Biologically active zone is created in the path of the plume by injecting
    nutrients and electron acceptors (could use oxygen-releasing
    compounds, or inject compressed air and form an air curtain)




                                                   Treated Water

                                              Air Curtain
             Benzoate addition as auxiliary substrate (1 mg/L)
         stimulated benzene attenuation through 1-D “biobarrier”




                                             200
                   Effluent Benzene (µg/L)




                                                                                             Sterile control
                                             150

                                                                           Not amended
                                             100
                                                               COO-


                                             50
                                                           with benzoate
                                               0
                                                   0   1   2      3   4     5   6        7       8    9    10
                                                                      Time (days)

Alvarez P.J.J., L. Cronkhite, and C.S. Hunt (1988). Environ. Sci. Technol. 1998; 32(5) 634-639
Bioremediation Market

 According to the Organization for Economic Cooperation
  & Development), the global market potential for
  environmental biotechnology doubled in the past 10
  years to $75 billions in the year 2000

 In USA, we have 400,000 highly contaminated sites, and
  NRC estimates the cleanup cost to be on the order of
  $1,000 billions

 In USA, the current bioremediation market is only about
  $0.5 billions
Bioremediation experienced many up- and downturns

 1950’s: Microbial infallibility hypothesis (Gayle, 1952)

 1970’s: Regulatory pressure stimulates development. Adding bacteria
  to contaminated sites becomes common practice. Failure to meet
  expectations (e.g., DDT accumulation) prompts a major downturn.

 1980’s: It becomes clear that fundamental processes need to be
  understood before a successful technology can be designed. This
  realization, along with the fear of liability and Superfund, stimulates the
  blending of science and engineering to tackle environmental problems.

 1990’s: Many bioremediation and hybrid technologies are developed.
  However, decision makers insist on pump and treat, and Superfund is
  depleted. Poor cleanup record and high costs stimulate paradigm shift
  towards natural attenuation and RBCA.
                                     Aerobic Unsaturated Zone


                               Volatilization

                                       Oxygen Exchange


             Dissolution   Anaerobic core       Advection

Aerobic
uncontaminated
                                    Aerobic Processes
groundwater      Mixing, Dilution
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
        Atenuação Natural




                            PE




Fluxo da água subterrânea
Plume



Source
What is Monitored Natural Attenuation?
MNA is the combination of natural biological, chemical and
 physical processes that act without human intervention to reduce
 the mass, toxicity, mobility, volume, or concentration of the
 contaminants (e.g., biodegradation, dispersion, dilution, sorption,
 and volatilization).

Success depends on adequate site characterization, a long-term
 monitoring plan consistent with the level of knowledge regarding
 subsurface conditions at the site, control of the contaminant
 source, and a reasonable time frame to achieve the objectives.

MNA should not be a default technology or presumptive remedy.
 The burden of proof (e.g., loss of contaminants at field scale, and
 geochemical foot-prints) should be on proponent, and evidence
 of its effectiveness should emphasize biodegradation.
Plume Dimensions Reflect Natural Attenuation
                      MEDIAN PLUME DIMENSIONS

         BTEX Plumes
         (604 Sites)
                                                      132 ft
                                                      1000 ft
         TCE Plumes
         (88 Sites)



        Other chlorinated
        solvent plumes                                500 ft
        (29 Sites)


        Salt Water Plumes
                                                      700 ft
        (chloride)
        (25 Sites)


                             0   200   400    600   800   1000
                                             Feet
Concentration




“safe”
                What is Risk-Based Corrective Action?
                Clean source only to a level that will result in an acceptable risk
                 at the potential receptor’s location (e.g., property boundary)
                Need a mathematical model to calculate the required Co



                                                                             receptor

                 Co =?
Concentration




                                                                               “safe”
     Analytical Solution of the Advection-Dispersion-Sorption
 Equation with First-Order Decay, for Constant Rectangular Source
                         (Domenico, 1987)


            Co   x - vt (1+ 4l a x / v)   ( y + Y / 2)
                                                                          ( y -Y / 2)   (Z ) 
                                                                                                                (-Z )       
                                          1/ 2
 C(x, t) =  erfc                            erf              - erf               erf          - erf               
            8          2(a x vt)
                                    1/ 2
                                                 2 (a y x) 
                                                
                                                             1/ 2
                                                                          2(a y x)   2 (a z x) 
                                                                          
                                                                                    1/ 2
                                                                                          
                                                                                                    1/ 2
                                                                                                                 2(a z x) 
                                                                                                                 
                                                                                                                           1/ 2
                                                                                                                                
                          x   4l a 1 / 2 
                                                 
                     exp 
                                1- 1+
                                 
                                            x
                                                 exp[- k s (t - x / v)]
                          2a x  
                                        v  




 Models are useful analytical tools, and can be used to demonstrate
  that natural attenuation is occurring

 Limited predictive capability (order-of-magnitude accuracy):
  groundwater flow and microbial behavior rarely follow simplifying
  assumptions.
                                    Sensitivity Analysis:
        Effect of Doubling a Variable on Plume Length (Lp)

                            Variable      Baseline Value                                            DLp (%)
                               l (day-1)         0.0005                                                       -24
                               Co (ppb)          25,000                                                        +7
                               Z (m)                3                                                          +7
                               Y (m)               10                                                          +7
                               ax (m)              10                                                          -1
                               foc                 0.01                                                       -17
                               n                    0.3                                                       +17
                               b (g/cm3)          1.86                                                       -17
                               Vw (m/day)         0.044                                                       +33

Lovanh, N., Y.-K. Zhang, and P.J.J. Alvarez (1999). Proc. 6th International Petroleum Environmental Conference, Houston, TX.
              How variable are biodegradation rates in the field, and
              What are “reasonable” parameters for RBCA models?
                                    Frequency Distribution for l (n=79)



                              100                           Mean = 0.0112 day -1
                    Density




                                                            Median = 0.005 day-1
                                                                      (t1/2 = 139 days)

                              50




                               0

                                     0.00                         0.05                            0.10

                                                               l (day-1)
Lovanh, N., Y.-K. Zhang, and P.J.J. Alvarez (1999). Proc. 6th International Petroleum Environmental Conference, Houston, TX.
      Current Status of Bioremediation
 We have made significant advances towards understanding the
  biochemical and genetic basis for biodegradation. However,
  bioremediation is still an underutilized technology.

 Bioremediation is not universally understood, or trusted by those who must
  approve it. To take full advantage of its potential, we need to communicate
  better the capabilities and limitations of bioremediation, and answer:

    What is being done in the subsurface, Why, How, and Who is doing what?

    How fast is it being done, and can we control it and make it go faster?

    When can we meet cleanup standards in a cost-effective manner?

    Can we reasonably predict that what we want to happen, will happen?
 EPISTEMOLOGY OF BIOREMEDIATION
  episteme = knowledge
  Theory of the method and basis we use to acquire knowledge, including the
  possibility and opportunity to advance fundamental understanding, sphere of
  action, and the philosophy of the scientific disciplines that we rely upon.

  Reductionism:
 System analysis through separation of its components
  (eliminates complexity to enhance interpretation).
 Based on the premise that a system can be known by studying its components,
  and that an idea can be understood if we understand its concepts separately.
 Used increasingly in bioremediation research to investigate mechanisms.

  Holism:
 The totality of a system is greater than the sum of its parts
  (synergism & antagonism)
         Epistemology’s Uncertainty Principle
 Reductionism simplifies the system, enhances hypothesis testing, and interpretation
 It also augments lab artifacts and hinders the relevance of the information we obtain

                  High                                                High




                  Low                                                 Low
                           Holism                  Reductionism
           Scale:        Field   Microcosms        Cells      Extracts      Genes
      Disciplines:       Ecology
                         Biogeochemistry
                                 Physiology
                                                      Biochemistry
                                                              Genetics
                                                            Molecular Biology
Implications
  Quantitative extrapolation from the lab to the field is taboo.
   (interpolate but do not extrapolate)

  Rely more on holistic disciplines (e.g., ecology,
   biogeochemistry) and iterate more between the field and
   the lab, between basic and applied research.

  Multidisciplinary Research (interstices)

  Aurea mediocridad (San Ignacio de Loyola)
 Pay attention to detail. You never know who is watching your
 work, and where your next promotion or demotion will come from.




 Bioremediation is seldom a straight line to an imagined goal (many branching
  decision points requiring flexibility and versatility)
 Remedial technologies are rapidly evolving. Be committed to life-long learning, and
  be aware that imagination and creativity could more important than knowledge
 Conclusions
Indigenous microorganisms can often destroy BTEX and
 other common groundwater contaminants, making
 bioremediation (often) technically feasible.
The pendulum recently swung towards natural attenuation.
 This can save money but take much longer to achieve
 cleanup and appear as if officials are walking away from
 contaminated sites. Early public involvement is critical to
 minimize such controversy.
Lets Take a Break!
                    TYPES OF MICROBES USED
A. Indigenous Microorganisms
      Used in most applications (99%)
      Pseudomonas have wide catabolic capacity
      May need to enhance proliferation/enzyme induction

B. Acclimated Strains
      Preselected naturally occurring bacteria
      Generally not needed for BTEX
      Often fail to function in situ; common reasons:
              - Conc. of target compound too low to support growth
              - Other substances and organisms inhibit growth
              - Microbe uses other food than target contaminant
              - Target compound not accessible to microbe

C. Genetically Engineered Microbes (GEMs)
     Could combine desirable traits from different microbes:
              - Ability to withstand stress & degrade recalcitrant compounds
              - Not needed for BTEX, many technical & political constraints
How does one prove biodegradation is occurring in situ?
1. Document loss of contaminants at field scale.

 Show that decrease in concentrations is not solely the result of plume migration
  and dilution. e.g., show that contaminant flux decreases along flowpath, using
  well transects.

                            Flux
                            =vC                            Multilevel
                                                           Well
                                                           Cluster




2. Geochemical indicators to demonstrate indirectly the type of degradation
   processes active at the site.
   Look for O2, NO3-, and SO42- levels below background in the core of the plume,
    and Fe(II) and CH4 levels above background. Also, higher CO2 and alkalinity.

   H2 levels can reflect dominant redox processes:
             0.1 nM            denitrification
             0.2 – 0.8 nM      Fe(III) reduction
             1.0 – 4.0 nM      sulfate reduction
             >5.0 nM           methanogenesis
 Stable Isotope Analysis:

Carbon atoms in BTEX are mainly 12C with some heavy isotopes (13C)

Isotope fractionation results because light ( 12C) isotope bonds are
preferentially biodegraded compared to heavy isotope ( 13C) bonds

No isotope fractionation results from abiotic processes (dilution,
sorption, etc.).

Thus, biodegradation results in isotopical depletion of 13C for dissolved
inorganic carbon and daughter products (13C = -20 to –30 per mil), and get
and enrichment of 13C for residual contamination of parent compound.


3. Laboratory or in situ microcosms showing BTEX degradation
   (look for mineralization or accumulation of metabolites)
  Análisis de varianza de las interacciones
                   BTEXN
 Las capacidades de degradación fueron mas amplias
  cuando los BTEXN fueron alimentados como mezcla
  que separadamente (particularmente cuando el T
  estaba presente)

 Las interacciones negativas (e.g., inhibición
  competitiva, toxicidad) fueron estadísticamente
  significativas cuando se alimentó 1 mg/L a cada una.

 Por estadística de Kappa se encontró una correlación
  significativa entre las habilidades para degradar T y E,
  p-X y m-X, y p-X y o-X. La falla de degradar B fue
  correlacionada con la inhabilidad para degradar o-X.
Specific degradation rate          Monod’s Equation

                            k
         dC/dt/X




                            k            dC    kCX
                                            =-
                            2            dt    KS + C



                                KS Contaminant Concentration, C
Why First-Order Degradation Rates?
     Monod’s Equation, When C << KS

           dC= - k XC  - k X
                   +C         C
           dt KS          KS

               dC= - lC
               dt

                   kX
                l= K
                    S
                (not constant)
Also, Mass Transfer Limitations Are Conducive to
       First-Order Kinetics (even if C > Ks)


                                 qt = k Cbulk - Ccell 
                                                                .
                                                                0
                                                                1
 concentration




                                                                .
                                                                8
                                                                0

                                                                .
                                                                6
                                                                0




                                                           q/ max
                                                                    .
                                                                    4
                                                                    0

                                                                    .
                                                                    2
                                                                    0

                                                                                 0
                                                                                 3
                 qdeg = qmax
                                    Ccell                           .
                                                                    0
                                                                    0
                                                                                1
                                                                                -
                                                                                5
                                                                                2
                                                                                 ]
                                                                                sd
                                                                                1a
                                                                                -e
                                                                               3b
                                                                      5
                                                                      1        0
                                                                               2
cell                           K1 / 2 + Ccell                                 15
                                                                     Cm
                                                                      []
                                                                       M
                                                                        10
                                                                             -m
                                                                             10
                                                                              4
                                                                            [1
                                                                      b
                                                                      ul
                                                                       k 5
                     distance                                              0
                                                                           k
                                                                           a
                                                                           e
                                                                           b
                                                                            1
                                                                            0
                                                                            d
                                                                             0
                                                                             5
Alta concentración microbiana = Taza más rápida

                                              Simulaciones empleadas:
                          80                  k = 0.28 g-T/g-células/día
                                              KS = 8.6 mg-T/L
         TOLUENO (mg/L)
                                              Y = 0.6 g- células/g-T
                          60
                                   107 células/mL

                          40

                                        102 células/mL
                          20


                          0
                               0   30    60        90      120       150
                                        Tiempo (Días)
                ¿Por qué es tan difícil limpiar acuíferos?




Detectar la contaminación en aguas subterráneas es como buscar una aguja en un pajar. Los puertos
de muestreo pueden ser demasiado profundos, no muy profundos o en un lugar equivocado.

				
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